Variegate porphyria in Western Europe: identification of PPOX gene mutations in 104 families, extent of allelic heterogeneity, and absence of correlation between phenotype and type of mutation

Abstract

Variegate porphyria (VP) is a low-penetrance, autosomal dominant disorder characterized clinically by skin lesions and acute neurovisceral attacks that occur separately or together. It results from partial deficiency of protoporphyrinogen oxidase encoded by the PPOX gene. VP is relatively common in South Africa, where most patients have inherited the same mutation in the PPOX gene from a common ancestor, but few families from elsewhere have been studied. Here we describe the molecular basis and clinical features of 108 unrelated patients from France and the United Kingdom. Mutations in the PPOX gene were identified by a combination of screening (denaturing gradient gel electrophoresis, heteroduplex analysis, or denaturing high-performance liquid chromatography) and direct automated sequencing of amplified genomic DNA. A total of 60 novel and 6 previously reported mutations (25 missense, 24 frameshift, 10 splice site, and 7 nonsense) were identified in 104 (96%) of these unrelated patients, together with 3 previously unrecognized single-nucleotide polymorphisms. VP is less heterogeneous than other acute porphyrias; 5 mutations were present in 28 (26%) of the families, whereas 47 mutations were restricted to 1 family; only 2 mutations were found in both countries. The pattern of clinical presentation was identical to that reported from South Africa and was not influenced by type of mutation. Our results define the molecular genetics of VP in western Europe, demonstrate its allelic heterogeneity outside South Africa, and show that genotype is not a significant determinant of mode of presentation.

Figure 1

Distribution of mutations in the human PPOX gene. Missense mutations (•) are shown below the gene diagram, with chain termination and splice mutations (frameshift [×], nonsense [▪], and splice defects [□]) above it. Unshaded areas of exons denote noncoding regions.

Figure 2

Missense mutations and intraexonic SNPs in human PPOX. The amino acid sequence for the human enzyme (1) was compared with those of the mouse (2), Saccharomyces cerevisiae (3), Myxococcus xanthus (4), Bacillus subtilis (5) Mycobacterium tuberculosis (6), Propionibacterium freundenreichii (7), Nicotiana tabacum I (plastid) (8), Nicotiana tabacum II (mitochondria) (9), and Arabadopsis thaliana (10). Comparisons are shown only for mutated residues; numbers refer to the human sequence; letters above numbers denote mutant residues. Intraexonic polymorphisms (P256R [256] and R304H [304]) are in italics. A pair of vertically stacked asterisks (**) denotes a conserved residue, and a single asterisk (*) denotes a highly conserved residue. Sequences were aligned by Clustal W1.7 (Thompson et al. 1994).