Developmental basis of severe neural tube defects in the loop-tail (Lp) mutant mouse: use of microsatellite DNA markers to identify embryonic genotype.

Mouse embryos homozygous for the mutation loop-tail (Lp) develop lethal defects in which the neural tube remains open from the hindbrain to the caudal extremity, a condition that closely resembles the human malformation craniorachischisis. Heterozygotes develop tail defects and occasional spina bifida, but are generally viable. In order to study the early development of these defects, it is necessary to determine the genotype of embryos at stages prior to the first appearance of the morphological abnormalities. We used a microsatellite DNA sequence, Crp, that is closely linked to the Lp locus and which segregates polymorphic variants in matings between Lp/+ mice, thus permitting identification of embryos of Lp/Lp, Lp/+ and +/+ genotypes. We found that the severe phenotype craniorachischisis is present at 9.5 and 10.5 days of gestation only in Lp/Lp embryos in utero, whereas Lp/+ and +/+ littermates show neural tube closure throughout most of the body axis. The open neural tube phenotype also develops in Lp/Lp embryos growing in whole embryo culture. A small proportion of Lp/+ embryos were found to develop this phenotype in vitro, but only when culture conditions were suboptimal. Analysis of 8.5-day embryos revealed that the initial defect in Lp/Lp embryos is failure to initiate neural tube closure at the cervical/hindbrain boundary when the embryo has 6-7 somites. Thereafter, the neural tube remains open throughout the body axis, with the exception of the midbrain and forebrain where neural tube closure is initiated independently. Closure at the midbrain/forebrain boundary does not appear to be defective in Lp/Lp embryos. Heterozygous Lp/+ embryos initiate neural tube closure at the cervical/hindbrain boundary with a slight delay compared with +/+ littermates. Moreover, at 10.5 days of gestation, Lp/+ embryos undergo delayed closure of the posterior neuropore. Thus, Lp/+ embryos are defective in several aspects of the neurulation process. The pattern of delayed neuropore closure in Lp/+ embryos resembles that caused by the ct and Sp mutations and is likely to be responsible for the development of tail defects (i.e., looped tails) and spina bifida in Lp/+ mice. The use of microsatellite markers to determine the genotype of mutant embryos has general application: microsatellites are widespread throughout the mouse genome, so that informative sequences are likely to be available with close linkage to the majority of mutant genes. Moreover, polymorphisms can be detected using the polymerase chain reaction, making it possible to determine the genotype of very early embryos when only small amounts of material are available.