Congenital valvular defects associated with deleterious mutations in the PLD1 gene.

BACKGROUND: The underlying molecular aetiology of congenital heart defects is largely unknown. The aim of this study was to explore the genetic basis of non-syndromic severe congenital valve malformations in two unrelated families. METHODS: Whole-exome analysis was used to identify the mutations in five patients who suffered from severe valvular malformations involving the pulmonic, tricuspid and mitral valves. The significance of the findings was assessed by studying sporulation of yeast carrying a homologous Phospholipase D (PLD1) mutation, in situ hybridisation in chick embryo and echocardiography and histological examination of hearts of PLD1 knockout mice. RESULTS: Three mutations, p.His442Pro, p.Thr495fs32* and c.2882+2T>C, were identified in the PLD1 gene. The mutations affected highly conserved sites in the PLD1 protein and the p.His442Pro mutation produced a strong loss of function phenotype in yeast homologous mutant strain. Here we show that in chick embryos PLD1 expression is confined to the forming heart (E2-E8) and homogeneously expressed all over the heart during days E2-E3. Thereafter its expression decreases, remaining only adjacent to the atrioventricular valves and the right ventricular outflow tract. This pattern of expression follows the known dynamic patterning of apoptosis in the developing heart, consistent with the known role of PLD1 in the promotion of apoptosis. In hearts of PLD1 knockout mice, we detected marked tricuspid regurgitation, right atrial enlargement, and increased flow velocity, narrowing and thickened leaflets of the pulmonic valve. CONCLUSIONS: The findings support a role for PLD1 in normal heart valvulogenesis.

A dynamic analysis of muscle fusion in the chick embryo.

Skeletal muscle development, growth and regeneration depend upon the ability of muscle cells to fuse into multinucleated fibers. Surprisingly little is known about the cellular events that underlie fusion during amniote development. Here, we have developed novel molecular tools to characterize muscle cell fusion during chick embryo development. We show that all cell populations arising from somites fuse, but each with unique characteristics. Fusion in the trunk is slow and independent of fiber length. By contrast, the addition of nuclei in limb muscles is three times more rapid than in trunk and is tightly associated with fiber growth. A complex interaction takes place in the trunk, where primary myotome cells from the medial somite border rarely fuse to one another, but readily do so with anterior and posterior border cells. Conversely, resident muscle progenitors actively fuse with one another, but poorly with the primary myotome. In summary, this study unveils an unexpected variety of fusion behaviors in distinct embryonic domains that is likely to reflect a tight molecular control of muscle fusion in vertebrates.