INTERACTION BETWEEN SPLOTCH (SP) AND CURLY TAIL (CT) MOUSE MUTANTS INTHE EMBRYONIC-DEVELOPMENT OF NEURAL-TUBE DEFECTS

The mouse mutations splotch (Sp) and curly tail (ct) both produce spin al neural tube defects with closely similar morphology, but achieve th is by different embryonic mechanisms. To determine whether the mutants may interact during development, we constructed mice carrying both mu tations. Double heterozygotes exhibited tail defects in 10% of cases, although the single heterozygotes do not express this phenotype. Backc rosses of double heterozygotes to ct/ct produced offspring with an ele vated incidence of neural tube defects, both spina bifida and tail def ects, compared with a control backcross in which Sp was not involved. Use of the deletion allele Sp2H permitted embryos carrying a splotch m utation to be recognised by polymerase chain reaction assay. This expe riment showed that only embryos carrying Sp2H develop spina bifida in the backcross with ct/ct, suggesting that the genotype Sp2H/+, ct/ct i s usually lethal around the time of birth as a result of severe distur bance of neurulation. The interaction between Sp and ct was investigat ed further by examining embryos in the backcross for developmental mar kers of the Sp/Sp and ct/ct genotypes. Sp/Sp embryos characteristicall y lack neural crest derivatives, such as dorsal root ganglia, and die on day 13 of gestation. Double mutant embryos from the backcross did n ot exhibit either of these characteristics suggesting that homozygosit y for ct does not cause Sp/+ embryos to develop as if they were of gen otype Sp/Sp. The angle of ventral curvature of the posterior neuropore region is enhanced in affected ct/ct embryos whereas it was found to be reduced in Sp/Sp embryos compared with their normal littermates. Do uble mutant embryos from the backcross had an angle of curvature that resembled the ct/ct pattern but was less exaggerated. We conclude that the non-allelic mutations Sp and ct interact to promote the developme nt of neural tube defects in a manner that does not involve exacerbati on of the specific developmental effects of either gene. The presence of enhanced curvature of the caudal region, which is responsible for n eurulation disturbance in ct/ct embryos, summates with the reduced neu rulation potential of the neuroepithelium in the Sp/+ genotype leading to the development of severe spina bifida. This study demonstrates th at the finding of a significant interaction between genes in double mu tant mice cannot be assumed to indicate that the genes operate in the same genetic pathway.