Quantitative linkage analysis for pancreatic B-cell function and insulin resistance in a large twin cohort

Human pluripotent stem cell-based models suggest preadipocyte senescence as a possible cause of metabolic complications of Werner and Bloom Syndromes

Werner Syndrome (WS) and Bloom Syndrome (BS) are disorders of DNA damage repair caused by biallelic disruption of the WRN or BLM DNA helicases respectively. Both are commonly associated with insulin resistant diabetes, usually accompanied by dyslipidemia and fatty liver, as seen in lipodystrophies. In keeping with this, progressive reduction of subcutaneous adipose tissue is commonly observed. To interrogate the underlying cause of adipose tissue dysfunction in these syndromes, CRISPR/Cas9 genome editing was used to generate human pluripotent stem cell (hPSC) lacking either functional WRN or BLM helicase. No deleterious effects were observed in WRN^−/− or BLM^−/− embryonic stem cells, however upon their differentiation into adipocyte precursors (AP), premature senescence emerged, impairing later stages of adipogenesis. The resulting adipocytes were also found to be senescent, with increased levels of senescent markers and senescence-associated secretory phenotype (SASP) components. SASP components initiate and reinforce senescence in adjacent cells, which is likely to create a positive feedback loop of cellular senescence within the adipocyte precursor compartment, as demonstrated in normal ageing. Such a scenario could progressively attenuate adipose mass and function, giving rise to “lipodystrophy-like” insulin resistance. Further assessment of pharmacological senolytic strategies are warranted to mitigate this component of Werner and Bloom syndromes.

A Low-Frequency Inactivating AKT2 Variant Enriched in the Finnish Population Is Associated With Fasting Insulin Levels and Type 2 Diabetes Risk

To identify novel coding association signals and facilitate characterization of mechanisms influencing glycemic traits and type 2 diabetes risk, we analyzed 109,215 variants derived from exome array genotyping together with an additional 390,225 variants from exome sequence in up to 39,339 normoglycemic individuals from five ancestry groups. We identified a novel association between the coding variant (p.Pro50Thr) in AKT2 and fasting plasma insulin (FI), a gene in which rare fully penetrant mutations are causal for monogenic glycemic disorders. The low-frequency allele is associated with a 12% increase in FI levels. This variant is present at 1.1% frequency in Finns but virtually absent in individuals from other ancestries. Carriers of the FI-increasing allele had increased 2-h insulin values, decreased insulin sensitivity, and increased risk of type 2 diabetes (odds ratio 1.05). In cellular studies, the AKT2-Thr50 protein exhibited a partial loss of function. We extend the allelic spectrum for coding variants in AKT2 associated with disorders of glucose homeostasis and demonstrate bidirectional effects of variants within the pleckstrin homology domain of AKT2.

Metabolomic profiling to dissect the role of visceral fat in cardiometabolic health

OBJECTIVE

Abdominal obesity is associated with increased risk of type 2 diabetes (T2D) and cardiovascular disease. The aim of this study was to assess whether metabolomic markers of T2D and blood pressure (BP) act on these traits via visceral fat (VF) mass.

METHODS

Metabolomic profiling of 280 fasting plasma metabolites was conducted on 2,401 women from TwinsUK. The overlap was assessed between published metabolites associated with T2D, insulin resistance, or BP and those that were identified to be associated with VF (after adjustment for covariates) measured by dual-energy X-ray absorptiometry.

RESULTS

In addition to glucose, six metabolites were strongly associated with both VF mass and T2D: lactate and branched-chain amino acids, all of them related to metabolism and the tricarboxylic acid cycle; on average, 38.5% of their association with insulin resistance was mediated by their association with VF mass. Five metabolites were associated with BP and VF mass including the inflammation-associated peptide HWESASXX, the steroid hormone androstenedione, lactate, and palmitate. On average, 29% of their effect on BP was mediated by their association with VF mass.

CONCLUSIONS

Little overlap was found between the metabolites associated with BP and those associated with insulin resistance via VF mass.

Metabolic syndrome in patients taking clozapine: prevalence and influence of catechol-O-methyltransferase genotype

RATIONALE

Metabolic syndrome (MetS) has consistently been identified as an adverse effect of long-term treatment with atypical antipsychotics (AAPs) such as clozapine. Elevated serum homocysteine concentration has been found to act as an independent risk factor for MetS, and catechol-O-methyltransferase (COMT) catalyzes the homocysteine metabolism. We accordingly hypothesized that COMT dysregulation may confer the susceptibility to MetS induced by AAPs, potentially in a gender-specific manner, because the interaction effects of COMT and gender have been consistently reported.

OBJECTIVES

This study aimed at determining the prevalence and influence of COMT on MetS among a population undergoing long-term clozapine treatment.

METHODS

A total of 468 schizophrenia patients taking clozapine were divided into two groups, those experiencing MetS and non-MetS. We genotyped three functional variants (rs4633, rs4680, and rs4818) in COMT and measured the serum levels of fasting homocysteine, glucose, triglyceride (TG), and high-density lipoprotein cholesterol.

RESULTS

MetS was found in 202/468 (43.2 %) of all the patients, with 40.2 % prevalence (138/343) in males and 51.2 % (64/125) in females. Patients with MetS had notably higher metabolic parameters than those without MetS. The mean levels of homocysteine in patients with MetS were significantly higher than those without MetS. We found a positive association between the rs4680 polymorphism and the serum triglyceride levels (corrected P = 0.024). Further analysis revealed that the rs4680 Met allele was significantly associated with increased triglyceride levels among female patients (P = 0.009), but not among males (P = 0.07).

CONCLUSIONS

Our findings suggest a potential association between rs4680 in COMT and elevated TG levels, particularly among female patients.

Genetics in endocrinology: genetic forms of severe insulin resistance: what endocrinologists should know

'Insulin resistance' (IR) is a widely used clinical term. It is usually defined as a state characterised by reduced glucose-lowering activity of insulin, but it is also sometimes used as a shorthand label for a clinical syndrome encompassing major pathologies such as type 2 diabetes, polycystic ovary syndrome, fatty liver disease and atherosclerosis. Nevertheless, the precise cellular origins of IR, the causal links among these phenomena and the mechanisms underlying them remain poorly understood or contentious. Prevalent IR usually results from a genetic predisposition interacting with acquired obesity; however, even in some lean individuals, very severe degrees of IR can be observed. It is important to identify these people as they often harbour identifiable single-gene defects and they may benefit from molecular diagnosis, genetic counselling and sometimes tailored therapies. Observation of people with known single-gene defects also offers the opportunity to make inferences about the mechanistic links between IR and common pathologies. Herein, we summarise the currently known monogenic forms of severe IR, with an emphasis on the practical aspects of their recognition, diagnosis and management. In particular, we draw distinctions among the biochemical subphenotypes of IR that arise from primary adipose tissue dysfunction or from primary insulin signalling defects and discuss the implications of this dichotomy for management.

Extent, causes, and consequences of small RNA expression variation in human adipose tissue

Small RNAs are functional molecules that modulate mRNA transcripts and have been implicated in the aetiology of several common diseases. However, little is known about the extent of their variability within the human population. Here, we characterise the extent, causes, and effects of naturally occurring variation in expression and sequence of small RNAs from adipose tissue in relation to genotype, gene expression, and metabolic traits in the MuTHER reference cohort. We profiled the expression of 15 to 30 base pair RNA molecules in subcutaneous adipose tissue from 131 individuals using high-throughput sequencing, and quantified levels of 591 microRNAs and small nucleolar RNAs. We identified three genetic variants and three RNA editing events. Highly expressed small RNAs are more conserved within mammals than average, as are those with highly variable expression. We identified 14 genetic loci significantly associated with nearby small RNA expression levels, seven of which also regulate an mRNA transcript level in the same region. In addition, these loci are enriched for variants significant in genome-wide association studies for body mass index. Contrary to expectation, we found no evidence for negative correlation between expression level of a microRNA and its target mRNAs. Trunk fat mass, body mass index, and fasting insulin were associated with more than twenty small RNA expression levels each, while fasting glucose had no significant associations. This study highlights the similar genetic complexity and shared genetic control of small RNA and mRNA transcripts, and gives a quantitative picture of small RNA expression variation in the human population.

A twin study of mitochondrial DNA polymorphisms shows that heteroplasmy at multiple sites is associated with mtDNA variant 16093 but not with zygosity

The mitochondrial theory of ageing proposes that damage to mitochondria and diminished mitochondrial DNA (mtDNA) repair are major contributors to cellular dysfunction and age-related diseases. We investigate the prevalence of heteroplasmy in the mtDNA control region in buccal swab and blood derived samples for 178 women from the TwinsUK cohort (41 DZ pair 39 MZ pairs, 18 singletons, mean age 57.5 range 28–82) and its relationship to age, BMI and fasting insulin and glucose serum levels. The overall estimated prevalence of heteroplasmy for both tissues in the control region measured for 37 sites was 17%. The prevalence of heteroplasmy was higher among the older half of the study subjects than in the younger half (23% vs 10% p<0.03), primarily reflecting the increase in the prevalence of a heteroplasmic dinucleotide CA repeat in variable region II (VRII) with age. The VRII 523–524 heteroplasmic site (heteroplasmic in 25 subjects) was also associated with a decrease in BMI. In addition, concordance rates for common heteroplasmy were observed to be near complete for both dizygotic (DZ = 94%) and monozygotic twin pairs (MZ = 100%), consistent with previous reports that suggest variation in heteroplasmy rates between generations are determined by bottlenecks in maternal transmission of mitochondria. Differences in the prevalence of heteroplasmy were observed overall between samples derived from buccal swabs (19%) and blood (15%, p<0.04). These were particularly marked at position 16093 of hypervariable region I (HVI, 7% vs 0%, respectively, p<4×10⁻¹¹). The presence of the C allele at position 16093 in blood was associated with the presence of heteroplasmy in buccal swabs at this position (p = 3.5×10⁻¹⁴) and also at VRII (p = 2×10⁻⁴) suggesting a possible predisposing role for this site in the accumulation of heteroplasmy. Our data indicate that BMI is potentially associated with control region heteroplasmy.

Genetic syndromes of severe insulin resistance

Insulin resistance is among the most prevalent endocrine derangements in the world, and it is closely associated with major diseases of global reach including diabetes mellitus, atherosclerosis, nonalcoholic fatty liver disease, and ovulatory dysfunction. It is most commonly found in those with obesity but may also occur in an unusually severe form in rare patients with monogenic defects. Such patients may loosely be grouped into those with primary disorders of insulin signaling and those with defects in adipose tissue development or function (lipodystrophy). The severe insulin resistance of both subgroups puts patients at risk of accelerated complications and poses severe challenges in clinical management. However, the clinical disorders produced by different genetic defects are often biochemically and clinically distinct and are associated with distinct risks of complications. This means that optimal management of affected patients should take into account the specific natural history of each condition. In clinical practice, they are often underdiagnosed, however, with low rates of identification of the underlying genetic defect, a problem compounded by confusing and overlapping nomenclature and classification. We now review recent developments in understanding of genetic forms of severe insulin resistance and/or lipodystrophy and suggest a revised classification based on growing knowledge of the underlying pathophysiology.

Identification of an imprinted master trans regulator at the KLF14 locus related to multiple metabolic phenotypes

Genome-wide association studies have identified many genetic variants associated with complex traits. However, at only a minority of loci have the molecular mechanisms mediating these associations been characterized. In parallel, whereas cis regulatory patterns of gene expression have been extensively explored, the identification of trans regulatory effects in humans has attracted less attention. Here we show that the type 2 diabetes and high-density lipoprotein cholesterol-associated cis-acting expression quantitative trait locus (eQTL) of the maternally expressed transcription factor KLF14 acts as a master trans regulator of adipose gene expression. Expression levels of genes regulated by this trans-eQTL are highly correlated with concurrently measured metabolic traits, and a subset of the trans-regulated genes harbor variants directly associated with metabolic phenotypes. This trans-eQTL network provides a mechanistic understanding of the effect of the KLF14 locus on metabolic disease risk and offers a potential model for other complex traits.

Agreement among type 2 diabetes linkage studies but a poor correlation with results from genome-wide association studies

AIMS/HYPOTHESIS

Little of the genetic basis for type 2 diabetes has been explained, despite numerous genetic linkage studies and the discovery of multiple genes in genome-wide association (GWA) studies. To begin to resolve the genetic component of this disease, we searched for sites at which genetic results had been corroborated in different studies, in the expectation that replication among studies should direct us to the genomic locations of causative genes with more confidence than the results of individual studies.

METHODS

We have mapped the physical location of results from 83 linkage reports (for type 2 diabetes and diabetes precursor quantitative traits [QTs, e.g. plasma insulin levels]) and recent large GWA reports (for type 2 diabetes) onto the same human genome sequence to identify replicated results in diabetes genetic 'hot spots'.

RESULTS

Genetic linkage has been found at least ten times at 18 different locations, and at least five times in 56 locations. All replication clusters contained study populations from more than one ethnic background and most contained results for both diabetes and QTs. There is no close relationship between the GWA results and linkage clusters, and the nine best replication clusters have no nearby GWA result.

CONCLUSIONS/INTERPRETATION

Many of the genes for type 2 diabetes remain unidentified. This analysis identifies the broad location of yet to be identified genes on 6q, 1q, 18p, 2q, 20q, 17pq, 8p, 19q and 9q. The discrepancy between the linkage and GWA studies may be explained by the presence of multiple, uncommon, mildly deleterious polymorphisms scattered throughout the regulatory and coding regions of genes for type 2 diabetes.