Muscle spindle alterations precede onset of sensorimotor deficits in Charcot-Marie-Tooth type 2E

Charcot-Marie-Tooth (CMT) is the most common inherited peripheral neuropathy, affecting approximately 2.8 million people. The CMT leads to distal neuropathy that is characterized by reduced motor nerve conduction velocity, ataxia, muscle atrophy and sensory loss. We generated a mouse model of CMT type 2E (CMT2E) expressing human neurofilament light E396K (hNF-L^E396K ), which develops decreased motor nerve conduction velocity, ataxia and muscle atrophy by 4 months of age. Symptomatic hNF-L^E396K mice developed phenotypes that were consistent with proprioceptive sensory defects as well as reduced sensitivity to mechanical stimulation, while thermal sensitivity and auditory brainstem responses were unaltered. Progression from presymptomatic to symptomatic included a 50% loss of large diameter sensory axons within the fifth lumbar dorsal root of hNF-L^E396K mice. Owing to proprioceptive deficits and loss of large diameter sensory axons, we analyzed muscle spindle morphology in presymptomatic and symptomatic hNF-L^E396K and hNF-L control mice. Muscle spindle cross-sectional area and volume were reduced in all hNF-L^E396K mice analyzed, suggesting that alterations in muscle spindle morphology occurred prior to the onset of typical CMT pathology. These data suggested that CMT2E pathology initiated in the muscle spindles altering the proprioceptive sensory system. Early sensory pathology in CMT2E could provide a unifying hypothesis for the convergence of pathology observed in CMT.