An 18-month Prospective Natural History Study to Gain Insight Into FSHD2 Pathophysiology and Disease Progression

Inclusion criteria: * Genetically confirmed FSHD2: pathogenic mutation in SMCHD1 gene and at least one D4Z4 4qA allele; * Age 18-75 years * Symptomatic limb weakness * Clinical severity score of 2 to 5 (RICCI score; range 0-5), inclusive, at screening: * Group ambulant patient with a RICCI score of 2 to 4 * Group non-ambulant patient with a RICCI score of 5 * Patient giving written consent after written and oral information * Patient affiliated to a social security system * If taking over the counter supplements, willing to remain consistent with supplement regimen throughout the course of the study Non inclusion criteria: * Patients with comorbidity not related to the disease that can modify the natural evolution of the disease or would interfere with safe testing in the opinion of the Investigator * Regular use of available muscle anabolic/catabolic agents such as corticosteroids, oral testosterone or derivatives, or oral beta agonists * Contraindication to muscle MRI as per clinic standard practice * Patients who has been to a tropical or subtropical country during the last 3 months * Patients who has practiced physical exercise within 10 hours before blood test * Patients declaring not to be fasting for at least 10 hours * Patients following a particular diet for medical reasons and after prescription by a doctor or dietitian * Patients who regularly consumes large quantities of alcohol * Patients having consumed an illicit recreational drug during the last 3 months * Patients having been vaccinated during the last 3 months * Patients having received a blood transfusion or immunoglobulins during the last 3 months * Patients declaring to be seropositive for HIV, HBV or HCV * Patients having had an infectious episode during the 3 weeks preceding the visit * Use of an experimental drug in an FSHD clinical trial within the past 30 days * Participation in others clinical trials * Pregnant women, breastfeeding women, women of childbearing age without contraception Pregnancy * Patient with legal protection measures (future protection mandate, family empowerment, guardianship, curators) under Article L. 1122-2 of the French Public Health Code * Patient refusing to participate in the study or expressing opposition to participation