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

Area of Science:

• Genetics and Epigenetics
• Developmental Biology
• Molecular Medicine

Background:

• Structural Maintenance of Chromosomes flexible hinge domain-containing 1 (SMCHD1) is crucial for epigenetic silencing and mammalian development.
• Heterozygous SMCHD1 mutations are linked to facioscapulohumeral muscular dystrophy type 2 (FSHD2) and Bosma arhinia microphthalmia syndrome (BAMS).
• The functional impact of SMCHD1 mutations in BAMS remains unclear, with FSHD2 mutations causing loss-of-function.

Purpose of the Study:

• To investigate the functional consequences of SMCHD1 missense mutations from FSHD2 and BAMS patients.
• To assess the effects of these mutations on ATP hydrolysis activity, protein conformation, and craniofacial development.
• To differentiate between loss-of-function and potential gain-of-function mechanisms in BAMS.

Main Methods:

• Assessed ATP hydrolysis activity and protein conformation of SMCHD1 variants.
• Utilized a Xenopus model to evaluate the impact of BAMS mutations on craniofacial development, specifically eye size.
• Analyzed mutations within the extended ATPase domain of SMCHD1.

Main Results:

• FSHD2 mutations were associated with decreased SMCHD1 ATP hydrolysis.
• Many BAMS mutations led to elevated ATPase activity and reduced eye size in Xenopus.
• A mutation found in both FSHD2 and BAMS patients exhibited increased ATPase activity and smaller Xenopus eye size.

Conclusions:

• SMCHD1 mutations have differential functional effects, with FSHD2 mutations causing loss-of-function and some BAMS mutations potentially causing gain-of-function.
• Elevated ATPase activity in BAMS mutations correlates with developmental defects, such as reduced eye size.
• Targeting the extended ATPase domain may offer therapeutic strategies for FSHD by modulating SMCHD1 activity.