Plasma fluorescence scanning and fecal porphyrin analysis for the diagnosis of variegate porphyria: precise determination of sensitivity and specificity with detection of protoporphyrinogen oxidase mutations as a reference standard.

BACKGROUND: Variegate porphyria (VP) is the autosomal dominant disorder associated with deficiency of the enzyme protoporphyrinogen oxidase (PPOX). Plasma fluorescence scanning has been reported to be a more sensitive test for VP than traditional fecal chromatography. Previous comparisons of these techniques predated identification of the PPOX gene. We assessed these techniques in a large group of patients characterized for VP at the DNA level. METHODS: We evaluated all patients for whom the genotype and a plasma scan or fecal porphyrin result were available. Mutations were detected by restriction digest analysis. Plasma fluorescence scanning was conducted according to published methods. Fecal porphyrins were identified and quantified by thin-layer chromatography. RESULTS: Plasma fluorescence scanning was assessed in 679 patients (205 with VP who were carriers of a PPOX mutation, either with disease symptoms or asymptomatic) and fecal analysis in 473 (190 with VP). Sensitivity and specificity of both tests were higher in adults than in children and higher for adults with disease symptoms than for asymptomatic carriers. In a direct comparison in 168 adults (73 with VP), plasma scanning was significantly more sensitive than fecal porphyrin analysis [sensitivity, 0.96 (95% confidence interval, 0.89-0.99) vs 0.77 (0.66-0.85)]. Fecal coproporphyrin [area under the curve, 0.87 (0.83-0.90)] was a better predictor of VP than protoporphyrin [0.80 (0.76-0.84)]. CONCLUSIONS: Plasma scanning is a more sensitive and specific test for VP than fecal porphyrin analysis. Neither test is sensitive in children, and both are less sensitive in asymptomatic carriers than in symptomatic cases. DNA analysis therefore remains the preferred method for the identification of carriers, particularly in children.

A mouse model for South African (R59W) variegate porphyria: construction and initial characterization.

Variegate porphyria is inherited as an autosomal dominant disease with variable penetrance. It is characterized clinically by photocutaneous sensitivity and acute neurovisceral attacks, and biochemically by abnormal porphyrin excretion in the urine and feces. While the world-wide incidence of variegate porphyria is relatively low, in South Africa it is one of the most common genetic diseases in humans. Due to the large number of patients with variegate porphyria in South Africa, and the fact that variegate porphyria is representative of both the so-called "acute" and the "photocutaneous" porphyrias, it would be valuable to have an animal model in which to study the disease. In this study we have produced a mouse model of "South African" variegate porphyria with the R59W mutation in C57/BL6 mice via targeted gene replacement. Hepatic protoporphyrinogen oxidase activity was reduced by approximately 50% in mice heterozygous for the mutation. Urine and fecal samples from these mice, in the absence of exogenous inducers of hepatic haem synthesis, contain elevated concentrations of porphyrins and porphyrin precursors in a pattern similar to that found in human variegate porphyric subjects. Bypassing the rate-limiting step in haem biosynthesis by feeding 5-aminolevulinic acid to these mice, results in an accentuated porphyrin excretory pattern characteristic of the variegate porphyric phenotype and urinary porphobilinogen is increased significantly. This initial characterization of these mice suggest that they are a good model for variegate porphyria at the biochemical level.