TWO MUTATIONS IN UROPORPHYRINOGEN DECARBOXYLASE (UROD) GENE LEADING TO MILD PHENOTYPE OF HEPATOERYTHROPOIETIC PORPHYRIA (HEP) IN ITALY

A deficiency in uroporphyrinogen decarboxylase (URO-D; EC 4.1.1.37) enzyme activity, the fifth enzyme of the heme biosynthetic pathway, is found in patients with sporadic porphyria cutanea tarda (s-PCT), familial porphyria cutanea tarda (f-PCT), and hepatoerythropoietic porphyria (HEP). Subnormal URO-D activity is due to mutations of the UROD gene in both f-PCT and HEP, but no mutations have been found in s-PCT. Genetic analysis has determined that f-PCT is transmitted as an autosomal dominant trait. In contrast, HEP, a severe form of cutaneous porphyria, is transmitted as an autosomal recessive trait. HEP is characterized by a profound deficiency of URO-D activity, and the disease is usually manifest in childhood, characterized by severe photosensitivity, skin fragility and hypertrichosis. Biochemical and genetic investigations suggested that HEP is the homozygous form of f-PCT. This hypothesis has been recently confirmed by the molecular analysis of the UROD gene. So far, 60 different mutations have been identified, 4 of whom (P62L, A80G, V134Q, G281E) detected either in HEP (in homozygosity or in compound heterozygosity) and in f-PCT, with 1 predominant mutation (G281E) detected in Spain. In this study we performed the molecular characterization in 4 subjects with coutaneous porphyiria at early onset. Patient 1 showed early onset, severe URO-D deficiency, marked hypertrichosis and ascertained familiarity; patients 2 and 4 both showed severe URO-D deficiency associated with a f-PCT-like phenotype. HEP in family 3 was suspected mainly by family history: the proband and her sister have a similar severe phenotype associated with early onset whereas the mother has a milder symptomatology. UROD gene mutations are detected by PCR-SSCP followed by nucleotide sequencing. Putative splicing mutations are analyzed by RT-PCR. The results obtained by molecular analysis are summarized in the following table.

The mutation screening has been extended, whenever possible, to other family members. In the case of pt. 1 we found the same two defects in a male twin with similar phenotype, R120C in the father and paternal aunt and L192P in the mother. In family 2, both the proband's daughters were carriers of the G318R mutation who are, at present, asymptomatic. The 756 C>G mutation (pt3), found in homozygosis in two sisters with the same severe phenotype, was detected in one allele of the mother. RT-PCR experiments showed that the correct splicing of exon 7 was not completely abolished by the mutation, possibly explaining the mild URO-D deficiency observed in RBCs. This study confirms, that HEP is the result of homozygosity or double heterozygosity for mutations occurring in f-PCT: among the 6 different mutations detected in this series, G318R has been previously detected in f-PCT heterozygotes in Northern Europe and R142Q was found by us in an unrelated patient of Italian ancestry.