Smchd1 haploinsufficiency exacerbates the phenotype of a transgenic FSHD1 mouse model

Abstract

In humans, a copy of the DUX4 retrogene is located in each unit of the D4Z4 macrosatellite repeat that normally comprises 8–100 units. The D4Z4 repeat has heterochromatic features and does not express DUX4 in somatic cells. Individuals with facioscapulohumeral muscular dystrophy (FSHD) have a partial failure of somatic DUX4 repression resulting in the presence of DUX4 protein in sporadic muscle nuclei. Somatic DUX4 derepression is caused by contraction of the D4Z4 repeat to 1–10 units (FSHD1) or by heterozygous mutations in genes responsible for maintaining the D4Z4 chromatin structure in a repressive state (FSHD2). One of the FSHD2 genes is the structural maintenance of chromosomes hinge domain 1 (SMCHD1) gene. SMCHD1 mutations have also been identified in FSHD1; patients carrying a contracted D4Z4 repeat and a SMCHD1 mutation are more severely affected than relatives with only a contracted repeat or a SMCHD1 mutation. To evaluate the modifier role of SMCHD1, we crossbred mice carrying a contracted D4Z4 repeat (D4Z4–2.5 mice) with mice that are haploinsufficient for Smchd1 (Smchd1^MommeD1 mice). D4Z4–2.5/Smchd1^MommeD1 mice presented with a significantly reduced body weight and developed skin lesions. The same skin lesions, albeit in a milder form, were also observed in D4Z4–2.5 mice, suggesting that reduced Smchd1 levels aggravate disease in the D4Z4–2.5 mouse model. Our study emphasizes the evolutionary conservation of the SMCHD1-dependent epigenetic regulation of the D4Z4 repeat array and further suggests that the D4Z4–2.5/Smchd1^MommeD1 mouse model may be used to unravel the function of DUX4 in non-muscle tissues like the skin.

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