AN X/AUTOSOME TRANSLOCATION IN A GIRL WITH DUCHENNE MUSCULAR DYSTROPHY (DMD): EVIDENCE FOR DMD GENE LOCALIZATION

Abstract

A 16 year old girl, an only child, was found to have Duchenne Muscular Dystrophy (DMD) on the basis of clinical history and course, elevated CPK and aldolase levels, typical EMG and muscle biopsy histology consistent with a dystrophy picture. A chromosome analysis from her skin and blood revealed a reciprocal X/11 translocation: 46,Xt(X;11) (Xqter→Xp21::11q13→11qter) (11pter→11q13::Xp21→Xpter), i.e., the breakpoint occurred in the X short arm at band p21; there is no evidence for mosaicism. The father's chromosomes were normal (46,XY). The mother's study revealed 46,XX, with 20% breakage and clinically she displayed untreated polycythema vera of perhaps 4 years duration. Auto-radiography of the subject's blood cells indicated non-random inactivation of the normal X. The subject exhibited normal 10 and 20 sexual development, including menses. Repeated CPK levels in the family suggests that the mother is not a DMD carrier. Additional genetic markers were determined, including PGK, Xg^a, G6PD and color vision. The occurrence of DMD in this girl may be explained by either the “uncovering” of a spontaneous mutant DMD gene on the translocated X chromosome, while the normal X is selectively inactivated; or that the break in Xp21 has caused a mutation of the normal locus leading to a null gene condition. The latter explanation would localize a regulatory or structural DMD gene to the region of the Xp21 band. Future efforts to identify the DMD gene product may find this X/11 translocation cell line useful.

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