MOLECULAR DEFECTS OF UROPORPHYRINOGEN DECARBOXYLASE IN A PATIENT WITHMILD HEPATOERYTHROPOIETIC PORPHYRIA

The molecular defect of uroporphyrinogen decarboxylase (UROD) was exam ined in a patient with mild hepato-erythropoietic porphyria. To elucid ate the UROD defect, we cloned UROD cDNAs from EBV-transformed lymphob lastoid cells of the proband using reverse transcriptase-polymerase ch ain reaction. Nucleotide sequence analysis of the cloned UROD cDNAs re vealed two separate missense mutations, each occurring in a separate a llele. One mutation was a Val(134) --> Gln transition, and was due to three sequential point mutations (T(417)G(418)T(419) --> CCA); the oth er mutation was a His(220) --> pro transition (A(677) --> C). UROD phe notype studies demonstrated that the TGT --> CCA mutation was inherite d from the father, and the A --> C mutation was inherited from the mot her. In contrast to the null activity previously described for a mutan t UROD from a patient with familial porphyria cutanea tarda, these mut ant URODs had subnormal but substantial enzyme activities, when expres sed in Chinese hamster ovary cells. This is the first demonstration of a mutation caused by three sequential base substitutions.