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Muscle dystrophin mRNAs from Duchenne (DMD) and Becker (BMD) patients with internal deletion of the DMD gene were quantitated and sequenced. In all cases (eight DMD and three BMD), truncated mature transcripts were found, and their amount was correlated to the clinical phenotype and to the reading frame. We focused on four cases that were apparently not in agreement with the reading frame rule. In two DMD cases, slightly reduced amounts of in-frame truncated mRNA are present but no dystrophin is detected, suggesting impaired translation and/or instability of the protein. In two BMD patients with out-of-frame deletions, the presence of minor in-frame alternatively spliced mRNA species is congruent with the observed truncated dystrophin and the mild phenotype.

Duchenne muscular dystrophy (DMD) is an X-linked recessive genetic disorder for which the biochemical defect is as yet unknown. Recently, two cloned segments of human X-chromosome DNA have been described which detect structural alterations within or near the genetic locus responsible for the disorder. Both of these cloned segments were described as tightly linked to the locus and were capable of detecting deletions in the DNA of boys affected with DMD. In an attempt to determine more precisely the occurrence of these deletions within a large population of DMD patients and the accuracy of one of the segments, DXS164 (pERT87), in determining the inheritance of the DMD X chromosome, the subclones 1, 8 and 15 were made available to many investigators throughout the world. Here we describe the combined results of more than 20 research laboratories with respect to the occurrence of deletions at the DXS164 locus in DNA samples isolated from patients with DMD and Becker muscular dystrophy (BMD). The results indicate that the DXS164 locus apparently recombines with DMD 5% of the time, but is probably located between independent sites of mutation which yield DMD. The breakpoints of some deletions are delineated within the DXS164 locus, and it is evident that the deletions at the DMD locus are frequent and extremely large.

The single X chromosome of a girl with Turner syndrome (45,X) and typical Duchenne muscular dystrophy was investigated at the chromosomal and DNA levels. No visible abnormality of the residual X chromosome was found upon high-resolution R-banding. The DNA was analysed by Southern blotting and hybridization with seven cloned probes mapping in the Xp21 region where the Duchenne locus is thought to be located. A molecular deletion was detected with probes pERT 87.1, pERT 87.8, and pERT 87.15. The other probes (754, C7, 99.6, and RC8) gave a normal signal. The DNA alleles seen in the two parents indicated that the deletion found in the propositus had occurred de novo on a maternal X chromosome.