Insights into the Role of a Cardiomyopathy-Causing Genetic Variant in ACTN2

Area of Science:

• Cardiovascular Genetics
• Molecular Cardiology
• Developmental Biology

Background:

• Pathogenic variants in ACTN2, encoding alpha-actinin 2, are rare causes of Hypertrophic Cardiomyopathy (HCM).
• The precise disease mechanisms underlying ACTN2-associated HCM remain largely unknown.
• Alpha-actinin 2 is a crucial sarcomeric protein involved in cardiac muscle structure and function.

Purpose of the Study:

• To investigate the pathogenic mechanisms of the ACTN2 p.Met228Thr variant.
• To determine the phenotypic consequences of heterozygous and homozygous ACTN2 p.Met228Thr variants in a mouse model.
• To elucidate the molecular basis of embryonic lethality and cardiac abnormalities.

Main Methods:

• Phenotyping of adult heterozygous Actn2 p.Met228Thr mice using echocardiography.
• Analysis of homozygous E15.5 embryonic hearts via High Resolution Episcopic Microscopy and wholemount staining.
• Unbiased proteomics, qPCR, and Western blotting to assess molecular changes.

Main Results:

• Heterozygous mice showed no overt phenotype, with only mature males exhibiting molecular signs of cardiomyopathy.
• Homozygous Actn2 p.Met228Thr variants resulted in embryonic lethality by E15.5, with significant cardiac morphological abnormalities.
• Molecular analyses revealed destabilization of mutant alpha-actinin, increased ubiquitin-proteasomal system activity, sarcomeric defects, cell-cycle abnormalities, and mitochondrial dysfunction.

Conclusions:

• The ACTN2 p.Met228Thr missense variant destabilizes alpha-actinin 2, leading to increased protein degradation via the ubiquitin-proteasomal system.
• Combined effects of sarcomeric dysfunction, cell-cycle defects, and mitochondrial impairment contribute to embryonic lethality in homozygous mutants.
• These findings provide critical insights into the molecular pathogenesis of ACTN2-related cardiomyopathies.