Structural and functional consequences of SMCHD1 mutations associated with arhinia and muscular dystrophy

Abstract

SMCHD1 (Structural Maintenance of Chromosomes Flexible Hinge Domain Containing‐1) encodes an epigenetic repressor that silences a subset of X‐linked and autosomal genes. SMCHD1 contains a conserved N‐terminal GHKL‐type ATPase domain and a C‐terminal SMC‐hinge domain important for dimerization and chromatin association. Heterozygous missense mutations in SMCHD1 cause two extremely rare disorders: Bosma arhinia microphthalmia syndrome (BAMS), which is characterized by arhinia, the absence of an external nose, as well as ocular and reproductive defects, and facioscapulohumeral muscular dystrophy type 2 (FSHD2), a late onset neuromuscular disorder. Patients with both BAMS and FSHD2 show hypomethylation at SMCHD1 binding sites (e.g., the 4q35 D4Z4 macrosatellite repeat array), but it is unknown how point mutations in SMCHD1 perturb protein structure and function at the molecular level and ultimately result in these diverse disease phenotypes.To begin to address this question, we solved the crystal structure of a dimeric human N‐terminal SMCHD1 construct. We discovered two novel functional domains and observed ATP‐dependent dimerization in the absence of the SMC‐hinge domain. BAMS and FSHD2 missense mutations mapped either to the protein surface, dimer interfaces, domain interfaces, or to hydrophobic pockets with complete sparing of the enzyme active site. As previously observed in studies that used the monomeric form of SMCHD1, we found that mutations in the dimeric form had variable and subtle effects on SMCHD1 ATPase activity and ATP binding affinity. Thermal stability and dimerization were generally preserved in BAMS mutant constructs, as determined by differential scanning fluorimetry and chemical cross‐linking.In summary, we find that human disease mutations map to multiple functional domains of SMCHD1 yet have only modest effects on the two most well‐described activities of this protein: ATP hydrolysis and dimerization. Work is ongoing to determine the effect of disease mutations on chromatin association, SMCHD1 interactome, and the dynamics underlying monomer‐dimer equilibrium.Support or Funding InformationThis work was supported by the Intramural Research Program of the NIH/NIEHS and equipment grants from the NIH.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.