Improved characterization of FSHD mutations

Facioscapulohumeral muscular dystrophy (FSHD) is caused by the shortest all eles of the 3.3kb-tandem repeat array D4Z4 at 4q35. Molecular diagnosis of FSHD depends upon the separation of unusually large alleles by pulse-field electrophoresis after EcoRI and ECORI/BlnI. digestion. The exact number of alleles could not however be directly inferred from the size of DNA fragmen ts owing to polymorphisms in the telomeric region of the locus. Knowing the exact repeat number of disease causing alleles may benefit genetic counsel ling, help to understand the mechanism of this singular disease and the pop ulation dynamics of subtelomeric sequences variations. We present here a pa rtial digestion mapping method giving the exact number of repeats for disea se causing alleles, and we suggest that most inaccuracies induced by common polymorphisms could be reduced by using EcoRV in place of ECoRI. After stu dying more than 300 DNA samples with both the standard method and this new method, we show that alleles size can be evaluated with a precision of less than one half repeat, and that the variations in length of the truncated r epeat in the telomeric region of the D4Z4 locus can be evaluated. The resul ts suggest that at least one intact chromosome 4 type repeat at 4q35 is nee ded to cause FSHD. (C) 2001 Editions scientifiques et medicales Elsevier SA S.