Assessing non-Mendelian Inheritance in Inherited Axonopathies

PURPOSE: Inherited axonopathies (IA) are rare, clinically and genetically heterogeneous diseases that lead to length-dependent degeneration of the long axons in central (hereditary spastic paraplegia (HSP)) and peripheral (Charcot-Marie-Tooth type 2 (CMT2)) nervous systems. Mendelian high-penetrance alleles in over one hundred different genes have been shown to cause IA; yet, about 50% of IA cases do not receive a genetic diagnosis. A more comprehensive spectrum of causative genes and alleles is warranted, including causative and risk alleles, as well as oligogenic multilocus inheritance. METHODS: Through international collaboration, IA exome studies are beginning to be sufficiently powered to perform a pilot rare variant burden analysis. After extensive quality control, our cohort contained 343 CMT cases, 515 HSP cases, and 935 non-neurological controls. We assessed the cumulative mutational burden across disease genes, explored the evidence for multilocus inheritance, and performed an exome-wide rare variant burden analysis. RESULTS: We replicated the previously described mutational burden in a much larger cohort of CMT cases, and observed the same effect in HSP cases. We identified a preliminary risk allele for CMT in the EXOC4 gene (p-value= 6.9×10−6, OR=2.1) and explored the possibility of multi-locus inheritance in IA. CONCLUSIONS: Our results support the continuing emergence of complex inheritance mechanisms in historically Mendelian disorders.