Residual C-peptide in type 1 diabetes: what do we really know?

Transcription

1 Pediatric Diabetes 2014: 15: doi: /pedi All rights reserved 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd. Pediatric Diabetes Review Article Residual C-peptide in type 1 diabetes: what do we really know? VanBuecken DE, Greenbaum CJ. Residual C-peptide in type 1 diabetes: what do we really know?. Pediatric Diabetes 2014: 15: Connecting peptide, or C-peptide, is a protein that joins insulin s α and B chains in the proinsulin molecule. During insulin synthesis, C-peptide is cleaved from proinsulin and secreted in an equimolar concentration to insulin from the β cells. Because C-peptide experiences little first-pass clearance by the liver, and because levels are not affected by exogenous insulin administration, it may be used as a marker of endogenous insulin production and a reflection of β-cell function. Residual β-cell function, as measured by C-peptide in those with type 1 diabetes (T1D), has repeatedly been demonstrated to be clinically important. The Eisenbarth model of type 1 diabetes postulated immune-mediated linear loss of β cells, with clinical diagnosis occurring when there was insufficient insulin secretion to meet glycemic demand. Moreover, the model also implied that all individuals with T1D rapidly and inevitably progressed to absolute insulin deficiency. Correspondingly, it was assumed that most people with longstanding T1D would show little to no residual C-peptide secretion. While more than a quarter century of data confirms that this model remains largely true and appropriately serves as the basis for prevention studies, accumulating evidence suggests that the natural history of β-cell function before, during and after diagnosis is more complex. In this review, we discuss the clinical benefits of residual insulin secretion and present recent data about the natural history of insulin secretion in those with, or at risk for T1D. Dana E. VanBuecken a and Carla J. Greenbaum b a Diabetes Clinical Research Program, Benaroya Research Institute, Seattle, WA USA; and b Diabetes Program Benaroya Research Institute Seattle WA USA Key words: C-peptide diabetes mellitus type 1 natural history β-cell function Corresponding author: Carla J. Greenbaum, MD, Diabetes Program, Benaroya Research Institute, th Ave, Seattle, WA 98101, USA. Tel: ; fax: ; cjgreen@benaroyaresearch.org Submitted 2 January Accepted for publication 5 February 2014 When C-peptide was discovered in 1967, it was considered to be little more than an inactive by-product of insulin synthesis (1 3). As recently reviewed (4), accumulating evidence suggests C-peptide may itself be a biologically active hormone with potentially direct effects on inflammation, microvascular circulation, endothelial function, as well as neuronal and glomerular structure and function. Regardless of whether due to a direct biological effect of C-peptide itself or as a marker of endogenous β-cell function, some, but not all studies have demonstrated an association between residual C-peptide and clinically relevant benefits in individuals with type 1 diabetes (T1D). Considerable data supporting a relationship between residual C-peptide and clinical measures comes from the Diabetes Control and Complications Trial (DCCT). This landmark trial demonstrated that intensive control of diabetes resulted in less 84 microvascular (5) and macrovascular (6) complications. Unfortunately, these benefits came at the expense of a marked risk of hypoglycemia in those receiving intensive therapy and this complication remains the limiting factor in instituting tight diabetes control in clinical practice today. DCCT investigators then explored the data further. Among the intensive-treated subjects, those who had 0.20 pmol/ml C-peptide initially or sustained over a year had markedly less complications a 79% decrease in the relative risk of retinopathy (7, 8) (Fig. 1). Importantly, these benefits were seen in the face of less hypoglycemia. Individuals in the intensive-treated group with 0.20 pmol/ml C-peptide had about the same frequency of severe hypoglycemia as those in the standard care group; a 30% reduction as compared to those in intensive therapy without this level of C-peptide. These initial DCCT reports led to the concept that 0.2 pmol/ml

2 Fig. 1. Cumulative incidence of sustained three or more step progression of retinopathy among baseline C-peptide responders vs. non-responders in the intensive treatment group of the Diabetes Control and Complications Trial (DCCT). Responders, DCCT participants with baseline C-peptide 0.2 nmol/l. Non-responders, DCCT participants with baseline C-peptide <0.2 nmol/l. Reprinted with permission from The American Diabetes Association (7). From Diabetes, 2004; 53: Copyright 2004 American Diabetes Association. C-peptide is a clinically significant value. However, a more recent analysis of the same DCCT data described an association between even lower levels of C-peptide and clinical benefit. That analysis found a linear relationship between frequency of retinopathy progression and C-peptide as low as 0.03 pmol/ml (9). No data is available as to the clinical significance of even lower levels of C-peptide as measured in an ultrasensitive assay with a detection limit of 1.5 pmol/l (10). The new DCCT data describe the lower boundary of clinically relevant C-peptide in individuals close to diagnosis; however, since the DCCT only enrolled subjects with stimulated C-peptide <0.5 pmol/ml, these data are not informative as to whether greater values might afford greater clinical benefit. Moreover, since the start of DCTT in 1982, the rate of diabetes complications has significantly fallen overall; thus how much more benefit preserved insulin secretion will bring to those currently being diagnosed with T1D is unknown. Islet transplant studies have also shown that even small amounts of residual β-cell function are clinically important. Post islet-cell transplant patients with higher as compared to absent or minimal C-peptide levels are more likely to maintain fasting blood glucose values in the mg/dl ( mmol/l) range, HbA1c values <6.5% (47.4 mmol/mol), and insulin independence after transplantation (11). Most pertinent are data pertaining to hypoglycemia. Subjects eligible for islet transplantation are largely individuals suffering from severe hypoglycemic unawareness. Vantyghem et al. showed that while significant β-cell function was required to improve mean glucose, lower glucose excursions, and result in insulin independence, participants who maintained minimal β-cell function (a stimulated C-peptide Residual C-peptide in T1D >0.30 pmol/ml) experienced almost no severe hypoglycemic events (12). In contrast to these data, other reports highlight the complexity in ascribing a reduction of microvascular complications with the presence of residual insulin secretion in those at varying times from diagnosis. After adjusting for age of diagnosis and duration of disease, an early large cross-sectional study involving more than 500 patients was unable to demonstrate a relationship between stimulated C-peptide and retinopathy (13). Additionally, Nakanishi et al. followed 150 patients for several decades from diagnosis and found no difference in the eventual incidence of retinopathy between those with and without C-peptide 5 yr from diagnosis (14). It was only when comparing those with and without C-peptide 15 yr from diagnosis that the marked relationship between residual β-cell function and the cumulative incidence of retinopathy was noted. In contrast to DCCT, these data suggest that prolonged preservation of C-peptide is needed to confer clinically meaningful benefits on microvascular disease. Additional studies are underway to evaluate the relationship of residual secretion and clinical course over a long duration of disease. Yet, even if associations are confirmed, whether the preservation of C-peptide is causal in the reduction of microvascular complications requires direct testing. If this is a causal relationship, the mechanism by which residual secretion would reduce microvascular complications or hypoglycemia is unknown. Current insulin preparations and administration do not allow for truly physiologic delivery of insulin, which often results in hypoglycemia when applying intensive therapy to control hyperglycemia. Thus, endogenous insulin secretion may reduce hypoglycemia simply by delivering insulin more physiologically. Alternatively the reduction in hypoglycemia could be due to sustained α-cell function in individuals with residual β-cell activity. However, the conflicting data as to the relationship between the glucagon response and residual β-cell function (15, 16) make this an unlikely explanation. As reported as early as 1979 (17), persistent C-peptide is associated with less hyperglycemia, which is in turn associated with reduced complications; suggesting that the relationship of endogenous secretion with complications could be secondary to effects on glycemic control. DCCT patients with C-peptide 0.2 pmol/ml had lower fasting glucose and lower HbA1c values; a 9-yr longitudinal analysis showed that for every 1 pmol/ml increase in baseline stimulated C-peptide, there was an associated 1% reduction in HbA1c among intensively treated DCCT participants (7). This somewhat circular discussion only serves to emphasize our incomplete understanding of the mechanisms underlying diabetes complications. Pediatric Diabetes 2014: 15:

3 VanBuecken and Greenbaum C-peptide in the peri-diagnosis period In 1986, Eisenbarth proposed his landmark model for the natural history of T1D, suggesting several progressive disease stages directly related to β-cell mass (2). He postulated that an unknown precipitating event leads genetically at-risk individuals to experience immunologic abnormalities, resulting in a linear loss of insulin release, worsening glucose tolerance, and eventually, clinically apparent T1D. Eisenbarth s model suggests that even after an individual meets the criteria for a diabetes diagnosis, he will persist in insulin and C-peptide production for a limited period of time. Since the initial proposal of this model, subsequent studies have further described C-peptide before, during, and after clinical diagnosis. Prior to diagnosis A significant amount of information on β-cell function prior to diagnosis is available from the Diabetes Prevention Trial type 1 (DPT-1) and Type 1 Diabetes TrialNet (18). These are clinical trial consortiums which screen relatives of those with T1D for the presence of autoantibodies in order to identify those at risk for disease. Those at risk are either enrolled in prevention trials to delay or prevent disease onset or monitored carefully with serial Oral Glucose Tolerance Testing (OGTT) so as to diagnose T1D onset prior to clinical symptoms and associated morbidity. Measuring insulin or C-peptide during the fasting state or during the OGTT provides information about how the β cell responds to oral stimulus over time. Additional information is obtained from measuring the insulin response to a bolus of IV glucose (intravenous glucose tolerance test; IVGTT). These data highlight several details not readily apparent from the Eisenbarth model. We know that disease progression is manifested first by rising postprandial glucose values such that some will have a diabetic OGTT (2-h value >200 mg/dl; 11.1 mmol/l) when fasting glucose in normal (19). This rise in postprandial glucose is associated with insulin resistance as modeled from OGTT (20). A key observation is that OGTT (and IVGTT) stimulated β-cell function appears stable for many years in antibody positive relatives, only to fall just around the time of diagnosis after an apparent change in the sensitivity of the β cell to glucose (20 22). It is possible that this drop in β-cell function is in response to coincident hyperglycemia and/or immune flaring around the islet and represents transient β-cell dysfunction rather than acute death. At time of diagnosis and the first few years postdiagnosis There is a wide spectrum of C-peptide values at the time of clinical diagnosis. When interpreting such data, it is important to understand that not all people with T1D are diagnosed at the same point in the disease course. Diabetes diagnosed in the community via recognition of the signs and symptoms of hyperglycemia looks very different than diabetes diagnosed through surveillance programs such as the DPT-1 or Type 1 Diabetes TrialNet. Participants in such diabetes surveillance programs will generally show higher levels of C-peptide at diagnosis than will individuals diagnosed in the community, as well as less hospitalization and diabetic ketoacidosis (DKA) (23). It may be surprising to realize that at the time of diagnosis, fasting C-peptide values in those diagnosed with diabetes in the community may be within the normal range of healthy individuals. The SEARCH for Diabetes in Youth study found that at the time of diagnosis, more than 40% of youth with diabetes were within the 5th percentile of C-peptide of healthy teens in the National Health and Nutrition Examination Survey (NHANES) study and 10% were within the 50% percentile (24). Factors related to C-peptide levels at time of diagnosis are the degree of insulin resistance and age. Since hyperglycemia occurs when the amount of insulin secretion is unable to meet insulin demand, an insulin resistant individual will present with hyperglycemia with more residual insulin secretion than someone who is insulin sensitive. This is likely the explanation for those with higher body mass index (BMI) presenting with higher C-peptide levels at time of diagnosis. In general, prepubertal children have less measurable C-peptide at the time of diagnosis than older individuals. While normative data in younger healthy children is not readily available, most consider this lower value to reflect that adult β-cell mass is not reached until early adolescence. Data supporting this concept come from longitudinal studies after diagnosis which demonstrate that even though younger children (7 11 yr) present with less β-cell function, the rate of fall over time is similar for those aged yr (25) (Fig. 2). Additional evidence comes from counting β cells in autopsy studies of those without diabetes; while there are some variations, children have less β cells than adults (26). Although other studies suggest faster loss of C-peptide in the very young (<age 4) (27), there is limited data in this age group. While individuals with T1D usually have considerable C-peptide at the time of diagnosis, in an analysis of subjects enrolled in multiple TrialNet sponsored clinical trials, we reported that the rate of fall of stimulated C-peptide is about 40% during the first year postdiagnosis in T1D individuals ages 7 45 (25). Similar to that noted prior to diagnosis, the rate of fall postdiagnosis is not linear; the rate of fall of C-peptide during the second year from diagnosis is about half that seen during the first year. As mentioned above, the rate of fall is similar in individuals over varying ages 86 Pediatric Diabetes 2014: 15: 84 90

4 Residual C-peptide in T1D AUC C-peptide Fig. 2. Model-based estimates of change in stimulated C-peptide within 2 yr of diagnosis according to age. Reprinted with permission from The American Diabetes Association (25). From Diabetes, 2012; 61: Copyright 2012 American Diabetes Association. diagnosed as children (more than 50% decline) whereas those diagnosed as adults loose about 20% of their stimulated C-peptide during the first year. It is possible that this slower process represents β-cell death as compared to dysfunction. Within these age categories there remains considerable variation; however, the important message is that persistent β-cell function is the norm. We found that 2 years after diagnosis 93% of T1D individuals have detectable C-peptide, 66% have C-peptide >0.2 pmol/ml, and 15% have no change from their baseline value. These values are similar from that reported (but underappreciated) in previous studies. In 1978, Madsbad reported that all, of the more than 400 subjects tested, had detectable C-peptide within the first 2 years from diagnosis (28) and 48% of patients screened for the DCCT between 1 and 5 yr after diagnosis had stimulated C-peptide levels 0.20 pmol/ml (7). These natural history data emphasize the need to interpret results of small or non-placebo-controlled immunotherapeutic intervention studies with caution. Aside from age, it is not clear what else influences the fall in C-peptide during the first few years from diagnosis. It has been suggested that poor glycemic control results in worsening β-cell function over time. Indeed, the DCCT found more β-cellfunctionin those randomized to intensive as compared with standard treatment groups (29). Another study employed an inpatient bedside glucose control device at time of diagnosis, and reported that this tight glucose control was associated with better β-cell function a year later (30). However, this hypothesis was recently retested in a study comparing the use of immediate inpatient closed loop insulin/glucose control combined with outpatient continuous glucose monitoring (CGM) and intensive therapy with standard current diabetes intensive care, and no difference in β-cell function was observed (31). Thus while early marked hyperglycemia likely results in β-cell dysfunction, this recent data suggest that glucose values achieved with current state of the art care have little impact on β-cell function in the first years after diagnosis. Like glucose, our understanding of the influence of BMI on β-cell function over time is evolving. As noted previously, an increased BMI results in a higher C- peptide value at the time of diagnosis; yet the impact of BMI on the rate of change of C-peptide over time is limited. While multiple other factors have been reported to be associated with the rate of loss of β-cell function including autoantibodies, human leukocyte antigen (HLA) type and other genotypes, such observations have not been consistently confirmed. Such studies may be confounded by the changing clinical presentation and population of those affected by T1D with reports noting that the increasing incidence of disease is associated with younger age at diagnosis and a spreading to less high-risk HLA genotypes (32). Since 2000, there have been three therapeutic approaches that at least transiently preserve β-cell function in recently diagnosed individuals; anti-t-cell therapies including Teplizumab (33) and otelixizumab (34), anti-b-cell therapy with Rituximab (35), and anti-co-stimulation therapy with Abatacept (36). Long-term follow up of many of these clinical trial participants is underway to evaluate the clinical impact of these interventions in the years after therapy (18). Teplizumab and Abatacept are also currently being tested in those at risk for T1D to assess whether preservation of β-cell function prior to clinical diagnosis delays or prevents the onset of hyperglycemia (18). Evidence supporting residual insulin and C-peptide secretion in longstanding T1D The newer information about β-cell function in the prediagnosis and peridiagnosis period has been accompanied by more reports in those with long duration of T1D. As initially reported in 1978 (28), there exists a considerable body of evidence to support that some individuals with T1D show persistent β-cell function and residual pancreatic β cells even decades after diagnosis. The Joslin 50-yr Medalist Study demonstrated that 67.4% of individuals who had disease duration of at least 50 yr had at least minimal (0.03 pmol/ml) random serum C-peptide levels (37). Of these individuals, 2.6% had random serum C-peptide 0.20 pmol/ml. To date, the Medalist Study has shown the highest prevalence of significant residual β-cell function in T1D patients among published studies, hinting that the preservation of C-peptide in this cohort contributed to the long-term survival of these individuals, some of whom were diagnosed soon after the discovery of insulin. For example, in Pediatric Diabetes 2014: 15:

5 VanBuecken and Greenbaum comparison, the DCCT reported that only 11% of patients screened by mixed meal tolerance testing (MMTT) with a mean duration of 2.3 ± 0.3 yr had an equivalent level of C-peptide (8). Other studies have demonstrated low, but detectable C-peptide long from diagnosis (10, 28, 38) and we recently reported that one third of about 1000 individuals 3 to >50 years from diagnosis have detectable C-peptide, a percentage that varied both with duration of disease as well as age at diagnosis (39). These functional data are consistent with examination of pancreata from individuals with longstanding T1D. Studies of pancreata from participants who have had T1D for at least 4 years have shown that residual (insulin-positive) β cells can be found in 40% of T1D pancreases upon autopsy (40, 41). In the Medalist Study, all nine participants who underwent postmortem analysis of islets had insulin-positive β cells with some evidence of apoptosis, even when they had undetectable C-peptide values. The majority of these medalists retained 1 2% of their original β-cell mass (37). It is unclear whether or not these residual β cells may have survived initial autoimmune destruction or formed at some later time. It is possible that β cells in T1D are in a steady state of apoptosis and proliferation. Collectively, the anatomic and functional data hold promise for therapies to increase β-cell function in those long from diagnosis. Similar tantalizing data that β-cell function may be able to increase in humans with long standing diabetes comes from studies of individuals with T1D during pregnancy. An increase in C-peptide during pregnancy which reverted to baseline levels soon after delivery was reported from a study of 90 pregnant women with T1D (42). As reviewed in 2009 (43), there were several caveats of this study including the lack of a standardized conditions for blood sampling. More recently, an elegant study by Murphy et al. employing standard meals and closed-loop insulin delivery on a clinical research setting found no changes in C-peptide during pregnancy in 10 women with T1D (44). Thus, determining whether or not β-cell function does increase during pregnancy will require additional careful studies in larger populations of subjects. New insights: implications and future questions The accumulating knowledge that almost all of those with T1D have some β-cell function even several years postdiagnosis and that a significant proportion retain some measurable C-peptide even with long standing disease leads to the tantalizing possibility that therapies can be developed to preserve or even augment endogenous function throughout the disease course. Reaching this elusive goal requires not only Beta cell function NGT A AGT Diagnosis B C D Months (pre-dx) Months (post-dx) Fig. 3. Hypothetical model of peri-diagnostic period incorporating natural history data in multiple antibody positive individuals progressing from normal glucose tolerance (NGT) to abnormal glucose tolerance (AGT) and finally diabetes (21, 25). (A) Gradual rate of fall of β-cell function due to β-cell death. (B) Immunologic flare and metabolic dysregulation resulting in acute β-cell dysfunction. (C) Recovery of acute β-cell dysfunction. (D) Resolving immunologic flare. Similar flares may occur long before and after diagnosis accounting for variable β-cell function within the T1D population. discovery and application of new therapies but also a greater understanding of the variable natural history and clinical course among those we classify as having T1D. For example, it is unknown whether those with longstanding diabetes and detectable C-peptide have ongoing destruction/remodeling of β cells, or just residual β cells from incomplete destruction early in disease. Longitudinal studies are underway to help unravel this question. It is hoped that the application of new technologies (e.g. epigenetics, metabolomics, and transcriptomics) and analytic approaches to big-data can be used to identify those most likely to benefit from such therapy and probe the mechanisms underlying differing clinical courses. Coincident with this work are efforts to better understand how much C-peptide and for what duration of time will result in optimum clinical benefit. In the future, for example, the clinical benefit of C-peptide preservation may not only be linked to reducing the complications of diabetes. Since most patients and clinicians feel that diabetes is easier to manage when individuals still have residual function; quantitation of this observation may someday be used to demonstrate clinical benefit of new therapies. In contrast, if clinical trials now underway show that preserving β-cell function prior to diagnosis prevents or delays the onset of disease in at-risk individuals, the clinical benefit is readily apparent and would likely lead to widespread screening of individuals for T1D risk. Yet, if trials indicate that therapies are only useful in the peri-diagnostic setting, it would suggest that therapy should be targeted during times of disease flares (Fig. 3), a hypothesis that will need additional testing. A quarter century since the publication of the Eisenbarth model of T1D, we continue to learn about β-cell function in the disease process. The future should enable us to exploit this information and alter the A 88 Pediatric Diabetes 2014: 15: 84 90

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