In facioscapulohumeral dystrophy (FSHD), hypomethylation is moderate and heterogeneous in patients. To date, a correlation between hypomethylation and disease presence or severity has not been demonstrated. Here, the authors investigated the link between DNA hypomethylation and clinical penetrance in 95 FSHD cases (37 FSHD1, 29 asymptomatic individuals carrying a shortened D4Z4 array, 9 patients with FSHD2, and 20 controls) by implementing 2 approaches: methylated DNA immunoprecipitation and sodium bisulfite sequencing. Both methods revealed statistically significant differences between asymptomatic carriers or controls and individuals with clinical FSHD, especially in the proximal region of the repeat. Absence of clinical expression in asymptomatic carriers was associated with a level of methylation similar to controls. This study provides proof of concept that the targeted approaches described could be applied systematically to patient samples in routine diagnosis and suggest that local hypomethylation within D4Z4 might serve as a modifier for clinical expression of FSHD phenotype.
