The porphyrias are group of mostly inherited disorders in which a specific spectrum of accumulated and excreted porphyrins and heme precursors are associated with characteristic clinical features. There are eight enzymes involved in the heme synthesis and defects in seven of them cause porphyria. Four of them are described as acute hepatic
porphyrias, which share possible precipitation of acute attacks with symptoms engaging the nervous system. Acute intermittent porphyria (AIP), caused by partial deficiency of the porphobilinogen deaminase (PBGD), is the most frequent among hepatic porphyrias. Clinical expression is highly variable and ~ 90 % of AIP heterozygotes remain
asymptomatic throughout life. During systematic genetic analysis of th
e AIP patients diagnosed in the Czech and Slovak Republics, we found a special case of AIP. In a 15-year-old boy with abdominal and subsequent neurological symptomatology, we identified de novo
mutation 966insA within the PBGD gene leading to a stop codon after 36 completely different amino acids compared to the wt-sequence. To establish the effects of this mutation on the protein structure, we expressed mutant constructs with described mutation in E. coli and analyzed their biochemical and enzymatic properties. Moreover, computer-assisted protein structure prediction was performed.
The porphyrias arise from predominantly inherited catalytic deficiencies of specific enzymes in heme biosynthesis. All genes encoding these enzymes have been cloned and several mutations underlying the different types of porphyrias have been reported. Traditionally, the diagnosis of porphyria
is made on the basis of clinical symptoms, characteristic biochemical findings, and specific enzyme assays. In some cases however, these diagnostic tools reveal overlapping findings, indicating the existence of dual porphyrias with two enzymes of heme biosynthesis being deficient simultane-ously. Recently, it was reported that the so-called Chester porphyria shows features of both variegate porphyria and acute intermittent porphyria. Linkage analysis revealed a novel chromosomal locus on chromosome 11 for the underly-ing genetic defect in this disease, suggesting that a gene that does not encode one of the enzymes of heme biosynthesis might be involved in the pathogenesis of the porphyrias. After excluding candidate genes within the linkage interval, we identified a nonsense mutation in the porphobilinogen deaminase gene on chromosome 11q23.3, which harbors the mutations causing acute intermittent porphyria, as the underlying genetic defect in Chester porphyria. However, we could not detect a mutation in the coding or the promotor region of the protoporphyrinogen oxidase gene that is mutated
in variegate porphyria. Our results indicate that Chester porphyria is neither a dual porphyria, nor a separate type of porphyria, but rather a variant of acute intermittent porphyria. Further, our findings largely exclude the possibility that a hitherto unknown gene is involved in the pathogenesis of the porphyrias.
The porphyrias are heterogeneous disorders arising from predominantly inherited catalytic deficiencies of specific enzymes along the heme biosynthetic pathway. Congenital erythropoietic porphyria is a very rare disease that is inherited as an autosomal recessive trait and results from a profound deficiency of uroporphyrinogen III cosynthase, the fourth enzyme in heme biosynthesis. The degree of severity of clinical symptoms mainly depends on the amount of residual uroporphyrinogen III cosynthase activity. In this study, we sought to characterize the molecular basis of congenital erythropoietic porphyria in Germany by studying four patients with congenital erythropoietic porphyria and
their families. Using PCR-based techniques, we identified four different mutations: C73R, a well-known hotspot mutation, the promoter mutation –86A that was also described previously, and two novel missense mutations, designated G236V and L237P, the latter one encountered in the homozygous state in one of the patients. Our data from the German population further emphasize the molecular heterogeneity of congenital erythropoietic porphyria as well as the advantages of molecular genetic techniques as a diagnostic tool and for the detection of clinically asymptomatic heterozygous mutation carriers within families.
Porphyria experts concur that acute attacks of AIP, VP and HCP, are invariably associated with increases in urinary PBG. Reports differ, however, as to the amount of increase indicative of an acute attack. Some authors consider excretion of at least 25-fold the upper level of normal, as indicative, whereas others regard a 10-fold or even a 2-fold
increase, as sufficient indication. An additional diagnostic difficulty arises from the fact that in many individuals known to have inherited one of the acute porphyrias, PBG is persistently raised also during remission. It may be markedly elevated even in asymptomatic carriers. In the absence of a universally accepted standard for interpreting PBG results, attribution of neurovisceral or neuropsychiatric symptoms in porphyrics to an acute attack of porphyria rather than to other causes, depends largely on clinical
assessment. The aim of this work was to identify reliable criteria, which will
enable establishing or excluding an acute attack, on a biochemical basis. The study summarizes and interprets data obtained during classical neurovisceral acute attacks and latent phases in 20 patients (10 with AIP, 6 with VP, and 4 with HCP). Calculated increases in urinary PBG, with the upper limit of normal excretion, (8.8 μmol/24 h), defined as
100 %, revealed an overlap between values in the acute and latent phases, (1 to 18.5-fold and 2.3 to 51-fold, respectively). This overlap indicates that the workup in each case needs to be individualized. We achieved this goal, by using another method of calculation, in which the PBG value measured during an acute attack in a particular patient was divided by the PBG value measured in that patient’s latent phase. Increases of 2.3 to 50.5-fold were obtained, leading to the conclusion that any increase, calculated as above, of 2.3-fold and higher, may be taken as indicative of an acute attack. An additional finding, demonstrated in the study, which might be useful for supporting the diagnosis of an acute attack, is the distinct emission peak observed at 404/621 nm, in the plasma fluorometric scan of AIP and HCP patients, during an acute attack. We conclude that comparison of the urinary PBG level and plasma fluorometric scan in the acute phase to those of the latent phase in the individual patient is the key to correct, accurate and reliable biochemical diagnosis of an acute attack in a patient previously diagnosed as a porphyric. The additional tests required for confirming a patient’s first acute attack, having no data to compare with, are discussed.