Abstract for presentation at 11th International Congress of Human Genetics

Genetic testing for Facioscapulohumeral muscular dystrophy: an update

  • Egbert Bakker, Leiden University Medical Center, The Netherlands
  • Michiel van der Wielen, Leiden university Medical Center, Dept of Human and Clinical Genetics, The Netherlands
  • Richard Lemmers, Leiden university Medical Center, Dept of Human and Clinical Genetics, The Netherlands
  • Silvére van der Maarel, Leiden university Medical Center, Dept of Human and Clinical Genetics, The Netherlands

    • The FSHD defect was elucidated (1993), as a D4Z4 3.3kb repeat array contraction from >11 to <10 units. While, recent evidence points to DNA-hypo-methylation of the D4Z4 array and allele specific deregulation of transcription, the pathogenesis of FSHD is still poorly understood. Genetic tests based are on determination of the D4Z4 array length. Complicating factors are; a homologous repeat array (10q26), de novo contractions, mosaicism, and a rare deletion of the p13E-11 probe. Here we provide an overview of the current practice in FSHD genetic testing. Genomic DNA is digested by use of EcoRI, EcoRI/BlnI (double digestion) and by XapI, after gelelectrophoresis and Southern blotting the DNA is hybridized with a 32P labeled Probe p13E-11, specific for the EcoRI D4Z4 fragment. FSHD1 is confirmed (99%) if an <38kb EcoRI P13E-11 fragment is detected AND is further reduced in size –3kb (to > 35kb) after digestion with BlnI. Upon XapI digestion the >38b EcoRI fragment should disappear. XapI digest the 4q35 repeats arrays while BlnI digests the 10q26 originated arrays. In >95% of the cases this test is conclusive. Diagnosis is not confirmed nor excluded (1%) if no short (<38kb) EcoRI/BlnI P13E-11 fragment is detected. If needed PFGE will separate the 4 alleles (2 of 4q35 and 2 of 10q26) and detect de novo rearrangements in mosaics. In the remaining 5% of cases were molecular diagnosis is not confirmative, but suggestive for FSHD, methylation study of the 4q35 repeat array is to be considered. This type of test is not easy and not operational yet. Over the last 12 years neurologists and clinical geneticists sent us families, sporadic cases, for molecular confirmation of the diagnosis FSHD1. Diagnosis was confirmed in over 600 cases (~50%), 10% of which are de novo rearrangements. In FSHD1 proven cases both pre-symptomatic tests (N= 150) and prenatal tests (N=27 in 21 pedigrees) were performed. Ten foetuses were diagnosed as affected and 17 were shown not to carry the defect.

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