Loss-of-function mutations in MRAP2 are pathogenic in hyperphagic obesity with hyperglycemia and hypertension

The G-protein-coupled receptor (GPCR) accessory protein MRAP2 is implicated in energy control in rodents, notably via melanocortin-4 receptor (MC4R)¹. Although some MRAP2 mutations have been described in people with obesity^1–3, their functional consequences on adiposity remain elusive. Using large-scale sequencing of MRAP2 in 9,418 people, we identified 23 rare heterozygous variants associated with increased obesity risk in both adults and children. Functional assessment of each variant shows that loss-of-function MRAP2 variants are pathogenic for monogenic hyperphagic obesity, with hyperglycemia and hypertension. This contrasts with other monogenic forms of obesity characterized by excessive hunger, including MC4R deficiency, that present with low blood pressure and normal glucose tolerance⁴. The pleiotropic metabolic effect of loss-of-function mutations in MRAP2 might be due to the failure of different MRAP2-regulated GPCRs in various tissues including pancreatic islets.

The expression of genes in top obesity-associated loci is enriched in insula and substantia nigra brain regions involved in addiction and reward

Genome-wide association studies (GWAS) have identified more than 250 loci associated with body mass index (BMI) and obesity. However, post-GWAS functional genomic investigations have been inadequate for understanding how these genetic loci physiologically impact disease development. We performed a PCR-free expression assay targeting genes located nearby the GWAS-identified SNPs associated with BMI/obesity in a large panel of human tissues. Furthermore, we analyzed several genetic risk scores (GRS) summing GWAS-identified alleles associated with increased BMI in 4,236 individuals. We found that the expression of BMI/obesity susceptibility genes is strongly enriched in the brain, especially in the insula (p = 4.7×10^-9) and substantia nigra (p = 6.8×10^-7), which are two brain regions involved in addiction and reward. Inversely, we found that top obesity/BMI-associated loci, including FTO, showed the strongest gene expression enrichment in the two brain regions. Our data suggest for the first time that the susceptibility genes for common obesity may have an effect on eating addiction and reward behaviors through their high expression in substantia nigra and insula, i.e. a different pattern from monogenic obesity genes that act in the hypothalamus and cause hyperphagia. Further epidemiological studies with relevant food behavior phenotypes are necessary to confirm these findings.

Beneficial effect of a high number of copies of salivary amylase AMY1 gene on obesity risk in Mexican children

AIMS/HYPOTHESIS

Childhood obesity is a major public health problem in Mexico, affecting one in every three children. Genome-wide association studies identified genetic variants associated with childhood obesity, but a large missing heritability remains to be elucidated. We have recently shown a strong association between a highly polymorphic copy number variant encompassing the salivary amylase gene (AMY1 also known as AMY1A) and obesity in European and Asian adults. In the present study, we aimed to evaluate the association between AMY1 copy number and obesity in Mexican children.

METHODS

We evaluated the number of AMY1 copies in 597 Mexican children (293 obese children and 304 normal weight controls) through highly sensitive digital PCR. The effect of AMY1 copy number on obesity status was assessed using a logistic regression model adjusted for age and sex.

RESULTS

We identified a marked effect of AMY1 copy number on reduced risk of obesity (OR per estimated copy 0.84, with the number of copies ranging from one to 16 in this population; p = 4.25 × 10(-6)). The global association between AMY1 copy number and reduced risk of obesity seemed to be mostly driven by the contribution of the highest AMY1 copy number. Strikingly, all children with >10 AMY1 copies were normal weight controls.

CONCLUSIONS/INTERPRETATION

Salivary amylase initiates the digestion of dietary starch, which is highly consumed in Mexico. Our current study suggests putative benefits of high number of AMY1 copies (and related production of salivary amylase) on energy metabolism in Mexican children.

Alternative human liver transcripts of TCF7L2 bind to the gluconeogenesis regulator HNF4α at the protein level

AIMS/HYPOTHESIS

Gene polymorphisms of TCF7L2 are associated with increased risk of type 2 diabetes and transcription factor 7-like 2 (TCF7L2) plays a role in hepatic glucose metabolism. We therefore addressed the impact of TCF7L2 isoforms on hepatocyte nuclear factor 4α (HNF4α) and the regulation of gluconeogenesis genes.

METHODS

Liver TCF7L2 transcripts were analysed by quantitative PCR in 33 non-diabetic and 31 type 2 diabetic obese individuals genotyped for TCF7L2 rs7903146. To analyse transcriptional regulation by TCF7L2, small interfering RNA transfection, luciferase reporter and co-immunoprecipitation assays were performed in human hepatoma HepG2 cells.

RESULTS

In livers of diabetic compared with normoglycaemic individuals, five C-terminal TCF7L2 transcripts showed increased expression. The type 2 diabetes risk allele of rs7903146 positively correlated with TCF7L2 expression in livers from normoglycaemic individuals only. In HepG2 cells, transcript and TCF7L2 protein levels were increased upon incubation in high glucose and insulin. Of the exon 13 transcripts, six were increased in a glucose dose-responsive manner. TCF7L2 transcriptionally regulated 29 genes related to glucose metabolism, including glucose-6-phosphatase. In cultured HepG2 cells, TCF7L2 did not regulate HNF4Α and FOXO1 transcription, but did affect HNF4α protein expression. The TCF7L2 isoforms T6 and T8 (without exon 13 and with exon 15/14, respectively) specifically interacted with HNF4α.

CONCLUSIONS/INTERPRETATION

The different levels of expression of alternative C-terminal TCF7L2 transcripts in HepG2 cells, in livers of normoglycaemic individuals carrying the rs7901346 type 2 diabetes risk allele and in livers of diabetic individuals suggest that these transcripts play a role in the pathophysiology of type 2 diabetes. We also report for the first time a protein interaction in HepG2 cells between HNF4α and the T6 and T8 isoforms of TCF7L2, which suggests a distinct role for these specific alternative transcripts.

TCF7L2 rs7903146 impairs islet function and morphology in non-diabetic individuals

AIMS/HYPOTHESIS

Transcription factor 7-like 2 (TCF7L2) is a Wnt-signalling-associated transcription factor. Genetic studies have clearly demonstrated that DNA polymorphisms within TCF7L2 confer the strongest known association with increased risk of type 2 diabetes. However, the impact of the TCF7L2 type-2-diabetes-associated rs7903146 T allele on biological function and morphology of human pancreatic islets is unknown.

METHODS

Paraffin sections of pancreases from 187 brain-deceased donors (HbA(1c) <6.5% [48 mmol/mol]) were used to genotype the TCF7L2 variant rs7903146 and evaluate its impact on islet morphology and alpha and beta cell subpopulations following immunostaining for glucagon and C-peptide. Following islet isolation, we investigated the correlation between TCF7L2 genotype and in vitro islet functional variables from our in-house pancreatic database.

RESULTS

TCF7L2 rs7903146 (T/T) was associated with reduced basal and glucose-stimulated insulin secretion in isolated human islets, and reduced islet density in whole pancreas. Morphological analysis demonstrated islet size was increased in T/T carriers. Furthermore, rs7903146 was associated with an increased glucagon/C-peptide ratio, especially in bigger islets.

CONCLUSION/INTERPRETATION

The TCF7L2 variant rs7903146 risk allele is associated with impaired insulin secretion, reduction of total islet number and quantitative as well as qualitative morphological changes in human islets. Understanding how the TCF7L2 genotype modulates its activity and how TCF7L2 impacts the islet morphology may aid the design of new therapeutic approaches for the treatment of type 2 diabetes.

Whole-exome sequencing and high throughput genotyping identified KCNJ11 as the thirteenth MODY gene

Background

Maturity-onset of the young (MODY) is a clinically heterogeneous form of diabetes characterized by an autosomal-dominant mode of inheritance, an onset before the age of 25 years, and a primary defect in the pancreatic beta-cell function. Approximately 30% of MODY families remain genetically unexplained (MODY-X). Here, we aimed to use whole-exome sequencing (WES) in a four-generation MODY-X family to identify a new susceptibility gene for MODY.

Methodology

WES (Agilent-SureSelect capture/Illumina-GAIIx sequencing) was performed in three affected and one non-affected relatives in the MODY-X family. We then performed a high-throughput multiplex genotyping (Illumina-GoldenGate assay) of the putative causal mutations in the whole family and in 406 controls. A linkage analysis was also carried out.

Principal Findings

By focusing on variants of interest (i.e. gains of stop codon, frameshift, non-synonymous and splice-site variants not reported in dbSNP130) present in the three affected relatives and not present in the control, we found 69 mutations. However, as WES was not uniform between samples, a total of 324 mutations had to be assessed in the whole family and in controls. Only one mutation (p.Glu227Lys in KCNJ11) co-segregated with diabetes in the family (with a LOD-score of 3.68). No KCNJ11 mutation was found in 25 other MODY-X unrelated subjects.

Conclusions/Significance

Beyond neonatal diabetes mellitus (NDM), KCNJ11 is also a MODY gene (‘MODY13’), confirming the wide spectrum of diabetes related phenotypes due to mutations in NDM genes (i.e. KCNJ11, ABCC8 and INS). Therefore, the molecular diagnosis of MODY should include KCNJ11 as affected carriers can be ideally treated with oral sulfonylureas.

Low-frequency variants in HMGA1 are not associated with type 2 diabetes risk

It has recently been suggested that the low-frequency c.136-14_136-13insC variant in high-mobility group A1 (HMGA1) may strongly contribute to insulin resistance and type 2 diabetes risk. In our study, we attempted to confirm that HMGA1 is a novel type 2 diabetes locus in French Caucasians. The gene was sequenced in 368 type 2 diabetic case subjects with a family history of type 2 diabetes and 372 normoglycemic control subjects without a family history of type 2 diabetes. None of the 41 genetic variations identified were associated with type 2 diabetes. The lack of association between the c.136-14_136-13insC variant and type 2 diabetes was confirmed in an independent French group of 4,538 case subjects and 4,015 control subjects and in a large meta-analysis of 16,605 case subjects and 46,179 control subjects. Finally, this variant had no effects on metabolic traits and was not involved in variations of HMGA1 and insulin receptor (INSR) expressions. The c.136-14_136-13insC variant was not associated with type 2 diabetes in individuals of European descent. Our study emphasizes the need to analyze a large number of subjects to reliably assess the association of low-frequency variants with the disease.