Investigation of the possible influences of candidate modifier genes on the clinical expression of variegate porphyria (VP)

[摘要] ENGLISH ABSTRACT:Variegate porphyria (VP, MIM 176200) is a low penetrance autosomal dominantdisorder that stems from mutations in the protoporphyrinogen oxidase (PPOX) gene.VP is found in most populations, but has a high prevalence in the South AfricanAfrikaner population with most patients inheriting the same PPOX mutation (R59W)from a common ancestor. The clinical manifestations of the disease include acuteneurovisceral attacks and/or cutaneous photosensitivity. Great variation in the clinicalpresentation of VP is observed; even in members of the same family that share acommon genetic background and that have been exposed to similar environmentalfactors.Candidate genes that may have an influence on phenotypic variation due to theregulatory function in the haem biosynthetic pathway include the two deltaaminolevulinicacid synthase (ALAS) genes and the porphobilinogen deaminase(PBGD) gene. Sequence homology searches between different species indicated thatthe ALAS-1, ALAS-2 and PBGD genes are highly conserved, indicating that thesegenes have an important function to fulfill in the haem biosynthetic pathway.The study population of 25 R59W individuals were divided in four categories accordingto their clinical presentation. The distribution of clinical symptoms observed in this studycorresponds with results from previous studies.Conformation sensitive gel electrophoresis (CSGE), conventional single strandedconformation polymorphism analysis (SSCP) and two buffer SSCP analysis wereimplemented to screen for possible sequence variants. The exons of all three genes aswell as the adjacent intronic sequences were investigated. A total of six sequencevariation sites were identified of which five had previously been described singlenucleotide polymorphisms (ALAS-1: 4713 T>C; PBGD: -64 C>T, 3581 A>G, 6479 G>T,7064 C>A)] and a novel 8bp deletion (PBGD: 4582_ 4589del). No sequence variant wasidentified in the ALAS-2 gene.The CSGE method proved to have the highest sensitivity (83%), identifying five of sixsequence variant sites. The conventional SSCP method identified only three (50%) sequence variant sites, while the two buffer system detected two (33%) of the sequencevariants.The 4713 T>C SNP in exon 4 of the ALAS-1 gene and the -64 C>T SNP in the PBGDgene were selected for further investigation due to their location in the respectivegenes. These sequence variants were typed in 50 patients and 50 control subjectsmatched for ethnic background. The relationship between variation at these loci andclinical features was investigated. No statistical significant association was observedfor either of the 4713 T>C SNP (P= 0.717) or the -64 C>T SNP (P= 0.931).Genetic modifying factors make a variable contribution to the total clinical picture andare difficult to identify in small populations. Due to the fact that we only had a limitednumber of VP samples, association cannot be ruled out. This study does, however,provide insight into investigational approaches that should be undertaken in futureresearch concerning the ALAS and PBGD genes. Further knowledge concerning thehaem biosynthetic pathway could ultimately lead to the understanding and assessmentof the clinical expression observed in individuals with VP.

[发布机构] Stellenbosch University