A STUDY OF THE POSSIBLE ROLE OF LOW EXPRESSED FUNCTIONAL PPOX ALLELES IN THE VARIABLE PENETRANCE OF VARIEGATE PORPHYRIA

Kimberg M, Warnich L

 

University of Stellenbosch, Stellenbosch, South Africa

 

 

The protoporphyrinogen oxidase (PPOX) gene is responsible for the penultimate step of the heme biosynthesis pathway where protoporphyrinogen-IX is oxidized to protoporphyrin. Mutations in the PPOX gene are associated with a diseased state known as variegate porphyria (VP). The term was chosen to describe the low penetrance and varied expression of the autosomal dominant inherited state, including the development of severe abdominal and neurological crises as well as cutaneous photosensitivity, with up to three-quarters of affected individuals remaining asymptomatic. South Africa has the highest incidence of VP in the world due to a founder effect. However, despite decreasing PPOX activity, the R59W founder mutation seems to play no role in the variable penetrance of the VP phenotype. It has recently been shown that where gene mutations are insufficient to convey a disease phenotype, symptoms may penetrate when the mutated allele is coinherited with a functional, yet low-expressed wild-type allele. This study investigates the possibility of a similar modulating mechanism for symptomatic penetrance in VP. Five conserved PPOX haplotypes have been identified based on four single nucleotide polymorphisms (SNPs) in the gene. Despite observed differences in haplotype frequencies between symptomatic, asymptomatic and control patients for at least two of the haplotypes, analysis of variance and association studies yielded negative results. However, due to the limited size of the sample population used the effects of varied expression of wild type haplotypes cannot be excluded. We are now focussing on a more causative approach to identify regulatory elements and possible low-expressed wild type alleles. The -1081G>A sequence variant in the promoter area of the PPOX gene has already been shown to reduce transcriptional activity relative to wild type alleles in in vitro expression assays, and allele specific quantification is now being used to confirm these results. Results from this study will additionally provide a better understanding of the mechanisms involved in PPOX gene regulation for future applications and possible therapies.