Expression of the neuropathy-associated MTMR2 gene rescues MTM1-associated myopathy

Abstract

Myotubularins (MTMs) are active or dead phosphoinositides phosphatases defining a large protein family conserved through evolution and implicated in different neuromuscular diseases. Loss-of-functionmutations inMTM1 cause the severe congenital myopathy calledmyotubularmyopathy (or X-linked centronuclearmyopathy) whilemutations in the MTM1-related protein MTMR2 cause a recessive Charcot-Marie-Tooth peripheral neuropathy. Here we aimed to determine the functional specificity and redundancy ofMTM1 and MTMR2, and to assess their abilities to compensate for a potential therapeutic strategy. Using molecular investigations and heterologous expression of humanMTMs in yeast cells and in Mtm1 knockoutmice, we characterized several naturally occurring MTMR2 isoforms with different activities. We identified the N-terminal domain as responsible for functional differences between MTM1 and MTMR2. An N-terminal extension observed in MTMR2 is absent in MTM1, and only the short MTMR2 isoform lacking this N-terminal extension behaved similarly to MTM1 in yeast andmice.Moreover, adeno-associated virus-mediated exogenous expression of several MTMR2 isoforms ameliorates themyopathic phenotype owing to MTM1 loss, with increasedmuscle force, reducedmyofiber atrophy, and reduction of the intracellular disorganization hallmarks associated withmyotubularmyopathy. Noteworthy, the short MTMR2 isoformprovided a better rescue when compared with the long MTMR2 isoform. In conclusion, these results point to themolecular basis for MTMs functional specificity. They also provide the proof-of-concept that expression of the neuropathy-associated MTMR2 gene improves the MTM1- associatedmyopathy, thus identifying MTMR2 as a novel therapeutic target formyotubularmyopathy.