Nt mutation causing laterality defects associated with deletion of rotatin

No turning (nt), a recessive lethal mutation causing left-right and axial patterning defects, was recovered in outbred CD-1 mice. Homozygous mutant embryos die at E11.5, exhibiting randomized heart tube looping and a failure to undergo embryonic turning. Previous studies showed notochordal defects associated with abnormal expression of HNF3 beta, lefty, and nodal. To identify the genetic alteration underlying nt mutants, we intercrossed this mutation into a Mus musculus castaneus strain background and conducted a full-genome scan using polymorphic microsatellite DNA markers. We mapped the mutation to an 18-Mb interval in mouse chromosome 18, spanning marker D18mit189 and the distal end of the chromosome. RT-PCR analysis of known genes in the mapped interval showed no transcripts for four genes, including Rttn, CD226, Dok6, and Txndc10. Chromosome walking by serial PCR amplification of genomic DNA from homozygous mutants revealed a 1.6-Mb deletion spanning these four genes. A gene-trap mouse line with disruption of Rttn was previously also shown to have randomized heart tube looping, defects in embryonic turning, and abnormal expression of HNF3 beta, lefty, and nodal. Together these findings suggest that nt is likely elicited by Rttn deficiency.