Congenital Contracture Syndrome Caused by Mutation in Embryonic Myosin Heavy Chain Characterized by Significant Changes in Adult Muscle Contractility

Abstract

Congenital contracture syndromes affect 1 out of every 1000 live births, and of those syndromes, distal arthrogryposis (DA), characterized by contractures of the hands and feet, is the most predominant. In one subtype of DA, Freeman Sheldon Syndrome (FSS), 97% of the cases are caused by mutations in the embryonic myosin heavy chain gene, MYH3. To assess the effects of this mutation on adult muscle contractility, skeletal muscle was obtained from a needle biopsy of the gastrocnemius muscle in an FSS individual (MYH3 R672C) and a control subject were performed and skinned single muscle fibers were dissected for measurements of contractile performance as the [Ca2+] of physiological solutions was varied. The magnitude of passive stiffness was 2x greater for patient fibers. There was no difference in maximal Ca2+ activated force found in the affected adult muscle fibers (0.204uN ± 0.044) compared to normal adult muscle fibers (0.259uN ± 0.028). However specific force was 69% less; this was attributable to hypertrophy of the patient fibers (159um ± 8 as compared to normal control myofibers of 87um ± 3). Little to no change was observed in Ca2+ sensitivity (pCa50) or in cooperativity of the force-pCa relationship. Relaxation was dramatically slower in patient fibers, taking 4x longer to reach 50% relaxation and 10x longer to reach 90% relaxation. Control experiments suggested this is not due to the larger patient fiber size. Preliminary analysis, using a 12.5% agarose gel, and Western Blots, indicated that these differences were not fiber type dependent. Interestingly, we have identified that embryonic myosin (MYH3) is present in single adult muscle cells. This work was supported by HL65497 (Regnier) and HD48895 (Bamshad).