THU143 A Novel Maternally Inherited GNAS Variant In A Family With Hyperphagia And Obesity

Disclosure: P. Purushothaman: None. A. Ramakrishnan: None. E.F. Gevers: None. Introduction: Heterozygous inactivating mutations in the maternal allele of the GNAS gene typically result in pseudohypoparathyroidism (PHP) characterized by developmental delay, short stature, obesity, hormone resistance and bone abnormalities. Recently, GNAS variants were also described in 1% of patients in the UK Genetics of Obesity cohort, resulting in reduced MC4R signalling. Here, we report another novel GNAS variant in family with hyperphagia and obesity and only mild features of PHP. Case: The proband presented at 6 years with hyperphagia and obesity from age 3 years. BMI was 30.4 (4.1 SDS) with height +1.9 SDS, and head circumference (HC) + 3.0 SDS. She had subtle brachydactyly, short toes and a café au lait patch. Developmental milestones and cognitive achievements were mildly delayed. Her 12-year-old brother [ height +2.1 SDS, weight +3.1 SDS, BMI 29.55 (2.9 SDS), HC +3.6 SDS] had been followed in an obesity clinic for a few years with hyperphagia, obesity, delayed developmental milestones and a recent diagnosis of autism. He also had subtle brachydactyly, as did the mother who was also obese (BMI 38.9) with a normal height but had normal cognition. Results: In the proband, initial investigations did not suggest hormone resistance. However, at 8.7 years, PTH was high [9.5 pmol/L (0.7-5.6)] with normal calcium and Vitamin D; TSH 6.6 mU/L (<6), FT4 11.8 pmol/L (10.8 - 19.0). She had hyperlipidaemia and fatty liver. X-ray hand showed generalised mild brachydactyly with a short right fifth metacarpal. In the brother, PTH tested for the first time at 12.9 years was 18.4 pmol/L (1.6-6.9), calcium and vitamin D were normal; TSH 4.04 mU/L (<4.2) and FT4 13.4 pmol/L. Next Generation Sequencing using the Cambridge Obesity gene panel detected a novel heterozygous (c.791A>C, p.(Asn264Thr) variant in exon 10 of GNAS in the proband and subsequently in the mother and brother confirming maternal inheritance. This variant has not been reported in control databases (1000 Genomes, ESP, ExAC and gnomAD, Human Gene Mutation Database and ClinVar). The variant is in a highly conserved region susceptible to missense mutations and has been classified as pathogenic using ACMG and ACGS guidelines. Discussion: In conclusion, we describe a novel heterozygous inactivating GNAS variant c.791A>C, p.Asn264Thr resulting in hyperphagia, obesity, developmental delay, macrocephaly, mild brachydactyly, tall stature and variable biochemical mild hormone resistance in 2 siblings and resulting in obesity, mild brachydactyly without cognitive impairment in the mother. These cases demonstrate the clinical heterogeneity of the monogenic disease. Clinical and biochemical screening for PHP and genetic screening for GNAS variants should be performed in patients presenting with obesity, even in the absence of classical signs of PHP, to reduce diagnostic delay. Presentation: Thursday, June 15, 2023