MYPN Mutations and Myopathies

Satoko M, Satomi M, Yukiko K H, Enkhsaikhan P, Atsuko N, Eriko K et al. Biallelic Mutations in MYPN, Encoding Myopalladin, are Associated With Childhood Onset Slowly Progressive Nemaline Myopathy. The American Journal of Human Genetics 2017;100:1-10

Lornage X, Malfatti E, Chéraud C, Schneider R, Biancalana V, Cuisset JM, et. al. Recessive MYPN Mutations Cause Cap Myopathy With Occasional Nemaline Rods. Ann Neurol. 2017 Feb 21

Submitted by Rocio Carolina C. Garcia Santibanez, MD, News Science Editorial Board

Myopalladin is a protein that is encoded by the MYPN gene. These two reports found recessive MYPN mutations as a cause of rod myopathy and cap myopathy. The first group described four patients of Japanese origin with recessive mutations (three homozygous and one compound heterozygous) with disease onset in the first decade with severe diffuse weakness affecting lower more than upper limbs as well as facial weakness. Cardiac and respiratory involvement was seen in two patients. Creatine kinase (CK) was normal in all four patients. Biopsies showed nemaline rods and type 1 fiber predominance in all four patients and intranuclear rods in two patients.

The second group described three patients (French and Mauritanian origin) with homozygous MYPN mutations. The onset of symptoms was birth to early childhood. The distribution of weakness was limb girdle, distal leg (tibial) and arm (finger flexor and finger extensor) and facial. All three had respiratory involvement and one had cardiac involvement. Muscle biopsies showed subsarcolemmal caps in 10% of fibers and rods in 2% of fibers. No intranuclear rods were seen.

Comment: Myopalladin is a protein localized in the Z and I bands of the sarcomere (as well as the nucleus) of the cardiac and skeletal myocites. It binds to alpha actinin and nebulin/nebulette. Heterozygous mutations in MYPN have been reported as the cause of cardiomyopathies and cardiac conduction defects. These two reports expand our knowledge of genetic mutations causing rod and cap myopathies adding to the already known mutations affecting other component of the thin filaments (ACTA1, BEN, TPM3, TPM2 among others).
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