Diabetes mellitus and autoimmunity

Introduction

Diabetes is a complex metabolic disorder of the endocrine system which plagues nearly 250 million people worldwide. Each year, a further 7 million people develop diabetes.1 While this debilitating and life-threatening disease can be controlled, diabetes is often accompanied by serious complications, and still today there is no cure.
Insulin, which is secreted by the beta cells of the pancreas and essential for normal metabolism, is unavailable to the diabetic. Therefore, normal metabolism is interrupted and hyperglycemia occurs. This imbalance of glucose and insulin causes the gradual breakdown of many vital body functions, often leading to serious renal, neurologic, ophthalmic, and circulatory complications.

Diabetes has a significant, negative impact on the global economy. Billions of dollars are spent annually to treat and prevent diabetes and its complications. Therefore, the need for an early diagnosis -- with an ongoing goal of finding a cure for diabetes -- is of ever increasing importance.

Overview

Type 1 diabetes, formerly known as insulin dependent diabetes mellitus (IDDM), accounts for approximately 15 – 20% of the diabetic population and while it can strike at any age, is generally seen in children and young adults.1 Prior to clinical onset, Type 1 diabetes is characterized by lymphocytic infiltration of the islet cells and circulating autoantibodies against a variety of islet cell antigens, including glutamic acid decarboxylase (GAD), IA-2, and insulin (IAA).2-4 Insulin deficiency is a result of the autoimmune destruction of the insulin producing pancreatic beta cells; however, clinical onset of diabetes does not occur until 80-90% of these cells have been destroyed. This prolonged prediabetic stage, in which these disease markers are present and measurable, may allow the opportunity to predict and prevent the clinical onset of disease.5
Type 2 diabetes, formerly known as non insulin-dependent diabetes mellitus (NIDDM), stems from both a decreased secretion of insulin as well as the body's inability to effectively utilize the insulin produced (insulin resistance). Current research suggests that Type 2 diabetes is genetically inherited, with a concordance rate of greater than 90% in identical twins. Also referred to as adult-onset diabetes, Type 2 diabetes generally develops after the age of thirty, and is commonly associated with obesity.1

While the majority of patients fall under the classic definition of either Type 1 or Type 2 diabetes, there are at least two subgroups of patients that bridge these classical barriers. On one hand, studies indicate that as many as 10% of adult patients, initially diagnosed with Type 2 diabetes, eventually become insulin dependent.1,6 These patients, initially "masquerading" as Type 2, are in fact late-onset, or slow developing, Type 1 diabetics. Like classical Type 1, late-onset diabetes results from the autoimmune destruction of the beta cells. Autoimmune markers, such as antibodies to GAD, have been detected in the serum of these diabetic patients many years prior to insulin dependency.6 On the other hand, the incidence of childhood obesity has risen dramatically in recent years, and so also has the number of children afflicted with Type 2 diabetes. Although generally seen in patients over the age of thirty, there have been reports recently of children as young as four years old developing Type 2 diabetes.

Immune-Mediated Diabetes -- Searching for a Cure...

Several recent advances in the areas of immunology and endocrinology have led diabetes research into an exciting discovery phase. Several studies worldwide are underway to identify ways to control or, ideally, prevent beta-cell destruction and the ensuing, devastating disease.

Associated Risk Factors

• Race and Ethnicity. There is clear evidence that race and ethnicity represent the most important factors for Type 1 diabetes. The incidence of Type 1 diabetes is highest in Scandinavian countries and among Caucasian populations.
  
• Relatives With Type 1 diabetes. While approximately 80% of those diagnosed with Type 1 diabetes have no family history of the disease, there is an increased risk associated with siblings, parents and offspring of Type 1 diabetes patients. This overall risk has been reported between 1 and 15% (which can be compared to less than 1% for individuals without diabetic relatives).1
  
• HLA Antigens HLA phenotypes, which are responsible for immune response, are associated with susceptibility and resistance to Type 1 diabetes. For example, the risk of developing Type 1 diabetes is markedly increased in individuals homozygous for both DQB1*0302 and DQA1*0301 alleles, while the DQB1*0602 allele has shown to be protective.1
  
• Environmental Factors While HLA phenotypes contribute significantly to the risk of Type 1 diabetes, a concordance rate of less than 50% in identical twins suggests that genetic susceptibility alone is not sufficient to cause Type 1 diabetes. Environmental factors such as nutrition, viral exposure, and stress may prove to be "triggers" that initiate beta-cell destruction.1
  
• Autoantibodies Autoantibodies to various antigens have been detected in Type 1 diabetes patients, and several are thought to be directly responsible for pancreatic beta-cell destruction. As markers of autoimmune activity, the presence of one or more of these antibodies carries with it an increased risk of developing Type 1 diabetes.1
  
• Undoubtedly, these studies -- and numerous others -- will yield information of great value. However, until such time that a cure is found for this life-long disease, the medical community must rely on the variety of diagnostic tools available, with an aim towards minimizing the effects of diabetes through early identification/awareness, education, and appropriate therapeutic measures.

Detection of Those at Risk

The Past...

For many years, physicians have relied on the measurement of antibodies directed at pancreatic islet cells (ICA) by immunofluorescence (IFA) to aid in the detection of those at risk of diabetes. While a positive ICA result is predictive for the eventual onset of the disease, this method measures a wide range of antibodies, has limited sensitivity, is subject to interpretation, and is difficult to standardize. The development of immunoassays capable of measuring antibodies directed against individual islet cell antigens now allows for accurate and sensitive detection of those at risk.8

The Future...

The characteristic lymphocytic infiltration of the islets and associated autoantibodies against a variety of islet cell antigens including glutamic acid decarboxylase (GAD), IA-2, islet cell antigens (ICA), and insulin (IAA) provides early markers of autoimmune disease activity.

The measurement of these antibodies has been shown to be extremely useful in assisting the physician with the prediction, diagnosis, and management of patients with diabetes.

Diagram 1

In a study (see Diagram 1) measuring antibodies to GAD, IA-2 and insulin in 50 first degree relatives of Type 1 diabetes patients that eventually developed diabetes, 46 (90%) were found to have antibodies to GAD, 32 (64%) were positive for IA-2, and 38 (76%) were positive for insulin antibodies.3

Autoantibodies to Glutamic Acid Decarboxylase (GAD)

First detected in the serum of patients suffering from the rare neurological disorder, Stiff-Man Syndrome9, antibodies to the islet cell antigen glutamic acid decarboxylase (GAD) have been found in 70-90% of prediabetic and Type 1 diabetic patients (including approximately 7-10% of adult onset diabetics with Type 1 diabetes), and have been shown to be the most sensitive single marker for identifying persons at risk of developing diabetes. Autoantibodies to GAD are generally more prevalent in older children and late-onset Type 1 diabetics, and in some patients, have been detected as many as ten years prior to the onset of clinical disease.

Autoantibodies to IA-2

Autoantibodies to IA-2, a tyrosine phosphatase-like protein, are found in 50-75% of Type 1 diabetic patients at and prior to disease onset, are generally more prevalent in younger onset patients, and are associated with rapid progression to overt disease10. These autoantibodies, have been detected in some ICA positive/GADAb negative patients, and therefore can be considered independent markers of disease.

Autoantibodies to Insulin

The presence of autoantibodies to insulin, the only beta-cell specific autoantigen thus far identified, is evidence of ongoing destruction of islet cells. Autoantibodies to insulin (IAA) are found predominantly, though not exclusively, in young children (<5 years) developing Type 1 diabetes. In insulin-naive (untreated) patients, the prevalence of autoantibodies to insulin is almost 100% in very young individuals and almost absent in patients with adult onset of Type 1 diabetes11. (It should be noted that insulin autoantibodies are indistinguishable from insulin antibodies that commonly develop with insulin therapy).

Predicting The Onset of Diabetes

While the presence of a single autoantibody has been shown to be a strong predictive marker for the eventual onset of Type 1 diabetes, the presence of additional antibodies suggests an even greater risk. Among the 50 relatives of Type 1 diabetics who developed diabetes (from the study above), 98% had antibodies to one or more antigens, and 80% were positive for more than two antibodies.3

Positive predictive value (after 3 and 5 years of follow-up) was determined by survival analysis on 763 (of 882) relatives screened for autoantibodies and for whom follow-up information was available. Sensitivity was determined as the proportion with the marker among 50 relatives in whom diabetes developed during follow-up. From Verge et al.3

Autoantibody Measurement

Found in many patients years prior to disease onset, the measurement of GAD, IA-2 and insulin autoantibodies can provide vital information and insight with regards to the autoimmune progression of diabetes. In addition, antibody measurement by immunoassay, while providing optimal sensitivity and specificity, is suitable for use in routine screening.8,12 The detection of autoimmune markers by simple immunoassay allows for reliable, cost-effective laboratory confirmation of diabetes. When the patient's clinical presentation is not clear, as in the diagnosis of patients without symptomatic hyperglycemia, the measurement of autoantibodies can be of considerable value. Such is also the case in detecting Type 1 diabetes patients "masquerading" as Type 2. Additionally, the absence of autoantibodies at the time of Type 2 diagnosis provides a negative predictive value upwards of 97% of eventually becoming insulin dependent.6

Assay Design and Methodology

To be effective in a clinical setting, assays designed to measure autoantibodies to GAD, IA-2, and insulin must offer the highest degree of sensitivity and specificity. Assay characteristics such as these are found in radiobinding assays (RBA) and, more recently, in highly-sensitive, non-radioactive ELISA methods15.

Summary

An increased understanding of the etiology and progression of diabetes has allowed for considerable progress in the possible prevention and cure of diabetes. A critical step in that prevention will be the ability to correctly identify those at risk of developing the disease. Strong evidence indicates that the measurement of antibodies to GAD, IA-2 and insulin, will make early detection a reality. Until diabetes can be cured or prevented, the early identification of at risk individuals will provide the opportunity for early education. In turn, this education will be the foundation that will help ensure good diabetic management, minimizing future medical complications associated with diabetes.

References

1. International Diabetes Federation. World Wide Web; 2007.
2. Tuomilehto, Jaakko et al., "Antibodies to Glutamic Acid Decarboxylase as Predictors of Insulin-Dependent Diabetes Before Clinical Onset of Disease", The Lancet, Vol. 343, pp. 1383-85, 1994.
3. Verge, Charles F. et al., "Prediction of Type I Diabetes in First-Degree Relatives Using a Combination of Insulin, GAD and ICA512bdc/IA-2 Autoantibodies", Diabetes, Vol.45, pp. 926-933, 1996.
4. Gianani, Roberto et al., "ICA512 Autoantibody Radioassay", Diabetes,44, pp. 1340-1344, 1995.
5. Vandewalle, Christina L. et al., "High Diagnostic Sensitivity of Glutamic Acid Decarboxylase Autoantibodies in Insulin-Dependent Diabetes Mellitus with Clinical Onset Between Age 20 and 40 Years", Journal of Clinical Endocrinology and Metabolism, Vol. 80, No. 3, pp. 846-51, 1995.
6. Niskanen, Leo, K. et al., "GAD Antibodies in NIDDM, Ten-Year Follow-up From the Diagnosis", Diabetes Care, Vol. 18, pp. 1557-1565, 1995.
7. National Institute of Diabetes and Digestive and Kidney Diseases, NIH Publication: World Wide Web, 1996.
8. Gottsater, A. et al., "Glutamate Decarboxylase Antibody Levels Predict Rate of _-cell Decline in Adult-Onset Diabetes", Diabetes Research and Clinical Practice, Vol. 27, pp. 133-140, 1995.
9. De Aizpuria, Henry J. et al., "Glutamic Acid Decarboxylase Autoantibodies in Preclinical Insulin-Dependent Diabetes", Proceedings of the National Academy of Science, Vol. 89, pp. 9841-45, 1992.
10. Christie MR, Genovese S, Cassidy D, Bosi E, Brown TJ, Lai M, Bonifacio E, Bottazzo GF, "Antibodies to Islet 37k Antigen, but Not to Glutamic Acid Decarboxylase, Discriminate Rapid Progression to IDDM in Endocrine Autoimmunity", Diabetes, Vol. 42, pp. 1254-1259, 1994.
11. Bonifacio, E., Bingley, P.J., et al., "Islet Autoantibodies and Their Use in Predicting Insulin-dependent Diabetes", Acta Diabetol., Vol. 34, pp. 185-193, 1997.
    
12. Schmidli, Robert S. et al., "Disease Sensitivity and Specificity of 52 Assays for Glutamic Acid Decarboxylase Antibodies, The Second International GADAb Workshop", Diabetes, Vol. 44, pp. 636-40, 1995.
13. Greenbaum, Carla J., et al., "Insulin Autoantibodies Measured by Radioimmunoassay Methodology Are More Related to Insulin-Dependent Diabetes Mellitus Than Those Measured by Enzyme-Linked Immunosorbent Assay: Results of the Fourth International Workshop on the Standardization of Insulin Autoantibody Measurement", Journal of Clinical Endocrinology and Metabolism, Vol. 74, No. 5. pp. 1040-1044, 1992.
14. University of Alberta/Capital Health - Canada. "Clinical Islet Transplantation Program" overview; 2003. World Wide Web.
15. Brooking, Helen, et al., "A sensitive non-isotopic assay for GAD65 autoantibodies", Clinica Chimica Acta, 331 (2003) 55-59.

Additional Reading

1. Bingley PJ, Bonifacio E, Gale EAM. "Can We Really Predict IDDM?", Diabetes, Vol.42, pp. 213-20, 1993.
2. Bingley, P.J., Bonifacio, E., et al. "Prediction of IDDM in the General Population: Strategies Based On Combinations of Autoantibody Markers", Diabetes, Vol. 46, pp. 1701-1710, 1997.
   
3. Pietropaolo, M., Peakman, M., et al. "Combined Analysis of GAD65 and ICA512(IA-2) Autoantibodies in Organ and Non-organ-specific Autoimmune Diseases Confers High Specificity for Insulin-dependent Diabetes Mellitus", Journal of Autoimmunity, Vol. 11, pp. 1-10, 1998.
   
4. Borg, H., Fernlund, P., et al. "Protein Tyrosine Phosphatase-like Protein IA2-antibodies plus Glutamic Acid Decarboxylase 65 Antibodies (GADAb) Indicates Autoimmunity as Frequently as Islet Cell Antibodies Assay in Children with Recently Diagnosed Diabetes Mellitus", Clinical Chemistry, Vol. 43, No. 12, pp. 2358-2363, 1997.
   
5. Sepe, Vincenzo, et al. "Islet-Related Autoantigens and the Pathogenesis of Insulin-Dependent Diabetes Mellitus: A 1996 Update", Leslie RDG (ed): Molecular Pathogenesis of Diabetes Mellitus, Vol. 22, pp. 68-89, 1997.