PIK3R1 mutations cause syndromic insulin resistance with lipoatrophy

Abstract

Short stature, hyperextensibility of joints and/or inguinal hernia, ocular depression, Rieger anomaly, and teething delay (SHORT) syndrome is a developmental disorder with an unknown genetic cause and hallmarks that include insulin resistance and lack of subcutaneous fat. We ascertained two unrelated individuals with SHORT syndrome, hypothesized that the observed phenotype was most likely due to de novo mutations in the same gene, and performed whole-exome sequencing in the two probands and their unaffected parents. We then confirmed our initial observations in four other subjects with SHORT syndrome from three families, as well as 14 unrelated subjects presenting with syndromic insulin resistance and/or generalized lipoatrophy associated with dysmorphic features and growth retardation. Overall, we identified in nine affected individuals from eight families de novo or inherited PIK3R1 mutations, including a mutational hotspot (c.1945C>T [p.Arg649Trp]) present in four families. PIK3R1 encodes the p85α, p55α, and p50α regulatory subunits of class IA phosphatidylinositol 3 kinases (PI3Ks), which are known to play a key role in insulin signaling. Functional data from fibroblasts derived from individuals with PIK3R1 mutations showed severe insulin resistance for both proximal and distal PI3K-dependent signaling. Our findings extend the genetic causes of severe insulin-resistance syndromes and provide important information with respect to the function of PIK3R1 in normal development and its role in human diseases, including growth delay, Rieger anomaly and other ocular affections, insulin resistance, diabetes, paucity of fat, and ovarian cysts.

Figure 1

Appearance of Individuals with SHORT Syndrome, Test Results for Oral Glucose Tolerance, and Distribution of PIK3R1 Mutations (A–H) Photos of subjects P1 (A–C), P2 (D), and P3 (E) and familial pedigree and photos of subject P5 (F–H). Subjects presented with a typical triangular facial shape, deep-set eyes, thin and hypoplastic alae nasi, a small chin, mild midfacial hypoplasia, and lipoatrophy. We obtained written consent to publish photographs of the subjects. (I) Test results for oral glucose tolerance show hyperglycemia in subjects P3 (at 13.5 years of age) and P7 and increased serum insulin levels in all affected children (subjects P1, P2, and P3). (J) Schematic representation of the three isoforms encoded by PIK3R1, their functional domains, and the alterations identified in individuals with SHORT syndrome.

Figure 2

Cellular Insulin Resistance in Subjects with PIK3R1 Mutations (A and B) PIK3R1 levels and short-term effects of insulin on proximal PI3K-dependent insulin signaling in fibroblasts from affected individuals (P1 and P2) and controls. PIK3R1 levels (A) and the effect of insulin on AKT activation (B) were evaluated by immunoblotting on whole-cell lysates. Intensities were quantified by densitometry scanning. PIK3R1 levels and AKT phosphorylation levels were normalized to β-actin and total AKT, respectively. Representative immunoblots are shown. The following abbreviation is used: p-AKT-ser473, phospho-AKT-ser473. (C) Short-term effects of insulin on distal PI3K-dependent signaling. The effects of insulin on glycogen synthesis and glucose transport were evaluated by the incorporation of ¹⁴C-glucose into glycogen and the cellular uptake of ³H-2-deoxy-D-glucose, respectively. In fibroblasts from control subjects and individuals P1 and P2, basal glycogen synthesis was 22.3 ± 1.6, 54.6 ± 6.0, and 52.5 ± 0.4 pmoles of glucose incorporated into glycogen per mg protein per hour, respectively, and basal glucose transport was 10.5 ± 1.1, 23.2 ± 2.4, and 11.4 ± 1.5 pmole of cellular 2-deoxyglucose per mg protein per min, respectively. Results are expressed as means ± SEM. Asterisks indicate a statistically significant difference compared to control cells (p < 0.05, Student’s t test).