Rare Heterozygous Loss-of-Function Variants in the Human GLP-1 Receptor Are Not Associated With Cardiometabolic Phenotypes.

CONTEXT: Lost glucagon-like peptide 1 receptor (GLP-1R) function affects human physiology. OBJECTIVE: This work aimed to identify coding nonsynonymous GLP1R variants in Danish individuals to link their in vitro phenotypes and clinical phenotypic associations. METHODS: We sequenced GLP1R in 8642 Danish individuals with type 2 diabetes or normal glucose tolerance and examined the ability of nonsynonymous variants to bind GLP-1 and to signal in transfected cells via cyclic adenosine monophosphate (cAMP) formation and ß-arrestin recruitment. We performed a cross-sectional study between the burden of loss-of-signaling (LoS) variants and cardiometabolic phenotypes in 2930 patients with type 2 diabetes and 5712 participants in a population-based cohort. Furthermore, we studied the association between cardiometabolic phenotypes and the burden of the LoS variants and 60 partly overlapping predicted loss-of-function (pLoF) GLP1R variants found in 330 566 unrelated White exome-sequenced participants in the UK Biobank cohort. RESULTS: We identified 36 nonsynonymous variants in GLP1R, of which 10 had a statistically significant loss in GLP-1-induced cAMP signaling compared to wild-type. However, no association was observed between the LoS variants and type 2 diabetes, although LoS variant carriers had a minor increased fasting plasma glucose level. Moreover, pLoF variants from the UK Biobank also did not reveal substantial cardiometabolic associations, despite a small effect on glycated hemoglobin A1c. CONCLUSION: Since no homozygous LoS nor pLoF variants were identified and heterozygous carriers had similar cardiometabolic phenotype as noncarriers, we conclude that GLP-1R may be of particular importance in human physiology, due to a potential evolutionary intolerance of harmful homozygous GLP1R variants.

Identification of novel LEPR mutations in Pakistani families with morbid childhood obesity.

BACKGROUND: Mutations in the genes encoding leptin (LEP), the leptin receptor (LEPR), and the melanocortin 4 receptor (MC4R) are known to cause severe early-onset childhood obesity. The aim of the current study was to examine the prevalence of damaging LEP, LEPR, and MC4R mutations in Pakistani families having a recessive heritance of early-onset obesity. METHODS: Using targeted resequencing, the presence of rare mutations in LEP, LEPR, and MC4R, was investigated in individuals from 25 families suspected of having autosomal recessive early-onset obesity. Segregation patterns of variants were assessed based on chip-based genotyping. RESULTS: Homozygous LEPR variants were identified in two probands. One carried a deletion (c.3260AG) resulting in the frameshift mutation p.Ser1090Trpfs*6, and the second carried a substitution (c.2675C > G) resulting in the missense mutation p.Pro892Arg. Both mutations were located within regions of homozygosity shared only among affected individuals. Both probands displayed early-onset obesity, hyperphagia and diabetes. No mutations were found in LEP and MC4R. CONCLUSIONS: The current study highlights the implication of LEPR mutations in cases of severe early-onset obesity in consanguineous Pakistani families. Through targeted resequencing, we identified novel damaging mutations, and our approach may therefore be utilized in clinical testing or diagnosis of known forms of monogenic obesity with the aim of optimizing obesity treatment.

Impact of PTBP1 rs11085226 on glucose-stimulated insulin release in adult Danes.

BACKGROUND: The variant rs11085226 (G) within the gene encoding polypyrimidine tract binding protein 1 (PTBP1) was reported to associate with reduced insulin release determined by an oral glucose tolerance test (OGTT) as well as an intravenous glucose tolerance test (IVGTT). The aim of the present study was to validate the association of the rs11085226 G-allele of PTBP1 with previously investigated OGTT- and IVGTT-derived diabetes-related metabolic quantitative phenotypes, to conduct exploratory analyses of additional measures of beta-cell function, and to further investigate a potential association with type 2 diabetes. METHODS: PTBP1 rs11085226 was genotyped in 20,911 individuals of Danish Caucasian ethnicity ascertained from 9 study samples. Case control analysis was performed on 5,634 type 2 diabetic patients and 11,319 individuals having a normal fasting glucose level as well as 4,641 glucose tolerant controls, respectively. Quantitative trait analyses were performed in up to 13,605 individuals subjected to an OGTT or blood samples obtained after an overnight fast, as well as in 596 individuals subjected to an IVGTT. RESULTS: Analyses of fasting and OGTT-derived quantitative traits did not show any significant associations with the PTBP1 rs11085226 variant. Meta-analysis of IVGTT-derived quantitative traits showed a nominally significant association between the variant and reduced beta-cell responsiveness to glucose (ß = -0.1 mmol · kg(-1) · min(-1); 95% CI: -0.200.20 - -0.024; P = 0.01) assuming a dominant model of inheritance, but failed to replicate a previously reported association with area under the curve (AUC) for insulin. Case control analysis did not show an association of the PTBP1 rs11085226 variant with type 2 diabetes. CONCLUSIONS: Despite failure to replicate the previously reported associations of PTBP1 rs11085226 with OGTT- and IVGTT-derived measures of beta-cell function, we did find a nominally significant association with reduced beta-cell responsiveness to glucose during an IVGTT, a trait not previously investigated, leaving the potential influence of this variant in PTBP1 on glucose stimulated insulin release open for further investigation. However, the present study does not support the hypothesis that the variant confers risk of type 2 diabetes.

The FOXO3A rs2802292 G-allele associates with improved peripheral and hepatic insulin sensitivity and increased skeletal muscle-FOXO3A mRNA expression in twins.

OBJECTIVE: The minor G-allele of FOXO3A rs2802292 has been associated with longevity. We aimed to investigate whether a phenotype related to healthy metabolic aging could be identified in individuals carrying the longevity-associated FOXO3A rs2802292 G-allele. RESEARCH DESIGN AND METHODS: rs2802292 was genotyped in a phenotypically well-characterized population of young and elderly twins (n = 190) and in the population-based Inter99 cohort (n = 5768). All participants underwent oral glucose tolerance tests, and the twin population was additionally examined with an iv glucose tolerance test and a hyperinsulinemic, euglycemic clamp. Basal and insulin-stimulated FOXO3A mRNA expression was assessed in skeletal muscle biopsies from the twin population. RESULTS: In the twin sample, carriers of the minor G-allele of rs2802292 showed reduced fasting plasma insulin [per allele effect (ß) = -13% (-24; -1) (95% confidence interval), P = 0.03] and lower incremental area under the curve 0-120 min for insulin after an oral glucose load [ß = -14% (-23; -5), P = 0.005]. The G-allele was associated with increased peripheral insulin action [glucose disposal rate clamp, ß = 0.85 mg · kg(fat-free mass)(-1) · min(-1)() (0.049; 1.64), P = 0.04] and lower hepatic insulin resistance index [ß = -13% (-25; -1), P = 0.03]. Furthermore, carriers of the G-allele had increased basal FOXO3A mRNA expression in skeletal muscle compared with T-allele carriers [ß = 16% (0; 33), P = 0.047]. In the Inter99 sample, we found an association with reduced incremental area under the curve 0-120 min for insulin after an oral glucose load [ß = -3% (-5; -0.07), P = 0.04], but this association was not significant after adjustment for body mass index. CONCLUSION: Our data indicate that the minor G-allele of FOXO3A rs2802292 is associated with enhanced peripheral and hepatic insulin sensitivity in our small twin cohort, which may be mediated through increased FOXO3A mRNA expression, although no major metabolic impact of rs2802292 was found in the large Inter99 cohort.