Discovering and Rescuing Toxic Autophagy Derangements in CMT4B3 Using a Novel iPSC-derived Peripheral Nervous System Model of MTMR5 Biology
Jason Chua1, Sarah Stumpf1, Valina Dawson1, Ted Dawson2
1Johns Hopkins University, 2Institute for Cell Engineering, Johns Hopkins University School of Medicine
Objective:

To fully elucidate the pathophysiology of Charcot-Marie-Tooth disease type 4B3 (CMT4B3) and targetable mechanisms for therapy development, we have engineered a new human peripheral nervous system (PNS) cell model of CMT4B3.

Background:

CMT4B3 is a rare, hereditary, incurable, and devastating neurologic disorder with heterogeneous manifestations, ranging from PNS involvement with sensory loss, muscle atrophy, foot deformities, and gait instability, to central nervous system (CNS) involvement with cognitive impairment, microcephaly, and multiple cranial neuropathies. CMT4B3 is caused by loss-of-function (LOF) mutations in the SBF1 gene, encoding the pseudophosphatase, MTMR5. MTMR5 participates in the metabolism of phosphoinositides that serve as membranal scaffolds for recruiting protein complexes overseeing induction of autophagy. Although the native function of MTMR5 has been identified, the precise mechanisms by which MTMR5 exerts contrasting functions and modifies cellular health in different cell and tissue types, thereby leading to the protean and cell type-specific manifestations of CMT4B3, are poorly understood. Previously established models of CMT4B3 consist of non-PNS cells and rodents but incompletely recapitulate the full spectrum of disease phenotypes found in humans. 

Design/Methods:
We generated isogenic motor neurons and skeletal muscle (iMotor Neurons, iMuscle) from patient-derived iPSCs, and assessed markers of autophagy, cell viability, and treated cells with pharmacologic modulators of autophagy.  
Results:
In CMT4B3 cells, there is marked attenuation of MTMR5 expression, heightened autophagic flux, and lower cell viability that is partially rescued by pharmacologic inhibition of autophagy. These phenotypes are more severe in iMotor Neurons compared to iMuscle, pointing to cell type-specific vulnerability to, and cell autonomous toxic effects of, LOF in MTMR5. 
Conclusions:

Our results confirm the biologic fidelity of our novel human PNS cellular system for modeling CMT4B3, and the foundational insights provided by our experimental platform will be invaluable for ongoing mechanistic investigations of MTMR5-related pathophysiology and discovering therapies for CMT4B3 and neurological disorders marked by dysregulated autophagy.

10.1212/WNL.0000000000208301