Epithelial cell polarity genes are required for neural tube closure

Human neural tube defects (NTD) are a heterogeneous group that exhibit complex inheritance, making it difficult to identify the underlying cause. Due to the uniform genetic background, inbred mouse strains are a more amenable target for genetic studies. We investigated the loop-tail (Lp) mouse as a model for the severe NTD, craniorachischisis. A homozygous point mutation was identified in the transmembrane protein Vangl2, which in Drosophila has been shown to function in the planar cell polarity (PCP) pathway. Morphological analysis of the Lp mice shows that the defect results from an abnormally broad floor plate, most likely through a failure in convergent extension. The elevated neural folds remain too far apart to contact, inhibiting neural tube closure. Recently, two other mouse mutants (crash and circletail) were described with a similar phenotype to Lp and were investigated as potentially new alleles. Mapping studies, however, showed that both mutants segregated to distinct loci. In the crash (Crsh) mouse, a mutation was identified in Celsr1, a seven pass transmembrane receptor that encodes a protein orthologous to Drosophila Flamingo. Like Vangl2, this gene also functions in the PCP pathway. While in circletail, a point mutation was identified introducing a premature stop codon into the apical-basal cell polarity gene scribble (Scrb1). We subsequently demonstrated a genetic interaction between all three genes, where double heterozygotes exhibit the same homozygous NTD phenotype. This strongly suggests both a candidate gene pathway and that interaction between independent recessive alleles may be a possible explanation for the complex inheritance in severe human NTD.