Significance Understanding loci nominated by genome-wide association studies (GWASs) is challenging. Here, we show, using the specific example of Parkinson disease, that identification of protein–protein interactions can help determine the most likely candidate for several GWAS loci. This result illustrates a significant general principle that will likely apply across multiple diseases.

Mutations in the glucocerebrosidase gene (GBA), which cause Gaucher disease, are also potent risk factors for Parkinson's disease. We examined whether a genetic burden of variants in other lysosomal storage disorder genes is more broadly associated with Parkinson's disease susceptibility. The sequence kernel association test was used to interrogate variant burden among 54 lysosomal storage disorder genes, leveraging whole exome sequencing data from 1156 Parkinson's disease cases and 1679 control subjects. We discovered a significant burden of rare, likely damaging lysosomal storage disorder gene variants in association with Parkinson's disease risk. The association signal was robust to the exclusion of GBA, and consistent results were obtained in two independent replication cohorts, including 436 cases and 169 controls with whole exome sequencing and an additional 6713 cases and 5964 controls with exome-wide genotyping. In secondary analyses designed to highlight the specific genes driving the aggregate signal, we confirmed associations at the GBA and SMPD1 loci and newly implicate CTSD, SLC17A5, and ASAH1 as candidate Parkinson's disease susceptibility genes. In our discovery cohort, the majority of Parkinson's disease cases (56%) have at least one putative damaging variant in a lysosomal storage disorder gene, and 21% carry multiple alleles. Our results highlight several promising new susceptibility loci and reinforce the importance of lysosomal mechanisms in Parkinson's disease pathogenesis. We suggest that multiple genetic hits may act in combination to degrade lysosomal function, enhancing Parkinson's disease susceptibility.

Autosomal-recessive early-onset parkinsonism is clinically and genetically heterogeneous. The genetic causes of approximately 50% of autosomal-recessive early-onset forms of Parkinson disease (PD) remain to be elucidated. Homozygozity mapping and exome sequencing in 62 isolated individuals with early-onset parkinsonism and confirmed consanguinity followed by data mining in the exomes of 1,348 PD-affected individuals identified, in three isolated subjects, homozygous or compound heterozygous truncating mutations in vacuolar protein sorting 13C (VPS13C). VPS13C mutations are associated with a distinct form of early-onset parkinsonism characterized by rapid and severe disease progression and early cognitive decline; the pathological features were striking and reminiscent of diffuse Lewy body disease. In cell models, VPS13C partly localized to the outer membrane of mitochondria. Silencing of VPS13C was associated with lower mitochondrial membrane potential, mitochondrial fragmentation, increased respiration rates, exacerbated PINK1/Parkin-dependent mitophagy, and transcriptional upregulation of PARK2 in response to mitochondrial damage. This work suggests that loss of function of VPS13C is a cause of autosomal-recessive early-onset parkinsonism with a distinctive phenotype of rapid and severe progression.

Mobility problems are the commonest early feature in PSP and visual symptoms are often functionally disabling. Early falls, speech and swallowing problems, diplopia, and early insertion of a percutaneous gastrostomy predict reduced survival.

We performed a study to estimate the point prevalence of progressive supranuclear palsy (PSP) in the UK at national, regional and community levels. A 'Russian doll' design was used in which the population denominator for each of the three studies was successively smaller, whilst the method of case ascertainment became increasingly more rigorous. The NINDS-SPSP (National Institute of Neurological Disorders and the Society for Progressive Supranuclear Palsy) diagnostic criteria for PSP were applied throughout the study for case definition. The national study identified cases using passive referral mechanisms [e.g. the British Neurological Surveillance Unit (BNSU), PSP (Europe) Association patient register]. We identified 577 cases of PSP, giving a national prevalence estimate of 1.0 per 100 000 [95% confidence interval (CI) 0.9-1.1]. The North of England regional study used active 'multiple source' case ascertainment from a collaborative network of neurologists and non-neurologists. We identified 80 cases of PSP in this study, giving a crude and age-adjusted prevalence of 3.1 (95% CI 2.4-3.8) and 2.4 (1.9-3.0) per 100 000, respectively. Of these 80 cases, 51 patients (65%) were referred initially to non-neurologists and 10 patients (13%) had not seen a neurologist at any stage of their illness. The proportion of female cases was significantly greater in the regional than in the national study (61% versus 44%; P < 0.02). Cases referred to non-neurologists were significantly older than those referred to neurologists in the regional study (median age 73 versus 69.5 years; P < 0.01). Patients in the community study were identified via diagnostic and therapeutic registers from a representative sample of general practices in Newcastle upon Tyne. We identified 17 cases of PSP, yielding crude and age-adjusted prevalences of 6.5 (95% CI 3.4-9.7) and 5.0 (95% CI 2.5-7.5) per 100 000, respectively. Seven of the 17 cases (41%) had not previously been diagnosed as PSP. This study suggests that PSP is more common than previously considered, is commonly misdiagnosed and that the majority of cases are not initially referred to neurologists. The study also confirms the importance of active and detailed case ascertainment in ensuring reliable prevalence estimates.