Defining and rescuing pathomechanisms of myotubularin and autophagy disruption in a novel human cell model of Charcot-Marie-Tooth Type 4B3

Charcot-Marie-Tooth Type 4B3 (CMT4B3) is a genetic disorder leading to peripheral axon degeneration and clinical manifestations of distal weakness and gait impairment. CMT4B3 is caused by mutations in SBF1/MTMR5, a negative regulator of phosphoinositide signaling and autophagy. Although SBF1 mutations are ubiquitously expressed, how and why loss of SBF1/MTMR5 exerts deleterious effects predominantly in neurons of the peripheral nervous system (PNS) is unknown. To investigate the effects of mutant SBF1/MTMR5 on PNS neurons compared to non-neurons, we engineered a novel and unique model system of CMT4B3 using human induced pluripotent stem cells (iPSCs) differentiated into key components of the PNS: motor neurons (iMNs), sensory neurons (iSNs), or skeletal muscle (iMuscle). To model CMT4B3, we used iPSCs derived from a CMT43B patient, or genetically knocked down SBF1 in WT cells. Strikingly, CMT4B3 iMNs showed the highest degree of cell degeneration among all cell types, concordant with the clinical phenotype of patients. We also found that CMT4B3 iMNs and iSNs showed attenuated expression of MTMR5 and related paralogs MTMR2 and MTMR13. Knockdown of SBF1 most significantly augmented autophagy in iMNs than other cell types. Finally, we tested treatment with VPS34-IN1, a pharmacologic inhibitor of the Class III PI3-Kinase functioning in opposition to MTMR5 in regulating phosphoinositides, and found that VPS34-IN1 rescued cell death in CMT4B3 iMNs. Together, our results for the first time confirm PNS cell type-specific differences in myotubularin expression, autophagy, and vulnerabilities to SBF1 mutations, and identify a novel therapeutic strategy of high disease-modifying potential for CMT4B3.