RF13 | PMON48 A novel missense variant in the gene encoding Fatty Acid Synthase (FASN) associated with a unique multi-system disorder including hypopituitarism and hypoparathyroidism

A male patient presented with a unique complex phenotype including panhypopituitarism, short stature with failure to respond to GH [IGF-1 generation test: no response to 2.8 mg/m(2)/day GH], sensorineural deafness, hypoparathyroidism, retinal dystrophy, and intellectual disability. He was 127cm tall at the age of 21 and was completely prepubertal (LH 0.8 U/L, FSH 2.6 U/L, Testosterone <0.69 nmol/L) with a bone age of 12 years. A trial of recombinant IGF-1 failed to improve growth. He died at the age of 26 years following a collapse of unknown etiology. Exome sequencing and subsequent variant calling on this patient revealed a novel heterozygous de novo missense variant (c.6395C>T, p.A2132V) at a highly conserved residue in FASN encoding Fatty Acid Synthase. The variant was not present in any control database, including the GnomAD browser. Human embryonic brain expression analysis using in situ hybridisation, revealed FASN mRNA transcript staining in the diencephalon and hypothalamus at Carnegie stages (CS) 16, 19, 20 and 23, and in Rathke's pouch (primordium of the anterior pituitary) at CS16 only. Fatty acid synthase (FASN), a crucial multienzyme with seven different catalytic activities, converts acetyl-CoA and malonyl-CoA into long-chain saturated fatty acids such as palmitate, in the presence of NADPH. Murine null mutant Fasn-/- embryos die before implantation and Fasn+/- embryos at various stages in the early postnatal period. Fasn is expressed in multiple tissues including the brain, parathyroids, liver and adrenal. Biochemical investigation of the patient revealed high triglyceride concentrations (3.81-6.49 mmol/L; NR 0.38-2) at all time points during an 18 hour fast. He mobilised free fatty acids but did not generate an increase in 3-hydroxybutyrate, suggesting a defect in fatty acid oxidation or ketone body synthesis. He maintained normoglycaemia with normal hormone responses to fasting, except for an undetectable IGF-1 (<25 ng/ml) and IGFBP-3 (1.17 mg/L). We measured de novo fatty acid synthesis in cultured patient and control fibroblast cells with 13C-Glucose, with and without the C75 fatty acid synthase inhibitor. Cell pellets were collected at three time-points, and 13C-fatty acids derived from 13C-glucose were analysed by mass spectrometry. Preliminary data showed that more C13-glucose was incorporated into de novo synthesised palmitic acid in control cells, suggesting reduced fatty acid synthesis in the patient. Our data suggest that FASN p.A2132V is pathogenic and contributed significantly to the complex patient phenotype, consistent with recent studies suggesting that neural stem/progenitor cells (NSPCs) in the rodent brain are governed by Fasn-dependent de novo lipogenesis for proliferation. Presentation: Sunday, June 12, 2022 12:36 p.m. - 12:41 p.m., Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.