title: [Facioscapulohumeral muscular dystrophy type 2].
abstract: BACKGROUND
  In recent years, the advances of knowledge in clinical, genetic and epigenetic features of facioscapulohumeral muscular dystrophy (FSHD) allowed the identification of two forms of FSHD, the classical autosomal dominant FSHD type 1, and FSHD type 2 characterized by an identical clinical phenotype but associated with a different (epi)genetic defect.
  
  
  BACKGROUND
  In the large majority of FSHD1 patients, the identification of D4Z4 pathogenic contraction on a permissive chromosome 4 is sufficient for diagnosis, while FSHD2 diagnosis is complicated by the fact that the genetic defect associated with this disease is not known yet and a complete D4Z4 genotype and a D4Z4 specific methylation test are required. Indeed, FSHD2 patients display a non-contracted D4Z4 allele on chromosomes 4, at least one permissive chromosome 4QA and a profound hypomethylation of both chromosomes 4 and 10. A common pathophysiological pathway has been hypothesized for FSHD1 and FSHD2 in order to explain the identical clinical phenotype and the highly similar epigenetic changes found in patients affected by these diseases. According to this hypothesis, chromatin relaxation - due to pathogenic D4Z4 contraction in FSHD1 patients, and to important hypomethylation of this locus in FSHD2 patients - would allow the last D4Z4 unit to encode for a toxic DUX4 transcript. This transcript would be stable only when encoded from a permissive chromosome 4 carrying a polyadenylation signal immediately distal to the last D4Z4 unit on chromosome 4.
  
  
  CONCLUSIONS
  Since, to express clinical phenotype, FSHD2 patients have to carry both 4QA chromosome and hypomethylated D4Z4 on chromosomes 4 and 10, digenic transmission has been hypothesized for this disease. The identification of the gene(s) and the exact epigenetic pathway underlining this disease will be mandatory to increase the rate of diagnosis for FSHD2 patients and to confirm the hypothesis of a common FSHD1 and FSHD2 pathophysiological pathway involving DUX4 gene.
  
  
  CONCLUSIONS
  The identification, among patients carrying a FSHD phenotype, of FSHD2, a new disease with distinct (epi)genetic features but having a common pathophysiological pathway with FSHD1, suggests the possibility of developping new therapeutic strategies suitable for both diseases.
authors: Centre de référence maladies neuromusculaires, hôpital Archet 1, CHU de Nice, BP 3079, 151, route de Saint-Antoine-de-Ginestière, 06202 Nice cedex 3, France; UMR CNRS 7277, Inserm 1091, faculté de médecine, Tour Pasteur, avenue de Valombrose, 06189 Nice cedex, France. Electronic address: sacconi@unice.fr.
authors: S
authors: S
authors: Sacconi
authors: C
authors: C
authors: Desnuelle
mesh_terms: D002894:Chromosomes, Human, Pair 4; D044127:Epigenesis, Genetic / Q000502:physiology; D020022:Genetic Predisposition to Disease; D018398:Homeodomain Proteins / Q000502:physiology; D006801:Humans; D018598:Minisatellite Repeats; D020391:Muscular Dystrophy, Facioscapulohumeral / Q000145:classification / Q000175:diagnosis / Q000235:genetics
chemical_list: C121257:DUX4L1 protein, human; D018398:Homeodomain Proteins
keywords: DUX4; Digenic transmission; Dystrophie musculaire facio-scapulo-humérale de type 2; Epigenomics; Facioscapulohumeral muscular dystrophy; Hypomethylation; Hypométhylation; Hérédité digénique; Épigenetique