Evidence of mutations in RIC3 acetylcholine receptor chaperone as a novel cause of autosomal-dominant Parkinson's disease with non-motor phenotypes.

BACKGROUND: The known genetic determinants of Parkinson's disease (PD) do not explain all cases investigated to date. Contemporary sequencing technologies hold promise for enhanced causal variant discovery. We attempted to identify the putative causal variant in an Indian PD family by whole exome sequencing (WES). METHODS: WES data generated for two affected cousins from a 14-member PD family with some non-motor phenotypes were analysed. Variants prioritised were checked for segregation with disease by targeted sequencing. An independent PD cohort (n=280) was screened for additional mutations in the prioritised gene. Variants were functionally validated in PC12 cells differentiated into neurons. RESULTS: A heterozygous mutation c.169C>A, p.P57T in RIC3 acetylcholine receptor chaperone (11p15) segregated with disease in the family confirming an autosomal-dominant mode of inheritance. Another heterozygous mutation c.502G>C, p.V168L was detected in an unrelated PD case. Both mutations were absent in 144 healthy control and in 74 non-PD WES data available in-house and in 186 age and sex-matched controls screened by PCR sequencing. RIC3 is a known chaperone of neuronal nicotinic acetylcholine receptor subunit α-7 (CHRNA7). Dominant negative effect of RIC3 mutants in transfected PC12 cells was reflected by the reduced levels of endogenous CHRNA7 in the membrane fractions in western blots and lower colocalisation profiles in confocal micrographs. CONCLUSION: The novel demonstration of a chaperone-mediated receptor density alteration due to RIC3 mutants provides strong evidence for the role of cholinergic pathway for the first time in PD aetiology. This may also be insightful for some non-motor symptoms and personalised treatment.