Gore, Beninio

The human genome encodes about 20,000 genes and only a fraction of them are expressed in the heart. We hypothesized that annotation of this category of genes may facilitate our understanding of cardiac development and function. Recently, we reported a pilot insertional mutagenesis screening in zebrafish to annotate cardiac gene functions using a gene-break transposon (GBT). It demonstrated the feasibility of generation of a Zebrafish Insertional Cardiac (ZIC) mutant collection by screening gene expression in the embryonic heart. Here, we report an alternative strategy to identify cardiac mutants based on adult heart expression, and explore the application of expanded ZIC lines for understanding cardiac biology. From 203 GBT lines, we identified 34 lines with detectable expression in the dissected adult hearts. Various expression levels of the tagged genes were detected in embryonic hearts. Based on their tissue-specific expression, the candidate ZIC lines can be grouped into myocardium lines, endocardium lines, or both. Based on their subcellular expression, myocardium ZIC lines can be further categorized into sarcomeric lines, mitochondrial lines, and/or nucleic lines. Phenotypic analysis is being conducted to identify recessive mutants including embryonic lethal lines. We will present the current status of our ZIC collection, and discuss its application as a genomic resource platform for elucidating cardiac biology. Our data suggest that ~15% genes in the whole genome might have detectable cardiac expression. The sensitivity of screening for cardiac genes at adult stage is higher than that at embryonic stage (34/203 versus 18/322). We plan to expand the ZIC collection by screening 1,000 GBT lines that are being generated at Mayo Clinic. It is expected that this living mutant collection of cardiac genes will open doors to systematic annotation of novel cardiac genes and forward cardiac mutagenesis screening.

Background: Cardiomyopathy and heart failure affect millions of people worldwide. Because genetic modifiers contribute in large part to the highly variable phenotypic expression of cardiomyopathy in patients even with identical disease-causing mutations, the identification of modifier genes for this disease will greatly improve risk stratification, prognostic test development, and personalized therapy. However, only a rather limited number of modifier genes for cardiomyopathy have been identified sporadically. Objective: To identify genetic modifiers for cardiomyopathy using a novel insertional mutagenesis screening approach in adult zebrafish. Methods and Results: We screened 476 gene break-transposon (GBT) lines and isolated 44 zebrafish insertional cardiac (ZIC) mutants. Employing doxorubicin (DOX) stress to these ZIC mutants, we identified four candidate GBT lines that modified the progression of DOX-induced cardiomyopathy. Here, we report the detailed study of the GBT0411 mutant that exacerbated DOX-induced cardiomyopathy. GBT0411 mutant was tagged to the dnajb6b gene. Mutations in the short (sarcomeric) isoform of its human homologue gene DNAJB6 was recently reported to cause limb-girdle muscular dystrophy type 1D. Interestingly, our data showed that long (nuclei) isoform (dnajb6b[L]) was the major isoform expressed in the heart, and loss-of-function of which deteriorated the progression of DOX-induced cardiomyopathy. We further found that a cardiomyocyte-specific dnajb6b(L) transgene reverted the deleterious modifying effect of GBT0411 mutant, and exerted a cardioprotective function on chronic anemia induced cardiomyopathy. Mechanistically, Dnajb6b(L) could partially localize to endoplasmic reticulum (ER) upon ER stress, and function as an ER stress suppressor. Indeed, inhibition of ER stress by using a chemical chaperon mimics the cardioprotective effect of dnajb6b(L) transgene. Conclusions: By conducting an unbiased mutagenesis screening in adult zebrafish, we identified dnajb6b as a novel genetic modifier for cardiomyopathy. A cardioprotective function was identified by overexpressing its long isoform in cardiomyocytes, which might be conveyed by inhibition of ER stress response.