NG.O.13 - Recessive mutation in EXOSC9 disrupts the exosome complex resulting in a novel form of cerebellar hypoplasia/atrophy with early motor neuronopathy

The exosome is essential for RNA metabolism via degradation of unstable AU-rich elements containing mRNAs. Mutations in EXOSC3 and EXOSC8, coding for core exosome subunits, and RBM7, coding for an exosomal-related protein, result in abnormal RNA metabolism, manifesting as spinal muscular atrophy, demyelination of the central nervous system and pontocerebellar hypoplasia (PCH1). Through whole exome sequencing (WES) we identified rare, predicted to be damaging, recessive mutations in a novel exosomal gene EXOSC9, in 2 independent patients with severe progressive neurodegeneration. Patient 1 (P1) is a 21-month-old female with normal tone at birth, followed by severe progressive weakness, feeding and respiratory difficulties by 8 months of age. MRI showed cerebellar atrophy, and EMG was suggestive of an axonal motor neuronopathy. WES identified a homozygous mutation p.L14P in EXOSC9. Patient 2 was born with fractures, arthrogryposis, severe hypotonia and respiratory insufficiency. MRI revealed generalized volume loss with cerebellum hypoplasia and delayed myelination. Muscle biopsy showed neurogenic changes. The same p.L14P mutation, in compound heterozygosity with a p.R161X mutation, was identified in EXOSC9. In vitro studies in P1's fibroblasts revealed a reduction of exosomal EXOSC8 and EXOSC3 protein levels, indicating secondary destabilization of the complex. Morpholino knockdown of Exosc9 in zebrafish resulted in embryos with phenotypes ranging from mildly affected to severely abnormal. CRISPR/Cas9-induced mutations in zebrafish Exosc9 resulted in milder, but similar defects with failure of motor neuron development and migration. Our data suggest that mutations in EXOSC9 may cause defective RNA metabolism due to abnormal function of the exosome, resulting in a severe clinical presentation related to PCH1. We establish EXOSC9 as a new gene for this phenotype, while further elucidating the role of exosome-mediated mRNA degradation in neurodegenerative disease.