FSHD2- and BAMS-associated mutations confer opposing effects on SMCHD1 function

Alexandra D Gurzau,Kelan Chen,Shifeng Xue,Weiwen Dai,Isabelle S Lucet,Thanh Thao Nguyen Ly,Bruno Reversade,Marnie E Blewitt,James M Murphy

doi:10.1074/jbc.ra118.003104

Abstract

Structural maintenance of chromosomes flexible hinge domain-containing 1 (Smchd1) plays important roles in epigenetic silencing and normal mammalian development. Recently, heterozygous mutations in SMCHD1 have been reported in two disparate disorders: facioscapulohumeral muscular dystrophy type 2 (FSHD2) and Bosma arhinia microphthalmia syndrome (BAMS). FSHD2-associated mutations lead to loss of function; however, whether BAMS is associated with loss- or gain-of-function mutations in SMCHD1 is unclear. Here, we have assessed the effect of SMCHD1 missense mutations from FSHD2 and BAMS patients on ATP hydrolysis activity and protein conformation and the effect of BAMS mutations on craniofacial development in a Xenopus model. These data demonstrated that FSHD2 mutations only result in decreased ATP hydrolysis, whereas many BAMS mutations can result in elevated ATPase activity and decreased eye size in Xenopus Interestingly, a mutation reported in both an FSHD2 patient and a BAMS patient results in increased ATPase activity and a smaller Xenopus eye size. Mutations in the extended ATPase domain increased catalytic activity, suggesting critical regulatory intramolecular interactions and the possibility of targeting this region therapeutically to boost SMCHD1's activity to counter FSHD.

Highlights

Structural maintenance of chromosomes flexible hinge domain-containing 1 (Smchd1) plays important roles in epigenetic silencing and normal mammalian development
Whole-exome sequencing of over 50 patients revealed de novo heterozygous missense mutations in SMCHD1, all of which mapped to the N-terminal GHKL-type ATPase domain and its adjacent extension domain, suggesting an altered enzymatic function in patients [6, 7]. Using both in vitro and in vivo systems, we proposed that SMCHD1 mutations affecting Bosma arhinia microphthalmia syndrome (BAMS) patients display a gain-of-function phenotype [6], as opposed to facioscapulohumeral muscular dystrophy type 2 (FSHD2) patients, who have been shown to harbor loss-of-function mutations that contrastingly occur throughout the SMCHD1 gene [4]
We previously reported de novo heterozygous missense mutations restricted to the ATPase domain of SMCHD1 as causative of BAMS [6], as opposed to FSHD2 patients, who have been established to harbor loss-of-function mutations that contrastingly occur throughout the gene [4]

Summary

Introduction

Structural maintenance of chromosomes flexible hinge domain-containing 1 (Smchd1) plays important roles in epigenetic silencing and normal mammalian development. Heterozygous mutations in SMCHD1 have been reported in two disparate disorders: facioscapulohumeral muscular dystrophy type 2 (FSHD2) and Bosma arhinia microphthalmia syndrome (BAMS). We have assessed the effect of SMCHD1 missense mutations from FSHD2 and BAMS patients on ATP hydrolysis activity and protein conformation and the effect of BAMS mutations on craniofacial development in a Xenopus model. E. B.), Australian National Health and Medical Research Council Fellowship 1105754 Additional support was provided by Victorian State Government Operational Infrastructure Support, Australian National Health and Medical Research Council IRIISS Grant 9000433, and the Australian Cancer Research Foundation. Mutations in one copy of SMCHD1 have recently been reported in two distinct diseases, facioscapulohumeral muscular dystrophy type 2 (FSHD2)4 [4, 5] and Bosma arhinia microphthalmia syndrome (BAMS) [6, 7]. Unlike other members of the SMC family, Smchd has a putative ATPase domain at its N terminus that resembles a GHKL (gyrase, Hsp, histidine kinase, MutL) ATPase [19, 20] rather than an ABC type present in other family members

Methods

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Conclusion