Mutation of FIG4 causes a rapidly progressive, asymmetric neuronal degeneration

Recessive Charcot-Marie-Tooth disease type-4J (CMT4J) and its animal model, the pale tremor mouse (plt), are caused by mutations of the FIG4 gene encoding a PI(3,5)P₂ 5-phosphatase. We describe the 9-year clinical course of CMT4J, including asymmetric, rapidly progressive paralysis, in two siblings. Sensory symptoms were absent despite reduced numbers of sensory axons. Thus, the phenotypic presentation of CMT4J clinically resembles motor neuron disease. Time-lapse imaging of fibroblasts from CMT4J patients demonstrates impaired trafficking of intracellular organelles because of obstruction by vacuoles. Further characterization of plt mice identified axonal degeneration in motor and sensory neurons, limited segmental demyelination, lack of TUNEL staining and lack of accumulation of ubiquitinated protein in vacuoles of motor and sensory neurons. This study represents the first documentation of the natural history of CMT4J. Physical obstruction of organelle trafficking by vacuoles is a potential novel cellular mechanism of neurodegeneration.