STEADY-STATE TRANSCRIPT LEVELS OF THE PORPHOBILINOGEN DEAMINASE GENE IN PATIENTS WITH ACUTE INTERMITTENT PORPHYRIA

PCR-based solid-phase minisequencing method was used to analyze the st eady-state mRNA levels of the porphobilinogen deaminase gene in eight patients with acute intermittent porphyria. The patients had the earli er characterized mutations 517C --> T (R173W), 518G --> A (R173Q), 673 C --> G (R225G), 673C --> T (R225X), 713T --> G (L278P), and 1073delA (frame shift). All mutations, except the missense mutation 517C --> T in exon 10, affected the steady-state transcript levels of the mutant allele. The mutant mRNA levels in lymphocytes varied from 5% to 95% of the corresponding wild-type allele levels. In contrast to the CRIM-ne gative mutation 517C --> T, the CRIM-positive mutation in the same cod on 518G --> A resulted in reduction of the steady-state transcript lev el of the mutant allele to 65% of that of the normal allele. The two m utations, 673C --> G or T, affecting the same nucleotide in exon 12 al so differed considerably in their effect on mRNA levels: The transcrip t level of the allele with a missense mutation was decreased to 80% of that of the normal allele, whereas a nonsense mutation at the same po sition resulted in a dramatic decrease (fivefold) in the levels of the mutant transcript. Our data showed large variations between the level s of mutant transcript in AIP patients and these variations did not co rrelate either to CRIM class, to the location of the disease causing m utation in the PBGD gene, or to the clinical phenotype of AIP.