A MUTATION WITHIN INTRON-3 OF THE PAX-3 GENE PRODUCES ABERRANTLY SPLICED MESSENGER-RNA TRANSCRIPTS IN THE SPLOTCH (SP) MOUSE MUTANT

The splotch (Sp) mouse mutant displays defects in neural tube closure in the form of exencephaly and spina bifida. Recently, mutations in the Par-3 gene have been described in the radiation-induced Sp(r) and Sp2H alleles. This led us to examine the integrity of the Pax-3 gene and its cellular mRNA transcript in the original, spontaneously arising Sp allele. A complex mutation in the Par-3 gene including an A --> T transversion at the invariant 3' AG splice acceptor of intron 3 was identified in the Sp/Sp mutant. This genomic mutation abrogates the normal splicing of intron 3, resulting in the generation of four aberrantly spliced mRNA transcripts. Two of these Pax-3 transcripts make use of cryptic 3' splice sites within the downstream exon, generating small deletions which disrupt the reading frame of the transcripts. A third aberrant splicing event results in the deletion of exon 4, while a fourth retains intron 3. These aberrantly spliced mRNA transcripts are not expected to result in functional Pax-3 proteins and are thus responsible for the phenotype observed in the Sp mouse mutant.