Duchenne muscular dystrophy: Pathogenetic aspects and genetic prevention

Abstract

Duchenne muscular dystrophy (DMD) is the most common sex linked lethal disease in man (one case in about 4000 male live births). The patients are wheelchair bound around the age of 8–10 years and usually die before the age of 20 years. The mutation rate, estimated by different methods and from different population studies, is in the order of 7×10^-5, which is higher than for any other X-linked genetic disease. Moreover, unlike other X linked diseases such as hemophilia A or Lesh-Nyhan's disease, there seems to be no sex difference for the mutation rates in DMD. Several observations of DMD in girls bearing X-autosomal translocations and linkage studies on two X chromosomal DNA restriction fragment length polymorphisms indicate that the DMD locus is situated on the short arm of the X chromosome, between Xp11 and Xp22. It may be of considerable length, and perthaps consisting of actively coding and non-active intervening DNA sequences. Thus unequal crossing over during meiosis in females could theoretically account for a considerable proportion of new mutations. However, there is no structurally or functionally abnormal protein known that might represent the primary gene product, nor has any pathogenetic mechanism leading to the observed biochemical and histological alterations been elucidated. Among the numerous pathogenetic concepts the hypothesis of a structural or/and functional defect of the muscular plasma membrane is still the most attractive. It would explain both the excess of muscular constituents found in serum of patients and carriers, such as creatine kinase (CK), as well as the excessive calcium uptake by dystrophic muscle fibres, which, prior to necrosis, could lead to hypercontraction, rupture of myofilaments in adjacent sarcomeres and by excessive Ca uptake to mitochondrial damage causing crucial energy loss. The results of studies on structural and functional memthrane abnormalities in cells other than muscle tissue, e.g., erythrocytes, lymphocytes and cultured fibroblasts, indicate that the DMD mutation is probably demonstrable in these tissues. However, most of the findings are still difficult to reproduce or even controversial. DMD is an incurable disease; therefore most effort, in research as well as in practical medicine, is concentrated upon its prevention. Unfortunately the disease cannot yet be diagnosed prenatally. Potential DMD carriers among female relatives of the patients may be identified by pathological heterozygote tests, of which determination of serum CK activity is probably still the most reliable method, allowing the detection of about 70% of adult and probably up to 90% of carriers at school age. Because of the high mutation rate, assessment of individual heterozygote risks in female relatives of isolated DMD cases is of special importance. For the calculations a maximum of genealogical and phenotype information on unaffected male and on heterozygote tests in female relatives is needed to obtain credible risk figures. However, estimating a consultand's risk and passing on this information is only one aspect of genetic counselling in DMD. At least as important is information on the medical, psychological and social impacts of the disease (burden) and the possibility of maintaining a long-term contact between the couples at risk and the team involved in medical, genetic and social problems of the disease. Neonatal CK screening for DMD, although without any therapeutical consequence, could theoretically lead to the prevention of “secondary” cases, accounting for some 15% of all DMD patients born, but an almost equal prevention rate of such cases would be achieved if CK examinations were limited to all boys with delayed motor development during the first 2 years of life. Finally, it is believed that the two most important preventive problems in DMD, carrier detection and prenatal diagnosis, will ultimately be solved by means of the rapidly advancing DNA technology.