An abundant, truncated human sulfonylurea receptor 1 splice variant has prodiabetic properties and impairs sulfonylurea action

Abstract

An alternatively spliced form of human sulfonylurea receptor (SUR) 1 mRNA lacking exon 2 (SUR1Δ2) has been identified. The omission of exon 2 caused a frame shift and an immediate stop codon in exon 3 leading to translation of a 5.6-kDa peptide that comprises the N-terminal extracellular domain and the first transmembrane helix of SUR1. Based on a weak first splice acceptor site in the human SUR1 gene (ABCC8), RT-PCR revealed a concurrent expression of SUR1Δ2 and SUR1. The SUR1Δ2/(SUR1 + SUR1Δ2) mRNA ratio differed between tissues, and was lowest in pancreas (46%), highest in heart (88%) and negatively correlated with alternative splice factor/splicing factor 2 (ASF/SF2) expression. In COS-7 cells triple transfected with SUR1Δ2/SUR1/Kir6.2, the SUR1Δ2 peptide co-immunoprecipitated with Kir6.2, thereby displacing two of four SUR1 subunits on the cell surface. The ATP sensitivity of these hybrid ATP-sensitive potassium channels (K(ATP)) channels was reduced by about sixfold, as shown with single-channel recordings. RINm5f rat insulinoma cells, which genuinely express SUR1 but not SUR1Δ2, exhibited a strongly increased K(ATP) channel current upon transfection with SUR1Δ2. This led to inhibition of glucose-induced depolarization, calcium flux, insulin release and glibenclamide action. A non-mutagenic SNP on nucleotide position 333 (Pro69Pro) added another exonic splicing enhancer sequence detected by ASF/SF2, reduced relative abundance of SUR1Δ2 and slightly protected from non-insulin dependent diabetes in homozygotic individuals. Thus, SUR1Δ2 represents an endogenous K(ATP)-channel modulator with prodiabetic properties in islet cells. Its predominance in heart may explain why high-affinity sulfonylurea receptors are not found in human cardiac tissue.

Marked clinical heterogeneity in congenital hyperinsulinism due to a novel homozygous ABCC8 mutation

“…13 In our case series, all patients had the same homozygous mutation: a 1-bp deletion around the splice acceptor site at exon 35 of ABCC8 which was reported as an unpublished pathogenic variant in a recent review. 17 Some alternatively spliced forms of human sulfonylurea receptor mRNA, which can be translated into a group of diverse proteins with different roles and structures in various tissues, were reported, 18 and clinical heterogeneity in carriers with the same mutation was speculated to be due to differences in splicing factor machinery. 13 Therefore, we analysed expression of exon 35 in normal human islets and two patient's lymphocytes by ddPCR to quantitatively detect alternative or abnormal splice variants.…”

Abnormalities in alternative splicing in diabetes: therapeutic targets

“…There are genes whose function is linked to obesity and insulin resistance that are regulated by AS (Pihlajamaki et al 2011) (Table 1). Examples of such aberrantly spliced genes in diabetes are ANO1, HNF-1a, IPF-1, GCK, SUR1, TCF7L2, VEGF and NOVA1 (Garin et al 2008, Costantini et al 2011, Schmid et al 2012.…”

“…ABCC8 is a member of the MRP multi-drug resistance sub-family and is involved in ATP-sensitive potassium channel modulation. Pancreatic β-cells express the KATP channels that play an important role in insulin release and ABCC8 encodes SUR1, a regulatory subunit of the KATP channels (Schmid et al 2012). Different isoforms of the channels are found expressed in the heart, pancreatic islet cells, brain, blood vessels, etc.…”

“…One such isoform, found in the pancreas of diabetic patients, is SUR1DM2 which lacks exon 2 as a result of a weak first splice acceptor site (Fig. 4) (Schmid et al 2012). The truncated transcript is thought to decrease the sensitivity of the channels to ATP resulting in insulin inhibition.…”

“…The truncated transcript is thought to decrease the sensitivity of the channels to ATP resulting in insulin inhibition. The presence of the SNP rs1048099 (Pro69Pro, T > C) on the gene of healthy individuals, generates an exonic splicing enhancer that results in the inclusion of exon 2 which confers resistance to diabetes (Sakamoto et al 2007, Schmid et al 2012.…”