Background: GNAS (guanine nucleotide-binding protein, alpha stimulating) is an imprinted gene that encodes Gs alpha, the alpha subunit of the heterotrimeric stimulatory G protein. This subunit mediates the signalling of a diverse array of G protein-coupled receptors (GPCRs), including the melanocortin 4 receptor (MC4R) that serves a pivotal role in regulating food intake, energy homoeostasis, and body weight. Genetic or epigenetic alterations in GNAS are known to cause pseudohypoparathyroidism in its different subtypes and have been recently associated with isolated, early-onset, severe obesity. Given the diverse biological functions that Gs alpha serves, multiple molecular mechanisms involving various GPCRs, such as MC4R, beta 2- and beta 3-adrenoceptors, and corticotropinreleasing hormone receptor, have been implicated in the pathophysiology of severe, early-onset obesity that results from genetic or epigenetic GNAS changes. Scope of review: This review examines the structure and function of GNAS and provides an overview of the disorders that are caused by defects in this gene and may feature early-onset obesity. Moreover, it elucidates the potential molecular mechanisms underlying Gs alpha deficiency-induced early-onset obesity, highlighting some of their implications for the diagnosis, management, and treatment of this complex condition. Major conclusions: Gs alpha deficiency is an underappreciated cause of early-onset, severe obesity. Therefore, screening children with unexplained, severe obesity for GNAS defects is recommended, to enhance the molecular diagnosis and management of this condition.

The kinase suppressor of Ras 2 (KSR2) gene is associated with monogenic obesity, and loss-of-function variants in KSR2 have been identified in individuals with severe early-onset obesity. This study investigated KSR2 variants in 9 pediatric patients with severe early-onset obesity in Qatar using whole genome sequencing among a cohort of 240 individuals. We focused on KSR2 variants with a minor allele frequency (MAF) below 1% and a Combined Annotation Dependent Depletion (CADD) score above 13 to identify potential causative variants. Our analysis identified four KSR2 variants: one intronic (c.1765-8G>A) and three missense variants (c.1057G>A, c.1673G>A, and c.923T>C) in nine patients. The intronic variant c.1765-8G>A was the most frequent (seen in six individuals) and had a CADD score of 21.10, suggesting possible pathogenicity. This variant showed a significantly higher allele frequency in the Qatari population compared to the Genome Aggregation Database (gnomAD), indicating a possible founder effect. Molecular modeling of the missense variants revealed structural changes in the protein structure. The study concludes that these four KSR2 variants are associated with monogenic obesity, with an autosomal dominant inheritance pattern. The c.1765-8G>A variant’s prevalence in Qatar underscores its importance in genetic screening for severe obesity. This research advances the understanding of genetic factors in severe early-onset obesity and may inform better management strategies.

GNAS (Guanine Nucleotide-Binding Protein, Alpha Stimulating) is a complex gene that encodes the alpha subunit of the stimulatory G protein (G_sα), critical for signaling through various G protein-coupled receptors. Inactivating genetic and epigenetic changes in GNAS, resulting in G_sα deficiency, cause different variants of pseudohypoparathyroidism, which may manifest features of Albright hereditary osteodystrophy (AHO, a syndrome characterized by early-onset obesity and other developmental defects). Recent findings have linked G_sα deficiency with isolated, severe, early-onset obesity, suggesting it as a potential, underrecognized cause of monogenic, non-syndromic obesity. This study was prompted by identifying several GNAS variants of uncertain significance (VUSs) in pediatric patients presenting with unexplained, severe, early-onset obesity at Sidra Medicine in Qatar. To functionally characterize these variants, we developed the first zebrafish model of G_sα deficiency, offering numerous advantages over other model systems. This was achieved by knockdown of the ortholog through microinjection of translation-blocking Morpholino antisense oligonucleotides into the yolks of 1-8-cell-stage zebrafish embryos. The morphant larvae displayed an obese phenotype, marked by significantly enlarged yolk sacs, increased neutral lipid accumulation, and reduced metabolic rates, among other developmental abnormalities resembling those in AHO. This zebrafish model lays the foundation for efficient functional characterization of GNAS VUSs and paves the way for enhancing our understanding of G_sα deficiency-associated early-onset obesity.