GENETIC INTERACTIONS SUGGEST THAT DANFORTHS SHORT TAIL (SD) IS A GAIN-OF-FUNCTION MUTATION

Danforth's short tail (Sd) is a semidominant mutation on mouse chromos ome 2 that acts cell autonomously in the notochord and leads io iis di sintegration, and thus causes severe defects in somite patterning and which is severely reduced or absent in Sd heterozygotes, is vertebral column development. The molecular nature of the Sd gene and mutation i s unknown, and ii is unclear whether Sd is a loss-of-function mutation and the semidominant inheritance of the Sd phenotype is due io haploi nsufficiency, or whether Sd represents a gain-of-function mutation in a gene essential for notochord development and maintenance. Here, we r eport on the genetic interaction between Sd and an insertional mutatio n called Etl4(lacZ), which provides genetic evidence that Sd is a gain -of-function mutation. Etl4(lacZ) is an enhancer trap insertion, which gives rise to lacZ expression in distinct cell types, including the n otochord. In homozygosity, the lacZ insertion leads to abnormal verteb rae in the caudal part of the vertebral column. Etl4(lacZ) maps approx imately 0.75 cM distal to Sd, and in double heterozygotes modifies the Sd phenotype contrarily, depending on the chromosomal configuration o f the Sd and Etl4(lacZ) mutations. when Etl4(lacZ) is present on the c hromosome wild type for Sd (Sd +/+ Etl4(lacZ); trans configuration), t he Sd phenotype is enhanced, i.e., Vertebral malformations extend io m ore anterior positions and the vertebral body of the axis is further r educed. Conversely, when Etl4(lacZ) is present on the same chromosome as Sd (Sd Etl4(lacZ) /+ +; cis configuration), the Sd phenotype is att enuated, i.e., vertebral malformations are confined io more posterior levels, and the dens axis, restored. The different effect of the Etl4( lacZ) insertion on Sd, depending oil iis presence in trans or cis, sug gests a direct interaction of the transgene insertion with the Sd gene . Additionally, the attenuation of;he Sd phenotype by Etl4(lacZ) in ci s suggests that Sd is a gain-of-function mutation and lends support to the idea that Etl4(lacZ) is a new allele of Sd. (C) 1998 Wiley-Liss, Inc.