Abstract 326 Cardiac Development and Gene Regulation Platform, Sunday, 5/2

From the time the cardiac tube is formed, many aspects of vertebrate cardiac development proceed in a left-right (L-R) asymmetric manner. Misspecification of the L-R axis in humans can result in severe defects of the cardiovascular system as seen in asplenia and polysplenia. Recent studies have begun to dissect the genetic pathways involved in the determination of the L-R axis. We present here the first genetic evidence that a member of the Fibroblast Growth Factor family, Fgf8, is required for the specification of the L-R axis in mice and that misspecification of L-R results in significant cardiac defects similar to those seen in humans.

Fgf8-/- (Fgf8Δ2,3/Δ2,3) null embryos die early in embryogenesis due to a gastrulation defect and therefore never form a cardiac tube. To investigate the role of Fgf8 in heart development we utilized a mouse line that carries a hypomorphic allele (Fgf8neo) of Fgf8 to generate Fgf8-/neo (compound heterozygous) embryos. Compound heterozygous embryos display features of the human asplenia syndrome including the predicted complex cardiac defects. Randomization of cardiac looping, transposition of the great arteries, double outlet right ventricle, pulmonary atresia and other defects are all seen at high frequency. In addition, right atrial isomerism, pulmonary isomerism (bilateral trilobed lungs), and abdominal situs abnormalities are present. Cardiac defects were seen in 42/63 (67%) of mutant embryos with 30/63 (48%) demonstrating right pulmonary isomerism. 2/63 (3%) had complete situs inversus while none of the embryos had evidence of left pulmonary/atrial isomerism. These data indicate a requirement for Fgf8 in the specification of left sided structures and that loss of Fgf8 function results in right sided isomerism. To establish the position of Fgf8 in the genetic hierarchy of left-right axis determination, we analyzed the expression of the early asymmetric markers Nodal, Lefty1, Lefty2, and Pitx2B in Fgf8 compound heterozygous embryos. The patterns of expression of these markers demonstrate abnormalities consistent with the phenotypic bilateral right sided features, placing Fgf8 upstream of these genes and suggests it is involved very early in the determination of the L-R axis. These data implicate Fgf8 as one candidate gene in the development of the human asplenia syndrome.