Expression and characterization of six mutations in the protoporphyrinogen oxidase gene among Finnish variegate porphyria patients.
BACKGROUND
Variegate porphyria (VP) is an inherited disorder of heme biosynthesis that results from a partial deficiency of protoporphyrinogen oxidase (PPOX). Patients with VP may experience acute neurovisceral attacks and cutaneous photosensitivity. To date we have characterized 109 VP patients representing 19 VP families in the Finnish population of 5 million, both biochemically and clinically.


METHODS
Mutations were identified by direct sequencing of the patients' genomic DNA. The effect of the mutations was determined by sequencing the reverse transcriptase polymerase chain reaction (RT-PCR) product amplified from total RNA extracted from the patients' lymphoblast cell lines and expressing the mutations in E. coli and COS-1 cells.


RESULTS
Of the six mutations identified in the PPOX gene, three mutations (IVS2-2a-->c, 338G-->C, and 470A-->4C) caused splicing defects, one produced a frameshift (78insC) and two mutations (R152C and L401F) caused amino acid substitutions. In RT-PCR, the IVS2-2a-->c mutation caused a retention of a 36-bp fragment in the 3' end of intron 2, the 338G-->C mutation caused an exon 4 deletion, and the 470A-->C mutation caused an exon 5 deletion with retention of a 19-bp fragment of the 3' end of intron 5. In both prokaryotic and eukaryotic expression systems, the PPOX activities of five mutants were decreased to 0-5% of the normal activity.


CONCLUSIONS
This study describes five novel mutations and one earlier described major mutation among Finnish VP patients. All mutations produced detectable transcripts, but resulted in decreased PPOX activity confirming the causality of the mutations and the biochemical defects in these patients.
von und zu Fraunberg
M
M
Department of Medicine, University Central Hospital of Helsinki, Finland. mikael.fraunberg@hus.fi
Tenhunen
R
R
Kauppinen
R
R
D000818:Animals; D019556:COS Cells; D004252:DNA Mutational Analysis; D004926:Escherichia coli; D029968:Escherichia coli Proteins; D005091:Exons; D005260:Female; D005387:Finland; D005420:Flavoproteins; D016368:Frameshift Mutation; D006580:Genetic Carrier Screening; D006579:Heterozygote; D006801:Humans; D008297:Male; D024101:Mitochondrial Proteins; D009154:Mutation; D020125:Mutation, Missense; D010088:Oxidoreductases / Q000172:deficiency / Q000235:genetics*; D044925:Oxidoreductases Acting on CH-CH Group Donors; D010375:Pedigree; D017354:Point Mutation; D017094:Porphyrias, Hepatic / Q000175:diagnosis / Q000235:genetics*; D050768:Protoporphyrinogen Oxidase; D015183:Restriction Mapping; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D014158:Transcription, Genetic
D029968:Escherichia coli Proteins; D005420:Flavoproteins; D024101:Mitochondrial Proteins; D010088:Oxidoreductases; D044925:Oxidoreductases Acting on CH-CH Group Donors; C493977:HemG protein, E coli; C493978:PPOX protein, human; D050768:Protoporphyrinogen Oxidase