Molecular genetic testing of patients with monogenic diabetes and hyperinsulinism
Introduction
Genetic sequencing is a critical part of the diagnosis of certain forms of pancreatic beta cell dysfunction, including Maturity Onset Diabetes of the Young (MODY), Neonatal Diabetes (NDM), and Congenital Hyperinsulinism (CHI). MODY is the most common form of monogenic diabetes, accounting for 2–5% of all patients with diabetes [1]. Initially defined by the presence of a positive family history, absence of insulin dependence, and early onset of symptoms (< 25 years), the diagnosis of MODY is now defined by molecular genetic testing [2]. Mutations in 11 genes have now been described as causing MODY, although mutations in just three (HNF1A, GCK, HNF4A) account for over 70% of patients [3]. Only 5–20% of patients with MODY are correctly diagnosed [4]. Because patients with MODY are frequently young and non-obese, they are often misdiagnosed as having type 1 diabetes and treated with insulin [2], [5]. Misdiagnosis with type 2 diabetes is possible as well [2], [5]. Since the majority of MODY patients can be managed with oral hypoglycemic agents or diet alone, accurate molecular diagnosis of these patients is critical [2], [6], [7].
NDM is a disorder of pancreatic beta cell function characterized by hyperglycemia within the first 6 months of life. There are a variety of syndromic forms, which are categorized by the presence of other abnormalities, but most patients with NDM are non-syndromic and are classified as having either transient or permanent NDM. Transient NDM patients typically have hyperglycemia that resolves by 18 months; ~ 70% of these patients have an imprinting abnormality at 6q24 [5]. The hyperglycemia in patients with permanent NDM typically does not resolve, and mutations in 4 different genes (KCNJ11, ABCC8, INS, GCK) account for ~ 70% of patients [8], [9], [10], [11].
CHI is characterized by hypoglycemia with inappropriately elevated insulin. CHI can present in the neonatal period with severe hypoglycemia, although milder, childhood-onset presentations occur as well. Mutations in eight different genes, all of which function in the regulation of insulin secretion, have now been described as causing CHI, although mutations in just three (ABCC8, KCNJ11, GCK) account for ~ 50% of patients [12].
Here we report the results of diagnostic testing for pancreatic beta cell disorders at the Seattle Children's Molecular Genetics Laboratory (SCHMGL) over a 4-year period. SCHMGL was the first laboratory in North America to offer such testing. Reportable variants were identified in 115 of 331 (35%) probands referred to us. A total of 91 variants were identified in 6 genes sequenced (HNF1A, GCK, HNF4A, ABCC8, KCNJ11, or INS), 23 of which are novel and not previously reported. We provide evidence for digenic and mosaic MODY presentations. Re-analysis of all variants using the latest available databases and literature altered variant interpretation in about 30% of cases. These results expand the spectrum of known variants associated with MODY, NDM, and CHI, and provide a resource for variant interpretation based on the largest North American cohort reported to date. In addition, they highlight the importance of periodic review of molecular testing results, in particular of variants of uncertain significance.
Section snippets
Study design
The SCHMGL is a reference diagnostic laboratory that has been performing molecular diagnostic testing for MODY, NDM, and CHI since September 2009. Referrals come from physicians and other laboratories within North America. After approval by the Seattle Children's Hospital Institutional Review Board, all molecular test results sent to SCHMGL for MODY, NDM, and CHI testing from 9/2009 to 10/2013 were reviewed. A total of 331 probands were referred for testing during this period. Reasons for
Molecular genetic testing
A total of 331 probands were referred to the SCHMGL for molecular genetic sequencing of HNF1A, GCK, HNF4A, ABCC8, KCNJ11, or INS during the 4-year review period. For 184 probands (55%) the indication for diagnostic testing was some form of hyperglycemia; for 30 (9%) probands the indication was hypoglycemia/hyperinsulinism, and for 117 probands (35%) no clinical information was available. Ages of the patients at the time of testing ranged from birth to 67 years; 53% of the cohort was female.
Discussion
We present the results of molecular genetic testing for disorders of pancreatic beta cell function (MODY, NDM, and CHI) from 331 probands, the largest North American cohort of these patients reported to date. Reportable variants were identified in 35% of patients. In addition to providing a useful resource for variant classification, our results highlight some of the major hurdles facing molecular diagnostic testing, including challenges with variant interpretation, paucity and heterogeneity of
Contributions
M.Z. and J.N. performed DNA extractions, PCR and sequencing analysis. J.T.B., V.V., and S.H. researched data and performed variant interpretation. P.G. contributed clinical information. J.T.B., V.V., and S.H. wrote and edited the manuscript. S.H. developed the diagnostic test and supervised the project, and serves as the guarantor of the manuscript.
Conflict of interest
There are no conflicts of interests.
Acknowledgments
This work was funded by the NIH/NIGMS Medical Genetics Training Grant T32 GM007454 (J.T.B.). The authors would like to thank all of the providers, in particular Drs. David Cobb, who referred their patients to us and the patients and their families for participating. We also thank Drs. Catherine Pihoker and Lisa Gilliam at the University of Washington and Dr. Sian Ellard for their help in validating the clinical tests.
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