Molecular genetics and mechanisms of disease in distal hereditary motor neuropathies: insights directing future genetic studies

The distal hereditary motor neuropathies (dHMNs) are a clinically and genetically heterogeneous group of disorders that primarily affect motor neurons, without significant sensory involvement. New dHMN genes continue to be identified. There are now 11 causative genes described for dHMN, and an additional five genetic loci with unidentified genes. This genetic heterogeneity has further delineated the classification of dHMN, which was previously classified according to mode of inheritance, age at onset, and additional complicating features. Some overlap between phenotypically distinct forms of dHMN is also apparent. The mutated genes identified to-date in dHMN include HSPB1, HSPB8, HSPB3, DCTN1, GARS, PLEKHG5, BSCL2, SETX, IGHMBP2, ATP7A and TRPV4. The pathogenesis of mutations remains to be fully elucidated, however common pathogenic mechanisms are emerging. These include disruption of axonal transport, RNA processing defects, protein aggregation and inclusion body formation, disrupted calcium channel activity, and loss of neuroprotective signalling. Some of these dHMN genes are also mutated in Charcot-Marie-Tooth (CMT) disease and spinal muscular atrophy (SMA). This review examines the growing number of identified dHMN genes, discusses recent insights into the functions of these genes and possible pathogenic mechanisms, and looks at the increasing overlap between dHMN and the other neuropathies CMT2 and SMA.