Mouse Gli1 mutants are viable but have defects in SHH signaling in combination with a Gli2 mutation

The secreted factor Sonic hedgehog (SHH) is both required for and sufficien t to induce multiple developmental processes, including ventralization of t he CNS, branching morphogenesis of the lungs and anteroposterior patterning of the limbs. Based on analogy to the Drosophila Hh pathway, the multiple GLI transcription factors in vertebrates are likely to both transduce SHH s ignaling and repress Shh transcription. In order to discriminate between ov erlapping versus unique requirements for the three Gli genes in mice, we ha ve produced a Gli1 mutant and analyzed the phenotypes of Gli1/Gli2 and Gli1 /3 double mutants. Gli3(xt) mutants have polydactyly and dorsal CNS defects associated with ectopic Shh expression, indicating GLI3 plays a role in re pressing Shh, In contrast, Gli2 mutants have five digits, but lack a floorp late, indicating that it is required to transduce SHH signaling in some tis sues. Remarkably, mice homozygous for a Gli1(zfd) mutation that deletes the exons encoding the DNA-binding domain are viable and appear normal. Transg enic mice expressing a GLI1 protein lacking the zinc fingers can not induce SHH targets in the dorsal brain, indicating that the Gli1(zfd) allele cont ains a hypomorphic or null mutation. Interestingly, Gli1(zfd/zfd);Gli2(zfd/ +), but not Gli1(zfd/zfd);Gli3(zfd/+) double mutants have a severe phenotyp e; most Gli1(zfd/zfd);Gli2(zfd/+) mice die soon after birth and all have mu ltiple defects including a variable loss of ventral spinal cord cells and s maller lungs that are similar to, but less extreme than, Gli2(zfd/zfd) muta nts. Gli1/Gli2 double homozygous mutants have more extreme CNS and lung def ects than Gli1(zfd/zfd);Gli2(zfd/+) mutants, however, in contrast to Shh mu tants, ventrolateral neurons develop in the CNS and the limbs have 5 digits with an extra postaxial nubbin,These studies demonstrate that the zinc-fin ger DNA-binding domain of GLI1 protein is not required for SHH signaling in mouse. Furthermore,Gli1 and Gli2, but not Gli1 and Gli3, have extensive ov erlapping functions that are likely downstream of SHH signaling.