Novel splice, missense, and nonsense mutations in the fumarylacetoacetase gene causing tyrosinemia type I

In six unrelated patients with hereditary tyrosinemia type 1 (HT1), three different disease-causing mutations were found by DNA sequencing. Two Pakistani patients, with acute and intermediate forms of HT1, were homozygous for a G¹⁹² → T mutation in the last nucleotide of exon 2. This caused aberrant splicing with partial intron 2 retention and premature termination. Three Turkish patients with chronic and intermediate forms of HT1 were homozygous for an A⁶⁹⁸ → T mutation substituting aspartic acid 233 with valine. A Norwegian patient with an intermediate clinical phenotype was heterozygous for G⁷⁸⁶ → A, introducing a TGA stop codon for Trp262 (W262X). Site-directed mutagenesis and expression in a rabbit reticulocyte lysate system demonstrated that the nonsense and missense mutations abolished fumarylacetoacetase activity and gave reduced amounts of a truncated and a full-length protein, respectively. Simple tests were established to identify the three mutations by restriction digestion of PCR-amplified genomic DNA. Among 30 additional HT1 patients investigated, 2 were found to be homozygous and 1 heterozygous for G¹⁹² → T. Two other patients were homozygous and one was heterozygous for W262X.