A case of erythropoietic protoporphyria

Abstract

A 53-year-old Texas rancher developed a blistering skin rash that was sensitive to exposure to sunlight. He was referred to hematology with a presumptive diagnosis of porphyria. His peripheral blood counts were within normal limits, and a bone marrow examination revealed erythroid dyspoiesis and ringed sideroblasts. Serum, plasma, and erythrocyte protoporphyrin levels were elevated, the findings of which are consistent with a diagnosis of erythropoietic protoporphyria. This paper discusses the diagnosis and etiology of the porphyrias.

CASE REPORT

A 53-year-old rancher from South Texas presented to his primary care physician with a blistering skin rash. He noted that his rash was predominantly on unclothed skin surfaces, namely his face and arms. The rash was very painful and seemed to be worsened by exposure to sunlight. Prior to this presentation he had been in very good health, with no diagnosed chronic conditions. He was evaluated initially by dermatology. In addition to the photosensitive skin rash, he was found to have a mild anemia. His blood counts included a hemoglobin of 11.4 g/dL, hematocrit of 32.2%, white blood cell count of 4.8 K/μL, and platelet count of 160 K/μL. The differential count was normal. He was referred to hematology with a presumptive diagnosis of new-onset porphyria. A bone marrow biopsy was performed.

The peripheral blood smear was unremarkable. The bone marrow aspirate and trephine biopsy showed trilineage hypercellularity (Figure 1a). Dyspoietic features were noted in the erythroid and megakaryocytic lineages in the form of irregular nuclear features and nuclear to cytoplasmic dyssynchrony. A Prussian blue stain performed on an aspirate smear was remarkable for numerous ring sideroblasts (Figure 1b). The ancillary studies revealed no specific pathology. Flow cytometry found no evidence of a lymphoproliferative or myeloproliferative disease. Conventional cytogenetics grew a normal complement of chromosomes. Fluorescence in situ hybridization for common myelodysplasia alterations highlighted normal patterns.

Figure 1.

The patient's bone marrow aspirate showing (a) hypercellular bone marrow with dyspoietic changes (hematoxylin and eosin, 400×) and (b) ringed sideroblasts (Prussian blue stain, 1000×).

Because of the clinical suspicion and the pathologic findings of erythroid dyspoiesis and ring sideroblasts, a peripheral blood sample was sent for testing of protoporphyrin levels. The total plasma porphyrins were 46.9 μg/dL (reference range, <1.0), total serum porphyrins were 784 nmol/L (reference range, 0–15), erythrocyte protoporphyrins were 300 umol/mol heme (reference range, <70), and the fractionation of plasma porphyrins (emission spectrum) was at a wave length suggestive of erythropoietic protoporphyria (EPP). With the positive results, he was considered to have EPP secondary to a myelodysplastic syndrome, refractory anemia with ring sideroblasts.

The patient's peripheral blood was sent for sequencing of his ferrochelatase gene, which showed a c.913G>T change in exon 9 which altered the canonical splice site and was therefore predicted to be deleterious. His skin disease has been well controlled with Lumitene, a high-dose beta-carotene supplement, and sun avoidance. The patient has been monitored with complete blood counts and metabolic profiles every 2 to 3 months since diagnosis and has required no other treatment. His liver enzymes have been normal except for bilirubin, which has mildly risen to 2.2 mg/dL. A bone marrow transplant is planned.

DISCUSSION

The porphyrias are a collection of diseases caused by inborn and acquired errors of heme synthesis. Heme is necessary for several types of hemoproteins, including liver and respiratory cytochromes, but hemoglobin synthesis for red blood cells accounts for the vast majority of heme synthesis in humans. The synthesis of the heme molecule, a heterocyclic organic ring surrounding an iron ion, is catalyzed in eight steps from succinyl coenzyme A and glycine (Figure 2). These eight enzymes are encoded on nine different genes, each with described mutations. Two genes encode isoenzymes that catalyze the first step in the pathway; one is erythroid specific, aminolevulinic acid synthase 2 (ALAS2). A loss of function mutation at any step marks a stop in the synthetic pathway and an accumulation of precursor molecules. These precursors have no physiologic function, are not normally detected at significant levels, and are toxic to varying degrees. Tight regulatory control is necessary and is cell type specific. In erythroid precursors, iron availability is required for the translation of the ALAS2 mRNA. The eighth step is also iron dependent, as ferrochelatase must add an iron ion to each protoporphyrin. In the liver, heme itself represses the translation of the first and rate-limiting enzyme, ALAS1 (1).

Figure 2.

Heme synthesis.

Except in rare cases, EPP is an inherited form of ferrochelatase deficiency or malfunction or an erythroid-specific ALAS2 enzyme gain of function mutation (Figure 2). The genetics of these diseases are complex, with forms with low penetrance and late onset. Acquired forms of EPP are caused by somatic mutations of the ferrochelatase gene often associated with the genetic instability of an underlying myelodysplastic or myeloproliferative disorder (2). The immediate pathology of EPP is the accumulation of protoporphyrin, the last substrate in the synthesis of heme. Loss of function mutation in the enzyme ferrochelatase, found on chromosome 18, is usually the culprit (3). Rarely, a gain of function mutation of the erythroid-specific enzyme ALAS2, found on the X chromosome, increases the turnover of the pathway (1). As the mutant ALAS2 continues to drive the biosynthesis without regard to the availability of iron, the synthesis stalls at the last step when ferrochelatase has insufficient iron to add to the accumulating protoporphyrins (2).

The protoporphyrins deposit in the liver, erythrocytes, and plasma and are highly toxic. The mechanism of the direct toxicity is poorly understood; however, the photodynamic reactions are. The molecule is hydrophobic, favoring lipid bilayers, like cell membranes. In the superficial vasculature of the skin, protoporphyrins in erythrocytes and plasma are exposed to light. Absorbing light at 320 to 595 nm, protoporphyrin is excited to a triplet state. The energy transfer is propagated to oxygen, resulting in reactive oxygen species. The pathogenic consequence is oxidative damage of proteins, lipids, and nucleic acids. Among its effects, this phenomenon can activate complement and degranulate mast cells. The histopathologic findings are consistent with the cycle of damage and repair. The blood vessels of the basal lamina thicken with perivascular hyaline material and proliferate for the sake of perfusion (2).

The liver is tasked with the excretion of the excess protoporphyrins. If its capacity is overwhelmed, the protoporphyrins are deposited in hepatocytes. Birefringent crystals, characterized by Maltese cross shapes, are visible within the deposits. The liver reacts as it would to any chronic insult with regeneration and fibrosis potentially leading to cirrhosis (2). The conventional histologic bone marrow pathology is not as striking. Increased ring sideroblasts by light microscopy have been described, but in a greater number of cases there is ultrastructural evidence of iron deposition in the mitochondria of erythroblasts (4). In acquired forms of the disease, the apparent histopathology will be that of the underlying myelodysplastic or myeloproliferative disease. A microcytic anemia may be seen in the inherited X-linked dominant EPP with decreases in iron stores and hemoglobin (5).

The laboratory investigation of EPP requires finding elevated protoporphyrins in erythrocytes, plasma, and feces, as in this case. Protoporphyrins are not excreted in urine. Protoporphyrins are exquisitely light sensitive. Since plasma concentrations fall rapidly upon exposure to light, measurement of erythrocyte protoporphyrin is usually preferred (6).