We report a Japanese Becker muscular dystrophy (BMD) patient with occasional myalgia and cramps during normal activity that developed at the age of 28 months. His family history was negative for neuromuscular diseases. Muscle biopsy analyses, including dystrophin immunostaining, disclosed no clinically relevant findings. The diagnosis of BMD was initially made at the age of 10 years, when indications of persistent high serum levels of CK prompted us to screen deletions in the dystrophin gene by amplification of 19 deletion-prone exons from the genomic DNA by the polymerase chain reaction (PCR). Among the exons examined, exons 13 and 17 were deleted. To clarify the size of the deletion, the dystrophin transcript was analyzed by reverse transcription PCR. The determined nucleotide sequence of the amplified product encompassing exons 10 to 20 disclosed that the entire segment corresponding to exons 13 to 18 (810 bp) was absent, a deletion that would be expected to cause the production of a dystrophin protein lacking 270 amino acids from the rod domain. This result indicates that occasional myalgia and cramps could be early clinical manifestations of mild BMD, especially in patients who have a deletion in the rod domain, and that deletion screening of the dystrophin gene might be the only reliable method to diagnose such cases.

Two Japanese brothers with Becker muscular dystrophy were shown by polymerase chain reaction (PCR) and cDNA sequence analysis to produce a dystrophin gene transcript lacking a single exon; that is, number 13. Despite having the same deletion mutation, the brothers showed clearly different clinical phenotypes: the younger brother developed cardiac failure at the age of nine, while the elder brother was asymptomatic. As alternative splicing was not responsible for this clinical difference, the amount of dystrophin transcript was examined by using reverse transcription semi-nested and parallel PCR. The results showed that the amount of the dystrophin transcript in the younger brother was 20% of that of the elder brother. This finding suggested that lesser amount of dystrophin transcript in the younger brother was responsible for the early onset of cardiac failure. This would represent a novel molecular mechanism for dystrophinopathy. Key wordsBecker muscular dystrophy, cardiac failure, dystrophin transcript.Duchenne and Becker muscular dystrophies (DMDBMD) are allelic diseases caused by mutation of the dystrophin gene, which extends over 3000 kb on Xp21 and has 79 exons. While DMD patients show very severe clinical symptoms and usually die at around 20 years old, BMD has a slower rate of progression: affected boys will remain ambulatory beyond the age of 16 years and a few may lead near-normal lives. Cardiac failure is one of the leading causes of death of DMD/BMD patients.Although cardiac involvement in dystrophinopathy has long been demonstrated, the pathogenesis of cardiac dysfunction