Phenotype‐genotype relations in facioscapulohumeral muscular dystrophy type 1

To determine how much of the clinical variability in facioscapulohumeral muscular dystrophy type 1 (FSHD1) can be explained by the D4Z4 repeat array size, D4Z4 methylation and familial factors, we included 152 carriers of an FSHD1 allele (23 single cases, 129 familial cases from 37 families) and performed state-of-the-art genetic testing, extensive clinical evaluation and quantitative muscle MRI. Familial factors accounted for 50% of the variance in disease severity (FSHD clinical score). The explained variance by the D4Z4 repeat array size for disease severity was limited (approximately 10%), and varied per body region (facial muscles, upper and lower extremities approximately 30%, 15% and 3%, respectively). Unaffected gene carriers had longer repeat array sizes compared to symptomatic individuals (7.3 vs 6.0 units, P = 0.000) and slightly higher Delta1 methylation levels (D4Z4 methylation corrected for repeat size, 0.96 vs -2.46, P = 0.048). The D4Z4 repeat array size and D4Z4 methylation contribute to variability in disease severity and penetrance, but other disease modifying factors must be involved as well. The larger effect of the D4Z4 repeat array on facial muscle involvement suggests that these muscles are more sensitive to the influence of the FSHD1 locus itself, whereas leg muscle involvement seems highly dependent on modifying factors.