Functional Characterization of Obesity-Associated Variants Involving the α and β Isoforms of Human SH2B1

Abstract

We have previously reported rare variants in sarcoma (Src) homology 2 (SH2) B adaptor protein 1 (SH2B1) in individuals with obesity, insulin resistance, and maladaptive behavior. Here, we identify 4 additional SH2B1 variants by sequencing 500 individuals with severe early-onset obesity. SH2B1 has 4 alternatively spliced isoforms. One variant (T546A) lies within the N-terminal region common to all isoforms. As shown for past variants in this region, T546A impairs SH2B1β enhancement of nerve growth factor-induced neurite outgrowth, and the individual with the T546A variant exhibits mild developmental delay. The other 3 variants (A663V, V695M, and A723V) lie in the C-terminal tail of SH2B1α. SH2B1α variant carriers were hyperinsulinemic but did not exhibit the behavioral phenotype observed in individuals with SH2B1 variants that disrupt all isoforms. In in vitro assays, SH2B1α, like SH2B1β, enhances insulin- and leptin-induced insulin receptor substrate 2 (IRS2) phosphorylation and GH-induced cell motility. None of the variants affect SH2B1α enhancement of insulin- and leptin-induced IRS2 phosphorylation. However, T546A, A663V, and A723V all impair the ability of SH2B1α to enhance GH-induced cell motility. In contrast to SH2B1β, SH2B1α does not enhance nerve growth factor-induced neurite outgrowth. These studies suggest that genetic variants that disrupt isoforms other than SH2B1β may be functionally significant. Further studies are needed to understand the mechanism by which the individual isoforms regulate energy homeostasis and behavior.

The Effect of SH2B1 Variants on Expression of Leptin- and Insulin-Induced Pathways in Murine Hypothalamus

“…The mutated amino acid is located at a key position of the nuclear localization domain of SH2B1 . This domain leads to nuclearization of βSH2B1 which drives the enhancement of NGF-induced neurite outgrowth [26]. …”

“…Although nuclearization of SH2B1 has been shown for the beta, but not the alpha, splice variant of the gene, the differential expression pattern does not have to be a direct outcome of SH2B1 acting as a transcription factor. Whether SH2B1 has to bind specific partners for nuclear shuttling is still unclear [26], although binding of STATs could well be one of them [42]. …”

“…SH2B1 mutations were detected among 800 obese children and adolescents with insulin resistance (Pro90His, Thr175Asn, Pro322Ser, Phe344Leufs*20, Thr564Ala, Ala663Val, Ala723Val) [25,26]. All variants impaired GH-induced cell motility when present in the alpha isoform of SH2B1.…”

“…All variants impaired GH-induced cell motility when present in the alpha isoform of SH2B1. Except for the frameshift variant (Phe344Leufs*20) that lacks the SH2 domain, all variants interfered with insulin- or leptin-induced IRS2 phosphorylation [26]. …”

A compendium of human genes regulating feeding behavior and body weight, its functional characterization and identification of GWAS genes involved in brain-specific PPI network

“…In addition, patients with chromosomal aberrations resulting in disruption or deletion of the single-minded homologue 1 gene ( SIM1 ), which is essential for proper development of the paraventricular nucleus of hypothalamus, have early-onset obesity [11, 12]. Monogenic obesity also results from mutations in some other genes involved in eating behavior and energy balance regulation: (1) brain-derived neurotrophic factor ( BDNF ) [13]; (2) its receptor, tyrosine receptor kinase B ( NTRK2 ) [14]; (3) SH2B adaptor protein 1 ( SH2B1 ), involved in the regulation of leptin signaling [15, 16]; (4) KSR2, which encodes a scaffolding protein kinase suppressor of Ras 2, participating in signaling pathways relevant to glucose homoeostasis and food intake control [17]; (5) TUB, encoding Tubby bipartite transcription factor [18]. Although not formally defined as a syndrome, the clinical features of TUB deficiency in humans may be consistent with a novel ciliopathy [5].…”