Facioscapulohumeral muscular dystrophy 1 (FSHD1) is caused by a contraction in the number of D4Z4 repeats on chromosome 4, resulting in relaxation of D4Z4 chromatin causing inappropriate expression of DUX4 in skeletal muscle. Clinical severity is inversely related to the number of repeats. In contrast, FSHD2 patients also have inappropriate expression of DUX4 in skeletal muscle, but due to constitutional mutations in SMCHD1 (structural maintenance of chromosomes flexible hinge domain containing 1), which cause global hypomethylation and hence general relaxation of chromatin. Thirty patients originally referred for FSHD testing were screened for SMCHD1 mutations. Twenty-nine had 411 D4Z4 repeats. SMCHD1 c.1040 þ 1G4A, a pathogenic splice-site variant, was identified in a FSHD1 family with a borderline number of D4Z4 repeats (10) and a variable phenotype (in which a LMNA1 sequence variant was previously described), and SMCHD1 c.2606 G4T, a putative missense variant (p.Gly869Val) with strong in vitro indications of pathogenicity, was identified in a family with an unusual muscular dystrophy with some FSHD-like features. The two families described here emphasise the genetic complexity of muscular dystrophies. As SMCHD1 has a wider role in global genomic methylation, the possibility exists that it could be involved in other complex undiagnosed muscle disorders. Thus far, only 15 constitutional mutations have been identified in SMCHD1, and these two sequence variants add to the molecular and phenotypic spectrum associated with FSHD.

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