IDENTIFICATION OF A FREQUENT PSEUDODEFICIENCY MUTATION IN THE FUMARYLACETOACETASE GENE, WITH IMPLICATIONS FOR DIAGNOSIS OF TYROSINEMIA TYPE-I

In healthy individuals, fumarylacetoacetase (FAH) activities close to the range found in hereditary tyrosinemia type 1 (HT1) patients indica ted the existence of a ''pseudo-deficiency'' allele. In an individual homozygous for pseudodeficiency of FAH and in three HT1 families also carrying the pseudodeficiency allele, western blotting of fibroblast e xtracts showed that the pseudodeficiency allele gave very little immun oreactive FAH protein, whereas northern analysis revealed a normal amo unt of FAH mRNA. Sequencing revealed an identical mutation, C-1021-->T (Arg341Trp), in all the pseudodeficiency alleles. Site-directed mutag enesis and expression in a rabbit reticulocyte lysate system demonstra ted that the C-1021-->T mu- tation gave reduced FAH activity and reduc ed amounts of the full-length protein. BsiEI restriction digestion of PCR products distinguished between the normal and the mutated sequence s. Among 516 healthy volunteers of Norwegian origin, the C-1021-->T mu tation was found in 2.2% of the alleles. Testing for the C-1021-->T mu tation may solve the problem of prenatal diagnosis and carrier detecti on in families with compound heterozygote genotypes for HT1 and pseudo deficiency.