Positional Cloning of Skeletal Mouse Mutation, Tail Short(Ts).

Abstract

Tail short (Ts) was initially found as a mutant which exhibits shortened kinked tail in heterozygotes. They also have numerous skeletal abnormalities throughout the entire body. Homozygous embryos die at around the time of implantation. This is probably due to a retardation in forming morulae because pale-staining embryos have been found at this stage in homozygotes. These various phenotypes indicates that the gene for Ts plays a crucial role throughout development including the early embryonic stage and skeletal formation. Ts gene was mapped to teromeric region of the chromosome 11. The Sry-related gene Sox9 has been proposed as the gene resposible for the mouse skeletal mutant Tail short (Ts) , on the basis of its expression in skeletogenic mesenchymal condensations in the mouse embryo, and its chromosomal location in the region of Ts on distal chromosome 11. Studies in humans have inforced the hypothesis that Sox9 may be the Ts gene. A link has been proposed between Ts in mice and a human skeletal dysmorphology known as campomelic dysplasia (CMPD1) . CMPDI has been mapped to human Chr17q23-25 and this region is homologous, syntenic, to distal mouse chr 11.We present here detailed mapping of Ts locus using an intersubspecific cross. Amongst 524 backcross progeny, 16 recombinants were detected between Sox9 and Ts, suggesting a separation of 3.5±0.01 cM, and excluding Sox9 as a candidate for Ts. Microsatellite markers were co-localized to the Ts locus, providing reagents for positional cloning of Ts. We screened the mouse YAC library and obtained YAC clones which carries microsatellite markers co-localized to the Ts. Further screening of the mouse genomic library is underway to make a contig between the two markers.We also explored the characterization of the morphological phenotype of Ts heterozygous mice in embryonic development and skeletal patterning. Detailed analysis of the skeleton of Ts/+ mice revealed that a number of homeotic transformations occur along the anteroposterior axis. In particular, transformation of the seventh cervical vertebrae to the first thoracic vertebrae was highly reproducible in Ts heterozygotes generated from the cross of (C57BL/6J-Ts/+ × MSM) F1 × C57BL/6J.It is of interest to note that this transformation was observed asymmetrically, namely, the left side of vertebrae was predominantly transformed. Besides the above skeletal anomalies, a neural tube defect in the rhombomere region was frequently observed. Thus it is possible that the Ts gene plays some roles providing segmental identity in mouse embryogenesis.