Myo18b-related nemaline myopathy null mutation causes dysfunctional sarcomere contractility (S22.006)

Objective: To explore the impact on muscular sarcomeric function of a homozygous premature nonsense mutation in MYO18B (c.6496G>T; p.Glu2166*) associated with severe nemaline myopathy with cardiomyopathy. Background: Nemaline myopathy (NM) is one of the most frequent forms of congenital myopathy defined by the presence of nemaline bodies (rods) in muscle biopsy. Clinical spectrum encompasses lethal forms and less severe diseases with later onset. We recently described clinical and histopathological features of a sporadic case with severe NM and cardiomyopathy and a homozygous premature nonsense mutation in MYO18B (c.6496G>T; p.Glu2166*). Protein and immunohistochemical studies demonstrated the presence of a C-terminally truncated protein. MYO18B codes for a non-conventional myosin protein and it is mainly expressed in skeletal and cardiac muscles. MYO18B null zebrafish mutants showed defects on sarcomeric organization suggesting a role of MYO18B in the integration of thick and thin filaments into the sarcomere. Design/Methods: We performed contractile measurements in individual permeabilized fibers obtained from muscle biopsy of our MYO18B mutated patient. Fibers were set at a sarcomere length of 2.5 μm and were then activated with exogenous calcium. The force response was measured. Results: Our findings reveal that patients muscle fibers have a severely reduced maximal active tension (i.e. force per cross sectional area at pCa 4.5). This force reduction was caused by a reduced number of myosin-actin interactions during activation (as indicated by a reduced active stiffness), due to changes in cross bridge cycling kientics and probably a lower number of intact myofibrils in the patient’s muscle fibers. The calcium sensitivity of force was reduced in the fibers of the patient (i.e. lower pCa 50 ), which was accompanied by a reduced cooperativity of activation (lower n H ). Conclusions: We demonstrated the deleterious effect of the MYO18B (c.6496G>T; p.Glu2166*) mutation on sarcomeric contractility, possibly contributing to muscle weakness confirming its pathogenic role in our NM patient. Disclosure: Dr. Malfatti has nothing to disclose. Dr. Joureau has nothing to disclose. Dr. de winter has nothing to disclose. Dr. Bohm has nothing to disclose. Dr. Laporte has nothing to disclose. Dr. Romero has nothing to disclose. Dr. Ottenheijm has nothing to disclose.