Brothers with Becker muscular dystrophy, born a year apart, show marked difference in atrophy of the glutes maximus and vastus femoris muscles at adulthood
Authors: Nambu Yoshinori , Shirakawa Taku , Osawa Kayo , Nishio Hisahide , Nozu Kanadai , Matsuo Masafumi , Awano Hiroyuki 
Published: December 2022

Background
Becker muscular dystrophy (BMD) is characterized by a progressive muscular atrophy caused by a truncated dystrophin produced by the mutant DMD gene. The onset and progression patterns of muscle atrophy in BMD are diverse, and previous reports attribute this to differences in the structure of the dystrophin produced. Therefore, it is expected that patients with BMD in the same family express the same truncated dystrophin and grow up in a similar environment, resulting in a similar progression of muscle atrophy. However, we report on brothers with BMD, who were only one year apart in age, showed marked differences in atrophy of the gluteus maximus and vastus femoris muscles on computed tomography (CT) scan at adulthood.
Case presentation
A retrospective analysis of clinical data from nine pairs of BMD siblings under follow-up at the Department of Pediatrics, Kobe University Hospital, revealed significant differences in clinical findings in only one pair. Brothers who were 1 year apart had identical deletions of exon 45-47 (Δ45-47) in the DMD gene, yet serum creatine kinase (CK) levels were higher in the older brother than in the younger brother at most ages from infancy to adulthood, and the median CK level was significantly higher in the older brother (4763 versus 3513 U /L, p < 0.01). Both the older brother (age 20) and younger brother (age 19) were able to walk independently, and motor function did not differ markedly between the brothers. To examine skeletal muscle damage in the brothers, CT examinations were performed to measure and compare cross-sectional area (CSA) and average CT values of a total of 10 different muscles in the upper arm, gluteal, and thigh. No significant differences in CSA and mean CT values were found between the brothers in the two upper arm muscles, but there were marked differences in CT values between the brothers in the gluteus maximus muscle in the gluteal region and the vastus femoris muscle in the thighs. The CSA of the gluteus maximus and vastus femoris muscles of the older brother was only 1/2 (2,380 vs. 4,756 mm2) and 1/3 (1,506 vs. 4,507 mm2) of that of the younger brother, respectively, and his gluteal and vastus atrophy was more pronounced than that of his younger brother. The mean CT values of the gluteus maximus and vastus femoris of the older brother were very low compared to those of the younger brother (-9.9 vs. 11.8 HU and -18.1 vs. 33.5, respectively), and despite the 1-year age difference, the adiposity of the gluteus maximus and vastus femoris of the older brother was more pronounced than that of the younger brother.
Conclusions
Brothers with BMD having identical DMD mutations, a 1-year age difference, and nearly identical growing environment developed significant differences in the gluteus maximus and vastus femoris atrophy. It strongly suggests that muscle atrophy in BMD may be regulated by factors other than DMD genotype or environmental factors.