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Abstract Background Myostatin is a negative regulator of skeletal muscle growth. Truncating mutations in the myostatin gene have been reported to result in gross muscle hypertrophy. Duchenne muscular dystrophy (DMD), the most common lethal muscle wasting disease, is a result of an absence of muscle dystrophin. Although this disorder causes a rather uniform pattern of muscle wasting, afflicted patients display phenotypic variability. We hypothesized that genetic variation in myostatin is a modifier of the DMD phenotype. Methods We analyzed 102 Japanese DMD patients for mutations in the myostatin gene. Results Two polymorphisms that are commonly observed in Western countries, p.55A>T and p.153K>R, were not observed in these Japanese patients. An uncommon polymorphism of p.164E>K was uncovered in four cases; each patient was found to be heterozygous for this polymorphism, which had the highest frequency of the polymorphism observed in the Japanese patients. Remarkably, two patients were found to be heterozygous for one of two novel missense mutations (p.95D>H and p.156L>I). One DMD patient carrying a novel missense mutation of p.95D>H was not phenotypically different from the non-carriers. The other DMD patient was found to carry both a novel mutation (p.156L>I) and a known polymorphism (p.164E>K) in one allele, although his phenotype was not significantly modified. Any nucleotide change creating a target site for micro RNAs was not disclosed in the 3' untranslated region. Conclusion Our results indicate that heterozygous missense mutations including two novel mutations did not produce an apparent increase in muscle strength in Japanese DMD cases, even in a patient carrying two missense mutations.

The dystrophin gene, which is mutated in Duchenne and Becker muscular dystrophy, is characterized by its extremely large introns. Seven cryptic exons from the intronic sequences of the dystrophin gene have been shown to be inserted into the processed mRNA. In this study, we have cloned seven novel cryptic exons embedded in dystrophin introns that were amplified from dystrophin mRNA isolated from lymphocytes. All of these sequences, which ranged in size from 27 to 151 bp, were found to be cryptic exons because they were completely homologous to intronic sequences (introns 1, 18, 29, 63, 67, and 77), and possessed consensus sequences for branch points, splice acceptor sites, and splice donor sites. Compared with the 77 authentic dystrophin exons, the 14 cryptic exons were characterized by (1) lower Shapiro’s splicing probability scores for the splice donor and acceptor sites; (2) smaller and larger densities of splicing enhancer and silencer motifs, respectively; (3) a longer distance between the putative branch site and the splice acceptor site; and (4) with one exception, the introduction of premature stop codons into their respective transcripts. These characteristics indicated that the cryptic exons were weaker than the authentic exons. Our results suggested that a mutation deep within an intron that changed these parameters could cause dystrophinopathy. The cryptic exons identified provide areas that should be examined for the detection of mutations in the dystrophin gene, and they may help us to understand the roles of large dystrophin introns. [ABSTRACT FROM AUTHOR]