Founder effect in the Horn of Africa for an insulin receptor mutation that may impair receptor recycling

Genome-wide analyses disclose the distinctive HLA architecture and the pharmacogenetic landscape of the Somali population

African populations are underrepresented in medical genomics studies. For the Somali population, there is virtually no information on genomic markers with significance to precision medicine. Here, we analyzed nearly 900,000 genomic markers in samples collected from 95 unrelated individuals in the North Eastern Somalia. ADMIXTURE program for estimation of individual ancestries revealed a homogenous Somali population. Principal component analysis with PLINK software showed approximately 60% East African and 40% West Eurasian genes in the Somali population, with a close relation to the Cushitic and Semitic speaking Ethiopian populations. We report the unique features of human leukocyte antigens (HLA) in the Somali population, which seem to differentiate from all other neighboring regions compared. Current study identified high prevalence of the diabetes type 1 (T1D) predisposing HLA DR-DQ haplotypes in Somalia. This finding may explain the increased T1D risk observed among Somali children. In addition, ethnic Somalis were found to host the highest frequencies observed thus far for several pharmacogenetic variants, including UGT1A4*2. In conclusion, we report that the Somali population displays genetic traits of significance to health and disease. The Somali dataset is publicly available and will add more information to the few genomic datasets available for African populations.

Insulin Receptor Trafficking: Consequences for Insulin Sensitivity and Diabetes

Insulin receptor (INSR) has been extensively studied in the area of cell proliferation and energy metabolism. Impaired INSR activities lead to insulin resistance, the key factor in the pathology of metabolic disorders including type 2 diabetes mellitus (T2DM). The mainstream opinion is that insulin resistance begins at a post-receptor level. The role of INSR activities and trafficking in insulin resistance pathogenesis has been largely ignored. Ligand-activated INSR is internalized and trafficked to early endosome (EE), where INSR is dephosphorylated and sorted. INSR can be subsequently conducted to lysosome for degradation or recycled back to the plasma membrane. The metabolic fate of INSR in cellular events implies the profound influence of INSR on insulin signaling pathways. Disruption of INSR-coupled activities has been identified in a wide range of insulin resistance-related diseases such as T2DM. Accumulating evidence suggests that alterations in INSR trafficking may lead to severe insulin resistance. However, there is very little understanding of how altered INSR activities undermine complex signaling pathways to the development of insulin resistance and T2DM. Here, we focus this review on summarizing previous findings on the molecular pathways of INSR trafficking in normal and diseased states. Through this review, we provide insights into the mechanistic role of INSR intracellular processes and activities in the development of insulin resistance and diabetes.

A novel insulin receptor mutation in an adolescent with acanthosis nigricans and hyperandrogenism

Insulin receptor mutations cause extreme insulin resistance resulting in acanthosis nigricans and hyperandrogenism. We report a pre-menarchal adolescent female with normal weight, with severe acanthosis nigricans, acne, and hirsutism. Initial investigation revealed elevated fasting and post-prandial insulin and high testosterone and androstenedione levels. Her father had frequent complaints of hypoglycemia. Coding sequence and splice junction analysis of the INSR gene, in our patient and her father, revealed a heterozygous missense mutation in the β subunit of the insulin receptor (Arg1131Trp), resulting in receptor loss of function. Metformin therapy and carbohydrate control improved acanthosis and menarche ensued within 3 months. Our case highlights the importance of distinguishing insulin resistance commonly associated with obesity from monogenic defects. Although, there is no consensus on treatment of children with monogenic forms of insulin resistance due to its rarity, dietary and lifestyle modifications and insulin-sensitizing agents play a key role in management.

Insulin Receptor and the Kidney: Nephrocalcinosis in Patients with Recessive INSR Mutations

BACKGROUND/AIMS

Donohue and Rabson-Mendenhall syndrome are rare autosomal recessive disorders caused by mutations in the insulin receptor gene, INSR. Phenotypic features include extreme insulin resistance, linear growth retardation, paucity of fat and muscle, and soft tissue overgrowth. The insulin receptor is also expressed in the kidney, where animal data suggest it plays a role in glomerular function and blood pressure (BP) regulation, yet such a role in the human kidney is untested. Patients with biallelic INSR mutations provide a rare opportunity to ascertain its role in man.

METHODS

Retrospective review of patients with INSR mutations. Data for BP, renal imaging, plasma creatinine and electrolyte levels, as well as urine protein, albumin and calcium excretion were sought from the treating clinicians.

RESULTS

From 33 patients with INSR mutations, data were available for 17 patients. Plasma creatinine was low (mean ± SD: 25 ± 9 μmol/l) and mean plasma electrolyte concentrations were within the normal range (n = 13). Systolic BP ranged between the 18th and 91st percentile for age, sex, height and weight (n = 9; mean ± SD: 49 ± 24). Twenty-four-hour urinary calcium data were available from 10 patients and revealed hypercalciuria in all (mean ± SD: 0.32 ± 0.17 mmol/kg/day; normal <0.1). Nephrocalcinosis was present in all patients (n = 17). Urinary albumin excretion (n = 7) ranged from 4.3-122.5 μg/min (mean ± SD: 32.4 ± 41.0 μg/min; normal <20).

CONCLUSIONS

INSR dysfunction is associated with hypercalciuria and nephrocalcinosis. No other consistent abnormality of renal function was noted. Normotension and stable glomerular function with only moderate proteinuria is in contrast to genetically modified mice who have elevated BP and progressive diabetic nephropathy.

Rabson-Mendenhall syndrome with recurrent cerebral infarcts caused by a novel INSR mutation

Rabson-Mendenhall syndrome (RMS) is an autosomal recessive disorder characterized by extreme insulin resistance and certain characteristic phenotypic features. The primary defect lies in the insulin receptor and involves biallelic mutations that lead to a loss of function through various postulated defects. We present a 9-year-old female patient with RMS complicated by multiple cerebral infarcts, in whom genetic analysis revealed a novel insulin receptor mutation.

Molecular characterization of a novel p.R118C mutation in the insulin receptor gene from patients with severe insulin resistance

CONTEXT

Mutations of the insulin receptor gene (INSR) can cause genetic syndromes associated with severe insulin resistance.

OBJECTIVES

We aimed to analyse INSR mutations in Saudi patients with severe insulin resistance.

DESIGN

Ten patients with Type A insulin resistance syndrome from five unrelated Saudi families were investigated. The entire coding region of INSR was sequenced. The founder effect was assessed by microsatellite haplotype analysis. The functional effect of the mutation was investigated by in vitro functional assays.

RESULTS

A novel biallelic c.433 C>T (p.R118C) mutation was detected in all patients. The c.433 C>T (p.R118C) sequence variation was not found in 100 population controls. The arginine residue at position 118 is located in the ligand-binding domain of INSR and is highly conserved across species. Microsatellite haplotype analysis of these patients indicated that p.R118C was a founder mutation created approximately 2900 years ago. The wild-type and mutant (R118C) INSR were cloned and expressed in CHO cells for functional analysis. Specific insulin binding to the mutant receptor was reduced by 83% as compared to wild-type (WT), although the mutant receptor was processed and expressed on the cell surface. Insulin-mediated receptor autophosphorylation was also significantly reduced in CHO(R118C) cells.

CONCLUSIONS

Biallelic c.433 C>T (p.R118C) mutation of INSR causes significant damage to insulin binding and insulin-mediated signal transduction. p.R118C is a founder mutation frequently present in the Saudi patients with severe insulin resistance.