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.

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.

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

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.

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

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

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

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

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.

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

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.

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

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.