UROPORPHYRINOGEN DECARBOXYLASE - COMPLETE HUMAN GENE SEQUENCE AND MOLECULAR STUDY OF 3 FAMILIES WITH HEPATOERYTHROPOIETIC PORPHYRIA

A deficiency in uroporphyrinogen decarboxylase (UROD) enzyme activity, the fifth enzyme of the heme biosynthetic pathway, is found in patien ts with sporadic porphyria cutanea tarda (s-PCT), familial porphyria c utanea tarda (f-PCT), and hepatoerythropoietic porphyria (HEP). Subnor mal UROD activity is due to mutations of the UROD gene in both f-PCT a nd HEP, but no mutations have been found in s-PCT. Genetic analysis ha s determined that f-PCT is transmitted as an autosomal dominant trait. In contrast, HEP, a severe form of cutaneous porphyria, is transmitte d as an autosomal recessive trait. HEP is characterized by a profound deficiency of UROD activity, and the disease is usually manifest in ch ildhood. In this study, a strategy was designed to identify alleles re sponsible for the HEP phenotype in three unrelated families. Mutations of UROD were identified by direct sequencing of four amplified fragme nts that contained the entire coding sequence of the UROD gene. Two ne w missense mutations were observed at the homoallelic state: P62L (pro line-to-leucine substitution at codon 62) in a Portuguese family and Y 311C (tyrosine-to-cysteine substitution at codon 311) in an Italian fa mily. A third mutation, G281E, was observed in a Spanish family. This mutation has been previously described in three families from Spain an d one from Tunisia, In the Spanish family described in this report, a paternal uncle of the proband developed clinically overt PCT as an adu lt and proved to be heterozygous for the G281E mutation. Mutant cDNAs corresponding to the P62L and Y311C changes detected in these families were created by site-directed mutagenesis. Recombinant proteins prove d to have subnormal enzyme activity, and the Y311C mutant was thermola bile.