CAPZB: A Potential Novel Key Player in Cardiac Development and Congenital Heart Defects

Objectives: Despite congenital heart defects (CHD) are one of the most common birth defects and often life-threatening their etiology is still poorly understood. Here we present a new candidate gene for CHD - CAPZB -, which was identified in a patient with hypoplastic left heart syndrome (HLHS). CAPZB encodes the β subunit of the fast growing-end actin binding protein, which belongs to the F-actin capping protein family. It regulates the assembly and disassembly of actin filaments, modulates the cytoskeleton as well as tethers actin filaments to the Z-line of the sarcomere and is therefore essential for muscle development.

Methods: Using exome sequencing, we identified CAPZB in a patient with HLHS. To understand the functional relevance of the mutation we used a mutant zebrafish model carrying a retroviral insertion in the second intron. We performed cardiac output measurements on zebrafish larvae at different developmental stages. Additionally, to identify physiological and morphological defects, we performed immunofluorescence staining, electron microscopy and optical mapping. To determine differentially regulated genes influenced by CAPZB mutation we used the differential expression transcript counting technique (DeTCT) on 48 hours (hpf) old zebrafish larvae.

Results: Zebrafish larvae being homozygous for the mutation (capzb−/−) died after 100 hours post fertilization (hpf). Heterozygous zebrafish (capzb+/−) survived until adulthood and were used for husbandry. Mutant capzb−/− zebrafish larvae developed microcephaly, short body length, hypoplastic jaw and an elongated, unlooped heart at 96 hpf compared with their heterozygous capzb+/− and wildtype siblings. Cardiac output measurements showed an impaired contractility in capzb−/− compared with capzb+/− and wildtype larvae at 72 hpf, which was significantly reduced after 96 hpf. Moreover, capzb−/− larvae showed physiological and morphological defects after 72 hpf compared with capzb+/− and wildtype siblings. RNA-Seq revealed differentially expressed actin binding genes in capzb−/− larvae.

Conclusion: Loss of CAPZB in zebrafish results in physiological and morphological defects, which phenocopy partly the human HLHS phenotype of the identified patient. These results highlight the importance of CAPZB as a new candidate gene for CHD and show its role during early cardiac development.

No conflict of interest has been declared by the author(s).