Sylvia S. Bottomley

The porphyrias are diverse disorders that arise from various inherited enzyme defects in the heme biosynthesis pathway (Fig. 26.1). The first authentic cases of porphyria were described in 1874.2, 3 Since that time, eight genetically distinct forms of porphyria have been characterized (Fig. 26.1; Table 26.1), each caused by a partial deficiency, and in one type by activation, of a different enzyme within the pathway (Table 26.2). The genes encoding these enzymes have been cloned, their chromosomal location is defined, and DNA analyses have revealed many heterogeneous molecular defects in all the porphyrias. The biosynthetic blocks resulting from the defective enzymes are expressed either in the liver or in the bone marrow, the sites where most of body heme is produced. The clinical and pathologic phenotype of each porphyria is dictated by effects of the associated enzyme defect and the mode of inheritance (Table 26.2) and is often influenced by certain metabolic and environmental factors that affect the tightly regulated heme pathway. In some “late-onset porphyrias,” acquired clonal hematopoietic disorders may harbor a somatic mutation involving a heme synthesis enzyme that contributes to the expression of the porphyria.

Although the pathophysiologic mechanisms of the clinical manifestations of the porphyrias are only partly understood, two cardinal features prevail: cutaneous photosensitivity and neurologic symptoms of intermittent autonomic neuropathy, motor nerve palsies, and central nervous system disturbances. The cutaneous photosensitivity is a manifestation of the unique fluorescent properties of the porphyrins (the oxidized forms of the natural porphyrinogens in the heme synthesis pathway) that accumulate in those porphyrias in which the enzyme defects cause porphyrin accumulation (Fig. 26.1). The neurologic manifestations are associated with increased production of the porphyrin precursors 5-aminolevulinic acid (ALA) and porphobilinogen (PBG), which characterize the acute or inducible porphyrias. Among some of these, however, photosensitivity occurs as well if the enzyme defect also leads to accumulation of porphyrins. The distinguishing clinical manifestations of the porphyrias form the basis for the classification in Table 26.1. The diagnosis of a specific porphyria is ascertained by its characteristic profile of accumulated and excreted metabolic intermediates of the heme synthesis pathway (Table 26.2). It can be further confirmed by the activity of the affected enzyme but best by identification of the molecular defect causing the porphyria.

FIGURE 26.1. The heme biosynthetic pathway, its enzymes, and the eight forms of human porphyria associated with genetic defects of these enzymes. The major compound accumulated and excreted in excess in the seven porphyrias beyond the ALA synthase step is the substrate of the respective deficient enzyme; genetic defects in the erythroid ALA synthase (ALAS2) causing increased enzyme activity lead to increased flux of intermediates through the pathway and accumulated protoporphyrins. In the acute or inducible porphyrias (*), increased hepatic ALA production results from release of the normal negative feedback on ALA synthase exerted by heme,1 indicated by the trail of arrows. ALA, 5-aminolevulinic acid; Fe++, ferrous iron.


I. Porphyrias with only cutaneous photosensitivity

Congenital erythropoietic porphyria (CEP)

Porphyria cutanea tarda (PCT); hepatoerythropoietic porphyria (HEP)

Erythropoietic protoporphyria (EPP)

X-linked protoporphyria (XLPP)

II. Acute porphyrias with only neurologic manifestations

Acute intermittent porphyria (AIP)

ALA dehydratase porphyria (ADP)

III. Acute porphyrias with both neurologic manifestations and cutaneous photosensitivity

Variegate porphyria (VP)

Hereditary coproporphyria (HCP)

IV. Dual porphyrias


Congenital Erythropoietic Porphyria

Congenital erythropoietic porphyria (CEP; Gunther porphyria), the second least common of the porphyrias, is characterized by a marked accumulation in erythroid precursors of uroporphyrin and coproporphyrin, which are predominantly of the isomer I type because defects reside in the gene for the enzyme uroporphyrinogen III synthase (UROS; Fig. 26.1). Many marrow erythroblasts and immature circulating erythrocytes exhibit intense red fluorescence in ultraviolet light. The large amounts of porphyrins released from these cells cause the most intense photosensitivity of all the porphyrias.


Type of Porphyria


Defective Enzyme

Gene (Symbol/Locationa)

Enzyme Activity (%)b

Porphyrin/Precursor Accumulated/Excretedd

Route of Excretion

Tissue Source



X-linked protoporphyria


5-Aminolevulinate synthase 2


2 to 3 times normal

Free and zinc protoporphyrin


Bone marrow

ALA dehy-dratase

Autosomal recessive

ALA dehydratase (PBG synthase)




ALA, coproporphyrin



Acute intermittent

Autosomal dominant

PBG deaminase (HMB synthase)







Congenital erythropoietic

Autosomal recessive

Uroporphyrinogen III synthase

UROS/10q25. 2-q26.3



Uroporphyrin I, coproporphyrin I


Bone marrow

Cutanea tarda

Autosomal dominant, or acquired

Uroporphyrinogen decarboxylase




Uroporphyrin I + III, 7-COOH porphyrin



Hereditary coproporphyria

Autosomal dominant

Coproporphyrinogen oxidase




Coproporphyrin IIIf, PBGf, ALAf

Feces, urine



Autosomal dominant

Protoporphyrinogen oxidase




Protoporphyrin Coproporphyrinf PBGf, ALAf

Feces Urine



Autosomal recessive





Free protoporphyrin


Bone marrow







Free protoporphyrin


Bone marrow

ALA, 5-aminolevulinic acid; PBG, porphobilinogen; HMB, hydroxymethylbilane.

a Gene symbol and chromosomal location as listed in the Human Gene Mutation Database, located at www.hgmd.org.

b Percentage of normal.

c Often a compound heterozygote.

d The major metabolite and route are shown in boldface. The route of porphyrin excretion is determined by the number of carboxyl groups on the porphyrin and hence by its water solubility.

e In harderoporphyria the activity is 18% to 24% of normal (see text).

f Increased in urine during acute attack; in VP urine coproporphyrin is increased when symptoms are only cutaneous.

g One allele is mutant and the other allele is usually underexpressed (see text).

Probably first recognized by Schultz2 and Baumstark3 in 1874, CEP was described comprehensively and distinguished from other porphyrias by Gunther in 1911.4 It is a very rare disease, with less than 200 cases reported. It occurs with equal frequency in males and females and in a wide variety of racial groups. Usually, the illness is first detected in infancy5 but, in a number of instances, was not apparent until later in life.6 All evidence is consistent with transmission of the disorder as an autosomal recessive trait.5, 7

Molecular Basis and Pathogenesis

The pattern of porphyrin accumulation in CEP reflects a defect in the conversion of PBG to uroporphyrinogen III. This conversion requires two enzymes: PBG-deaminase (PBGD) and UROS (Fig. 26.1). Affected individuals are homozygotes or compound heterozygotes for mutations in the UROS gene that result in markedly reduced activity of UROS (1% to 20% of normal), leading to the overproduction of uroporphyrinogen I through nonenzymatic cyclization of the substrate hydroxymethylbilane in erythroid precursors.5, 7 In heterozygotes (parents and some siblings of
patients with the disease), the activity of the synthase is intermediate (approximately 50%) between that found in affected individuals and that in normal subjects.8 The UROS gene resides on chromosome 10 (10q25.2-q26.3)9 and has alternative promoters that generate identical housekeeping and erythroid-specific transcripts.7 Forty-six distinct mutant alleles have been identified and molecular defects include point mutations, deletions, insertions, splicing defects, intron branch chain mutations, and erythroid-specific promoter mutations.5, 7, 10 Approximately one third are homoallelic and about 20% of alleles remain undefined. The most common mutation (Cys73Arg) has occurred in one third of alleles (and in nearly one half of homoallelic cases) and correlates with the most severe phenotype of nonimmune hydrops fetalis, transfusion-dependent anemia from birth, or both, particularly in homoallelic cases.5 In most instances, mild forms of the disease have been heteroallelic or only one mutant allele could be identified. Residual enzyme activity of the mutant proteins expressed in prokaryotic systems, or gene promoter-reporter activities in cases of promoter mutations, have provided more precise genotype/phenotype correlations.5, 7, 11 However, in two instances marked divergence of phenotypic expression among siblings suggested undefined modifying factors.12, 13 In another family it was demonstrated that the clinical phenotype is markedly affected by co-inheritance of an activating (gain-of-function) mutation in the erythroid-specific 5-aminolevulinate synthase (ALAS2) gene.14

In addition, variant genetic forms of the CEP phenotype are described. (1) In two patients porphyrin patterns and enzyme assays were consistent with a uroporphyrogen decarboxylase (UROD) rather than a UROS defect, and the presence of a second dyserythropoietic defect was suggested to account for the clinical severity.15, 16 (2) A variant caused by a mutation in the X-linked erythroid transcription factor GATA binding protein 1 (GATA1) demonstrated that CEP can result from a genetic defect in a transacting factor.17 (3) CEP has occurred as a milder form in later life.6, 18 Of 16 such cases, 8 were associated with a myelodysplastic syndrome; the pattern of excess porphyrins was characteristic of childhood-onset CEP but at lower concentrations and erythrocyte UROS activity was normal.18 Presumably UROS mutations could not be identified if only a minor clone of uroporphyric cells harbors a somatic mutation; alternatively another gene that determines UROS activity remains a possibility. This late-onset form of CEP has been termed “erythropoietic uroporphyria secondary to myeloid malignancy.”18

Although defective UROS operating in the erythron characterizes the primary inherited abnormality, no overall deficiency of heme production is evident, perhaps because normal UROS activity appears to be the highest relative to all other heme biosynthetic enzymes in erythroid cells. It is more than 1,000 times greater than the activity of ALA synthase,19 the rate limiting enzyme, so that even very low residual enzyme activity seems to be sufficient to support normal or even increased rates of heme synthesis. Uroporphyrin I is the predominant urinary porphyrin, but increased excretion of uroporphyrin III also invariably occurs, implying up-regulation of the UROS or of enzymes higher up in the pathway. These findings are consistent with the idea that the pathophysiology of CEP is a result of the accumulation of uroporphyrin I rather than of subnormal production of uroporphyrinogen III and heme.20 However, it is unclear why uroporphyrin I, not coproporphyrin I, is the main porphyrin excreted, which may relate to the relative UROD activity in normal erythroid cells being on the order of one tenth of that of UROS.19

The large amounts of porphyrins released from erythroid cells and deposited in multiple tissues21 exert the principal damage in the skin and subcutaneous regions through oxygen-dependent phototoxic reactions on excitation by light (see section “Porphyria Cutanea Tarda”).22, 23 This relentless process leads in varying degrees to the formation of subepidermal bullae, secondary infection, scarring, epidermal atrophy, and resorption of acral structures. Laboratory parameters of hemolysis correlate with severity of anemia. Associated splenic sequestration leads to splenomegaly and variable leukopenia and thrombocytopenia. Whether photolysis of the porphyrin-laden erythrocytes occurs in vivo is not known.

Clinical Description

The first sign of this affliction is often discoloration of the infant’s diapers by the urine, which ranges in color from pink to deep burgundy and fluoresces under Wood’s light.24 The most prominent manifestation is pronounced cutaneous photosensitivity. Exposure to visible light is followed by the development of vesicular or bullous lesions containing a porphyrin-rich, fluorescent fluid. The lesions tend to heal slowly, leaving pigmented scars (Fig. 26.2A). Often, they become infected, ulcerated, and necrotic. Over a period of years, patients develop progressive mutilation and disfigurement with loss of portions of the fingers, nose, eyelids, or ears (Fig. 26.2B). Corneal scarring can lead to blindness.5 Skin not exposed to light is unaffected. Hypertrichosis is prominent and fine hair growth may cover much of the face and extremities. The patients often adopt extreme precautions to avoid the sun. Deposition of porphyrin in the dentin of the teeth causes them to appear red (erythrodontia), brown, or yellowish. Even if discoloration is not apparent in ordinary light, the teeth exhibit red fluorescence in ultraviolet light. At necropsy, the entire skeleton has this red fluorescence.21 Osteopenia and osteolytic lesions can occur.25

Anemia, the result of both hemolysis and ineffective erythropoiesis, is detected in most patients, and usually the spleen is enlarged. The most severe phenotypes are transfusion-dependent.

Laboratory Findings

The anemia is normocytic, normochromic, and of variable severity. Detailed morphologic descriptions of the blood and bone marrow have been reported in a few patients. Anisocytosis, poikilocytosis, polychromasia, basophilic stippling, and nucleated erythrocytes are fairly common features of the peripheral blood.26 Following splenectomy, needlelike fluorescent red cell inclusions that may represent precipitated porphyrin have been observed.27 Morphologic abnormalities of the bone marrow range from erythroid hyperplasia to striking dyserythropoiesis.5, 17, 26, 28 Nuclear inclusions containing hemoglobin may be present.15, 28 Results of studies of the marrow with light and fluorescence microscopy suggest the coexistence of normal and abnormal erythropoietic cells.15, 28 Fluorescence is restricted to the morphologically abnormal cells and is most marked in cell nuclei.15, 28, 29 Evidence of a dual nature of the erythroid precursors has been supported by ultrastructural studies in two cases.15, 30 Kinetically, the anemia is characterized by both a shortened red cell survival and ineffective erythropoiesis.31 Its morphologic and kinetic features closely resemble those of congenital dyserythropoietic anemia type I (Chapter 40).

The most characteristic metabolic abnormality is greatly increased urinary excretion of uroporphyrin I, a biologically useless isomer that cannot be converted to heme. Urinary excretion of uroporphyrin III and coproporphyrins I and III is also increased but to a lesser extent than uroporphyrin I.5, 20 Total urinary porphyrin excretion may exceed 100,000 µg daily (normal is <300 µg), and the urine usually fluoresces on exposure to ultraviolet light. Fecal excretion of porphyrins, especially coproporphyrin I, is greatly increased.20 The concentration of uroporphyrin I is increased in erythrocytes and plasma, but the marrow porphyrin content exceeds that of the peripheral blood or other tissues.20

Congenital Erythropoietic Porphyria in Animals

CEP (pink tooth) in cattle appears to be milder than the human disorder. The disease is inherited as an autosomal recessive trait, and heterozygous animals are clinically and biochemically normal,47, 48 although an increased ratio of coproporphyrin I to coproporphyrin III in the urine was reported.49 Photosensitivity of the skin areas not covered by pigmented hair and hemolytic anemia are observed. The teeth and bones are stained red. UROS activity is severely impaired in erythrocyte lysates.50

A similar disorder has been observed in pigs and cats.51, 52 A naturally occurring feline CEP was recently characterized by clinical phenotype, biochemical analysis, and molecular studies.53 In the fox squirrel (Sciurus niger), as part of the apparently normal physiology of the species, large amounts of uroporphyrin I are deposited in the tissues and excreted in the urine.54 As in human CEP, the normoblasts and young erythrocytes show a red fluorescence; however, hemolytic anemia is lacking, and the fox squirrel seems to suffer no ill effects. Erythroid UROS activity is lower than that found in the closely related but nonporphyric gray squirrel (Sciurus carolinensis).54

In knockin mouse models of patient mutations, the phenotypes closely resemble the human disorder.55, 56 These mice would appear suitable for studies of pathogenesis and therapies in CEP.

Porphyria Cutanea Tarda

Porphyria cutanea tarda (PCT) is the most commonly encountered porphyric disorder. It results from reduced activity of UROD (Fig. 26.1) in the liver, leading to accumulation of uroporphyrins (oxidized uroporphyrinogens), their release into plasma, and excretion in the urine.20 Clinical manifestations are limited to the skin in the form of a photosensitive bullous dermatosis as a consequence of the circulating uroporphyrins. Neurovisceral symptoms never occur. Symptoms usually arise in mid- or later life (hence the name tarda) and are nearly always brought on by genetic and environmental factors, most often hepatic siderosis, alcohol abuse, hepatitis C virus, and estrogen therapy. Since the first comprehensive description of the disease in 1937.57, three variant forms of the illness have been recognized: familial, sporadic, and toxic.

Molecular Basis and Pathogenesis

Among the variants of the disorder, a genetic basis has been established for one, designated familial PCT (type II), but it accounts on average for only 20% to 30% of cases.58, 59, 60, 61 Studies of patients with subnormal UROD activity in both hepatic and extrahepatic tissues, and their relatives, revealed that the enzyme defect is inherited as an autosomal dominant trait.62, 63, 64, 65 Enzyme activity is one-half normal in all tissues, as is the amount of UROD protein detected with antibodies.66, 67, 68 In the rare homozygous form, designated hepatoerythropoietic porphyria (HEP), residual enzyme activity ranges from 3% to 27% of normal,20, 68, 69, 70, 71 but immunoreactive enzyme is variable.67, 68, 72 Since the cloning of the human UROD gene,73 located on chromosome 1 (1p34),74 at least 113 different mutations in the gene have been identified.10, 75 Usually, a given mutation is found in one or in a few kindreds only. The majority are various point mutations in the coding region of the gene or splice site mutations, resulting in changes of amino acids, frameshifts, or deletions; in some instances, large nucleotide deletions were detected. The defects usually result in the production of an unstable or inactive enzyme from the mutant allele. Functional consequences of many of the mutations have been defined or predicted on the crystal structure of the enzyme.76, 77 In the homozygous version among about 40 cases reported, 15 different mutations have been identified.10, 75, 78 The mutations are homoallelic or heteroallelic, causing instability or impaired catalytic activity of the enzyme, and survival hinges on residual enzyme activity encoded by the alleles.

When reduced UROD activity is restricted to the liver and a genetic basis for the disorder is not evident, it is called sporadic PCT (type I).64, 66, 79 Both the catalytic activity and the immunoreactivity of erythrocyte UROD are normal in this variant.67, 79 In the liver, catalytic activity of UROD is subnormal, but immunoreactive enzyme is present in normal or increased amounts.79 After prolonged remission is induced by phlebotomy therapy, both catalytic activity and immunoreactivity become normal.80 These findings, coupled with lack of a family history in most cases of sporadic PCT, have implied that this form of the disease is acquired and not inherited. At the molecular level, no mutations could be found in the UROD gene or its promoter region.81 However, in a few families, clinically manifest PCT was associated with decreased UROD activity in liver and normal activity in erythrocytes and other tissues.75 These cases were called type III PCT; molecular studies have not identified UROD mutations in them.82

The cutaneous damage produced by the accumulated uroporphyrins in plasma and skin of patients with PCT is the consequence of the fundamental properties of the porphyrins. As they absorb light at 400 to 410 nm (Soret band), not only does their photoexcitation yield energy as fluorescence, but in the aerobic environment of tissues, reactive oxygen species (superoxide anion and other reactive metabolites) are produced44 that are damaging to cells. In vivo, superoxide anions are generated by activation of xanthine oxidase.83 The complement system is light activated in vitro in sera containing porphyrins and in vivo in the skin of porphyric patients, promoting release of proteases from dermal mast cells.23 Chemotactic activity is also generated under these conditions, and the peroxides produced by photoexcitation of the porphyrin and those arising from polymorphonuclear leukocytes with activation of the complement system may act synergistically to contribute to the development of the cutaneous lesions. As a consequence of these events, the dermal-epidermal junction becomes disrupted and leads to skin fragility and the formation of vesicles and bullae that easily rupture. Sclerodermoid changes of the skin result from a light-independent effect (the “dark effect”) of the uroporphyrin on collagen synthesis by skin fibroblasts.84 The pathogenesis of the hyperpigmentation, hypopigmentation, and hypertrichosis is not understood.

Most family members carrying a defective gene of familial PCT do not have clinically apparent disease but a significant number have increased excretion of porphyrins;77 in occasional pedigrees, overt manifestations of the disease were noted in several generations.63 Clinical expression of both familial and sporadic PCT is nearly always associated with prevalent confounding factors that cause hepatic injury, so the disorder remains silent without these. In large series of PCT, most patients had more than one of four cardinal hepatotoxic risk factors, namely hepatic iron excess, excessive alcohol use, viral infection (hepatitis C), and medicinal estrogens,60, 61, 85 although these are far more common than the estimated frequency of PCT (approximately 1 per 25,000).20

Hepatic Iron

The central role of iron in the clinical expression of PCT has been recognized for 52 years.86, 87 Numerous studies documented hepatocellular siderosis in most patients with significant uroporphyrinuria.88, 89, 90 The iron deposits generally are moderate in amount, 1.5 to 4 times normal, but are usually greater with co-inheritance of hemochromatosis susceptibility alleles (Chapter 25).88, 91, 92, 93 Except for the geographic variation in the frequency of the hemochromatosis HFE alleles,75, 94 on average, 35% of patients with PCT are heterozygous for the Cys282Tyr mutation, and 15% are homozygous or double heterozygotes with the His63Asp mutation.60, 61, 85, 94 The cause of excess hepatic iron in the remaining cases appears to result from down-regulation of hepcidin expression regardless of iron status and consequent to the oxidative stress associated with the other risk factors for PCT.95 However, in another study serum hepcidin levels were found increased.96 Transfusional iron overload also promotes clinical expression of PCT.97

Depletion of iron by repetitive phlebotomy98 or by administration of deferoxamine99 uniformly induces both clinical and biochemical remissions. Replenishment of iron stores promptly reignites symptoms in patients in whom a remission has been induced by phlebotomy.88, 100 The explanation for the provoking influence of iron is based on studies in mouse models, which indicate that the hepatic UROD activity of 50% in UROD-deficient animals must be reduced to 20% or less of normal in order for PCT to be manifest and is achieved by concomitant iron overload.101, 102 An inhibitor of the enzyme was found in liver cytosol of these animals, as well as of PCT patients, and is a porphomethene that is generated by iron-dependent partial oxidation of uroporphyrinogen via catalysis by cytochrome P4501A2 (CYP1A2) or another enzyme.103, 104 This is consistent with the diminished hepatic UROD activity without reduction in enzyme protein in symptomatic PCT patients and the restoration of hepatic UROD activity to its genetically determined level after iron depletion.60, 79, 80 The mechanism for the generation of sufficient inhibitor in patients with sporadic PCT that does not occur in iron overload states in general remains to be defined. The oxidative environment in the
hepatocyte generated by the common environmental risk factors accompanying clinical expression of the disorder, as well as other traits such as the allele variants of CYP1A2 and G51M1A,105 probably play an important role.94


Ethanol exacerbates PCT. Heavy alcohol intake was found in 25% to 100% of patients in many studies,60, 61, 85, 89, 94, 106 and hepatic cirrhosis is not uncommon.89, 90 Yet the porphyria is an uncommon complication of alcoholic liver disease, occurring in only approximately 2% of cases.107 The association of clinically expressed PCT with alcoholism would relate to its hepatotoxicity and its effect of stimulating iron absorption.108 In this regard, PCT is particularly common among the Bantu population of South Africa, where it is associated with excessive consumption of alcoholic beverages brewed in iron containers,109 thus providing a double threat to susceptible people. Studies have suggested that an undefined hemochromatosis gene plays a role in the iron overload of this population when iron intake is excessive (Chapter 25).110

Hepatitis C

A striking association between symptomatic PCT, familial and sporadic, and hepatitis C is well recognized but is variable around the world. Although most patients with hepatitis C infection do not have the porphyria, as many as 80% of PCT patients are chronically infected with the virus in some locations.60, 61, 75, 94, 111 This infection elicits oxidative stress in hepatocytes as well as down-regulation of hepcidin expression94 and would explain, at least in part, the liver damage often found in PCT. Association of PCT with human immunodeficiency virus (HIV) infection also occurs,112 and such patients may be infected with both HIV and the hepatitis C virus.94, 112, 113 Dual infection causes more severe hepatic disease; whether the HIV virus per se plays a role in the expression of PCT is not known. Thus, patients should be evaluated for the presence of hepatitis C and HIV at the time of diagnosis.


The association of estrogen ingestion and expression of PCT has been reported numerous times but occurs only in a very small percentage of patients who ingest estrogens, consistent with an underlying predisposition to the disease. The cutaneous symptoms have occurred with the use of estrogen as a post-menopausal supplement,60 for contraception,114 and for prostatic carcinoma.115 Uncommonly, patients present with the disorder during pregnancy,116 but pregnancy has also not exacerbated PCT.117 The mechanism of the estrogen effect is thought to relate to its adverse effects on the liver such as steatosis and steatohepatitis. It is generally a lesser factor, as venesection alone has brought about full recovery. Most women needing estrogen supplements at the usual dose can probably use them, provided that storage iron is removed and maintained at a low level.

Other Susceptibility Factors

Additional susceptibility factors that are positively associated with PCT are smoking, allele variants of CYP1A2 and GSTM, and ascorbic acid deficiency.61, 105, 118, 119 Several diverse drugs and radiation have also been implicated in precipitating PCT in a few cases.

Renal Failure

PCT occurs in patients with renal failure undergoing hemodialysis.120 Symptoms may first arise in this setting for several reasons. Iron overload is not uncommon in such patients, and intravenous iron used in conjunction with erythropoietin for the anemia may precipitate clinical PCT.121 Hemodialysis or peritoneal dialysis does not effectively clear circulating plasma uroporphyrins,122, 123 presumably because porphyrins are bound to proteins.124 The uremic state may also contribute to an inhibited UROD and uroporphyrin accumulation independent of an underlying enzyme defect or inhibition.125 In most reported cases, the lack of family or molecular studies makes it difficult to classify the porphyria.

Clinical Description

Cutaneous changes represent the clinical manifestations of PCT.20, 89 Skin lesions are found predominantly on light-exposed areas such as the face and hands and, in women, on the legs and feet as well. There is little discomfort with sun exposure per se, and blue (visible) light triggers only an insidious cutaneous damage. The most common complaint is marked skin fragility in areas subjected to repeated minor trauma, such as the hands and forearms (Fig. 26.3). Vesicles and bullae form primarily on the dorsa of the hands and may erode, leaving atrophic scars that often display zones of both hyperpigmentation and hypopigmentation. Small, 1- to 2-mm, firm, whitish papules (milia) are commonly noted on the hands and, at times, on the face.

Facial hypertrichosis occurs in most patients, is generally more noticeable in women, and may be an isolated presenting feature.126 Hypertrichosis, sometimes striking, is occasionally observed on areas of the skin that are rarely exposed to the sun, such as the trunk and legs. Other findings include hyperpigmentation of facial skin, alopecia, and scleroderma-like changes on the skin of the face, neck, and hands. The histologic appearance of the scleroderma-like lesions is identical to that seen in patients with systemic scleroderma.89 Occasionally, patients have overt signs and symptoms of underlying liver disease, but no good correlation exists between the degree of liver disease and the occurrence or severity of the porphyria.

In the homozygous form of familial PCT, or HEP, the skin changes just described usually occur before the age of 5 years and photosensitivity may be severe, resembling CEP and including disfigurement.75, 78 Developmental delay and seizures have also been reported. Other features are pink urine, dental fluorescence, and occasionally hepatosplenomegaly. Severity of clinical manifestations can vary in affected children within a family; in some instances the clinical phenotype is unusually mild.78, 127

Laboratory Findings

Patients with symptomatic PCT excrete greatly increased amounts of porphyrins in the urine. Uroporphyrin and heptacarboxylic porphyrin predominate, with lesser amounts of coproporphyrin and small amounts of 5- and 6-carboxylate porphyrins.89, 128 Usually, the daily urinary excretion of uroporphyrin is approximately 3,000 µg (normal, <50 µg), but considerably higher values may be found. Uroporphyrin in the urine is predominantly isomer I, whereas the heptacarboxylic porphyrin is predominantly isomer
III.20, 89 Photosensitive cutaneous symptoms rarely occur when the daily urinary excretion of uroporphyrin is <1,000 µg. Urinary coproporphyrin rarely exceeds 600 µg daily.89 An unusual and distinctive tetracarboxylic porphyrin, isocoproporphyrin, is excreted in feces,129 and a slight increase may be noted in total fecal porphyrin excretion.20 Plasma porphyrin level is typically increased, with a fluorescence peak at neutral pH near 619 nm. Homozygotes for the defect have mild anemia; it was severe in three cases.75 The erythrocyte protoporphyrin was found to be moderately increased when measured and is primarily zinc protoporphyrin.75, 78

FIGURE 26.3. Porphyria cutanea tarda in a 60-year-old man. Note denuded skin areas over the fingers, an erosion, blisters, and milia.

The serum iron concentration and, hence, transferrin saturation are commonly increased as is the serum ferritin level.60, 89 Values for liver function tests vary considerably from one case to another. Some patients have mild degrees of jaundice and slight to moderate elevations in serum transaminase levels. Liver biopsy specimens display characteristic reddish-pink fluorescence.89, 90, 130 On microscopic examination, siderosis, periportal inflammation, focal necrosis, fatty infiltration, and some evidence of fibrosis are common findings.89, 90, 130, 131 Often, fluorescent and birefringent needle inclusions are noted.131 Electron microscopic studies reveal needlelike lucent areas, which appear to be in lysosomes.132 Microscopic examinations of skin biopsy specimens show bullae formed by the separation of the epidermis from the dermis; periodic acid-Schiff-positive, diastase-resistant material around vessel walls in the upper dermis; and a sparse inflammatory cell infiltrate.89, 133 Immunofluorescence studies display deposition of immunoglobulin G (and less often immunoglobulin M) or complement around upper dermal vessels and at the dermal-epidermal junction.133

Toxic Porphyria Cutanea Tarda

In Turkey between 1956 and 1961, an epidemic of acquired porphyria affecting more than 3,000 people occurred as the result of exposure to flour contaminated with a seed-wheat fungicide, hexachlorobenzene.149, 150, 151 Members of both sexes were affected, and many of the subjects were children. Affected people developed hepatomegaly, hypertrichosis, hyperpigmentation, uroporphyrinuria, and a photosensitive dermatosis. Uroporphyrinuria and a variety of other signs and symptoms persisted in some individuals for more than 25 years,152, 153 and no effective therapy has been devised for this toxic porphyria. Studies in animals demonstrated that hexachlorobenzene, or one of its metabolites, is a potent inhibitor of hepatic UROD.154, 155 Iron magnifies this inhibitory effect,154 and clear genetic susceptibility to this type of acquired porphyria has been shown in mice.156

Other polyhalogenated aromatic hydrocarbons also have produced a toxic porphyria in humans. 2,3,7,8-Tetrachlorodibenzo-p-dioxin, a by-product in the manufacture of the herbicides 2,4-dichlorophenoxyacetic acid, 2,4,5-trichlorophenoxyacetic acid, and some polychlorinated biphenyls have proved porphyrinogenic for humans.157 Animal studies indicate that compounds
of this class inhibit hepatic UROD activity.158 As in the case of hexachlorobenzene-induced porphyria, iron magnifies the inhibition of UROD, and iron depletion minimizes the inhibitory effect of these agents.159 The parallel between the permissive effects of iron in these toxic porphyria models and the role of iron in the pathogenesis of both familial and sporadic PCT is striking.

Hepatocellular Carcinoma

An increased frequency of hepatocellular carcinomas is recognized in patients with PCT,160, 161, 162 and such tumors are associated with a long symptomatic period before the start of treatment as well as with the presence of cirrhosis or chronic active hepatitis.161 The body iron status is not known in these cases, but iron overload could also play a role. Thus, in the presence of hepatitis or cirrhosis, surveillance for hepatocellular carcinoma with regular hepatic imaging is indicated.

In contrast, documented evidence supports a PCT-like illness as a manifestation of porphyrin-producing hepatoma in an otherwise normal liver (paraneoplastic PCT) in several cases.163, 164, 165, 166 When the tumor could be surgically removed, the cutaneous symptoms and biochemical abnormalities remitted.163, 166

Animal Models of Porphyria Cutanea Tarda

The first genetically designed animal model of PCT and HEP was developed in zebrafish, designated yquem.167 Heterozygous and homozygous mutants have 67% and 36% of wild-type UROD activity, respectively. The enzyme deficiency was linked to a missense mutation in the UROD gene, predicting a Met38Arg substitution that involves a conserved amino acid across all species examined, including humans. The mutant phenotype could be rescued by transient and germline expression of the wild-type allele.

Transgenic mice with one disrupted UROD allele (UROD+/−) have half the wild-type hepatic UROD protein and enzyme activity but no accumulation of porphyrins.101 However, in response to iron loading, hepatic porphyrins accumulate, and UROD activity declines to 20%. When bred to HFE-/- mice, UROD+/-/HFE-/- animals developed a porphyric phenotype with further reduction of UROD activity to 14%, resembling the human condition.

HFE-/- mice that are fed ethanol also develop uroporphyria and reduced hepatic UROD activity, seemingly mediated by effects of ethanol on hepatic iron metabolism.168 Additional mouse models have been informative for understanding the susceptibility factors in PCT. CYP1A2 knockout mice are highly resistant to developing uroporphyria induced by chemicals.169 In mice that are genetically unable to synthesize ascorbic acid, a threshold of body iron was demonstrated at which a biphenyl-induced uroporphyrin accumulation occurs despite ascorbate repletion.170 Extensive study of such animal models of hepatic uroporphyria has provided a paradigm of the complex interactions among genetic factors, chemicals, drugs, and endogenous factors including iron homeostasis.171

PCT was identified in a flock of German Blackface sheep with clinical features similar to human PCT.172 It is associated with a point mutation in the ovine gene, predicting the amino acid substitution Leu131Pro, which is located in the active site cleft of the UROD protein.

Erythropoietic Protoporphyria

Erythropoietic protoporphyria (EPP), the third porphyric disorder with only cutaneous manifestations, is not uncommon, with an estimated prevalence of 1 per 75,000 to 200,000. Since the first clear description of the disease by Magnus et al. in 1961,173 hundreds of cases have been reported throughout the world.20, 174, 175, 176 Defects in the gene encoding ferrochelatase (FECH), the last enzyme in the heme biosynthetic pathway (Fig. 26.1), underlie most cases and result in accumulation of free protoporphyrin, mainly in erythroid tissue. The cutaneous manifestations are distinctive, but considerable individual variation is noted in clinical severity as well as in biochemical abnormalities. Three patterns of inheritance are recognized for the disorder (Table 26.2), and two variants occur.

Molecular Basis and Pathogenesis

Subnormal FECH activity is found in all tissues examined from patients with EPP, namely bone marrow,177 reticulocytes,178 liver,179 cultured skin fibroblasts,179 and lymphocytes.180 However, the protoporphyrin accumulates principally, if not entirely, in erythroid cells,181 whose contribution to heme production far exceeds that of all other tissues. Although the normal relative activity of FECH in erythroid tissue, in contrast to liver, is the second lowest among the enzymes of the heme biosynthetic pathway and is only three times higher than 5-aminolevulinate synthase,19 defective FECH in EPP does not appear to be rate limiting until late in erythroid development.177 Patients often exhibit a mild hypochromic-microcytic anemia as a consequence of the FECH deficiency that in turn appears to limit iron assimilation.177, 182, 183, 184 Protoporphyrin begins to accumulate in bone marrow erythroblasts just before the nucleus is lost,185 and reticulocytes and young erythrocytes probably are the major source of protoporphyrin in plasma on its rapid release from these cells.186, 187 Because of its low water solubility, the protoporphyrin is not excreted in urine but is taken up by the liver and excreted exclusively through the biliary tract.

Based on clinical features and FECH activity in many pedigrees, inheritance of this porphyria was first believed to most often follow an autosomal dominant pattern with incomplete penetrance.183, 188 In a few families, autosomal recessive inheritance was evident.174, 189 However, FECH activity in tissue lysates from patients with clinical EPP is always only 20% to 30% of normal,20, 181 not the 50% value expected for an autosomal-dominant enzyme deficiency. Neither levels of FECH activity nor levels of erythrocyte and fecal protoporphyrin consistently correlate with severity of symptoms. Moreover, most obligate carriers of the disorder have no symptoms. Their erythrocyte and fecal protoporphyrin levels are usually normal, and tissue FECH activity is approximately 50% of normal.20, 179, 181 To explain these findings, it was proposed that more than one allele is involved in the full expression of the disease.174, 190

Upon the cloning and characterization of the human FECH gene,191 which is located on chromosome 18 (18q21.3),192 at least 187 different molecular defects have been identified and are highly heterogeneous, commonly as missense/nonsense mutations, nucleotide deletions and insertions, and intronic point mutations near intron/exon splice sites in one FECH allele.10, 192a Most often, the defects lead to frameshifts or deletions, resulting in a truncated protein (“null allele”). The entire FECH gene was absent in one patient as a result of a chromosome 18q deletion.193 Two types of regulatory defects in the gene have also been reported, impairing transcription due to hypermethylation of the promoter region194 or a point mutation affecting transcription factor binding in the promoter.195 In about 4% of cases a mutation is present on each allele, defining recessive inheritance,10, 196 where the majority are missense mutations. Although these few double heterozygotes clarify the phenotypic differences between symptomatic and asymptomatic family members, all other manifesting cases usually coinherit a “low-expression” normal FECH allele trans to a deleterious mutant allele.197, 198 The low-expression allele is highly associated with a specific ancestral haplotype involving a single nucleotide polymorphism site in the FECH gene (IVS3-48T/C) that influences the use of a constitutive aberrant acceptor splice site in the gene.199 The aberrantly spliced RNA fraction is degraded, decreasing the steady-state level of FECH mRNA expressed by the allele by ≥20%. Hence the term
“pseudodominant EPP” came into use for patients with a FECH mutation trans to a hypomorphic IVS3-48C allele, to distinguish them from those with autosomal recessive EPP who are hetero-or homoallelic for FECH mutations. The frequency of the hypomorphic IVS3-48C allele differs widely between ethnic groups, ranging from 67.8% in Japanese to <1% in West Africans, and in general correlates with prevalence of clinical EPP among individuals with a mutant FECH allele in the different populations.176 When a low-expression allele is not found, a dominant-negative effect of FECH mutants may be operative.200 Because the enzyme activity is restricted to its homodimers,201 a mutant monomer may generate nonfunctional homodimers and impaired or unstable heterodimers, resulting in residual enzyme activity of <50% of normal.202, 203 Thus, the ultimate effect of a specific mutation in the FECH gene depends on how it affects the integrity of the protein and whether the nonmutated allele is expressed at a lower level to reduce FECH activity to below a threshold of around 30% of normal as found in clinically overt EPP.

More recently a third pattern of inheritance of EPP that is X-linked was described and is referred to as X-linked protoporphyria (XLPP).204 In the United Kingdom and in the US, the disorder accounts for 2% and 10% of unrelated EPP patients, respectively.192a, 205 FECH enzyme activity is normal but erythrocyte protoporphyrin concentrations are higher than in EPP due to FECH deficiency. Around 40% is zinc protoporphyrin, indicating that the supply of the iron substrate as well as ferrochelatase activity becomes rate limiting. To date, five distinct mutations have been identified in exon 11 of the erythroid-specific 5-aminolevulinate synthase gene (ALAS2) (Fig. 26.1), which resides on the X chromosome, and lead to predicted amino acid sequence disruption or deletion of up to 40 C-terminal amino acids of the enzyme.192a, 204, 205a The recombinant mutants have two- to three-fold increased activity,205b explaining the protoporphyrin overproduction. This gain-of-function in the enzyme contrasts with all other previously described ALAS2 mutations that decrease enzyme activity and cause X-linked sideroblastic anemia (Chapter 24). It also reflects a critical role of the C-terminal structure of ALAS2 for its activity and thus erythroid heme synthesis.

About 5% of EPP families in a large cohort were mutation-negative for FECH and ALAS2.205 However, in these the disease was strongly associated with inheritance of the IVS3-48C allele and decreased FECH activity, suggesting that most may have mutations in regions of the FECH gene that are not included in current strategies for mutation detection. In one case mild photosensitivity was ascribed to the IVS3-48CC genotype alone.205

The pathophysiology of EPP is mediated by the accumulated protoporphyrin. As it leaks out of erythroid cells into plasma, it gains entry into tissues. It is extracted solely by the liver and secreted unchanged into the bile. The liver is capable of clearing large amounts of protoporphyrin, but its secretion across the canalicular membrane and into the bile appears to be rate limiting.206 Yet despite microscopic evidence of hepatobiliary changes in many EPP patients, cholestasis leads to cirrhosis and hepatic failure in only a few (<5%).207 This complication tends to be an abrupt event, is unrelenting, and is not predictable from prior biochemical features or the clinical course of the patient. However, it occurs more commonly in compound heterozygotes for two mutations196 and in XLPP204 than in heterozygotes. Profoundly reduced ferrochelatase activity predisposes to liver failure.208 In an analysis of 112 EPP patients, all 18 who developed liver disease carried a null allele mutation, whereas none of 20 patients having missense mutations had liver disease as yet.209 Undetected molecular derangements194 or undefined hepatic factors,210 including a greater hepatic source of the excess protoporphyrin related to the defect, can be postulated. Excess alcohol intake211 and viral hepatitis,212 as well as hyperthyroidism,212a also accentuate the genetic disorder.

In the skin, the hydrophobic protoporphyrin transfers to endothelial cells of capillaries to produce the light-induced skin damage in patients with EPP.213, 214 Porphyrin-sensitized, oxygen-dependent histochemical reactions and the activation of complement eliciting an inflammatory response are involved in the pathogenesis.22, 23 The extensive double-bond structure of the protoporphyrin renders it particularly photoactive, resulting in the unique acute epidermal phototoxicity,173 in contrast to the porphyrins that accumulate in the other porphyrias with cutaneous photosensitivity. The activated porphyrin also stimulates fibroblast proliferation, accounting for a characteristic waxy thickening of the sun-exposed skin 23

Variant Erythropoietic Protoporphyria

The presence of an abnormal transcript of mitoferrin 1 (MFRN1) associated with reduced FECH activity was identified in a series of 7 patients with the EPP phenotype but without genetic defects in FECH in 6 of them, although no cause for the aberrant splicing of MFRN1 mRNA was found in the MFRN1 sequence.215 Four patients also had a previously reported C-terminal deletional ALAS2 mutation, and 4 cases had advanced liver disease. MFRN1, which transports iron into mitochondria, is highly expressed and regulated in erythroid cells.216 A molecular complex of FECH protein, MFRN1 and ABCB10 (a MFRN1 stabilizer) integrates mitochondrial iron import with heme synthesis.217 Thus, aberrant MFRN1 can contribute to the EPP phenotype in some patients, apparently by reducing FECH activity.

Late-onset Erythropoietic Protoporphyria Variant

EPP that is clinically indistinguishable from the inherited forms has developed in later years in the setting of a myelodysplastic syndrome218 including the sideroblastic anemia subtype (Chapter 24),19 or a myeloproliferative disorder.219 Two cases were associated with severe cholestatic liver disease.219, 260 In several instances acquired deletion of one FECH gene was demonstrated.218, 219, 220 An associated low-expression allele or a mutation in the other FECH allele, or altered erythroid heme biosynthesis in the dysplastic clone, likely contributes to the clinical expression in such cases. One case with late-onset XLPP has also been reported.220a

Clinical Description

EPP typically first manifests in childhood or early adolescence.20, 183, 221 The cutaneous features of photosensitivity are unlike those seen in patients with other porphyrias; bullae, scarring, sensitivity to trauma, hypertrichosis, and hyperpigmentation are extremely unusual.213 In some cases, the symptoms are only subjective.222,223 Exposure to the sun (or to UV light) for periods of a few minutes up to several hours induces diffuse, intensely unpleasant sensations of prickling, itching, or burning (photoparesthesias) followed by the development of erythema, urticaria, and edema.183, 213 Less commonly, solar urticaria, a confluent, hivelike rash, occurs within minutes of exposure and lasts approximately 30 minutes.173, 213 In general, the signs and symptoms involve only the uncovered areas of the body and subside in the course of hours or days without significant sequelae.

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Oct 21, 2016 | Posted by in HEMATOLOGY | Comments Off on Porphyrias
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