Clinical implications of the molecular biology of erythropoietic protoporphyria

Production and characterization of erythropoietic protoporphyric heterodimeric ferrochelatases

Mutations resulting in diminished activity of the dimeric enzyme ferrochelatase are a prerequisite for the inherited disorder erythropoietic protoporphyria (EPP). Patients with clinical EPP have only 10% to 30% of normal levels of ferrochelatase activity, and although many patients with EPP have one mutant allele and one "low-expression" normal allele, the possibility remains that, for some, low ferrochelatase activity may result from an EPP mutation that has an impact on both subunits of the wild-type/mutant heterodimer. Here we present data for 12 ferrochelatase wild-type/EPP mutant heterodimers showing that some mutations result in heterodimers with the residual activity anticipated from individual constituents, whereas others result in heterodimers with significantly lower activity than would be predicted. Although the data do not allow an a priori prediction of heterodimeric residual activity based solely on the in vitro activity of EPP homodimers or the position of the mutated residue within ferrochelatase, mutations that affect the dimer interface or [2Fe-2S] cluster have a significantly greater impact on residual activity than would be predicted. These data suggest that some EPP mutations may result in clinically overt EPP in the absence of a low-expression, wild-type allele; this is of potential significance for genetic counseling of patients with EPP.

[Photodermatoses during childhood]

Actinic dermatitis and phototoxic and idiopathic photodermatoses occur relatively often during childhood. Photodermatoses stemming from enzyme defects and genetic disorders are much less common, while photoallergic and chronic actinic dermatitis have not been dealt with to date. A specific sensitivity of a child's skin to light is often the first manifest clinical symptom of a photodermatosis, the key is to ensure early diagnosis. However, it is often difficult to reach a diagnosis, as the lesions are barely perceptible or have even healed by the time the patient is examined. This is why photoprovocation tests are so important in diagnosing photodermatoses, particularly during childhood. In addition to early diagnosis of these illnesses and offering the appropriate treatment, it is also crucial that the parents be well informed and aware of what symptoms to look for so that they can protect their children from overexposure to UV light and thereby avoid later actinic injuries that can become as serious as malignancies.

Vascular changes in erythropoietic protoporphyria: histopathologic and immunohistochemical study

BACKGROUND

Erythropoietic protoporphyria (EPP) is an inherited disease caused by deficient activity of ferrochelatase in the heme biosynthetic pathway. Accumulation of protoporphyrins and light exposure results in acute phototoxic skin reactions. The histopathologic findings of the light-exposed skin are thickening of the superficial dermal vessel walls and amorphous deposits around the vessels, but the origin and detailed composition of the perivascular material have been unclear.

OBJECTIVE

The vascular morphology and composition of the perivascular material were studied in the skin samples of patients with EPP.

METHODS

Skin biopsy specimens of 8 patients with EPP representing 7 Finnish EPP families with different genotypes were studied by means of light and electron microscopy and immunohistochemical methods.

RESULTS

The characteristic finding was thickened, periodic acid-Schiff-positive vessel walls caused by concentric reduplication of basal lamina and excess of fine granular material at the basal membrane zone in the superficial dermis. The perivascular deposits in the vicinity of vessel walls had a homogeneous or fine granular appearance without filaments. Direct immunofluorescence showed constant IgG deposits together with IgA, IgM, and C3 in the vessel walls. In immunohistochemistry, collagen IV and laminin could be demonstrated at the vascular basal membrane together with serum amyloid P protein, kappa and lambda light chains, and a 90-kd glycoprotein.

CONCLUSION

The vascular involvement indicates that the blood vessel walls in the papillary dermis are the primary tissues affected during an acute photoreaction. The repeated acute damage and repair processes in the basement membrane zone result in thickening of the vessel walls. Perivascular deposits are a secondary and irreversible phenomenon resulting from the leakage and accumulation of different serum components. These changes were not found in the nonexposed skin, indicating that an increased level of erythrocyte protoporphyrin per se is not responsible for the cutaneous manifestations, but the interaction of solar radiation is mandatory. Amorphous deposits distinguish EPP from variegate porphyria and porphyria cutanea tarda; a histopathologic examination may be a helpful tool in differentiating porphyric and nonporphyric photosensitivity.