Absence of dystrophin and spectrin in regenerating muscle fibers from Becker dystrophy patients

Muscular dystrophy: centronucleation may reflect a compensatory activation of defective myonuclei

Muscular dystrophy has long been believed to be characterized by degeneration and abortive regeneration of muscle fibers (the muscle degeneration theory), but unfortunately its pathogenesis is still unclear and an effective treatment has yet to be developed. As a challenge to the theory, we have proposed an alternative muscle-defective-growth theory and a further bone muscle growth imbalance hypothesis supposing possible defects in bone-growth-dependent muscle growth based on our findings in hereditary dystrophic dy mice (C57BL/6J dy/dy). This review presents some new insights into the pathogenesis of the disease along with our hypothesis, focusing on the physiological meaning of centronucleation, one of the major pathological changes commonly observed in dystrophic muscles of man and experimental animals.

Regeneration in sarcoglycanopathies: expression studies of sarcoglycans and other muscle proteins

We have studied the immunohistochemical expression of 14 different muscle proteins of the basal lamina, sarcolemma and cytoskeleton in primary sarcoglycanopathies (13 cases) and compared it with Duchenne dystrophy (6 cases) and myositis (5 cases). Sarcolemmal proteins (i.e. 4 sarcoglycans, beta-dystroglycan, dystrophin, beta-spectrin) were reduced both in sarcoglycanopathies and Duchenne dystrophy, because of structural and functional impairment of the plasma membrane. Sarcolemmal proteins are poorly expressed in regenerating fibers of all muscle disorders, due to a developmental delay or to an abnormal assembly. Laminins (alpha2 and beta chains) were preserved in all cases while utrophin was expressed in Duchenne muscle but not in sarcoglycanopathies. Regenerating fibers were studied with different markers (i.e. fetal myosin, desmin, vimentin, laminin alpha1). Fetal myosin positive fibers (as well as desmin and laminin alpha1), were significantly higher in Duchenne dystrophy (25%) than in age-matched sarcoglycanopathies (7%). Vimentin, a marker of early regeneration, was expressed at higher level in sarcoglycanopathies than in Duchenne dystrophy, suggesting in the former a lower extent of regeneration or a shorter regeneration cycle.

Myocardial involvement is very frequent among patients affected with subclinical Becker's muscular dystrophy

BACKGROUND

Several cases of Becker's muscular dystrophy (BMD) have been reported, which showed mild or subclinical skeletal muscle involvement with an overt dilated cardiomyopathy. Here, for the first time, a group of 28 patients with BMD who had a subclinical or benign myopathy have been studied through a thorough cardiological assessment.

METHODS AND RESULTS

Each patient underwent ECG and echocardiographic examinations. Molecular analyses of the dystrophin gene and protein were performed. An unexpectedly high incidence of myocardial involvement was observed among patients affected with subclinical (72%) or benign (60%) BMD. The cardiac involvement appears to develop early from the right ventricle. Both the increase in left ventricular end-diastolic volume and the reduction in the ejection fraction appeared to be age related. Severe left ventricular dilation with reduced ejection fraction, which could be complicated by life-threatening arrhythmias, may occur. Contrary to previous reports, which indicated the involvement of 5'-end mutations in cardiomyopathies as a result of dystrophin gene alterations, this study shows that despite the apparent concentration of deletions in two regions (5'-end and exons 47 through 49), no general conclusions can be drawn regarding the involvement of specific gene mutations in the development of cardiomyopathy.

CONCLUSIONS

Cardiomyopathy is the main clinical feature and complication in patients affected by subclinical or mild BMD. The cardiac manifestation is characterized by early right ventricular involvement and is later associated with left ventricular impairment. In mild BMD, myocardial damage may develop because the patients, who are unaware of a possible cardiac involvement, are still able to perform strenuous muscle exercise and, through pressure or volume overload, may induce mechanical stress, which is harmful for dystrophin-deficient myocardial cells.

Prognostic factors in mild dystrophinopathies

One hundred twenty five patients from 105 families were considered, showing in-frame intragenic deletion or duplication of the dystrophin gene and/or abnormal dystrophin on muscle biopsy. According to clinical status of patients, the affection was classified as subclinical, benign, moderate or severe. Significant decrease of dystrophin abundance was observed with increasing clinical severity (p < 0.05). Detailed clinical data were available in 68 patients in whom a long-term follow-up (6-39 years) was obtained. Functional performance at different ages and disease endpoints were recorded in order to analyze the rate of disease progression. We identified three different disease courses: stable, slow and rapid progression. We observed a significantly lower level of dystrophin and immunohistochemical score (p < 0.05 vs. the other courses) in patients with rapid course. Deletion or duplication in the 5' end of the gene was associated with poor prognosis. Prognosis was substantially better, showing a stable course, in patients with large deletions or duplications in the proximal rod region. These subjects often suffered from a cramps/myalgia syndrome or experienced rhabdomyolisis. Cardiac involvement was detected in 65% of cases. A significant increase of right ventricular volume was seen in all clinical groups (p < 0.05). A left ventricular dilation was observed in 25% and a decreased ejection fraction in 29% of our patients. The reduction of ejection fraction and the increase of left ventricular volume were age-related. Since sudden death may occur as a consequence of cardiomyopathy, severe left ventricular dysfunction in dystrophinopathic patients is another important adverse prognostic factor, although not always directly correlated with skeletal muscle impairment.

Duchenne phenotype with in-frame deletion removing major portion of dystrophin rod: threshold effect for deletion size?

In a 9-year-old boy with Duchenne muscular dystrophy we found a large in-frame deletion, spanning exons 10 to 53 of the dystrophin gene. The deletion removed almost all of the central rod domain of dystrophin. Using carboxyterminal dystrophin antibodies the immunohistochemical reaction was normal in all muscle fibers. In immunoblot studies we found dystrophin of abnormal size (160 kDa) and normal amount (about 100%). The immunochemical features and the reading frame deduced from DNA analysis are usually associated with Becker muscular dystrophy, but the clinical characteristics were those of the severe Duchenne phenotype. All the cases of in-frame dystrophin deletions reported so far, which involved more than 36 exons, invariably resulted in a severe phenotype. Therefore, a threshold effect for dystrophin length may be reasonably suspected. Very short dystrophin molecules might induce a severe disarray of the dystrophin network.

Dystrophin-positive fibers in Duchenne dystrophy: origin and correlation to clinical course

In 132 DMD muscle biopsies we investigated the presence of dystrophin-positive fibers and the relationship of dystrophin immunohistochemical pattern to the clinical severity of the disease. Reverted fibers were detected in 37% of patients; their prevalence increased significantly in each biopsy with age of patients. We suggest that reversion occurs in satellite cells, when muscle differentiation is completed. The longitudinal extent of dystrophin-positive domain spans a maximum length of 900 microns. No correlation was found between the presence of reverted fibers and the clinical severity of DMD, whereas a milder form of Duchenne dystrophy was observed in patients showing a faint reaction in all fibers. The occurrence of reverted fibers is independent of the type of gene mutation; however, a higher proportion of cases with reverted fibers was found among patients with gene duplications.