Constantinescu, F.; Szczurek, A.; Nesterova, T. B.; Wei, G.; Pustygina, V.; Uphoff, S.; Cawte, A. D.; Brockdorff, N.

The chromosomal protein SMCHD1 is a GHKL ATPase that plays important roles in epigenetic silencing, including on the inactive X chromosome (Xi) and at the D4Z4 macrosatellite linked to regulation of DUX4 expression in the disorders facioscapulohumeral muscular dystrophy (FSHD2) and Bosma arrhinia micropthalmia syndrome (BAMS). In this study we use live-cell and single-molecule imaging approaches to investigate SMCHD1 interactions with chromatin and its function in epigenetic silencing. We show that chromatin binding of SMCHD1 genome-wide, including on the Xi, is critically dependent on the protein LRIF1 that mediates interaction with H3K9me2/3 modified nucleosomes. Using engineered mutations in the GHKL ATPase domain we show that ATP hydrolysis is required for selective enrichment of SMCHD1 at specific chromatin regions, which is critical for gene silencing on the Xi. A gain-of-function mutation, G137E, that occurs in BAMS patients, results in accelerated Xi recruitment and greater Xi chromosome compaction. Together, our findings advance mechanistic understanding of SMCHD1 function on the Xi and at other target sites in the genome.