Genetics of Gestational Diabetes Mellitus and Type 2 Diabetes

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Abstract
The successful application of positional cloning, the process of disease susceptibility gene identification using gene mapping techniques, to identify the gene underlying cystic fibrosis (1) opened the era of gene hunting. The subsequent identification of susceptibility genes underlying numerous monogenic disorders led to the possibility that susceptibility genes for complex diseases could also be identified using the positional cloning approach. This, coupled with advances in both genotyping technology and the identification of large numbers of microsatellite markers across the genome, led to numerous genome-wide scans to identify susceptibility genes for various forms of diabetes. However, despite the genetic screening of genes known to be involved in the biology of diabetes (candidate genes) and positional cloning approaches, to date, few susceptibility genes for diabetes have been identified. Even less is known about the genetic basis for gestational diabetes mellitus (GDM). The inability to readily identify susceptibility genes for diabetes can be attributed to a variety of issues, including insufficient statistical power, etiologic heterogeneity, and the confounding effect of interactions with environmental factors. These same problems will likely apply to GDM, which appears to represent early stages of many forms of diabetes outside of pregnancy. Here, we briefly review the current state of knowledge regarding the genetics of diabetes and discuss specific issues regarding the genetics of GDM. ### Type 1 diabetes and rare/monogenic forms of diabetes The HLA region on chromosome 6 was identified very early on as a major susceptibility gene for type 1 diabetes (2–4), with haplotypes within the HLA region accounting for as much as 50% of cases of type 1 diabetes in Caucasians (4). While the contribution of HLA to genetic susceptibility to type 1 diabetes was readily identified, numerous genome scans have also identified at least 16 additional loci across the genome that may harbor susceptibility genes for type 1 diabetes (5). However, …