Whole exome sequencing performed on a male patient with a unique complex phenotype revealed a novel de novo missense variant in FASN (c.6395C>T, p.A2132V), encoding Fatty Acid Synthase. The patient presented with panhypopituitarism (GH, TSH, LH, FSH and ACTH deficiencies), short stature, sensorineural deafness, hypoparathyroidism, retinal dystrophy, and developmental delay. He was 127 cm tall at the age of 21 and failed to respond to GH treatment [IGF-1 generation test: no response to 2.8mg/m2/day GH]. He was completely prepubertal (LH 0.8U/l, FSH 2.6U/l, Testosterone <0.69nmol/L) with a bone age of 12 years. A trial of recombinant IGF-1 failed to improve his growth. He died at 26 years of age following a collapse of unknown etiology. The variant was not present on control databases, including the GnomAD browser. Human embryonic brain expression analysis using in situ hybridisation revealed FASN mRNA transcripts in the diencephalon and hypothalamus at Carnegie stages (CS) 16, 19, 20 and 23, and in Rathke’s pouch (the primordium of the anterior pituitary) at CS16 and CS19 only. FASN is a crucial multienzyme with seven different catalytic activities, essentially converting acetyl-CoA and malonyl-CoA into long-chain saturated fatty acids such as palmitate. Murine null Fasn-/- embryos die before implantation and Fasn+/- embryos die in early postnatal stages. Murine Fasn is expressed in multiple tissues including the brain, parathyroids, liver and adrenal. Biochemical investigation of the patient revealed high triglyceride concentrations (3.81-6.49mmol/L; NR 0.38-2) throughout an 18 hour fast. He mobilised free fatty acids but 3-hydroxybutyrate failed to increase, 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 (<25ng/ml) and IGFBP-3 (1.17mg/L). A glucose tolerance test was abnormal; plasma glucose did not decrease at the expected rate. We measured de novo fatty acid synthesis in cultured patient and control fibroblast cells with 13C-labelled glucose, with and without a C75 fatty acid synthase inhibitor. The 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 the 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 (Bowers M et al.,2020) suggesting that neural stem/progenitor cells (NSPCs) in the rodent brain are governed by Fasn-dependent de novo lipogenesis for proliferation.
15 Sep 2022 - 17 Sep 2022