Characterization of a novel insertional mouse mutation, kkt: A closely linked modifier of Pax1

We describe a novel transgene insertional mouse mutant with skeletal abnorm alities characterized by a kinked tail and severe curvature of the spine. T he disrupted locus is designated Wt for "kyphoscoliosis kinked tail." Malfo rmed vertebrae including bilateral ossification centers and premature fusio n of the vertebral body to the pedicles are observed along the vertebral co lumn, and the lower thoracic and lumbar vertebrae are the most affected. So me of the homozygous Wt neonates displayed two backward-pointing transverse processes in the sixth lumbar vertebra (L6) that resembled the first sacra l vertebra, and some displayed one forward- and one backward-pointing trans verse process in L6. The fourth and fifth sternebrae were also fused, and t he acromion process of the scapula was missing in kkt mice. The skeletal ab normalities are similar to those observed in the mouse mutant undulated (un ). The transgene is integrated at the dis tal end of chromosome 2 close to the Pax1 gene, as revealed by FISH[ analysis. However, mutation of the Pax1 gene is responsible for the un phenotype, but the Pax1 gene in the kkt mic e is not rearranged or deleted. Pax1 is expressed normally in Wt embryos an d in the thymus of mature animals, and there is no mutation in its coding s equence. Thus, the skeletal abnormalities observed in the Wit mutant are no t due to a lack of functional Pax1. Mouse genomic sequences flanking the tr ansgene and PAC clones spanning the wild-type kkt locus have been isolated, and reverse Northern analysis showed that the PACs contain transcribed seq uence. Compound heterozygotes between un and Wt (un(+/-)/kkt(+/-)) display skeletal abnormalities similar to those of un or kkt homozygotes, but they have multiple lumbar vertebrae with a split vertebral body that is more sev ere than in homozygous un or kkt neonates. Furthermore, the sternebrae are not fused and no backward-pointing transverse processes are detected in L6. It is therefore apparent that these two mutations do not fully complement each other, and we propose that a gene in the kkt locus possesses a unique role that functions in concert with Pax1 during skeletal development. (C) 2 000 Academic Press.