Thomas Bartman and colleagues use the powerful tools afforded by zebrafish genetics to examine the early steps of heart valve formation. In the process, they provide evidence for a causal relationship between the early function of the heart and its final structure. Using a fluorescent molecular marker highly expressed in the developing heart, the authors found mutations that result in valve defects, and identified a fish mutant they named cardiofunk (cfk).
Genetic mapping of cfk showed that the abnormality was caused by a mutation in a gene encoding a novel actin molecule that is most closely related to the actins found in muscle cells. Actin is involved in muscle contraction; so these results suggest that muscle contraction in the embryonic heart is intimately involved in heart development. Blood flow might also be involved, although the authors find that the heart develops normally even in the presence of pharmacological compounds that abolish blood flow. The characterization of additional mutants will shed further light on this issue.
Valve or septal defects represent 40% of cardiac anomalies in humans. Bartman and colleagues suggest that, by analogy with zebrafish, some of these defects may result from congenital defects affecting very early heart function.
Nancy Hopkins of the Massachusetts Institute of Technology is using zebrafish to study cancer. Her group has created over 500 lines of zebrafish with lesions in key genes involved in development and used them to identi
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