Molecular Characterization of Pontocerebellar Hypoplasia

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PROJECT SUMMARY/ABSTRACT Pontocerebellar hypoplasia (PCH) is a heterogeneous group of mostly recessive pediatric brain disorders that show features of both impaired neurodevelopment and presence of neurodegeneration. PCH is characterized by severe age-dependent neurological impairment, and notable radiographic volume loss of the pons and cerebellum with loss of brainstem and cerebellar neurons. Currently there are 32 genes known mutated in PCH, but still more genes await to be discovered, and molecular mechanisms are poorly understood. Some of the genes implicate key steps in protein synthesis and genomic integrity including tRNA and mRNA splicing, suggesting disruption to homeostatic cellular functions, but many questions remain: 1] How many genetic subtypes remain to be discovered? 2] Why do loss of broadly expressed genes predispose specifically to neurons? 3] Are there convergent molecular pathways for PCH? Over the past 5 years, we have: 1] Grown our unique cohort of PCH patients, containing 248 families including 132 still without a molecular cause. 2] Applied a range of genomics and transcriptomics methods to uncovered mutations in several novel genes including TOE1, TBC1D23, PRP17 and PPIL1 leading to specific PCH subtypes. 3] Revealed defects in RNA splicing and genome integrity as underlying causes. 4] Uncovered the first spliceosome protein mutations. 5] Revealed new genotype phenotype correlations. In our preliminary data we have: 1] Secured resources to advance whole genome sequencing to evaluate our remaining unsolved cases. 2] Identified a further ten new genes as causes for PCH. 3] Remarkably, found that six of the novel causes encode spliceosome proteins. 4] Uncovered R-loop accumulation as a cause of DNA damage by which mutations lead to genotoxic stress. The goal of this application is to: 1] Identify the remaining ‘discoverable’ genes for PCH. 2] Functionally validate mutations within a pathogenic framework. 3] Test the hypothesis that PCH gene loss leads to neurons cell death through R-loop accumulation, DNA damage and genotoxic stress. This work will lead to insight into causes and mechanisms of an important cause of infantile encephalopathy, and uncover mechanisms of selective neuronal vulnerability and pediatric neurodegeneration underlying developmental brain disease.