Gene expression in muscle

This volume contains the edited transcript of an interdisciplinary colloquium held at Totts Gap Medical Research Laboratories, Bangor, Pennsylvania on October 12-14, 1983 under the sponsorship of the Muscular Dystrophy Association. The aim was to illuminate the pathogenic mechanism of Duchenne Muscular Dystrophy through a synthesis of available data on gene expression in muscle. In the informal give and take of the colloquium, the participants found themselves engaged in mutual education and enlightenment as they attempted to put together what is known and to highlight what is not known about the subject. Significant research into muscle as a tissue and muscle disease began only about 50 years ago although the description of muscular dystrophy by Guillaume Benjamin Amand Duchenne de Boulogne had been published in 1862. By 1943 it was clear that Duchenne muscular dystrophy was an X-linked genetic disorder. Up to the present, however, the offending gene has not been identified although its location on the short arm of the X chromosome has been approximately determined. The gene product associated with the initial disturbance in skeletal muscle has also remained elusive up to now. Moreover, investigations into the mechanisms of the muscle degeneration have been hampered by ignorance of the fundamental phenotypic expression of the genetic disorder. The pathological picture of muscle degeneration with fat and collagen replacement of muscle cells is familiar, but as yet there has been no clear identification of the initial lesion. It has not even been established whether the basic disturbance is impaired control of muscle growth, accelerated catabolism in muscle cells, or defective structural or contractile protein synthesis. Most investigators believe that the flagrant morphologic changes seen in muscle biopsies of even early cases of dystrophy are secondary to a more unitary and fundamental disorder of gene expression. It is known that approximately 1/3 of cases of Duchenne Muscular Dystrophy are the result of a new mutation, presumably in the grandparents, that is passed along to the patient's mother. This high rate of mutation encourages the speculation that the disorder involves a single gene. Although the clearest phenotypic marker, increased serum concentration of creatine kinase, is usually detectable at birth and often in the amniotic fluid of the fetus, morphologic changes in muscle have not been detected prior to the onset of symptoms at age 2-4. The elusiveness of the initial lesion in vivo has led investigators to seek it in cultures of developing muscle cells. Work with these cultures has uncovered much knowledge of myoblast differentiation and muscle cell maturation but has shown the process to be unexpectedly complex. Although gene expression in muscle proteins has been observed to vary from the embryonic state to the neonatal and to the adult form, the morphological characteristics of embryonic fibers are indistinguishable from their neonatal and adult counterparts. Nevertheless, the different muscle protein isoforms must represent the expression of different genes or at least different gene transcript processing for some proteins. The pertinent data and interpretations from a variety of approaches to these problems have been arranged in the following chapters in what we hope is a logical sequence. The editors acknowledge with thanks the invaluable assistance of Joy Colarusso Lowe, who with skill, patience and precision, produced the manuscript for publication.