Myosin myopathy. A new disease entity

Abstract

By the discovery and characterization of two familial myopathies caused by mutations in myosin heavy chaingenes, we introduce a new entity within the field of neuromuscular disorders: Myosin myopathyThe first myopathy affected a family in western Sweden. Previous linkage analysis had located the gene tochromosome 17p13.1, where a cluster of myosin heavy chain (MyHC) genes is located. By analysis ofmorphological changes in muscle tissue, MYH2 appeared to be the primary candidate gene since type 2A musclefibers, which express MyHC IIa, were mainly affected. Sequencing of MYH2 disclosed a heterozygous missensemutation (706Glu>Lys) affecting a highly conserved residue in the head of MyHC IIa. The mutated amino acid islocated in the SH1-helix, which is important for the conformational changes that take place in the myosin head athydrolysis of ATP and result in muscle contraction. We developed a new method to analyze the relativeexpression of the three major MyHC isoforms at mRNA level, which we combined with SDS-PAGE analysis tostudy the expression of myosin isoforms at the protein level and immuno-histochemistry to study the distributionof the different myosin isoforms. We demonstrated a clear correlation between the expression of the mutatedMyHC IIa and pathological changes in muscle. In some cases muscle fibers showed rimmed vacuoles withinclusions of 15-20 nm filaments identical to those in sporadic inclusion body myositis (s-IBM). All these fibersexpressed the mutant myosin isoform, indicating the possibility that breakdown of sarcomeric proteins is ofpathogenetic importance for the formation of rimmed vacuoles and filamentous inclusions.It is known that endurance training in normal individuals causes a gradual shift of myosin expression in skeletalmuscle from fast isoforms (MyHC IIa and MyHC IIx) to slow isoform (MyHC I). Since we had demonstratedthat the muscle pathology in MyHC IIa myopathy correlated with the expression of MyHC IIa, we investigated ifendurance training could alter the myosin expression in the patients with MyHC IIa myopathy. An eight-weektraining program caused a significant decrease in MyHC IIx and a significant increase in MyHC I expressionwhile MyHC IIa expression increased slightly in all patients. We conclude that physical training affects themyosin isoform expression in patients with myosin myopathy and speculate that a longer training period or othertype of training may reduce the MyHC IIa expression.On the background of the hypothesis that MYH2 mutations are a common cause of muscle disorder, weinvestigated eight families with hereditary myopathy of unknown cause by complete sequencing of MYH2, andwe also analyzed the normal nucleotide variation in MYH2 by sequencing the entire coding region of MYH2 inDNA obtained from 50 blood donors. We found extremely low sequence variability. Among the eight familieswith myopathy we found three novel missense mutations in MYH2, which were possibly associated with themyopathies. However, further studies are necessary to establish the functional effects of these mutations.The second type of myosin myopathy was identified in several members of a family with apparent autosomaldominant inheritance and in a sporadic case. Signs and symptoms started in childhood with proximal muscleweakness and there was slow progression. Muscle biopsies showed characteristic morphological changes withtype 1 fiber predominance and large inclusions in most type 1 fibers. Analysis by immuno-histochemistry,electron microscopy, SDS-PAGE and immunoblotting indicated that there was storage of slow/ß myosin (MyHCI) in the inclusions. Sequencing of MYH7, which encodes MyHC I, disclosed a heterozygous missense mutation,1845Arg>Trp, affecting a highly conserved residue in the distal rod region of the protein. The mutationsegregated with the disease in the familial cases and it was not identified in any of the relatives of the sporadiccase. In spite of the fact that MYH7 is expressed also in the heart, there was no cardiomyopathy. We conclude thatthe mutated amino acid is important for the assembly of thick filaments in the sarcomeres of skeletal muscle, andpropose the term Myosin storage myopathy for this disease, which should be included among disorders in thenovel entity Myosin myopathy.