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Facioscapulohumeral muscular dystrophy, known in genetic forms FSHD1 and FSHD2, is associated with D4Z4 repeat array chromatin relaxation and somatic derepression of  DUX4  located in D4Z4. A complete copy of  DUX4  is present on 4qA chromosomes, but not on the D4Z4-like repeats of chromosomes 4qB or 10. Normally, the D4Z4 repeat varies between 8 and 100 units, while in FSHD1 it is only 1–10 units. In the rare genetic form FSHD2, a combination of a 4qA allele with a D4Z4 repeat size of 8–20 units and heterozygous pathogenic variants in the chromatin modifier SMCHD1 causes  DUX4  derepression and disease. In this study, we identified 11/79 (14%) FSHD2 patients with unusually large 4qA alleles of 21–70 D4Z4 units. By a combination of Southern blotting and molecular combing, we show that 8/11 (73%) of these unusually large 4qA alleles represent duplication alleles in which the long D4Z4 repeat arrays are followed by a small FSHD-sized D4Z4 repeat array duplication. We also show that these duplication alleles are associated with  DUX4  expression. This duplication allele frequency is significantly higher than in controls (2.9%), FSHD1 patients (1.4%) and in FSHD2 patients with typical 4qA alleles of 8–20 D4Z4 units (1.5%). Segregation analysis shows that, similar to typical 8–20 units FSHD2 alleles, duplication alleles only cause FSHD in combination with a pathogenic variant in  SMCHD1 . We conclude that  cis  duplications of D4Z4 repeats explain  DUX4  expression and disease presentation in FSHD2 families with unusual long D4Z4 repeats on 4qA chromosomes.

Cis D4Z4 repeat duplications associated with facioscapulohumeral muscular dystrophy type 2