Selective degradation of nonsense beta-phosphodiesterase mRNA in the heterozygous rd mouse

PURPOSE. TO investigate the molecular mechanism relating phenotype and geno type in the rd mouse, mRNA and pre-mRNA levels derived from the wild-type a nd position-347 nonsense mutant beta-phosphodiesterase (beta-PDE) genes wer e determined and compared with the corresponding gene copy ratios. METHODS. Total RNA and genomic DNA was isolated from the retinas of three h eterozygous rd/+ mouse strains. For each, quantitative reverse transcriptio n-polymerase chain reaction (RT-PCR) was used to determine the ratio of wil d-type and rd beta-phosphodiesterase pre-mRNA and mature mRNA. The gene cop y ratio between wild-type and rd beta-PDE was also determined by quantitati ve PCR. RESULTS. The pre-mRNA ratio of wild-type versus nonsense mutant was close t o 1:I, whereas the corresponding mRNA ratio was greater than 3.1, even thou gh the gene copy ratio was confirmed to be 1:1. CONCLUSIONS. The equivalence of pre-mRNA ratio level for wild-type and nons ense mutant in the rd/+ retina indicates that both genes were transcribed a t similar levels. Thus, neither the nonsense mutation at position 347 nor t he intron 1 retroviral insertion also present in the rd gene seem to have a ffected gene transcription. In contrast, the strain-independent bias favori ng wild-type mature mRNA in vivo suggests a specific degradation of mutant transcript during or after pre-mRNA splicing. This allele-specific degradat ion set-yes to decrease mutant transcript levels dramatically in all rd str ains, and suggests that photoreceptor cells have the capacity to reduce the level of an mRNA containing a nonsense mutation.