Contrasting effects of ENU induced embryonic lethal mutations of the quaking gene

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 alleles qk(l-1), qk(kt1), qk(k2), and qk(kt3). The mutation in qk(l-1) creates a splice site in the terminal exon of the QKI-6 isoform, Missense mutations in the KH domain and the QUA1 domains in qk(k2) and qk(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(v) differ. Compound heterozygous qk(u) animals carrying qk(kt1), qk(k2),and qk(kt3) all exhibit a permanent quaking phenotype similar to that of qk(u)/qk(u) animals, whereas qk(u)/qk(l-1) animals exhibit only a transient quaking phenotype, The qk(l-1) mutation eliminates the QKI-5 isoform, showing that this isoform plays a crucial role in embryonic survival. The transient quaking phenotype observed in qk(u)/qk(l-1) mice indicates that the QKI-6 and QKI-7 isoforms function primarily during myelination, but that QKI-5 may have a concentration-dependerat 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. (C) 1999 Academic Press.