The zebrafish embryo is transparent and can tolerate absence of blood flow because its oxygen is delivered by diffusion rather than by the cardiovascular system1. It is therefore possible to attribute cardiac failure directly to particular genes by ruling out the possibility that it is due to a secondary effect of hypoxia. We focus here on pickwickm171 (pikm171), a recessive lethal mutation discovered in a large-scale genetic screen2. There are three other alleles in the pik complementation group with this phenotype (pikm242, pikm740, pikm186; ref. 3) and one allele (pikmVO62H) with additional skeletal paralysis4. The pik heart develops normally but is poorly contractile from the first beat. Aside from the edema that inevitably accompanies cardiac dysfunction, development is normal during the first three days. We show by positional cloning that the 'causative' mutation is in an alternatively-spliced exon of the gene (ttn) encoding Titin. Titin is the biggest known protein and spans the half-sarcomere from Z-disc to M-line in heart and skeletal muscle5. It has been proposed to provide a scaffold for the assembly of thick and thin filaments6 and to provide elastic recoil engendered by stretch during diastole7. We found that nascent myofibrils form in pik mutants, but normal sarcomeres are absent. Mutant cells transplanted to wildtype hearts remain thin and bulge outwards as individual cell aneurysms without affecting nearby wildtype cardiomyocytes, indicating that the contractile deficiency is cell-autonomous. Absence of Titin function thus results in blockage of sarcomere assembly and causes a functional disorder resembling human dilated cardiomyopathies, one form of which is described in another paper in this issue8.
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We thank M. McKee and D. Brown for transmission electron microscopy services. This work was supported in part by grants from the National Institutes of Health and a sponsored research agreement by Genentech (M.C.F.), the Foundation for Anesthesia Education and Research (S.E.M.) and a Postdoctoral Fellowship of American Heart Association (X.X.).
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Xu, X., Meiler, S., Zhong, T. et al. Cardiomyopathy in zebrafish due to mutation in an alternatively spliced exon of titin. Nat Genet 30, 205–209 (2002). https://doi.org/10.1038/ng816
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