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.