Biallelic Variants in ASNA1, Encoding a Cytosolic Targeting Factor of Tail-Anchored Proteins, Cause Rapidly Progressive Pediatric Cardiomyopathy.

BACKGROUND: Pediatric cardiomyopathies are a clinically and genetically heterogeneous group of heart muscle disorders associated with high morbidity and mortality. Although knowledge of the genetic basis of pediatric cardiomyopathy has improved considerably, the underlying cause remains elusive in a substantial proportion of cases. METHODS: Exome sequencing was used to screen for the causative genetic defect in a pair of siblings with rapidly progressive dilated cardiomyopathy and death in early infancy. Protein expression was assessed in patient samples, followed by an in vitro tail-anchored protein insertion assay and functional analyses in zebrafish. RESULTS: We identified compound heterozygous variants in the highly conserved ASNA1 gene (arsA arsenite transporter, ATP-binding, homolog), which encodes an ATPase required for post-translational membrane insertion of tail-anchored proteins. The c.913C>T variant on the paternal allele is predicted to result in a premature stop codon p.(Gln305*), and likely explains the decreased protein expression observed in myocardial tissue and skin fibroblasts. The c.488T>C variant on the maternal allele results in a valine to alanine substitution at residue 163 (p.Val163Ala). Functional studies showed that this variant leads to protein misfolding as well as less effective tail-anchored protein insertion. Loss of asna1 in zebrafish resulted in reduced cardiac contractility and early lethality. In contrast to wild-type mRNA, injection of either mutant mRNA failed to rescue this phenotype. CONCLUSIONS: Biallelic variants in ASNA1 cause severe pediatric cardiomyopathy and early death. Our findings point toward a critical role of the tail-anchored membrane protein insertion pathway in vertebrate cardiac function and disease.

Mechanistic basis for a molecular triage reaction.

Newly synthesized proteins are triaged between biosynthesis and degradation to maintain cellular homeostasis, but the decision-making mechanisms are unclear. We reconstituted the core reactions for membrane targeting and ubiquitination of nascent tail-anchored membrane proteins to understand how their fate is determined. The central six-component triage system is divided into an uncommitted client-SGTA complex, a self-sufficient targeting module, and an embedded but self-sufficient quality control module. Client-SGTA engagement of the targeting module induces rapid, private, and committed client transfer to TRC40 for successful biosynthesis. Commitment to ubiquitination is dictated primarily by comparatively slower client dissociation from SGTA and nonprivate capture by the BAG6 subunit of the quality control module. Our results provide a paradigm for how priority and time are encoded within a multichaperone triage system.