ASAH2 deficiency affects sphingolipid homeostasis and neuromotor control, causing a progressive neurological disorder

: Sphingolipids are integral components of cell membranes and modulate cell survival, proliferation, and apoptosis. ASAH2 is a brain- and gut-enriched gene encoding the neutral N-acylsphingosine amidohydrolase 2, a poorly characterized member of the human ceramidase family. This enzyme plays a pivotal role in maintaining the sphingolipid homeostasis, which is crucial for neurogenesis and synaptic function in central and peripheral nervous system. In fact, a dysregulated sphingolipid metabolism is associated with progressive neurological conditions, including Alzheimer's disease and Parkinson's disease. Here, we report the identification of biallelic ASAH2 variants in an individual with a neurodevelopmental condition featuring cognitive impairment, neuropathy, ophthalmoplegia, and progressive cerebellar and extraocular muscles atrophy. Through exome sequencing, we identified very rare missense ASAH2 variants, predicted to be deleterious by in silico analyses. Muscle biopsy histopathologic evaluation revealed features suggestive of neuropathic damage. Lipidomic profiling revealed a hyper-accumulation of glucosylceramide in the subject's cells. Then, the functional investigation of the ASAH2 variants in Drosophila showed the production of an unstable protein and consistent loss-of-function neuromotor phenotypes. Our findings support ASAH2 as a candidate gene for a previously uncharacterized neurodevelopmental disorder with neuropathic features and progressive cerebellar atrophy, underscoring the important role of this ceramidase in human nervous system.