Only the C73R missense mutation is common, occurring in approximately one-third of the CEP alleles studied, whereas the other mutations have been detected in only 1 to 3 presumably unrelated families. These include 24 missense mutations, 1 nonsense, 2 splice site (c.63 + 1G → A c.245-2A → T), 2 deletions, 4 insertions, and 1 complex rearrangement. To date, 38 mutations have been identified in unrelated CEP patients (Human Gene Mutation Database ). The approximately 34-kb human URO-synthase gene ( UROS), located at chromosome 10q25.3-26.3, has 10 exons and 2 alternative promoters that generate housekeeping- and erythroid-specific transcripts 5 that express the same enzyme polypeptide of 265 amino acids (molecular mass ∼ 29.5 kDa). Light activates the photocatalytic URO I and COPRO I isomers, resulting in tissue damage and the formation of bullous cutaneous lesions that rupture, often becoming infected, and lead to bone resorption and cutaneous deformity. The released porphyrins accumulate in tissues and bones and are excreted in the urine and feces. An excess of URO'gen I and COPRO'gen I and their oxidized forms, uroporphyrin (URO) I and coproporphyrin (COPRO) I, primarily in erythrocytes, leads to hemolysis. ![]() ![]() 2, 3 Deficient activity of URO-synthase in CEP patients results in the nonenzymatic cyclization of hydroxymethylbilane to URO'gen I. 1 This enzyme catalyzes the conversion of the linear tetrapyrrole, hydroxymethylbilane to uroporphyrinogen (URO'gen) III, the only isomer of URO'gen that can ultimately be converted to heme. This is the first BPS mutation in the last intron, presumably accounting for the observed 100% intron retention without exon skipping.Ĭongenital erythropoietic porphyria (CEP) is an autosomal recessive inborn error of heme biosynthesis resulting from markedly deficient, but not absent, activity of the fourth enzyme in the heme biosynthetic pathway, uroporphyrinogen III synthase (URO-synthase EC 4.2.1.75). Thus, the BPS mutation markedly reduced the wild-type transcript and enzyme activity, thereby causing the disease. Although the +81-nucleotide alternative transcript was in-frame, it only contributed approximately 0.2% of the lymphoblast URO-synthase activity. RT-PCR revealed aberrant transcripts in both wild-type and CEP lymphoblasts, whereas BPS mutation reduced the wild-type transcript and enzyme activity in CEP lymphoblasts to approximately 10% and 15% of normal, respectively. Sequencing intron 9 RT-PCR products and genomic DNA in each case revealed homozygosity for a novel BPS mutation (c.661-31T→G) and alternatively spliced transcripts containing 81, 246, 358, and 523 nucleotides from intron 9. Northern analyses of lymphoblast mRNAs from 2 patients and reverse-transcribed polymerase chain reaction (RT-PCR) of lymphoblast mRNAs from all 3 patients revealed multiple longer transcripts involving intron 9 and low levels of wild-type message. ![]() In 3 families with autosomal recessive congenital erythropoietic porphyria (CEP) resulting from uroporphyrinogen III synthase (URO-synthase) deficiency, sequencing the promoter, all 10 exons and the intron/exon boundaries did not detect a mutation. Splicing mutations account for approximately 10% of lesions causing genetic diseases, but few branchpoint sequence (BPS) lesions have been reported.
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