Genetic dissection of type 2 diabetes

Abstract

Compared to the successful probing of genetic causes of monogenic disorders, dissecting the genetics of complex polygenic diseases has until recently been a fairly slow and cumbersome process. With the introduction of whole genome wide association studies (WGAS) the situation dramatically changed in 2007. The results from several recent WGAS on type 2 diabetes (T2D) and obesity have identified at least eighteen genes consistently associated with T2D. Many of the genes implicate pancreatic beta-cell function in the pathogenesis of T2D whereas only one clearly associate with insulin resistance. The identified genes most likely merely represent the tip of the iceberg in the explanation behind T2D. Refined tools will have to provide a more complete picture of the genetic complexity of T2D over the next few years. In addition to common variants increasing susceptibility for the disease, rare variants with stronger effects, copy number variations, and epigenetic effects like DNA methylation and histone acetylation will become important. Nevertheless, today we are able for the first time to anticipate that the genetics of a complex disease like T2D really can be dissected. Key wordsgenetics, complex disease, monogenic, polygenic, linkage study, genome wide scan, association study, single nucleotide polymorphism, epigenetic, type 2 diabetes, obesity. Evidence that type 2 diabetes is inheritedThere is ample evidence that T2D has a strong genetic component. The concordance of T2D in monozygotic twins is approximately 70% compared with 20-30% in dizygotic twins (1,2). Given the age-dependent penetrance of the disease, it is clear that the longer the follow-up, the higher the concordance rate (2). T2D clusters in families. The life-time risk of developing the disease is about 40% in offspring of one parent with T2D (3), greater if the mother is affected (4), the risk approaching 70% if both parents have diabetes. Translated into a s-value, the recurrence risk for a sibling of an affected person divided by the risk for the general population, this means that a first-degree relative of a patient with type 2 diabetes has a 3-fold increased risk of developing the disease (5). Large ethnic differences in the prevalence of T2D have also been ascribed to a genetic component (6,7). The change in the environment towards a more affluent Western life style plays a key role in the epidemic increase in the prevalence of T2Dworldwide. This change has occurred during the last 50 years. Clearly, our genes have not changed during this period but this does not exclude an important role for genes in the rapid increase in T2D, since genes or gene variants explain how we respond to the environment. Page 4 of 25A c c e p t e d M a n u s c r i p t 4 Thrifty genotypes or phenotypes?A plausible explanation for the interaction between genes and environment comes from the thrifty gene hypothesis. Neel (8) proposed that individuals living in an environment with unstable food supply (as for hunters and nomads) would maximize their probabilit...

Evidence that BMI and type 2 diabetes share only a minor fraction of genetic variance: a follow-up study of 23,585 monozygotic and dizygotic twins from the Finnish Twin Cohort Study

“…As the FTO gene appears to increase diabetes risk mainly via obesity [20], no gene accounting for the observed r g between BMI and type 2 diabetes exists at present. Of the genes found for BMI, a significant proportion act through the central nervous system [4,5], whereas most diabetes genes implicate pancreatic beta cell function or insulin resistance [3]. In other words, despite the strong association between diabetes risk and obesity, only part of this association is explained by genetic similarity.…”

“…The results from several recent genome-wide association studies have identified at least 18 genes consistently associated with type 2 diabetes [3]. After the initial discovery of FTO and MC4R genes, 15 new loci have so far been found for BMI [4,5].…”

“…Thus, contrary to earlier expectations, confirmation of the existence of major genetic factors behind type 2 diabetes and obesity, or shared by them has proven to be a challenging task. In addition to allelic variations, copy number variation, epigenetic changes and gene-gene interaction may contribute to susceptibility to obesity and type 2 diabetes [3]. It is therefore unlikely that molecular genetic methods will be able to uncover the complete genetic make-up in the foreseeable future.…”

Association between FTO, MC4R, SLC30A8, and KCNQ1 gene variants and type 2 diabetes in Saudi population

“…In recently years, genomewide association studies have identified various susceptibility genetic loci associated with T2DM in several ethnic populations (Hinney et al, 2010;Ridderstråle and Groop, 2009;McCarthy, 2010). Among the implicated genes include fat mass and obesity associated (FTO) Frayling et al, 2007), melanocortin 4 receptor (MC4R) (Chambers et al, 2008;Loos et al, 2008), solute carrier family 30 member 8 (SLC30A8) (Scott et al, 2007;Sladek et al, 2007;Zeggini et al, 2007), and potassium voltage-gated channel, KQT-like subfamily, member 1 (KCNQ1) (Unoki et al, 2008;Yasuda et al, 2008).…”

Melatonin receptors in pancreatic islets: good morning to a novel type 2 diabetes gene

“…The recent advent of the genome-wide association studies of type 2 diabetes and its associated traits has proved to be a tremendous breakthrough in diabetes and metabolic research [1,2]. These studies have produced lists of genes, many of which were not suspected to be associated with the disease, to which the research field now needs to shift its attention.…”