No major contribution of TCF7L2 sequence variants to maturity onset of diabetes of the young (MODY) or neonatal diabetes mellitus in French white subjects

Abstract

To the Editor: Maturity onset diabetes of the young (MODY), and permanent and transient neonatal diabetes mellitus (NDM), are the most prevalent monogenic forms of diabetes. Dominant activating mutations in the KCNJ11 gene [1, 2], which encodes the inwardly rectifying Kir6.2 subunit of the ATP-sensitive potassium (KATP) channel expressed at the surface of the pancreatic beta cell, or in the ABCC8 gene [3], which encodes the SUR1 regulatory subunit of the channel, account for more than 30% of cases of NDM in the French population. So far, genes involved in the aetiology of MODY include glucokinase (GCK) [4] (also occasionally responsible for permanent NDM) and at least five beta cell-expressed transcription factors [5]. Common DNA polymorphisms in most of these genes have also been repeatedly associated with beta cell dysfunction and with type 2 diabetes in a polygenic context [6]. Recently, intronic single nucleotide polymorphisms (SNPs) of the transcription factor 7-like 2 (TCF7L2) gene have been associated with type 2 diabetes in different ethnic groups, including French white subjects [7, 8]. The physiological consequences of carrying the rs7903146 T allele, at risk for type 2 diabetes, remain unclear but there are some arguments supporting a primary effect on insulin secretion [8–10]. These data prompted us to evaluate the putative contribution of TCF7L2 in monogenic forms of diabetes. In this study, we sought variants by direct sequencing of all potential exons and intron–exon junctions of TCF7L2 in unrelated white individuals with still unexplained NDM (n=28) and MODY (n=17), as described in Table 1. Two other groups were also screened for mutations: 205 nonMODY early onset type 2 diabetes (EOD; age at diagnosis <40 years), and 93 normoglycaemic (NG) subjects (Table 1). The TCF7L2 gene (formerly known as TCF4) is subject to extensive alternative splicing [11], and its exon– intron structure was obtained using the Genbank sequences AJ270770 to AJ270778. Seventeen different exons have been described, eight of them being totally (exons 4, 13, 14, 15 and 16) or partially (exons 7, 9 and 17) alternatively expressed (Fig. 1). Alternative splicing events of exons 14 and 15 change the reading frame in exon 17, leading to three different COOH-terminal ends. When exons 14 and Diabetologia (2007) 50:214–216 DOI 10.1007/s00125-006-0505-z