Cardiomyopathy in zebrafish due to mutation in an alternatively spliced exon of titin

Abstract

The zebrafish embryo is transparent and can tolerate absence of blood flow because its oxygen is delivered by diffusion rather than by the cardiovascular system. 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 screen. 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 paralysis. 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 muscle. It has been proposed to provide a scaffold for the assembly of thick and thin filaments and to provide elastic recoil engendered by stretch during diastole. 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 issue.

Illuminating cardiac development: Advances in imaging add new dimensions to the utility of zebrafish genetics

“…Several of these mutations disrupt genes encoding components of the sarcomere, including pickwick (titin) [46], tell tale heart (cardiac myosin light chain 2) [47], weak atrium (amhc) [13], cardiofunk (actc1) [41], and silent heart (tnnt2) [48], and lead to problems with sarcomere assembly or maintenance. Other mutations in this category affect genes encoding components of the electrical conduction system that controls cardiomyocyte depolarization and repolarization.…”

Modeling human hematopoietic and cardiovascular diseases in zebrafish

“…Antisense inhibition or deletion of the TTN cardiac scaffold protein was previously shown to cause defective sarcomere assembly in cardiac cell lines (Person et al, 2000;van der Ven et al, 2000). The phenotype associated with pik resembles the defects seen in human familial dilated cardiomyopathy, a disease with a strong genetic contribution in which the genes responsible for the defect have been identified only in several rare cases (Xu et al, 2002). A companion study has shown that mutations in TTN cause human familial dilated cardiomyopathy (Gerull et al, 2002).…”

“…Sarcomeres are also the affected target in the mutation that causes the pickwick (pik) phenotype (Xu et al, 2002). Little if any blood is ejected from the heart in pik mutants due to a severe reduction in systolic pressure Xu et al, 2002).…”

Mécanotransduction des forces hémodynamiques et organogenèse

“…Chez les mutants pickweak (pik), island beat (isl), silent heart (sih) et valentine, les podocytes demeurent en position bilatérale, et les CE migrent anormalement pour finir par entourer les podocytes. Les gènes respectivement affectés chez ces mutants (pik/titin, codant pour une myofibrille cardiaque [15] ; isl/cacna1c, codant pour le canal calcique dépendant du potentiel, prédominant des cardiomyocytes [16]; sih/tnnt2, codant pour une troponine cardiaque [17]) ne sont pas exprimés dans les podocytes, suggérant qu'un effet indirect de ces mutations, probablement lié à l'absence de circulation sanguine qu'elles entraînent, est responsable de l'absence de migration des podocytes, et par conséquent, de l'anomalie de développement glomérulaire. En outre, la glomérulogenèse est affectée de façon semblable chez des embryons sauvages dont la circulation est bloquée pharmacologiquement ou par occlusion physique, confirmant que le développement rénal est intimement lié au bon fonctionnement de la circulation sanguine.…”