Nhlh1, a basic helix-loop-helix transcription factor, is very tightly linked to the mouse looptail (Lp) mutation

Looptail (Lp) is a mutation on the distal portion of mouse Chromosome (Chr) 1 that affects neurulation in mouse and is phenotypically expressed by appearance of an open neural tube along the entire antero-posterior axis of the embryo (craniorachischisis). Nhlh1, a member of the basic helix-loop-helix family of transcription factors, is expressed in the developing neural tube in structures affected by the Lp mutation and has been regionally assigned to the distal part of mouse Chr 1. Using a large panel of looptail animals from an (Lp/+ x SWR/J)F1 x SWR/J segregating backcross progeny, we have determined that Nhlh1 maps very close to Lp, with no recombinant detected in 500 informative animals tested; both map within a 0.6-cM segment defined as D1Mit113/Apoa2/Fcer1 gamma-(0.4 cM)-Nhlh1/Lp-(0.2 cM)-Fcer1 alpha/D1Mit149/Spna1. Nucleotide sequencing of Nhlh1 cDNA clones from wild type (WT) and Lp/Lp embryos failed to identify sequence alterations associated with the mutant phenotype. Southern hybridization of genomic DNA from WT and Lp/Lp embryos failed to identify specific rearrangements at or near the Nhlh1 locus, and Northern RNA blotting and RT-PCR evaluation of Nhlh1 mRNA expression indicated that both the levels and types of Nhlh1 mRNAs produced in WT and Lp/Lp embryos were indistinguishable. These studies suggest that Nhlh1 and Lp are not allelic. Nevertheless, Nhlh1 is the Chr 1 marker most tightly linked to Lp identified to date and can, therefore, be used as an excellent entry probe to clone the Lp region.