DNA Methylation-based diagnosis of Facio Scapulo Humeral Dystrophy

International audienceIntroduction: FSHD is an autosomal dominant myopathy characterized by a progressive and asymmetric weakening of facial, shoulder, and upper body muscles with a progression to the lower body. At the molecular level, for 95% of patients, the pathology is linked to the deletion of a number of repetitive macrosatellite elements at the 4q35 locus (D4Z4). D4Z4 is extremely GC rich (70%) and methylated in normal individuals but hypomethylated in patients with FSHD and in patients with phenotypic FSHD but without the D4Z4 deletion on both chromosomes 4 (Gaillard et al., 2014). In addition, some phenotypic patients have been linked to mutations in a gene carried by chromosome 18, SMCHD1. The role of this protein is poorly known in human cells but some studies in the mouse involves SMCHD1 in the DNA methylation maintenance pathway. Objective: We have studied the relationship between SMCHD1 mutations, the presence of the SMCHD1 functional domains at the protein level and the DNA methylation of D4Z4 and other repetitive sequences. Methods: We explored a cohort of FSHD patients, phenotypic FSHD patients and a population of non-carriers, using SMCHD1 sequencing, antibodies based functional domains detection, bisulfite sequencing, and high-throughput targeted bisulfite sequencing. Results: In our cohort of phenotypic patients, we demonstrated that some SMCHD1 mutations might be associated to a decrease of the expression of SMC functional domain and a decrease of D4Z4 DNA methylation. We also show that some patients carry SMCHD1 mutation, associated to a DNA methylation decrease but without any modification of SMCHD1 functional domains. Conclusion: We propose some translational methods and a decision tree where SMCHD1 sequencing, western blot SMCHD1 detection and DNA methylation help physician to diagnose FSHD patients without D4Z4 deletion