Genetic Determinants of Psoriatic Arthritis

Transcription

1 Genetic Determinants of Psoriatic Arthritis by Vinod Chandran A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Institute of Medical Sciences University of Toronto Copyright by Vinod Chandran 2013

2 Genetic Determinants of Psoriatic Arthritis Vinod Chandran Doctor of Philosophy Institute of Medical Sciences University of Toronto 2013 Abstract Psoriatic Arthritis (PsA) is an inflammatory arthritis associated with psoriasis that leads to progressive joint damage. Genetic variants in the Major Histocompatibility Complex (MHC) region on human chromosome 6p, especially Human Leucocyte Antigen (HLA), are the most important genetic risk variants associated with susceptibility to PsA. I aimed to investigate the heritability of PsA and to determine the association between HLA polymorphisms with susceptibility and severity of PsA. I first validated the new CASPAR classification criteria for PsA in patients in with both early and established disease. Subsequently, I demonstrated that PsA as defined by the CASPAR criteria has a high recurrence risk ratio. In a large case-control and family-based association study, I demonstrated that the class I HLA alleles, HLA-C*12/B*38, - B*27 and -C*06/B*57 are associated with increased susceptibility to PsA. HLA class I molecules biologically interact with Killer-cell Immunoglobulin-like Receptors (KIR) on Natural Killer (NK) to influence immune response. I demonstrated that KIR2DS2 and HLA C group 2 and HLA-B Bw4 were associated with PsA susceptibility. Further analyses of PsA cases with Type II psoriasis and with dactylitis suggested that HLA-C*02, -B*27, -B*38 and KIR2DS2 may be markers of musculoskeletal manifestations of PsA. Furthermore, using longitudinal data, I demonstrated that HLA-B*39, -B*27, -A*02 and KIR3DS1 are associated with peripheral joint ii

3 damage progression whereas the alleles DQB1*0604, -C*04 and B*60 are associated with less damage progression. HLA-C*02, -C*12, -DQB1*0609 and KIR2DS1 are associated with higher risk of sacroiliitis, and HLA-B*27 with syndesmophytes. HLA-A*29 was associated with reduced risk of development of both sacroiliitis and syndesmophytes. These studies indicate that HLA alleles and KIR genes are important in PsA susceptibility and severity and suggest that CD8+ T cells and NK cells that modulate the innate and adaptive immune response play an important role in the susceptibility and severity of PsA. iii

4 Acknowledgments At the outset I would like to acknowledge the support and guidance provided to me by my supervisor Dr. Dafna Gladman and my program advisory committee members Dr. Shelley Bull and Dr. Robert Inman. Their insightful and critical comments and suggestions have helped me greatly in directing my research efforts and have led to fruitful completion of the project. The tremendous support provided by colleagues at the University of Toronto Psoriatic Arthritis program including Fawnda Pellett, Renise Ayearst and Arane Thavaneswaran as well as fellow graduate students Remy Pollock and Lihi Eder is greatly appreciated. I would like to thank my collaborators, especially Dr. Vern Farewell (MRC Biostatistics Unit, Cambridge, UK), Dr. Brian Tom, (MRC Biostatistics Unit, Cambridge, UK), Dr. Richard Cook (University of Waterloo) and Dr. Proton Rahman (Memorial University of Newfoundland) for their invaluable support. The administrative support provided by Anne Mackinnon is greatly appreciated. I would also like to acknowledge the help provided by the administrative staff at the Institute of Medical Science. Of course, none of this would have been possible without the participation of patients with psoriatic arthritis. Their enthusiasm and willingness to participate in a longitudinal cohort study is the most important contributor to the success of my research project. Lastly, but most importantly, I would like to thank my partner in life, Shelly, for all the encouragement and support that she has provided without which completion of my graduate program would not have been possible. To her I dedicate this work. iv

5 Table of Contents Chapter Section Title Page Abstract Acknowledgements List of Tables List of Abbreviations List of figures ii iv ix xiii xv 1 Introduction Abstract Background Case definition Clinical features Radiographic features Descriptive epidemiology Study design in genetic epidemiology Genetic Epidemiology in Psoriatic Arthritis Rationale 24 2 Sensitivity of the Classification of Psoriatic Arthritis Criteria in Early Psoriatic Arthritis Abstract Introduction Patients and Methods Results 30 v

6 2.5 Discussion 31 3 Sensitivity and Specificity of the CASPAR Criteria for Psoriatic Arthritis in a Family Medicine Clinic Setting 36 4 Familial aggregation of psoriatic arthritis Abstract Introduction Methods Results Discussion 46 5 Human Leukocyte Antigen alleles and susceptibility to Psoriatic Arthritis Abstract Introduction Methods Results Discussion 62 6 Killer-cell Immunoglobulin-like Receptor gene polymorphisms and susceptibility to Psoriatic Arthritis Abstract Introduction Methods Results Discussion The association between HLA and KIR polymorphisms and Psoriatic Arthritis sub-phenotypes 107 vi

7 7.1 Abstract Introduction Methods Results Discussion Association of HLA and KIR Polymorphisms with Progression of Peripheral Joint Damage in Psoriatic Arthritis Abstract Introduction Methods Results Discussion Association of HLA And KIR Polymorphisms with the Development of Axial Arthritis in patients with Psoriatic Arthritis Abstract Introduction Methods Results Discussion Discussion Summary of findings and implications Limitations and considerations Future directions 169 vii

8 10.4 Concluding remarks 170 Bibliography 172 viii

9 List of Tables Chapter Table no. Table name Page The CASPAR criteria KIR and their ligand specificity Demographic and disease characteristics of study patients 2.2 Number of patients satisfying individual items of the CASPAR criteria Demographic and disease characteristics of all study participants (N=175) Demographic and disease characteristics of the 100 probands and their first degree relatives with psoriatic arthritis. 4.2 Recurrence risk ratio for Psoriatic Arthritis and psoriasis without arthritis in first degree relatives of probands with Psoriatic Arthritis Summary of results of HLA association studies in PsA published in the last 25 years on subjects of European descent 5.2 Summary of the results of the Principal Component Analysis showing the 24 outliers (6 cases, 18 controls) 5.3 Demographic and disease characteristics at first clinic visit of cases included in the case control study 5.4 Results of the univariate analyses investigating the association between HLA alleles and psoriatic arthritis. 5.5 Results of the haplotype analyses to evaluate the association between HLA -C, B haplotypes and psoriatic arthritis ix

10 5.6 Results of the haplotype analyses to evaluate the association between HLA C, B, DR haplotypes and psoriatic arthritis. 5.7 Results of the haplotype analyses to evaluate the association between HLA -A, C, B, DRB1, DQB1 haplotypes and psoriatic arthritis. 5.8 Results of the multivariate logistic regression analysis to determine independent association between HLA alleles and PsA case-control status 5.9 Demographic and disease characteristics at first clinic visit of the 283 probands with psoriatic arthritis included in family-based association study 5.10 Results for the family-based analyses of the association between HLA alleles and the presence of psoriatic arthritis 5.11 Summary of the comparisons between case-control and family-based association tests for the association between HLA alleles and PsA Results of the univariate analyses assessing the association between KIR gene polymorphisms and PsA. Bold script indicates statistical significance (p<0.05) 6.2 Results of the joint analysis assessing the association of KIR2DS2 polymorphisms and HLA-C group 1 alleles with PsA 6.3 Results of the joint analysis assessing the association of KIR2DS1 polymorphisms and HLA-C group 2 alleles with PsA Demographic and disease characteristics at first clinic visit of PsA patients with type I and type II psoriasis included in the case control study 7.2 Results of the univariate analyses of the association between HLA alleles and PsA with type II psoriasis 7.3 Results of the univariate analyses demonstrating association between KIR genes and the risk of having psoriatic arthritis with type II psoriasis x

11 7.4 Results of the joint analysis of KIR2DS2 and HLA-C group 1 alleles and the risk of having psoriatic arthritis with type II psoriasis 7.5 Demographic and disease characteristics at first clinic visit of PsA patients with and without dactylitis 7.6 Results of the univariate analyses of the association between HLA alleles and dactylitis in patients with PsA 7.7 Results of the joint analysis of KIR2DS2 and HLA-C group 1 alleles and the risk of having dactylitis Demographic and disease characteristics at first clinic visit of 649 PsA patients included in the study 8.2 Results of univariate analyses of the association between HLA alleles and clinical joint damage progression in psoriatic arthritis, adjusted for sex and age at diagnosis of PsA 8.3 Results of the multivariate analysis of the association between HLA alleles and clinical joint damage progression in psoriatic arthritis, adjusted for sex and age at diagnosis of PsA 8.4 Results of the univariate analyses of the association between KIR genes and clinical joint damage progression in psoriatic arthritis 8.5 Results of the analyses of the association between combined HLA and HLA-KIR genotypes and clinical joint damage progression in psoriatic arthritis Demographic and disease characteristics at first clinic visit of 633 PsA patients included in the study 9.2 Results of the univariate analyses of the association between HLA alleles and each of time to development of sacroiliitis and time to development of at least 1 syndesmophyte, adjusted for sex and age at diagnosis of PsA 9.3 Results of the multivariate analyses of the association between HLA alleles and time to development of sacroiliitis adjusted for sex and age at onset of xi

12 psoriatic arthritis 9.4 Results of univariate analyses of the association between KIR gene polymorphisms and each of time to development of sacroiliitis and time to development of at least 1 syndesmophyte, adjusted for sex and age at diagnosis of PsA Summary of the association analyses between PsA (and its sub-phenotypes) and HLA and KIR gene polymorphisms xii

13 List of Abbreviations AIC AS CASP CASPAR CD CT DAP DMARD DNA ELISA ESR ESSG FDR FDR FBAT GEE GWAS HIV HLA HNPCC HR ITAM ITIM KIR LD LILR LIR Akaike information criterion Ankylosing Spondylitis Collaborative Association Study of Psoriasis Classification of Psoriatic Arthritis Cluster of Differentiation Computerized Tomography DNAX activation protein Disease Modifying Anti-Rheumatic Drug Deoxyribonucleic Acid Enzyme-linked Immunosorbent Assay Erythrocyte Sedimentation Rate European Spondyloarthropathy Study Group False Discovery Rate First Degree Relatives Family-Based Association Test Generalized Estimating Equation Genome-Wide Association Study Human Immunodeficiency Virus Human Leukocyte Antigen Hereditary Non-Polyposis Colorectal Cancer Hazard Ratio Immunoreceptor tyrosine-based Activation Motif Immunoreceptor Tyrosine-based Inhibition Motif Killer-cell Immunoglobulin-like Receptor Linkage Disequilibrium Leukocyte Immunoglobulin-like Receptors Leukocyte Immunoglobulin-like Receptors xiii

14 LOD MHC MODY MRI NCR NK OR PASI PCR PsA PSORS RA RF SNP SpA SSO SSP TDT TNFi WHO Logarithm of the Odds Major Histocompatibility Complex Maturity-Onset Diabetes of Young Magnetic Resonance Imaging Natural Cytotoxicity Receptors Natural Killer Odds Ratio Psoriasis Area and Severity Index Polymerase Chain Reaction Psoriatic Arthritis Psoriasis Susceptibility locus Rheumatoid Arthritis Rheumatoid Factor Single Nucleotide Polymorphism Spondyloarthritis Sequence-Specific Oligonucleotide Sequence Specific Primer Transmission Disequilibrium Test Tumour Necrosis Factor Inhibitor World Health Organization xiv

15 List of Figures Chapter Figure Title Page A model of the hierarchy of HLA-KIR interactions and the risk of inflammatory disease used to build statistical models Flow chart showing recruitment of study subjects Plot showing the distribution of cases (blue dots) and Controls (green dots) when plotting Eigenvalue 1 (x-axis) vs. 2 (y-axis) after outlier removal. 5.2 Plot showing the distribution of cases (blue dots) and controls (green dots) when plotting Eigenvalue 2 (xaxis) vs. 3 (y-axis) after outlier removal xv

16 Chapter 1 Introduction 1.1 Abstract Psoriatic arthritis (PsA) is a spondyloarthritis associated with psoriasis that has recently been better defined according to the CASPAR classification criteria. This chapter provides an overview of the descriptive epidemiology, case definition, the varied clinical and radiologic features as well as the course and prognosis of PsA. After a review of genetic epidemiology methods for investigating complex diseases, results of familial aggregation studies and genetic linkage and association studies on susceptibility to PsA are described with particular reference to studies investigating HLA and KIR polymorphisms. Results from recent genome-wide association studies are reviewed. Association studies that have identified genetic markers associated with specific disease manifestations of PsA are described. The chapter provides background information to the reader to set the stage for reviewing subsequent investigations on the genetics of PsA conducted as part of my graduate program that aims to- 1) validate the newly developed CASPAR criteria for classification of PsA in early disease; 2) quantitate the magnitude of the genetic component of PsA; 3) test hypotheses concerning association of HLA and KIR polymorphisms with susceptibility to PsA; and 4) test hypotheses concerning association of HLA and KIR polymorphisms with joint damage progression in PsA. 1.2 Background The spondyloarthritides (SpA) are a group of inflammatory rheumatic diseases that have common features including absence of rheumatoid factor, absence of subcutaneous nodules, peripheral as well as axial arthritis, radiological sacroiliitis, mucosal and skin inflammation and a tendency for familial aggregation (Zeidler H, Armor B, 2011). Psoriatic Arthritis (PsA) is a SpA that is associated with psoriasis, a chronic inflammatory skin disease (Gladman DD, 2009a). 1

17 Psoriasis itself has a strong genetic predisposition. PsA affects musculoskeletal structures such as axial and appendicular joints, entheses, and tendon sheaths. Eyes and mucous membranes are often involved. PsA thus has varied manifestations; a case definition is therefore difficult. The co-occurrence of cutaneous psoriasis with arthritis was first reported by the pioneering French dermatologist Baron Jean-Louis Marc Alibert in the year 1818 (O Neill T and Silman AJ, 1994). However, it was only in 1964 that the American Rheumatism Association (ARA) - now the American College of Rheumatology (ACR) - recognized this condition as a distinct clinical entity. John Moll and Verna Wright from Leeds, United Kingdom, provided a disease definition and described five disease patterns (Moll JM, Wright V, 1973b). They also carried out the seminal genetic epidemiologic study proving a genetic basis for PsA (Moll JM, Wright V, 1973a). With the advent of therapy with TNF inhibitors (TNFi), it was realized that the earlier disease definition and assessment tools were inadequate. Subsequently, researchers with an interest in PsA successfully formed the ClASsification of Psoriatic ARthritis (CASPAR) Study Group to work towards a better case definition. This group was then expanded to include dermatologists, methodologists and geneticists to form a larger international research group the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA) - leading to new developments in assessment and treatment of PsA (Mease PJ, Gladman DD and Krueger GG, 2005). PsA has a major impact on the patient as well as the society. Patients with PsA develop progressive joint damage as well as decline in physical function in spite of optimizing therapy (Siannis F et al 2006; Bond SJ et al 2007). The annual health care costs associated with PsA are considerable. In the United States, the mean annual direct cost per patient with PsA was estimated at $3,638 in (Williams JP, Meyers JA, 2002). In Germany, the mean annual direct cost per patient with PsA is estimated as 3,162, with the mean indirect cost per patient of 11,075 (Huscher D et al, 2006). In the UK, the total mean annual health care costs ranged from 11 to with a mean of 1446 (Poole CD, 2010). The annual health care cost is higher in those with severe disease. Patients with PsA have reduced quality of life and 2

18 increased work-related problems, health care resource utilization and comorbidities compared to those with PsC (Christophers E et al 2010, Husted JA et al 2011). They have an increased mortality risk compared to the general population (Wong K et al 1997; Ali Y et al 2007). Disease severity at presentation is a risk factor for mortality (Gladman DD et al, 1998b). Moreover, both health-related quality of life and function of patients with PsA are reduced compared to the general population (Husted JA et al, 2001; Husted JA et al, 2007) Case definition Despite the recognition of PsA as a distinct disease by the ARA in 1964, research into this disorder was confounded by the absence of a widely agreed-upon or validated case definition. The most acceptable early definition for PsA was provided by Moll and Wright in 1973 (Moll JM, Wright V, 1973b). The Moll and Wright definition was itself modified from that originally proposed by Wright in 1956 and defined PsA as psoriasis associated with inflammatory arthritis (peripheral arthritis and/or spondylitis) and usually a negative serologic test for rheumatoid factor (Wright V, 1956). Rheumatoid factor is a serologic marker for rheumatoid arthritis (RA); thus, the definition was probably meant to help distinguish PsA from RA, which at that time was a more recognized form of inflammatory arthritis. Subsequently, a number of research groups have proposed classification criteria, but none were universally accepted (Bennett RM 1979; Gladman DD et al, 1987; Vasey F, Espinoza LR, 1984; Dougados et al, 1991; McGonagle D et al, 1999; Fournie B et al, 1999). The CASPAR study group conducted a large multi-centre study to develop universally acceptable classification criteria. Data were collected prospectively worldwide by clinicians who were particularly interested in PsA. Using this data a new set of criteria for classification of PsA was developed (Taylor W et al, 2006) (Table 1.1). The CASPAR criteria had specificity of 98.7% and sensitivity of 91.4% in the original study (Taylor W et al, 2006). Using these criteria it is possible to classify patients even when they do not have current, past, or family history of psoriasis. However, these criteria were developed using data from patients with well-established disease. The mean disease duration was

19 years; only 5.3% of the patients had disease duration of less than a year. Therefore, it may not be appropriate to directly apply the criteria to patients with recent-onset disease without further validation. Thus the new CASPAR Classification criteria for PsA have to be further validated, especially in early PsA, before use for case definition in epidemiologic and genetic studies in PsA Clinical Features The CASPAR classification criteria define PsA as an inflammatory musculoskeletal disease that involves joints, spine or entheses. Clinical features of PsA include peripheral arthritis, axial arthritis (sacroiliitis and spondylitis), enthesitis, dactylitis and tenosynovitis. Typically early PsA is oligoarticular (affecting less than 5 joints). Early PsA often involves the joints of the lower limbs, but any joint of the body may be affected. Oligoarticular PsA may evolve into polyarthritis. The distribution of the joint involvement in PsA is often asymmetric. However, as the number of joints affected increases, there is a tendency towards symmetry (Helliwell PS et al, 2000). In addition to peripheral arthritis, PsA affects the axial joints in 30 to 50% of patients with PsA. Axial inflammatory arthritis presents with inflammatory back and/or neck pain, typically associated with stiffness worse after periods of prolonged inactivity such as night-time sleep. However, evidence of radiographic involvement of the axial joints may be present in a substantial proportion of patients presenting with peripheral PsA in the absence of axial symptoms (Chandran V et al, 2009a). Axial arthritis leads to restriction in the mobility of the spine. The process can lead to a completely fused and immobile spine, sometimes called bamboo spine. Dactylitis, defined as inflammatory swelling of an entire finger or toe, is a characteristic manifestation of PsA (Taylor W et al, 2006). This is due to inflammation of the joints, tendons, bones and soft tissues in the digit(s). Persistent dactylitis leads to destruction of the joints in that digit. Dactylitis is thus a marker of severity of PsA (Brockbank JE et al, 2005). Enthesitis, defined as inflammation at the entheses (sites where ligaments or tendons attach to bone), is 4

20 another important manifestation of PsA (Gladman DD et al, 2007a). The most common sites to be affected by enthesitis are the plantar fascia on the sole of the feet (plantar fasciitis) and Achilles tendon insertion at the back of the heel (Achilles enthesitis). Enthesitis can also affect other sites including tendon insertion sites on the patella, shoulder, elbows, pelvis, spinous processes and chest wall (Gladman DD et al, 2007a). Tenosynovitis, or inflammation of the tendon sheath, may affect tendons in the hands, wrists and around the ankles. Five patterns of PsA were originally described by Moll and Wright. These include: 1. Asymmetric oligoarthritis, 2. Symmetric polyarthritis similar to rheumatoid arthritis, 3. Spondyloarthritis, 4. Distal interphalangeal joint arthritis, and 5. Arthritis mutilans. The category of asymmetric oligoarthritis includes those patients with four or less joints affected by arthritis. The joints involved are usually of the lower limbs and there is lack of symmetry. In the category of symmetric polyarthritis, five or more joints are involved in a symmetric fashion. Therefore, it is sometimes difficult to distinguish it from RA. The spondyloarthritis category includes patients with predominant involvement of the spine (axial arthritis), similar to the involvement in patients with AS. As the name suggests, the category of distal interphalangeal joint arthritis includes those patients with predominant involvement of the DIP joints. Arthritis mutilans describes a category with severe arthritis leading to shortening and destruction of fingers and toes. Longitudinal studies have shown that these patterns change over time (Khan M et al, 2003; Kane D et al, 2003a; Mchugh NJ et al, 2003). The longer the disease duration, the higher the number of joints involved, and the more symmetric becomes the joint involvement. Distal joint involvement also is common and is seen in all categories. Arthritis mutilans is a manifestation of severity of the arthritic process and may not be an exclusive category. Therefore, experts nowadays tend to classify the disease as peripheral arthritis alone, peripheral arthritis with axial arthritis, and axial arthritis alone (Taylor WJ et al, 2005). 5

21 1.2.3 Radiologic features of PsA Musculoskeletal imaging is an important modality in the assessment of patients with PsA. Imaging complements clinical assessment and helps in confirming the diagnosis as well as in determining disease severity, since radiographic damage is associated with reduced physical function and disability. The various modalities used in assessment of PsA include X-rays, ultrasound, computerized-tomography scan (CT scan), magnetic resonance imaging (MRI) and bone scan. However, plain radiographs are the mainstay in determining disease progression and severity (Gladman DD and Chandran V, 2011c). Plain radiographs are the mainstay of radiologic assessment of PsA. Plain radiographs are relatively cheap, easily available and can be read by most physicians. Once PsA is suspected clinically, radiographs of the hands, feet, pelvis, spine and other affected joints are done to look for manifestations suggestive of PsA. Radiological evidence of juxta-articular new bone formation (excluding osteophyte formation) is the only manifestation that differentiates PsA from other inflammatory arthritides and is one of the CASPAR criteria (Taylor W et al, 2006). Plain radiographs are also used to assess disease severity, as well as to follow disease progression. In early disease, radiographs of the hands and feet show soft tissue swelling around the joints involved. If dactylitis is present, soft tissue swelling will involve the whole finger or toe. In more severe disease, erosions develop near the joint margin and are markers of disease severity. Erosions may be paramarginal, in contrast to RA were the erosions are usually marginal. In PsA erosions are often accompanied by new bone formation. The combination of erosions and new bone formation at joint margins is characteristic of PsA. Following the development of erosions there may be joint space narrowing, and finally total joint destruction may occur, either total joint lysis and the so called pencil-in-cup change or complete bony bridging through the joint termed ankylosis. Plain radiographs of the pelvis often show changes reflecting the presence of sacroiliitis. The earliest notable changes include widening of the joint space, which is often difficult to 6

22 appreciate. Subsequently, erosions develop, followed by sclerosis and subsequent bony bridging across the joints, ultimately leading to complete fusion of the joint. Plain radiographs of the neck and the back often show changes that reflect consequences of inflammation at the spinal joints. The earliest changes on lateral view of the spine include shiny vertebral corners, erosions and squaring of vertebrae. This is followed by bone bridging, or marginal syndesmophytes, beginning from the ends of the vertebrae across the disc space. Complete bony bridging can occur. If most of the vertebrae are bridged, it is called bamboo spine. These changes closely resemble the changes in AS. Often in PsA, the syndesmophytes can develop from sites away from the vertebral body. The presence of these non-marginal syndesmophytes is characteristic of PsA. The changes described can at occur at any site- cervical and lumbar vertebrae are frequently involved. In the cervical spine, occasionally atlanto-axial subluxation may be present and can lead to serious consequences, including sudden death from cervical cord compression. PsA has a variable course and prognosis. While some patients do well when a small number of joints are involved with no significant damage, others progress very quickly and develop marked joint damage and disability (Gladman DD, 1994). Gladman et al have shown that after over 10 years of follow up, 55% of the patients have at least 5 clinically damaged joints (Gladman DD et al, 1990b). By the time patients present to a PsA clinic, 67% have at least one erosion. Kane et al have shown that in early disease, out of 129 patients seen within 5 months of onset of symptoms, 47% developed erosions within the first 2 years (Kane D et al, 2003b). Factors associated with progression of joint damage include the number of actively inflamed and damaged joints at presentation, the ESR at presentation, and the number of actively inflamed joints at each visit (McHugh et al, 2003; Gladman DD et al, 1995b; Torre Alonso JC et al, 1991; Bond SJ et al, 2007). On the other hand, there are patients with PsA who achieve remission, defined as no actively inflamed joints for 12 months (Gladman DD et al, 2001). In an observational cohort study 17.6% of the patients achieved remission. The period of remission lasted for 2.6 years, after which 52% of the patients flared. 7

23 1.3 Descriptive epidemiology There are a number of challenges in conducting epidemiological studies in PsA. The most important problem, until recently, was the lack of validated classification criteria. Most epidemiological studies have used the co-occurrence of psoriasis and (inflammatory) arthritis or the European Spondyloarthropathy Study Group (ESSG) criteria to identify cases of PsA (Dougados M et al, 1991; Alamanos Y et al, 2008). Use of these criteria is not appropriate because even if patients with inflammatory arthritis were correctly identified, not all patients with psoriasis and inflammatory arthritis have PsA. Moreover, the ESSG criteria have poor sensitivity for PsA (Taylor W et al, 2006). The first reported population-based study was conducted by Hellgren in Sweden; an association between psoriasis and rheumatoid arthritis was shown and the population prevalence was 0.02% (Hellgren L, 1969). A subsequent survey of 3659 subjects greater than or equal to 20 years of age in the Netherlands showed a prevalence of 0.05% (van Romunde LK et al, 1984). Moll and Wright (1976) estimated the population prevalence as 0.1% in England. A similar estimate was obtained in a survey from France using a telephone questionnaire followed by physical examination (Saraux A et al, 2005). Population-based studies from Greece, have reported an estimated age and sex adjusted prevalence of PsA of 0.056% (Northwest Greece) and 0.17% (Alamanos Y et al, 2003; Trontzas P et al, 2005). Prevalence in Western Norway was reported as 0.195% (Madland TM et al, 2005). A similar prevalence of 0.164% was reported from Reykjavik, Iceland, and 0.15% in Denmark (using 3 Danish twin cohorts evaluated using a questionnaire, interview, clinical examination and scrutiny of medical records) (Love TJ et al, 2007; Pederson OB et al, 2008b). However, a much higher prevalence of 0.42% was reported from Italy based on a survey that included physical examination by rheumatologists on subjects from 16 general practices (Salaffi F et al, 2005; De Angelis R et al, 2007). Thus, there are variations in the estimated prevalence in various parts of Europe. This is probably due to ethnic variation, heterogeneity in study methods and criteria used for defining PsA. 8

24 In the United States, using the resources of the population-based Rochester Epidemiology Project, Shbeeb et al reported a population prevalence of 0.101% in Rochester, Minnesota (Shbeeb M et al, 2000). Using the newly developed CASPAR criteria, a subsequent study using the same resource showed an increased point prevalence of 0.158% in 2000 (Wilson FC et al, 2009a). In another study, based on telephone interview and patient report of physician diagnosis of PsA, Gelfand et al estimated PsA prevalence of 0.25% in the US population (Gelfand JM et al, 2005). Thus, the prevalence is similar to that observed in Europe. Studies outside of Europe and North America are few. However, it is striking that the prevalence in Japan is significantly lower and is reported to be % (Hukuda S et al, 2001). From population-based surveys in China, the prevalence of PsA appeared to be similar to the rest of the world, ranging from 0.01 to 0.1% (Zeng QY et al, 2008). Only one study has fully reported the prevalence of PsA in a population based sample of psoriasis subjects (Ibrahim G et al, 2009). Ibrahim et al reported a prevalence using the CASPAR criteria of 13.8% in a population-based sample from West Yorkshire, England (Ibrahim G et al, 2009). The frequency of patient reported physician diagnosis of PsA in psoriasis patients was 11% in the study in USA by Gelfand et al (Gelfand JM et al, 2005). Wilson et al reported in their population-based study from the Rochester Epidemiology Project that <10% of patients with psoriasis developed clinically recognized PsA during a 30-year period (Wilson FC et al, 2009b). Studies on the incidence of PsA are fewer. The incidence in Europe and North America ranges between 3 and 23.1 cases/100,000, whereas that in Japan was only 0.1 cases/100,000 (Alamanos Y et al, 2008; Hukuda S et al, 2001). Kaipiainen-Seppanen reported annual incidence of PsA of 6/100,000 in the adult population in Finland (Kaipiainen-Seppänen O, 1996). In the Finnish Kuopio 2000 arthritis survey, however, the incidence was higher at 23/100,000 (Savolainen E et al, 2003). In southern Sweden, PsA incidence was 8/100,000 (Soderlin MK et al, 2002). The incidence in northwest Greece, based on ESSG criteria was 3.02 cases per 100,000 adults (Alamanos Y et al, 2003). Shbeeb et al, from the Rochester Epidemiological Project in 9

25 Rochester, Minnesota showed in 2000 that the overall incidence of PsA was 6.59 per 100,000 between 1982 and 1991 (Shbeeb M et al, 2002). Subsequently, using the same resources but applying the CASPAR classification criteria, Wilson et al have shown that the overall annual incidence of PsA in Rochester, Minnesota was 7.2/100,000 (Wilson FC et al, 2009a). Interestingly, the incidence increased significantly from 3.6 in to 9.8 in (Wilson FC et al, 2009a). The incidence in patients with psoriasis is less well known. From the Rochester epidemiological project, Wilson et al reported a cumulative incidence of 1.7%, 3.1%, and 5.1% at 5, 10, and 20 years following psoriasis incidence, respectively (Wilson FC et al, 2009b). From a prospective cohort of patients with psoriasis in whom PsA was ruled out at baseline, the annual incidence rate was found to be 1.87 (95% CI ) PsA cases per 100 psoriasis patients, which is higher than previously reported (Eder L et al, 2011b). Thus, the estimate of the prevalence of PsA varies depending on the study methodology and the geographic location (Chandran V and Raychaudhuri SP, 2010b). Recent studies from Europe estimate a population prevalence of %, while those from the United States, estimate a prevalence of % (Chandran V and Raychaudhuri SP, 2010b). Epidemiologic studies have noted an association between Human Immunodeficiency Virus (HIV) infection and PsA. Psoriasis occurs with at least the same frequency in HIV infected individuals as in the general population. Pre-existing psoriasis may undergo severe exacerbation in HIV disease (Mallon E and Bunker CB, 2000). Interestingly, the incidence and prevalence of PsA is higher in patients with HIV infection. Prospective studies as well as the experience in Africa, where SpA, including PsA, were uncommon before the HIV epidemic, have confirmed a true association of PsA with HIV infection (Berman A et al, 1991; Medina-Rodriguez F et al, 1993; Buskila D et al, 1990; Calabrese LH et al, 1991; Mody GM et al, 2003). 10

26 Study design in genetic epidemiology Study design: Classic genetic epidemiology involves various steps beginning with descriptive epidemiological studies, followed by investigation of familial aggregation, segregation and linkage analysis in families and fine mapping and association studies (Thomas DC, 2004). Familial aggregation studies look for evidence of clustering within families. If the phenotype is a binary trait, familial aggregation is assessed by the recurrence risk ratio (λ R ) defined as risk ratio for a type R relative of an affected individual compared with population prevalence (Risch NJ, 2000). It can be estimated by: λ R = Number of affected relatives of type R/Number of relatives of type R Number of affected individuals in population/population size Any value of λ R >1.0 is considered to provide evidence for familial component in a disease, be it shared genetic and environmental factors. In traits in which, in addition to environmental components, a genetic component does indeed contribute to familial aggregation, λ R is a mixture of genetic and environmental contributions (Guo SW, 2002). Traditionally, once familial aggregation was demonstrated subsequent genetic investigations would include segregation analysis and linkage analysis in families followed by association analysis and fine mapping/positional cloning of linkage peaks in order to identify the disease susceptibility locus/loci and risk variant(s) (Thomas DC, 2004). Although this approach has been successful in unravelling the susceptibility loci of Mendelian diseases including cystic fibrosis, Huntington s disease and an uncommon subset of common disorders such as breast cancer (BRCA-1 and -2), colon cancer (familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC)), Alzheimer s disease (b-amyloid precursor protein (APP) and presenilin-1 and -2) and diabetes (maturity-onset diabetes of young (MODY)-1, -2 and -3), this approach has not been very successful in identifying genes with small effects underlying complex traits such as diabetes and psychiatric disorders (Risch NJ, 2000; Baron M, 2001). Compared to Mendelian disorders that have a large effect size, the modest nature of the effect size of loci underlying complex traits could explain the lack of replication and plethora of false 11

27 positive associations. Risch and Merikangas showed that linkage analysis in sib-pairs has limited power to detect genes of modest effect, but that association studies that utilize large number of markers in known linkage regions or candidate genes have far greater power than linkage studies for mapping complex diseases, even if one needs to test every gene in the genome (Risch N and Merikangas K, 1996). Therefore, over the last decade studies to identify disease susceptibility loci in complex diseases has reverted back to studying candidate genes on a large scale or using high-density genome scans dependent on linkage disequilibrium using association analyses. Identifying disease susceptibility loci using association studies involves comparing allele or haplotype frequencies between samples of affected and unaffected individuals using tests such as the χ 2 test. If an allele or haplotype has a higher frequency in affected versus unaffected individuals this may indicate that it contributes to the disease in the population, or is in linkage disequilibrium with a disease susceptibility locus. Association studies have many practical advantages over linkage studies. In addition to having higher statistical power for loci with modest effects, association studies have the advantage of not requiring ascertainment of families with multiple affected individuals and specifying mode of inheritance (Risch NJ, 2000; Baron M, 2001; Risch NJ and Merikangas K, 1996; Morton NE, 1998). Over the last few years, association studies using markers across the whole genome- genome-wide association study (GWAS) have been reported. GWAS are based upon the principle of linkage disequilibrium (LD) at the population level. The genomic distance at which LD decays determines how many genetic markers are needed to tag a haplotype, and the number of such tagging markers is much smaller than the total number of segregating variants in the population (Visscher PM et al, 2012). The HapMap project provided information on the LD structure of human populations (International HapMap Consortium, 2005). This information was exploited to develop dense SNP arrays to interrogate the human genome for common SNPs. Association studies using these SNP arrays using large numbers of cases and controls have provided considerably more information on the genetics of a number of common diseases, particularly in populations of European descent (Visscher PM et al, 2012). 12

28 However, low statistical power, multiple-hypothesis testing, variability in study designs, phenotype definition and/or statistical modelling and population substructure result in low replication rates of positive hits identified in candidate gene association studies (Clayton DG et al; Freedman ML et al, 2004). Family-based designs have unique advantages over populationbased designs, as they are robust against population admixture and stratification and allow both linkage and association to be tested. The transmission disequilibrium test (TDT) (Spielman RS et al, 1993) and its non-parametric extensions (family-based association tests- FBAT) (Laird NM and Lange C, 2006; Van Steen K and Lange C, 2005) provide an alternative approach to population based gene-association studies. These can also allow the study of parent-of-origin effect (Laird NM and Lange C, 2006). However, family based study designs have a number of disadvantages. The recruitment of probands and their relatives usually requires more resources in terms of time and money than that of unrelated subjects, and more genotyping is required. Parents who are homozygous for a marker are uninformative, leading to loss of power. For common diseases, they generally have less power than a case-control study design (in the absence of population stratification) although for rare diseases a study design with trios is more powerful (Laird NM and Lange C, 2006). Given the advantages and disadvantages of both designs, family-based and population-based studies are complementary approaches to determine the genetics of complex traits. Genetics of disease expression: Genetic factors also may play a role in how disease manifestations evolve over time. The most obvious examples are pharmacogenetic studies that aim to identify genetic variants that influence response to pharmacotherapy. Genetic factors can also influence pattern, severity and progression of disease. Non-conventional study designs such as case-only studies may be used to evaluate gene-gene or gene-environment interaction, to investigate contribution of complex genotypes to disease susceptibility, and to assess heterogeneity in outcomes (Botto LD et al, 2004; Begg CB and Zhang ZF, 1994). However, several methodological issues should be considered when using a case-only approach to analysis of gene-gene or gene-environment interaction- cases should be chosen carefully from a representative population, and genotype and exposure should be independent in the source population (Liu X et al, 2004). The case-only approach does not allow evaluation of the 13

29 independent effects of the exposure alone or the genotype alone. Rather, the measure obtained can be interpreted as a departure from a multiplicative effect. Interestingly, case-only studies using population-based disease registries together with the concept of population attributable fraction are useful in identifying combinations of genes of potential aetiologic importance (Botto LD et al, 2004). Case-only studies may also be used to evaluate aetiologic and prognostic heterogeneity of complex diseases. Genotype-phenotype correlations can be examined among subsets of cases defined clinically or by other biologic features; a notable example is Crohn s disease in which genetic variants of the CARD15 gene are associated with disease of the ileum but not the colon (Lesage S et al, 2002) Genetic Epidemiology of Psoriatic Arthritis: There are few published studies on the familial aggregation of PsA. In a study from the early 1970 s, the prevalence of PsA among first-degree relatives of probands with PsA was 5.5% compared to the calculated prevalence in the UK population of 0.1% (Moll JM and Wright V, 1973a). The calculated λ 1 according to Risch s method is therefore 55 (Risch N, 1990). In a Canadian study of 182 sibling pairs, 51% were concordant for psoriasis and 25% for PsA (Gladman DD et al, 2003). Myers et al. reported a sibling concordance rate of 14% for all types of PsA and 21% for psoriasis (Myers A et al, 2005). They reported that the heritability score using Falconer s simple graph for estimating heritability was greater than 100%, indicating high heritability (Myers A et al, 2005; Falconer DS, 1965). Strong heritability was also demonstrated in a recent study from Iceland (Karason A et al, 2009). By linking 220 living Icelanders in Reykjavik to the Icelandic genealogy database, it was demonstrated that the relative risk of PsA among first-, second-, third- and fourth-degree relatives of patients with PsA was 39, 12, 3.6 and 2.3, respectively. This decrease in risk by more than a factor of 2 is consistent with a genetic model of multiple genes contributing to susceptibility with some interaction effects (Risch N, 1990). The only twin study in PsA confirmed that genes are important for psoriasis but the study did not have the power to detect a genetic effect on PsA (Pederson OB et al, 2008a). No segregation study in PsA has been reported. A parent-of-origin effect has also been 14

30 demonstrated, where the proportion of probands with an affected father (0.65) was greater than the expected proportion of 0.5 (p=0.001) (Rahman P et al, 1999). Only one genome-wide linkage scan has been reported for PsA (Karason A et al, 2003). The study was conducted in Iceland, where 178 patients with PsA were identified from 906 patients included in a genetic study of psoriasis (Karason A et al, 2005). 100 out of the 178 patients with PsA were connected into 39 families using a genealogy database. A linkage with a LOD score of 2.17 was observed on 16q close to the PSORS8 locus for psoriasis. When the linkage analysis was conditioned on paternal transmission to affected individuals, a LOD score of 4.19 was obtained, whereas a LOD score of only 1.03 was obtained when conditioned on maternal transmission. A number of candidate genes have been tested in case-control association studies in PsA. Associations with variants of TNFA, ILIA, IL23R, MICA and KIR2DS1 have been independently replicated (Rahman P et al, 2006a; Rahman P, et al, 2006b; Liu Y et al, 2008; Filer C et al, 2008; Gonzalez S et al, 1999; Nelson GW et al, 2004). Other genes with at least one positive association study include: CARD15, PPARγ, VEGF, PTPN22 and CYP1A1 (P Rahman et al, 2005a; Butt C et al, 2006a; Butt C et al, 2007; Butt C et al, 2006b; Yen JH et al, 2004). No family-based association tests have been reported in PsA and there have been very few reports of disease expression studies in PsA. Studies of disease expression are inherently challenging because of the complex nature of the disease and the need to follow up patients longitudinally. PsA disease features such as the number of joints involved, number of joints damaged, presence of axial arthritis, etc., are often difficult to assess, tend to evolve over time and are influenced by treatment. Detection of these changes with time is also influenced by the time point at which clinical and/or radiographic assessments are done, the duration of follow-up and patient attrition due to death or loss to follow-up. The TNFA -308 and TNFB +252 polymorphisms were found to be significantly associated with age at psoriasis onset, presence of joint erosions in PsA, and progression of joint erosions in early PsA (Balding J et al, 2003). The IL4R I50V polymorphism was also shown to be associated with erosive PsA (P Rahman et al, 2008). 15

31 Genome-wide Association studies: In the Collaborative Association Study of Psoriasis (CASP), the first comprehensive GWAS on patients of European ethnicity with psoriasis, 1755 cases were known to have psoriasis with PsA and 3523 had psoriasis alone (Nair RP et al, 2009). The authors were thus able to investigate associations with PsA and differences between PsA and psoriasis alone. In a secondary analysis, three loci were associated with PsA when compared to normal controls (HLA-C, IL12B and TNIP1). There was a statistically significant difference in allele frequencies between PsA and psoriasis alone at three loci (HLA-C, IL12B and IL23R). HLA-C and IL23R were more strongly associated with psoriasis alone compared to PsA, but IL12B was more strongly associated with PsA (Nair RP et al, 2009). A smaller GWAS identified a novel PsA locus on chromosome 4q27 that harbours the IL2 and IL21 genes (Liu Y et al, 2008). The largest GWAS on patients with PsA reported to date included 609 German individuals with PsA compared to 990 controls (Hüffmeier U et al, 2010). Their replication cohort included 6 European cohorts with a total of 5,488 individuals. PsA associations at HLA-C and IL12B were replicated and a new association at TRAF3IP2 was identified. An initial report from a British GWAS in PsA using 492 PsA cases and 5984 healthy controls was reported in an abstract format (Bowes J et al, 2011). The study confirmed association to previously identified PsA risk loci; HLA-C, IL12B, IL23R, and TRAF3IP2. Association of known psoriasis loci (IL28RA, TNIP1, IL23A, and RNF114) with PsA was confirmed. A number of novel loci not previously reported in either PsA or psoriasis (1p22, 5p13, 8q22, 14q12) was reported and await replication (Bowes J et al, 2011). A meta-analysis of three imputed genome-wide association studies (GWAS) on psoriasis, stratified for PsA was recently reported (Ellinghaus E et al, 2012). A total of 1,160,703 SNPs were analyzed in the discovery set consisting of 535 PsA cases and 3,432 controls. Two SNPs in 1,931 PsA cases and 6,785 controls were followed up. A genomewide significant association was detected at 2p16 near the REL locus encoding c-rel. GWAS in psoriasis and PsA have confirmed that the strongest genetic risk for PsA lies within the MHC region. The MHC is a gene-rich region that encodes genes involved in the immuneresponse. The proteins encoded by the classical HLA class I and class II genes in the MHC are highly polymorphic and play an essential role in self/non-self immune recognition. HLA variation plays a major role in determining transplant rejection and susceptibility to a large 16

32 number of infectious and autoimmune diseases (de Bakker PI et al, 2006). However, identification of causal variants is problematic due to linkage disequilibrium (LD) that extends across multiple HLA and non-hla genes in the MHC (Miretti MM et al, 2005; Walsh EC et al, 2003). Therefore, in addition to SNP typing, molecular HLA typing will help in better delineation of the genetic architecture of this region. The association between alleles of HLA Class I molecules and PsA indicates a potential role for both the adaptive (CD 8+ T cells) and the innate immune system in PsA pathogenesis (Traherne JA et al, 2008; Ritchlin CT 2005). One important innate immune mechanism involving HLA class I recognition is through activation of NK cells upon engagement of NK cell receptors, chiefly the Killer-cell Immunoglobulin-like Receptors (KIR), the locus of which is on chromosome 19q13.4 within the 1 Mb leukocyte receptor complex (Lanier LL, 2005). Therefore, HLA and KIR genes are of particular interest as candidate genes underlying susceptibility to PsA. HLA Studies: Polymorphisms in the genes coded for the Human Leukocyte Antigen (HLA) on the Major Histocompatibility Complex (MHC) region on chromosome 6p have been shown to be associated with psoriasis and PsA. Class I antigens HLA-B13, HLA-B57, HLA-B39, HLA-Cw6, HLA-Cw7 have consistently shown association with psoriasis and PsA in population studies, with the strongest association being with HLA-Cw6 (Gladman DD et al, 1986; Eastmond CJ 1994; Gladman DD et al, 1999). HLA antigens also appear to identify patients with particular patterns of PsA: HLA-B27 with spinal involvement, B38 and B39 with peripheral polyarthritis. None of the associated HLA antigens occurred with >50% frequency amongst the patients, which however suggests genetic heterogeneity in PsA or the fact that the identified HLA markers do not themselves play a causal role in the disease but may be increased in frequency as a result of linkage disequilibrium with the true disease gene(s) (Tomlinson IP et al, 1995). Increased sharing of HLA haplotypes was documented among sibling pairs concordant for PsA but not among those concordant for psoriasis only, supporting the role of HLA in susceptibility to PsA (Gladman DD et al, 2003). When patients with PsA were compared to patients with psoriasis in whom PsA was excluded, HLA -B*08, -B*27, -B*38 alleles were found to increase the odds of having PsA, whereas HLA-C*06 was protective (Eder L et al, 2012b). 17

33 HLA and disease expression in PsA: Studies in European populations have shown that genetic markers also appear to be able to identify individuals prone to develop either mild or severe disease. HLA-C*0602 was associated with an earlier age of onset of psoriasis in PsA patients (Gladman DD et al, 1999; Enerback C et al, 1997). HLA antigens were identified as prognostic factors in patients with PsA (Gladman DD et al, 1995a). HLA-B39 alone, HLA-B27 in the presence of HLA-DR7, and HLA-DQ3 in the absence of HLA-DR7, each conferred an increased risk for disease progression. HLA-B22 has also been found to be protective for disease progression (Gladman DD et al, 1998a). A study of HLA-DRB1 alleles in 158 patients with PsA found a relationship of the RA shared epitope with radiological progression among patients with PsA (Korendowych E et al, 2003). Recently, patients with PsA carrying both HLA-Cw6 and HLA-DRB1*07 alleles were found to have a less severe course of arthritis (Ho PY et al, 2007). In another study, patients with HLA-C*0602 were shown to have a longer psoriasisarthritis latency period, HLA-C*0701 and HLA-C*0802 were less prevalent in PsA than in controls, and HLA-C*0702 was found to be associated with psoriatic spondylitis (Queiro R et al, 2006). Killer-cell Immunoglobulin-like Receptor (KIR) genes and MHC ligands: Natural killer (NK) cells are lymphocytes activated during the early phase of the innate immune response and they eliminate pathogens or malignant cells (Vivier E et al, 2004). NK cell receptors include KIRs coded for on chromosome 19q13.4 and lectin related NK (NKG2) receptors coded on chromosome 12p13.1 (Lanier LL, 2005). NK cell function is regulated by a balance between positive-signaling receptors that initiate, and inhibitory receptors that suppress, cell activation. Both of these receptors recognize MHC class I ligands: KIRs recognize classical MHC class I HLA alleles while NKG2 bind the non-classical MHC Class I MICA alleles. KIRs are annotated according to the number of extracellular immunoglobulin domains (KIR2D or KIR3D) and the length of their cytoplasmic domains i.e. short (S) or long (L). KIR2D recognize HLA-C ligands. HLA-C alleles may be classified into 2 groups based on the presence of amino acid residues in certain positions at the α1 domain. Alleles that encode Ser77 & Asn80 are classified into HLA-C group 1 and those that encode molecules that have Asn77 and Lys80 are classified into HLA-C group 2. This polymorphism is recognized by NK cells using different members of the KIR2D. 18

34 Thus, HLA-C group 2 alleles are recognized by KIR2DL1 (an inhibitory KIR) as well as KIR2DS1 (an activating KIR). On the other hand, KIR2DS2, KIR2DL2, and KIR2DL3 recognize HLA-C group 1 alleles. Similarly, HLA-B alleles are classified into mutually exclusive categories Bw4 and Bw6 alleles based on serologically defined epitopes. HLA-B Bw4 is further classified into Bw4 80Ile and Bw4 80Thr, based on the presence of the amino acids isoleucine/threonine at position 80. HLA-Bw4 is the ligand for KIR3DL1. Receptor binding data suggests that HLA-B Bw4 80Ile is a more effective ligand for KIR3DL1. (See Table 1.2 for KIRs and their HLA ligands). Unlike the KIR family of receptors, NKG2D is encoded by a single gene that is non-polymorphic and is an activating receptor (Lanier LL, 2005). Independent segregation of HLA and KIR genes, along with KIR specificity for particular HLA allotypes, makes it possible that any given individual may express KIR molecules for which no ligand is present. The genes encoding inhibitory KIR2DL receptors are present in almost all individuals whereas the activating receptor genes, KIR2DS1 and KIR2DS2, are present in only 35 to 56% of people of European ethnicity. The ligands for the activating KIRs have not been elucidated. HLA-KIR interactions have been associated with viral infections, autoimmunity, pregnancyrelated disorders and cancer (Traherne JA, 2008; Khakoo SI and Carrington M, 2006). NK cells are activated by KIR2DS especially if the corresponding inhibitory KIR2DL genes and/or their HLA Class I ligands are not present (Nelson GW et al, 2004; Kulkarni S et al, 2008). Given the nature of this biological interaction between KIR2DS and HLA-C group 1 or 2, a hierarchy of risk for disease may be constructed (Figure 1). When examining KIR2DS2, the risk is expected to be lowest if KIR2DS2 is absent, intermediate if KIR2DS2 as well as KIR2DL2/2DL3 (corresponding inhibitory KIR) and HLA-C group 1 (ligand for KIR2DL2/2DL3) are present, and highest if KIR2DS2 is present along with KIR2DL2/2DL3 in the absence of HLA-C group 1 or if KIR2DS2 is present without KIR2DL2/2DL3. Similarly, when examining KIR2DS1, the risk is expected to be lowest if KIR2DS1 is absent, intermediate if KIR2DS1 as well as KIR2DL1 (corresponding inhibitory KIR) and HLA-C group 2 (ligand for KIR2DL1) are present, and highest if KIR2DS1 is present along with KIR2DL1 in the absence of HLA-C group 2 or if KIR2DS1 is present without KIR2DL1. However, the extensive polymorphisms of the KIR and HLA Class I ligands, the LD between variants within each gene family, incomplete knowledge 19

35 of KIR ligands and oversimplification of the structural complexities of their interactions, and limited understanding of KIR gene expression control complicate the interpretation of KIR-HLA disease association (Traherne JA, 2008). In general, KIR-HLA combinations with a tendency towards stronger NK cell activation or lower levels of inhibition are associated with increased risk of autoimmune diseases but tend to be protective against infectious diseases. Stronger inhibitory combinations are associated with protection against inflammatory diseases and disorders in pregnancy (Traherne JA, 2008). In rheumatic diseases, polymorphisms at the KIR gene complex have been associated with rheumatoid vasculitis, ankylosing spondylitis, systemic sclerosis and microscopic polyangiitis and systemic lupus erythematosus (Yen JH et al, 2001; Lopez-Larrea C et al, 2006; Diaz-Pena R et al, 2010; Momot T et al, 2004; Miyashita R et al, 2006; Pellett F et al, 2007; Toloza S et al, 2008). KIR and PsA: KIR genes are associated with both psoriasis and PsA. The activating KIRs KIR2DS1 and KIR2DS2 have been shown to be associated with PsA, particularly in the absence of the HLA ligands for the corresponding inhibitory KIRs (KIR2DL1 and KIR2DL2/3) (Martin MP et al, 2002; Williams F et al, 2005). Furthermore, it was shown that the susceptibility to PsA may be determined by the overall balance of activating and inhibitory composite KIR-HLA genotypes (Nelson GW et al, 2004). No studies on the association between KIRs and PsA disease expression (patterns, severity or progression) have yet been done, although KIRs have been shown to be associated with disease progression in diseases such as HIV/AIDS, hepatitis C, idiopathic bronchiectasis and autoimmune diseases like diabetes, and cancer (Boyton RJ et al, 2007). Thus, although PsA has been better defined recently and likely has a significant genetic component a systematic approach to quantifying the genetic component has not yet been done. Since it is closely related to SpA as well as psoriasis, HLA alleles, especially class I alleles are likely to be associated with PsA. The contribution of KIR genes that interact with Class I HLA alleles is also of interest. The influence of HLA and KIR genes in PsA disease expression (joint damage progression, axial arthritis) has also not been explored. 20

36 Genetic epidemiological and genetic studies on PsA are likely to identify risk factors for disease susceptibility. Identifying factors associated with disease progression and severity will help better disease management. Table 1.1 The CASPAR criteria* Inflammatory musculoskeletal disease (joint, spine, or entheseal), with 3 or more of the following: 1. Evidence of psoriasis (one of a, b, c) a. Current psoriasis b. Personal history of psoriasis c. Family history of psoriasis Psoriatic skin or scalp disease present today as judged by a dermatologist or rheumatologist A history of psoriasis that may be obtained from patient, family doctor, dermatologist or rheumatologist A history of psoriasis in a first or second degree relative according to patient report 2. Psoriatic nail dystrophy Typical psoriatic nail dystrophy including onycholysis, pitting and hyperkeratosis observed on current physical examination 3. A negative test for rheumatoid factor By any method except latex but preferably by ELISA or nephelometry, according to the local laboratory reference range 4. Dactylitis either a or b a. Current Dactylitis b. History of Dactylitis Swelling of an entire digit Recorded by a rheumatologist 5. Radiological evidence of juxtaarticular new bone formation Ill-defined ossification near joint margins (but excluding osteophyte formation) on plain x-rays of hand or foot * The CASPAR criteria have specificity of 98.7% and sensitivity of 91.4%. Current psoriasis is assigned a score of 2; all other features are assigned a score of 1. 21

37 Human Leucocyte Antigens Vinod Chandran Table 1.2 KIR and their ligand specificity*adapted from Carrington M, Norman P. The KIR Gene Cluster. Killer-cell Immunoglobulin-like Receptor KIR2DL1 & KIR2DS1 KIR2DL2/3 & KIR2DS2 KIR3DL1/S1 KIR3DL2 KIR2DL4 KIR2DS4 HLA- C HLA- C HLA-B Bw4 HLA-A HLA-G group 2 group 1 C*02 C*01 B*08 A*03 C*04 C*04 C*03 B*13 A*11 C*05 C*07 B*27 C*06 C*08 B*44 B*51 B*52 B*53 B*57 B*58 22

38 Increasing activation Vinod Chandran Figure 1.1 A model of the hierarchy of HLA-KIR interactions and the risk of inflammatory disease used to build statistical models Target Act. ligand C grp NK cell Least Activation Target Act. ligand 2DS NK cell Intermediate C grp 2DL Activation Target Act. ligand 2DS NK cell 2DL Maximum Activation Target Act. ligand 2DS NK cell C grp Act. Ligand = Activating ligand, C grp = HLA C group 1 or 2; N cell- Natural Killer cell; 2DS = KIR2DS1 or KIR2DS2; 2DL = KIR2DL1 or KIR2DL2/3 23