Prenatal Diagnosis and Carrier Detection. With gene based technologies, accurate carrier detection and prenatal diagnosis are available for most families with a history of DMD. In the 60% to 70% of families in whom the allele results from a deletion or duplication, the presence or absence of the defect can be assessed by examination of fetal DNA using methods that assess the gene’s genomic continuity and size (see Fig. 1).
Fig1. Diagnosis of Duchenne muscular dystrophy (DMD) involves screening for deletions and duplications by a procedure called multiplex ligation-dependent probe amplification (MLPA). MlPA allows the simultaneous analysis of all 79 exons of the DMD gene in a single DNA sample and can detect exon deletions and duplications in males or females. Each amplification peak represents a single DMD gene exon, after separation of the amplification products by capillary electrophoresis. (Top panel) The amplification profiles of 16 exons of a normal male sample. Control (C) DNAs are included at each end of the scan. The MlPA DNA fragments elute according to size, which is why the exons are not numbered sequentially. (Bottom panel) The corresponding amplification profile from a DMD patient with a deletion of exons 46 and 47. (Courtesy P. N. Ray, the Hospital for Sick Children, Toronto.)
In most other families, single nucleotide variants can be identified by sequencing of the coding region and intron-exon boundaries. Because the disease has a very high frequency of new mutations and is not manifested in carrier females, ~80% of Duchenne boys are born into families with no previous history of the disease. Thus the incidence of DMD will not decrease substantially until universal prenatal or pre conception screening for the disease is possible.
Maternal Mosaicism. If a boy with DMD is the first affected member of his family, and if his mother is not found to carry the variant in her lymphocytes, the usual explanation is that he has a new mutation at the DMD locus. However, ~5% to 15% of such cases appear to be due to maternal gonadal mosaicism, in which case the recurrence risk is significant.
Therapy. At present, only symptomatic treatment is available for DMD. The possibilities for rational therapy for DMD have greatly increased with the understanding of the normal role of dystrophin in the myocyte. Some of the therapeutic considerations are discussed in Chapter 14.
