Molecular analysis of Duchenne muscular dystrophy: past, present, and future

Novel mutation in exon 56 of the dystrophin gene in a child with Duchenne muscular dystrophy

Duchenne type muscular dystrophy (DMD) is an allelic X-linked recessive disorder caused by mutations in the gene encoding dystrophin. Genotype analysis has shown that deletion mutations account for approximately 65% of all cases, and 5-10% are duplications, while the remaining 30% of affected individuals may have smaller mutations, including point mutations, small deletions or small insertions. In this study, we present the case of a 4-year-old boy with typical clinical features of DMD, who developed normally until the age of 2. However, at age 3 he presented his first symptom, a tendency to fall, had difficulty in rising from the floor and in walking on his toes. At age 4 he had a waddling gait and could no longer climb stairs. A physical examination revealed proximal muscle weakness, calf hypertrophy, deep tendon hyporflexia and a positive Gower's sign. To identify the disease-causing gene in the proband, all coding regions (exons 1-79) of the dystrophin gene were PCR-amplified and sequenced. A novel duplication (c.8284dupA) in exon 56 of the dystrophin gene was identified, which was predicted to generate a frameshift mutation and create a premature termination codon (p.Ile2762Asnfs*10). This mutation was further confirmed by single-strand conformation polymorphism (SSCP) analysis, which revealed an extra band found in exon 56 of the dystrophin in the proband; however, this was not present in his family members or in the 100 matched normal controls. The data presented in this study may aid in expanding the spectrum of mutations causing DMD. To our knowledge, we demonstrate for the first time that a small duplication mutation can cause severe DMD.

Dystrophin and mutations: one gene, several proteins, multiple phenotypes

A large and complex gene on the X chromosome encodes dystrophin. Many mutations have been described in this gene, most of which affect the expression of the muscle isoform, the best-known protein product of this locus. These mutations result in the Duchenne and Becker muscular dystrophies (DMD and BMD). However, there are several other tissue specific isoforms of dystrophin, some exclusively or predominantly expressed in the brain or the retina. Mutations affecting the correct expression of these tissue-specific isoforms have been associated with the CNS involvement common in DMD. Rare mutations also account for the allelic disorder X-linked dilated cardiomyopathy, in which dystrophin expression or function is affected mostly or exclusively in the heart. Genotype definition of the dystrophin gene in patients with dystrophinopathies has taught us much about functionally important domains of the protein itself and has provided insights into several regulatory mechanisms governing the gene expression profile. Here, we focus on current understanding of the genotype-phenotype relation for mutations in the dystrophin gene and their implications for gene functions.

Effects of shaker-1 mutations on myosin-VIIa protein and mRNA expression

Numerous mammalian diseases have been found to be due to mutations in components of the actin cytoskeleton. Recently, mutations in the gene for an unconventional myosin, myosin-VIIa, were found to be the basis for the deafness and vestibular dysfunction observed in shaker-1 (sh1) mice and for a human deafness-blindness syndrome, Usher syndrome type 1B. Seven alleles of sh1 mice were analyzed to assess the affects of different myosin-VIIa mutations on both gene expression and tissue function. Myosin-VIIa is expressed in the inner ear and the retina, as well as the kidney, lung, and testis. Northern blot analysis indicated that myosin-VIIa mRNA expression, size, and stability were unaffected in the seven sh1 alleles. Immunoblot analysis showed that all seven alleles expressed some full-length myosin-VIIa protein. The range of expression, however, ran from sh1 [original], which expressed wild-type levels of protein, to two strains, sh1(4494SB) and sh1(4626SB), which expressed less than 1% of the normal level of myosin-VIIa protein. For the three alleles of sh1 that have been characterized and that have mutations in the motor domain, sh1 [original], sh1(816SB) and sh1(6J), the level of protein expression observed in these sh1 alleles correlated well with the predicted effects of the mutations on motor function. No change in retinal or testicular structure was observed at the light microscopic level during the life span of the seven sh1 alleles. Myosin-VIIa protein, when detectable, was observed to locate properly in the sh1 mice. On the basis of these results, we propose that the mutations in myosin-VIIa in the sh1 alleles leads to both motor dysfunction and to a protein destabilization phenotype.

The clinical and molecular genetic approach to Duchenne and Becker muscular dystrophy: an updated protocol

Detection of large rearrangements in the dystrophin gene in Duchenne and Becker muscular dystrophy is possible in about 65-70% of patients by Southern blotting or multiplex PCR. Subsequently, carrier detection is possible by assessing the intensity of relevant bands, but preferably by a non-quantitative test method. Detection of microlesions in Duchenne and Becker muscular dystrophy is currently under way. Single strand conformational analysis, heteroduplex analysis, and the protein truncation test are mostly used for this purpose. In this paper we review the available methods for detection of large and small mutations in patients and in carriers and propose a systematic approach for genetic analysis and genetic counselling of DMD and BMD families, including prenatal and preimplantation diagnosis.

Expression of myogenic regulatory factors in normal and dystrophic mice: effects of IGF-1 treatment

Myogenic regulatory factors (MRFs) promote differentiation of muscle cells from fibroblasts and are induced by insulin-like growth factor I (IGF-1). Prior studies have shown synthesis of new muscle protein and improved muscle morphology when mature dy mice with muscular dystrophy are treated with IGF-1. We investigated whether these salutary effects of IGF-1 might be attributable to stimulation of MRFs. Male dy (129ReJ) mice and controls (129J) were assigned to IGF-1 treatment (10 micrograms twice daily) or non-treatment at about 5 weeks of life and sacrificed 6 weeks later. RNA was extracted from skeletal muscles, reverse transcribed, and amplified by polymerase chain reaction (PCR) using primers specific for each MRF. Competitive PCR was performed to quantify MyoD expression in response to IGF-1 treatment. Transcripts for myf-5, MRF4, and myogenin were detected in both control and dy mouse muscles; no apparent differences were observed between treatment groups. Quantitative analysis of transcripts for MyoD indicated no significant basal differences between control and dy mice. There was, however, significantly higher MyoD expression in the dy group, and a trend toward significance in the control group, following IGF-1 treatment. These data suggest that IGF-1 exerts its in vivo effects in postembryonal muscle by stimulating MRFs.

A Drosophila dystrophin-related protein, MSP-300, is required for embryonic muscle morphogenesis

Proteins from the spectrin superfamily contribute to cell polarity and shape during the morphogenetic that accompany embryogenesis. Drosophila MSP-300, a member of the spectrin superfamily, is expressed in somatic, visceral and heart embryonic muscles. Cloning and sequence analysis of various spliced forms of MSP-300 reveals functional and structural similarities between MSP-300 and vertebrate Dystrophin, the product of the Duchenne Muscular Dystrophy gene. The identification of a strain mutant for the MSP-300 gene is described. Analysis of the somatic muscle phenotype in MSP-300 mutant embryos suggests that the protein contributes to the integrity of the somatic and visceral muscle during periods of significant morphogenetic change. Functional synergism between MSP-300 and laminin is demonstrated by the analysis of the phenotype of embryos mutant for both genes. The enhancement of aberrant muscle phenotype in the double mutants suggests a link between MSP-300 and laminin function in mediating proper extension of the myotube towards the epidermal muscle attachment site. In addition, both genes function to establish gut integrity. In view of the functional and structural similarities between MSP-300 and Dystrophin, it is postulated that Dystrophin is not only required for proper muscle function in adult life but also contributes to muscle morphogenesis during the development of the vertebrate embryo.