Different clinicopathological features between Japanese siblings with facioscapulohumeral muscular dystrophy 2 with a novel nonsense SMCHD1 mutation (Arg552 ∗ ).

Facioscapulohumeral muscular dystrophy (FSHD) 2 is caused by a combination of heterozygous structural maintenance of chromosomes flexible hinge domain containing 1 (SMCHD1) mutation plus DNA hypomethylation on D4Z4. Here we report two Japanese FSHD2 siblings (brother and sister) with a new SMCHD1 nonsense mutation (a heterogeneous c. 1654C > T substitution, leading to a stop codon Arg552∗ ). They showed the typical phenotype of FSHD2 such as asymmetric muscle weakness and atrophy in bilateral facial, scapular and humeral muscles, but different clinicopathological features between them. The brother and asymptomatic mother showed normal D4Z4 methylation plus the same SMCHD1 mutation, but the sister showed the SMCHD1 mutation plus D4Z4 hypomethylation, suggesting an interesting correlation of the new SMCHD1 nonsense mutation and D4Z4 hypomethylation.