A common Greenlandic TBC1D4 variant confers muscle insulin resistance and type 2 diabetes

Abstract

The Greenlandic population, a small and historically isolated founder population comprising about 57,000 inhabitants, has experienced a dramatic increase in type 2 diabetes (T2D) prevalence during the past 25 years. Motivated by this, we performed association mapping of T2D-related quantitative traits in up to 2,575 Greenlandic individuals without known diabetes. Using array-based genotyping and exome sequencing, we discovered a nonsense p.Arg684Ter variant (in which arginine is replaced by a termination codon) in the gene TBC1D4 with an allele frequency of 17%. Here we show that homozygous carriers of this variant have markedly higher concentrations of plasma glucose (β = 3.8 mmol l(-1), P = 2.5 × 10(-35)) and serum insulin (β = 165 pmol l(-1), P = 1.5 × 10(-20)) 2 hours after an oral glucose load compared with individuals with other genotypes (both non-carriers and heterozygous carriers). Furthermore, homozygous carriers have marginally lower concentrations of fasting plasma glucose (β = -0.18 mmol l(-1), P = 1.1 × 10(-6)) and fasting serum insulin (β = -8.3 pmol l(-1), P = 0.0014), and their T2D risk is markedly increased (odds ratio (OR) = 10.3, P = 1.6 × 10(-24)). Heterozygous carriers have a moderately higher plasma glucose concentration 2 hours after an oral glucose load than non-carriers (β = 0.43 mmol l(-1), P = 5.3 × 10(-5)). Analyses of skeletal muscle biopsies showed lower messenger RNA and protein levels of the long isoform of TBC1D4, and lower muscle protein levels of the glucose transporter GLUT4, with increasing number of p.Arg684Ter alleles. These findings are concomitant with a severely decreased insulin-stimulated glucose uptake in muscle, leading to postprandial hyperglycaemia, impaired glucose tolerance and T2D. The observed effect sizes are several times larger than any previous findings in large-scale genome-wide association studies of these traits and constitute further proof of the value of conducting genetic association studies outside the traditional setting of large homogeneous populations.

The Inactivation of RabGAP Function of AS160 Promotes Lysosomal Degradation of GLUT4 and Causes Postprandial Hyperglycemia and Hyperinsulinemia

“…G: glut4/Slc2a4 only affects the long isoform that is dominant in human skeletal muscle (19), whereas the AS160 E6/7KO , AS160 E10KO , and AS160 R917K knockin mutations affect both isoforms in mice. Nevertheless, GLUT4 levels were decreased in all of the AS160 mutant mice (14)(15)(16)(17) and human AS160 Arg684Ter patients (19). One possibility for this unexpected decrease of GLUT4 in mutant mice and human patients is that the putative N-terminal fragments of AS160 might be the cause for this phenomenon.…”

“…Surprisingly, deletion of AS160 decreases GLUT4 levels and causes severe insulin resistance in mice (14)(15)(16)(17). More importantly, human patients bearing truncation mutations on AS160 (AS160 Arg363Ter or AS160 Arg684Ter , in which arginine is replaced by a stop codon) have postprandial hyperglycemia and hyperinsulinemia (18,19). GLUT4 is also decreased in skeletal muscle of patients harboring the AS160 Arg684Ter mutation (19).…”

“…More importantly, human patients bearing truncation mutations on AS160 (AS160 Arg363Ter or AS160 Arg684Ter , in which arginine is replaced by a stop codon) have postprandial hyperglycemia and hyperinsulinemia (18,19). GLUT4 is also decreased in skeletal muscle of patients harboring the AS160 Arg684Ter mutation (19). Despite the severe insulin resistance, the AS160 knockout (KO) male mice exhibit either normal (14,15,17) or impaired (16) glucose clearance, and have normal plasma insulin levels when intraperitoneally administered with glucose (14,15,17).…”

The Effect of an Extreme and Prolonged Population Bottleneck on Patterns of Deleterious Variation: Insights from the Greenlandic Inuit

“…We based all of our analyses of the GI population on the highdepth whole exome sequencing data from Moltke et al (2014). This dataset consists of nine Greenlandic trios; however, we restricted our analyses to data from the 18 unrelated parents to avoid any effect of relatedness among individuals.…”

“…To generate it, Moltke et al (2014) performed SNP and indel calling followed by genotype calling for the extended exome target region, spanning 75 MB, captured using Agilent SureSelect from the 18 Greenlandic samples using Samtools version 0.1.18 (Li et al 2009). Reads with a mapping quality lower than 30 as well as bases with a base quality lower than 20 were removed.…”