REVIEW ARTICLE www.ijdr.in B Kavitha, Vijayashree Priyadharshini, B Sivapathasundharam, TR Saraswathi Department of Oral and Maxillofacial Pathology, Meenakshi Ammal Dental College and Hospital, Chennai, India Received : 31-07-09 Review completed : 08-09-09 Accepted : 23-01-10 PubMed ID : *** DOI: 10.4103/0970-9290.66646 ABSTRACT In oral cavity, the spectrum of diseases due to genetic alterations ranges from developmental disturbances of teeth to the pre-cancerous and cancerous lesions. Of late, significant progress has been made in the molecular analysis of tumors. With molecular genetic testing emerging as diagnostic, prognostic, and therapeutic approach, a review of genetic alterations ranging from the development of oro-facial structures to the tumors in the head and neck region are addressed in this article. The functional regulatory aspect of genes in relation to oro-facial structures are discussed separately, i.e., in relation to tooth genesis, tooth agenesis (non-syndromic, syndromic), tooth structural alterations, syndromic oro-facial defects, bone diseases, skin diseases (genodermatoses), and malignant tumors. In this literature, various genes involved in the development of the oro-facial structures and tooth in particular are discussed. The genetic basis of disorders in the tooth development (agenesis, hypodontia), tooth structural defects like amelogenesis imperfecta (AI), dentinogenesis imperfecta (DI), and oro-facial structural alterations (various syndromes) are explained. Key words: Dental diseases, genetics, tooth development Genetic disorders are far more common than is widely appreciated and the genetic diseases encountered in medical practice represent only the tip of the iceberg. The lifetime frequency of genetic diseases is estimated to be 670 per 1000. Humans have a mere 30,000 genes and in recent years the explosion of knowledge in this field resulted in the evolution of genetics (study of single or few genes and their phenotypic effects) into genomics (study of all the genes in the genome and their interactions). Progress in genetics and molecular biology has resulted in the emergence of new concepts to explain the etiology and pathogenesis of many human disease processes including oro-dental diseases. Technologic advances in molecular biology have provided tools to study the alterations in gene structure that are associated with a particular disease. Blotting techniques, PCR, in-situ hybridization, and cdna micro arrays are some of the technical advances in molecular biology. Cytogenetics and molecular biology techniques have revolutionized the field of genetics in the recent years. GENES INVOLVED IN TOOTH DEVELOPMENT (TOOTH GENESIS) Initiation, morphogenesis, and differentiation are the three fundamental processes involved in organogenesis. A group Address for correspondence: Dr. B Kavitha, E-mail: kavithabottu2@yahoo.com of cells interpret positional information provided by other cells to initiate organ formation both at the right place and time (initiation). This leads to the formation of an organ rudiment (morphogenesis), followed by the development of organ specific structures (differentiation) [Figure 1]. Tooth development is an excellent example for the reciprocal interaction between ectoderm and underlying mesenchyme. This results in sequential cell activities like proliferation, condensation, adhesion, migration, differentiation, and secretion and leads to the formation of a functional tooth organ. Recent advances in molecular aspects of odontogenesis indicate that the development of teeth is under strict genetic control. More than 300 genes are involved in determination of the position, number, and shape of different types of teeth. [1] Mutations in those genes encoding transcription factors and signaling molecules involved in odontogenesis is responsible for numerous abnormalities of the teeth. Most commonly studied genes in tooth development are homeobox genes. HOMEOBOX GENES A homeobox (HOX) is a DNA sequence of about 180 base pairs long, found within genes that are involved in the regulation of development (morphogenesis) of animals, fungi, and plants. Genes that have a homeobox are called homeobox genes and form a homeobox gene family. Homeobox genes encode transcription factors, which typically switch on cascades of other genes. HOX genes 270
Figure 1: Representation of role of genes at various stages of tooth development are a particular cluster of homeobox genes which function in patterning the body axis thereby providing the identity of particular body region and they determine where body segments grow in a developing fetus. Mutations in any one of these genes can lead to the growth of extra, typically non-functional body parts. Thus, mutations to homeobox genes can produce easily visible phenotypic changes. Humans generally contain homeobox genes in four clusters, called HOXA (or HOXI), HOXB, HOXC, or HOXD, on chromosomes 2, 7, 12, and 17, respectively. HOX gene network appears to be active in human tooth germs between 18 and 24 weeks of development. [2] PAX, MSX, DLX, LHX, BARX, and RUNX-2 are the important members of the homeobox genes involved in tooth development. PAX-9 gene PAX-9 belongs to a transcription factor family with nine members characterized by a DNA-binding domain called paired domain. They are important regulators of organogenesis that can trigger cellular differentiation. PAX-9 is widely expressed in the neural crest derived mesenchyme involved in craniofacial and tooth development. PAX-9 gene is mapped onto 14q12-q13 and mutations in this gene can lead to non-syndromic tooth agenesis. PAX-9-/- mice show cleft of secondary palate besides other skeletal alterations, lack thymus and parathyroid glands, and show absence of teeth. PAX-9 is expressed in the dental mesenchyme prior to the first morphological manifestation of odontogenesis. [3] Tooth development in homozygous PAX-deficient mouse embryos is arrested at the bud stage, indicating that PAX-9 is required for tooth development to proceed beyond this stage. PAX-9 is required for the mesenchymal expression of Bmp-4, MSX-1, Lef-1, suggesting that its function is essential to establish the inductive capacity of this tissue. 271 MSX-1 gene The MSX gene is a member of MSX homeobox gene family, a small family of homeobox genes related to the drosophila gene muscle segment homeobox (msh). [4] At present, two human MSX genes-msx-1 and MSX-2, have been isolated. [5] MSX-1 gene is mapped onto 4p16.1. MSX-1 and MSX-2 are found to be expressed in several embryonic structures including premigratory and migratory neural crest cells, as well as in the neural crest derived mesenchyme of the pharyngeal arches and median nasal process. The expression of this gene is observed very early in the odontogenic mesenchyme. They are expressed in undifferentiated multipotential cells that are proliferating or dying and they provide positional information, and regulate epithelial-mesenchymal signaling in cranio-facial development. MSX-1 gene encode a group of homeodomain transcription factors required in different stages of development, like patterning, morphogenesis, and histogenesis and they function as transcriptional repressors. It has been shown that MSX-1 inhibits cell differentiation by maintaining high levels of cyclin DI expression and Cdk-4 activity, thus preventing the exit from the cell cycle and enabling the cells to respond to proliferative factors. The loss of function mutation would lead these cells to differentiate earlier and stop proliferating, producing impaired morphogenesis. MSX-/- mice have cleft secondary palate, lack all teeth whose development is arrested at bud stage, and have skull, jaw, and middle ear defects. DLX gene DLX (Distal less) family of homeobox genes consists of six members (DLX 1-6) and is expressed in the epithelium and mesenchyme of the branchial arches, tooth bud mesenchyme, dental lamina, cranial neural crest, dorsal neural tube, and frontonasal process. Mutation in these
genes results in abnormalities affecting first four branchial arch derivatives including mandible and calvaria. DLX genes have been involved in the patterning of ectomesenchyme of the first brachial arch with respect to tooth development. Loss of function mutation of these genes apparently results in failure of development of upper molars. [6] LHX gene Lim homeodomain transcription factors (LHX-1 and LMX1-b) are expressed in neural crest derived ectomesenchyme of first branchial arch. Improper expression of this gene leads to abnormal development of first arch derivatives including tooth agenesis and cleft palate. Recently a Lim homeobox gene, LHX-8, is found to be expressed in murine embryonic palatal mesenchyme, and targeted deletion of this gene resulted in a cleft secondary palate in LHX-8 homozygous mutant embryos. [7] BARX gene Telencephalon, diencephalon, mesencephalon, hindbrain, spinalcord, cranial and dorsal root ganglia, craniofacial structures, and palate are the expression sites for Barx gene. Improper expression of this gene results in failure of nervous system to develop and cleft palate formation. BARX-1 is expressed in the mesenchyme of the mandibular and maxillary process and in the tooth primordial, while BARX-2 is expressed in the oral epithelium prior to the tooth development. [8] RUNX gene RUNX 2 (Runt related protein) is a transcription factor and a key regulator of osteoblast differentiation and bone formation. Also, analysis of RUNX-2 showed that it is restricted to dental mesenchyme between the bud and early bell stages of tooth development. Epithelium-mesenchymal recombinants demonstrated that the dental epithelium regulates mesenchymal RUNX-2 expression during the bud and cap stages. [9] At molecular level, the signal molecules bind to their respective target receptors and trigger responses through the activation of transcription factors leading to an altered gene expression profile in target cells. Tgf superfamily, Bmps, Fgf superfamily, hedgehog (Hh) superfamily, and TNF are the few signaling molecules involved in tooth genesis. TOOTH AGENESIS (NON-SYNDROMIC AND SYNDROMIC) This is the most common craniofacial malformation. Its prevalence in permanent dentition reaches 20% and its expressivity ranges from only one tooth, usually a third molar, to the whole dentition. [10] Tooth agenesis could be isolated and manifested as the only phenotypic alteration in a person (non-syndromic) or associated with other alterations as part of a syndrome (syndromic). Non-syndromic tooth agenesis Isolated, non-syndromic tooth agenesis can be sporadic or familial and may be inherited as an autosomal dominant, recessive, or X-linked mode. Molar oligodontia, second premolar and third molar hypodontia, incisor-premolar hypodontia exemplify nonsyndromic agenesis. Mutations in PAX-9 gene mapped to 14q12-q13 were found in patients affected by molar oligodontia. Mutations responsible for second premolar and third molar hypodontia were found in MSX-1 gene mapped to 4p16.1.The genetic cause for Incisor-premolar hypodontia has not been found yet but mutations in MSX-1, MSX-2, EGF, and EGFR have been excluded. [11] Syndromic tooth agenesis Tooth agenesis is associated with many syndromes because many genes take part in molecular mechanisms common to tooth and other organs development. The following are the commonly associated syndromes. ECTODERMAL DYSPLASIA Ectodermal dysplasias are a group of 192 distinct disorders that involve anomalies in at least two of the following ectodermal-derived structures: Hair, skin, nails, and teeth. The most common EDs are X-linked recessive hypohidrotic ED (Christ-Siemens-Touraine syndrome) and hidrotic ED (Clouston syndrome). Hypohydrotic ectodermal dysplasia This disease is produced by point mutations, deletions, or translocations in the EDA gene, mapped to Xq12-q13.1. EDA gene encodes ectodysplasin-a, a 391 amino acid protein that belongs to the TNF-ligand family. Ectodysplasin plays a vital role during development by promoting interaction between ectodermal and mesodermal layers. [12] Ectodermal-mesodermal interactions are essential for many structures derived from ectoderm, including skin, hair, nails, teeth, and sweat glands. [12] Mutated EDA gene leads to the production of a non-functional version of the ectodysplasin, a protein which in turn cannot trigger the normal signals needed for the normal ectodermalmesodermal interaction resulting in the defective formation of the corresponding derivatives. Hidrotic ectodermal dysplasia Hidrotic ED (Clouston syndrome) is an autosomal dominant disorder caused by mutations in GJB-6, which encodes the gap junction beta protein connexin 30, a component of intercellular gap junctions. [13] Connexon mediates the direction of diffusion of ions and metabolites between the cytoplasm of adjacent cells. Mutations in this gene deregulate the trafficking of the protein and are thus associated with defects like palmar-plantar hyperkeratosis, generalized alopecia, and nail defects. 272
Witkop tooth and nail syndrome The tooth-and-nail syndrome (Witkop syndrome) is a rare autosomal dominant ectodermal dysplasia manifested by defects of the nail plates of the fingers and toes and hypodontia with normal hair and sweat gland function. A nonsense mutation within MSXI homeobox has been responsible for this disorder. The protein produced from the mutated allele would be truncated, and lack the entire C-terminal region that is important for protein stability and DNA binding. The mutant protein would have no biological function, and the haploinsufficiency is probably the pathogenic mechanism. [14] Reiger syndrome This is characterized by hypodontia, malformation of the anterior chamber of the eyes, and umbilical anomalies. The maxillary deciduous and permanent incisors and second maxillary premolars are most commonly missing, and cleft palate may be present. The mandibular anterior teeth have usually conical crowns. Mutations responsible for this malformation have been found in PITX-2 (paired like homeodomain transcription factor 1), a gene mapped to 4q25-q26. PITX-2 is a gene involved in tooth development and is more restricted to dental lamina. PITX-2-null mice revealed that PITX-2 was both a positive regulator of Fgf-8 and a repressor of Bmp-4 signaling suggesting that PITX- 2 may function as a coordinator of craniofacial signaling pathways. [15] STRUCTURAL TOOTH DEFECTS Amelogenesis imperfecta Enamel consists of 96% inorganic and 4% organic matrix. The organic matrix is made of several enamel proteins and enzymes. The enamel proteins include amelogenins (90%) and non-amelogenins (10%). Enamelin, tuftelin, and ameloblastin are the non-amelogenin proteins. The enzymes include metalloproteinases, proteinases, and phosphatases. Genes that code amelogenin and enamelin are AMELX and ENAM. Amelogenin gene is located on X and Y chromosome. Apart from tooth enamel, amelogenin is found in bone, bone marrow, and brain cells. AMELX gene located on X-chromosome has a major role in enamel formation, whereas AMELY gene located on Y-chromosome is not needed for enamel formation. Mutations in the AMELX and ENAM genes are mainly demonstrated to result in Amelogenesis imperfecta. Recently, mutations of two genes encoding enamel proteases, Kallikrein-4 (KLK-4) and MMP-20 (metalloproteinases), have been reported. [16] Amelogenesis imperfecta can be inherited as autosomal dominant, recessive, or as X-linked recessive (Xp-22) trait. Mutations in AMELX gene cause X-linked AI, whereas 273 mutations in ENAM gene cause autosomal inherited forms of AI. Dentinogenesis imperfecta Dentin consists of 65% inorganic and 35% organic substance. The major portion of the organic substance is made of Type I collagen, a product of COLIAI and COLIA-2 genes. This trimeric collagen molecule forms the foundation for several mineralized tissues including bone and dentin. There are numerous non-collagenous proteins present in dentin, some of which interact with collagen to initiate and/or regulate mineralization. The most abundant non-collagenous protein is dentin sialophosphoprotein, which is a product of DSPP gene located on 4q21.3. Dentin sialophosphoprotein is a highly phosphorylated protein that attaches to the type 1 collagen fibril and helps in regulation of mineralization at specific sites within the collagen. [17] Mutations in either COL or DSPP genes can alter this interaction resulting in abnormal mineralization and a Dentinogenesis imperfecta phenotype. SYNDROME ASSOCIATED ORO-FACIAL DEFECTS Van der Woude syndrome Van der Woude syndrome is an autosomal dominant syndrome typically consisting of a cleft lip or palate and distinct pits of the lower lip. Most cases of V-W syndrome are due to deletion in chromosome 1q32-q41 and recently locus 1p34 is reported. [18] IRF-6 gene (interferon regulatory factor) mutations are responsible for this disorder but the exact mechanism of this mutation on craniofacial development is uncertain. Crouzon syndrome Crouzon syndrome is characterized by premature closing of the cranial sutures leading to cranial malformations. Maxillary hypoplasia and midline maxillary pseudocleft are the common oral manifestations. Mutations in the FGFR-2 gene, located on 10q24, cause Crouzon syndrome. The FGFR-2 gene provides instructions for making a protein called fibroblast growth factor receptor 2. This protein plays an important role in bone growth, particularly during embryo development. Immature osteoblasts respond to FGF treatment with increased proliferation, whereas in differentiating cells FGF does not induce DNA synthesis but causes apoptosis. [19] Mutations in FGFR-2 gene probably overstimulate signaling by the FGFR-2 protein, which causes the bones of the skull to fuse prematurely. Apert syndrome Apert syndrome is characterized by premature closing of the cranial sutures and characteristic limb defects. Mutations in the FGFR-2 gene (10q25-26) causes Apert syndrome. The FGFR-2 gene produces a protein called fibroblast growth factor receptor 2. Among its multiple functions, this protein signals immature cells to become bone cells in
a developing embryo and fetus. A mutation in a specific part of the FGFR-2 gene alters the protein and causes prolonged signaling, which can promote the premature fusion of bones in the skull, hands, and feet. Treacher collins syndrome Treacher Collins syndrome is characterized by defects of structures derived from the first and second branchial arches. Hypoplastic zygomas and mandible, coloboma, ear defects, lateral facial clefting, and cleft palate are seen in these patients. Mutations in the TCOF-1 (5q32 - q33.1) gene cause Treacher Collins syndrome. The TCOF-1 gene (Treacher Collins-Franceschetti syndrome) provides instructions for making a protein called treacle. Treacle plays a key role in pre-ribosomal processing and ribosomal biogenesis. In mice, haploinsufficiency of TCOF-I results in a depletion of neural crest cell precursors through high levels of cell death in the neuroepithelium, which results in a reduced number of neural crest cells migrating into the developing cranio-facial complex leading to the specific problems with facial development found in Treacher Collins syndrome. [20] Down syndrome Down syndrome is characterized by single transverse palmar crease, epicanthic folds, upslanting palpebral fissures, shorter limbs, hypotonic muscles, learning disabilities, and physical growth retardation. Trisomy 21, mosaicism, and tranlocation are the various genetic events that result in Down syndrome. And 95% of Down syndrome results from trisomy 21, 3-4% of cases from translocation, and 1-2% by mosaicism. Most cases of Down syndrome result from trisomy 21, which means each cell in the body has three copies of chromosome 21 instead of the usual two. 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