Contrasting Effects of ENU Induced Embryonic Lethal Mutations of thequakingGene

Multiple alleles of the quaking (qk) gene have a variety of phenotypes ranging in severity from early embryonic death to viable dysmyelination. A previous study identified a candidate gene, QKI, that contains an RNA-binding domain and encodes at least three protein isoforms (QKI-5, -6 and -7). We have determined the genomic structure of QKI, identifying an additional alternative end in cDNAs. Further we have examined the exons and splice sites for mutations in the lethal allelesqk^l-1, qk^kt1, qk^k2,andqk^kt3.The mutation inqk^l-1creates a splice site in the terminal exon of the QKI-6 isoform. Missense mutations in the KH domain and the QUA1 domains inqk^k2andqk^kt3,respectively, indicate that these domains are of critical functional importance. Although homozygotes for each ENU induced allele die as embryos, their phenotypes as viable compound heterozygotes with qk^vdiffer. Compound heterozygousqk^vanimals carryingqk^kt1, qk^k2,andqk^kt3all exhibit a permanent quaking phenotype similar to that ofqk^v/qk^vanimals, whereasqk^v/qk^l-1animals exhibit only a transient quaking phenotype. Theqk^l-1mutation eliminates the QKI-5 isoform, showing that this isoform plays a crucial role in embryonic survival. The transient quaking phenotype observed inqk^v/qk^l-1mice indicates that the QKI-6 and QKI-7 isoforms function primarily during myelination, but that QKI-5 may have a concentration-dependent role in early myelination. This mutational analysis demonstrates the power of series of alleles to examine the function of complex loci and suggests that additional mutant alleles of quaking could reveal additional functions of this complex gene.