Mutations in HHEX are not a common cause of monogenic forms of beta cell dysfunction

The influence of genetic variations in HHEX gene on insulin metabolism in the German MESYBEPO cohort

BACKGROUND

In the present study, we aimed to validate the type 2 diabetes (T2DM) susceptibility alleles identified in the first genome-wide association study in the hematopoietically expressed homeobox protein (HHEX) gene region (rs1111875 and rs7923837). Furthermore, we investigated quantitative metabolic risk phenotypes of these two variants for association with three key components of the insulin metabolism: insulin secretion, insulin sensitivity and insulin degradation.

METHODS

Two HHEX polymorphisms were genotyped in 1026 subjects from the German MESYBEPO cohort. Complete OGTT data were available for a subset of 420 with normal glucose tolerance (NGT), 282 with impaired glucose tolerance/impaired fasting glucose (IGT/IFG) and 146 diabetic subjects.

RESULTS

We validated association of both HHEX polymorphisms with T2DM. In the non-diabetic subcohort including NGT and IFG/IGT subjects, the risk alleles of rs7923837 and rs1111875 were significantly associated with decreased first and second phases of insulin secretion and lower insulinogenic index after oral glucose loading. In healthy, normal glucose-tolerant subjects, the same association of HHEX SNP rs1111875 with OGTT-derived phases of insulin secretion were detectable, however, rs7923837 was only weakly associated with reduced insulinogenic index. For both polymorphisms, no significant correlations with insulin sensitivity were obtained. Reduced insulin clearance was also observed in heterozygous carriers of rs1111875.

CONCLUSIONS

We validated the association of polymorphisms of the HHEX gene with T2DM in the MESYBEPO cohort. Importantly, variations within the HHEX gene conferred the impaired insulin secretion and changes of insulin degradation but no alteration in insulin sensitivity in carriers of risk alleles.

Glucokinase (GCK) and other susceptibility genes for β-cell dysfunction: the candidate approach

There are well-documented examples in the literature of where determining the genetic aetiology of a disorder has provided insights into important regulatory pathways and protein interactions, and, more recently, has led to improved treatment options for patients. The studies of monogenic forms of β-cell dysfunction are no exception. Naturally occurring mutations in the gene for the β-cell enzyme glucokinase (GCK) result in both hyper- and hypo-glycaemia. Over 200 mutations have been described, and careful study of the mutational mechanisms for a number of these has provided important insights into glucokinase regulation. Increased understanding of post-translational regulatory mechanisms holds the promise of novel pharmacotherapeutic options for the treatment of T2DM (Type 2 diabetes mellitus). It is well established that common genetic variation in genes involved in monogenic forms of β-cell dysfunction contributes to susceptibility to T2DM. Recent genome-wide scans for association have identified a number of novel T2DM susceptibility genes which probably influence β-cell mass and/or function. Their identification allows the investigation of the role of rare mutations in monogenic β-cell dysfunction. Current results indicate the importance of these genes in pancreatic development and suggest that mutations which result in a severe functional defect could be lethal.