Finding New Molecular Targets for Parkinson's Disease through Gene Hunting in Queensland Families with Inherited Parkinsonism

Parkinson’s disease is an insidious neurodegenerative disease, for which there are no reliable pre-symptomatic biomarkers or preventative therapies. While mostly sporadic, approximately 15% of PD cases report an affected first-degree relative. The study of these patients, and those that belong to larger multi-incident families, have previously identified a number of rare genetic causes of disease. The characterisation of these genes and proteins have uncovered novel underlying molecular mechanisms involved in disease, which has facilitated the investigation of novel therapeutic interventions. Further, the study of affected and asymptomatic carriers of these mutations benefit biomarker research. To this end, this study has screened and reported on subjects with known genetic causes of disease from the Queensland Parkinson’s Project (QPP). These include two VPS35 p.D620N kindreds, eleven LRRK2 p.G2019S kindreds and one phenocopy, two SNCA duplication kindreds and four kindreds with PARK2 mutations, two of which had seemingly dominant inheritance. Additionally, this study also conducted gene hunting analyses to identify novel high impact genetic mutations that are sufficient to cause parkinsonism. Initially, a prioritisation and genetic pre-screening model was established to select families that had the highest potential to identify novel causes of disease. The model considered the number of, and the relationships between, affected members, as well as the likelihood of recruiting more family cases. Then genetic pre-screening investigated copy number variations, polymorphic nucleotide expansions as well as employing initial whole exome sequencing to identify possible causes of disease. The results from this analysis identified eleven multi-incident families suitable for next generation sequencing in multiple affected members. Whole exome sequencing was coupled with supportive evidence such as variant frequency, differential expression in an in vitro disease model and genotype analysis of other cases to identify the most likely candidates for disease. While more evidence of causality is required, the KCNJ15 p.R28C mutation was found to segregate with disease in a kindred consisting of nine affected members, as well as being identified in two additional patients, one sporadic case from Queensland, Australia, and one Italian familial case. Further, the SIPA1L1 p.R236Q mutation was identified in members from one multiincident family and one familial case. While, analysis of de novo arising mutations in an early onset case identified the novel FAM134B p.D381V mutation as a candidate for disease. Analysis of a family with multiple movement disorders identified a number of possible candidates, which require further investigation in other multiple systems atrophy, essential tremor, dystonic and PD cases. Furthermore, this study has also collated putative PD genes from the literature, and examined their segregation within the multi-incident families of the QPP. A number of mutations in these genes were identified to segregate, at least partially, with disease. However, considerable more evidence is required before their role in disease can be determined. The study of multi-incident families provides valuable insights into genetic aberrations that are sufficient to cause disease. The investigation of these monogenic forms of disease allow for the characterisation of molecular processes that may lead to disease. Indeed, the putative genetic targets identified in this study may improve our understanding of molecular pathways involved in parkinsonism, which may consequently aid in the development of novel therapies or biomarkers.