👪

Inheritance

2

Autosomal recessive HP:0000007

FECH-related EPP is inherited as an autosomal recessive trait with a common hypomorphic modifier allele: clinically overt disease typically requires a severe loss-of-function FECH mutation on one allele together with the low-expression IVS3-48C polymorphism on the other allele.

Autosomal recessive inheritance

Show evidence (3 references)

PMID:30704898 SUPPORT Human Clinical

"EPP is autosomal recessive in inheritance"

This review states the autosomal recessive inheritance of FECH-related EPP.

PMID:10068685 SUPPORT Human Clinical

"EPP may thus be considered as an inherited disorder that does not strictly follow recessive or dominant rules."

The two-hit model (severe FECH mutation in trans to a low-expression allele) explains why EPP does not follow simple Mendelian rules.

PMID:23016163 SUPPORT Human Clinical

"In about 96% of cases an affected individual inherits a loss-of-function FECH allele from one parent and a low-expression FECH allele from the other parent."

GeneReviews genetic-counseling section quantifies the two-allele transmission model underlying autosomal recessive EPP.

X-linked HP:0001417

X-linked protoporphyria is caused by gain-of-function ALAS2 mutations on the X chromosome.

X-linked inheritance

Show evidence (2 references)

PMID:18760763 SUPPORT Human Clinical

"These gain-of-function mutations cause a previously unrecognized form of porphyria, X-linked dominant protoporphyria"

Gain-of-function ALAS2 mutations define the X-linked form of protoporphyria.

PMID:23409301 SUPPORT Human Clinical

"Affected males transmit the pathogenic variant to all of their daughters and none of their sons. Women with an ALAS2 pathogenic variant have a 50% chance of transmitting the variant to each child."

GeneReviews genetic-counseling section describes the X-linked transmission pattern of XLP.

◆

Subtypes

2

Autosomal Erythropoietic Protoporphyria (FECH-related)

The classic and most common form (~90% of cases), caused by deficient ferrochelatase activity. Overt disease usually requires a loss-of-function FECH allele in trans to the common hypomorphic IVS3-48C low-expression allele, so that total ferrochelatase activity falls below the threshold (roughly 30% of normal) for protoporphyrin accumulation.

X-linked Protoporphyria (ALAS2-related)

A phenotypically similar form caused by gain-of-function mutations (typically C-terminal frameshifts) in the erythroid-specific 5-aminolevulinate synthase ALAS2. Increased ALAS2 activity overproduces the protoporphyrin precursor pool, so protoporphyrin IX accumulates despite normal ferrochelatase, with a characteristically higher zinc-protoporphyrin fraction than FECH-related EPP.

⚙

Pathophysiology

7

Ferrochelatase Enzymatic Deficiency

Ferrochelatase (FECH), the terminal enzyme of heme biosynthesis, catalyzes insertion of ferrous iron into protoporphyrin IX to form heme. In FECH-related EPP, one severe loss-of-function allele in trans to the low-expression IVS3-48C allele reduces total ferrochelatase activity below roughly 30% of normal, the threshold needed to keep pace with protoporphyrin production.

erythroid progenitor cell CL:0000038

heme biosynthesis GO:0006783 ↓ DECREASED

Downstream

Erythroid Protoporphyrin IX Accumulation Failure to consume protoporphyrin IX allows the substrate to accumulate.

Show evidence (3 references)

PMID:30704898 SUPPORT Human Clinical

"EPP results from the deficient activity of ferrochelatase, the final enzyme in the heme-biosynthetic pathway"

Establishes ferrochelatase deficiency as the primary lesion of EPP.

PMID:30704898 SUPPORT Human Clinical

"When FECH is deficient to <30% enzyme activity, there is an increased accumulation of PPIX"

Quantifies the ferrochelatase activity threshold below which protoporphyrin accumulates.

PMID:10068685 SUPPORT Human Clinical

"coinheritance of a FECH gene defect and a wild-type low-expressed allele is generally involved in the clinical expression of EPP"

Documents the two-allele requirement that drives ferrochelatase activity below the disease threshold.

ALAS2 Gain-of-Function Precursor Overproduction

In X-linked protoporphyria, gain-of-function C-terminal mutations in the erythroid-specific 5-aminolevulinate synthase ALAS2 increase the rate of the first, committed step of heme biosynthesis. The resulting excess flux through the pathway overwhelms normal ferrochelatase capacity, so protoporphyrin IX (with a relatively higher zinc-protoporphyrin fraction) still accumulates.

erythroid progenitor cell CL:0000038

porphyrin biosynthesis GO:0006779 ↑ INCREASED

Downstream

Erythroid Protoporphyrin IX Accumulation Precursor overproduction drives protoporphyrin IX beyond ferrochelatase capacity.

Show evidence (2 references)

PMID:18760763 SUPPORT In Vitro

"Prokaryotic expression studies show that both mutations markedly increase ALAS2 activity."

In vitro expression demonstrates the gain-of-function increase in ALAS2 enzymatic activity.

PMID:30704898 SUPPORT Human Clinical

"the rate of ALA formation is increased, and the insertion of iron into PPIX by FECH becomes rate-limiting for heme synthesis in erythroid tissues resulting in accumulation of protoporphyrin"

Increased precursor flux makes normal ferrochelatase rate-limiting, so protoporphyrin accumulates in XLP.

Erythroid Protoporphyrin IX Accumulation

Whether from deficient ferrochelatase or excess precursor supply, metal-free protoporphyrin IX accumulates. It is generated chiefly by bone-marrow reticulocytes and late erythroid precursors, then loaded into circulating erythrocytes and released into plasma, from which it distributes to skin and the hepatobiliary system.

reticulocyte CL:0000558 erythrocyte CL:0000232

Downstream

Cutaneous Photoexcitation of Protoporphyrin IX Protoporphyrin IX distributed to dermal blood absorbs light.

Hepatobiliary Protoporphyrin Loading Plasma protoporphyrin IX is taken up by hepatocytes for biliary excretion.

Show evidence (1 reference)

PMID:30704898 SUPPORT Human Clinical

"Protoporphyrin is released from the bone marrow into the circulating erythrocytes and plasma where it is taken up by the liver and vascular endothelium including the superficial skin vasculature."

Describes the erythroid origin and distribution of accumulated protoporphyrin to skin and liver.

Cutaneous Photoexcitation of Protoporphyrin IX

Protoporphyrin IX circulating in dermal blood vessels absorbs visible violet light near its 400-410 nm Soret band. The photoexcited porphyrin transfers energy to molecular oxygen, so that light exposure of the skin converts an otherwise inert accumulated pigment into a localized photochemical source.

dermal microvascular endothelial cell CL:0002139

response to light GO:0009416

Downstream

Phototoxic Reactive Oxygen Species Dermal Injury Photoexcited protoporphyrin IX transfers energy to oxygen, generating reactive oxygen species.

Show evidence (2 references)

PMID:26132941 SUPPORT Human Clinical

"When the skin is exposed to sun or visible light, the accumulated phototoxic protoporphyrin in superficial vessels is activated by blue light (400 to 410 nm), triggering singlet oxygen free-radical reactions"

Identifies the 400-410 nm blue-light activation of protoporphyrin as the initiating photochemical event.

PMID:30704898 SUPPORT Human Clinical

"The protoporphyrin molecules are photodynamic and absorb light radiation in visible blue-violet light in the Soret band"

Confirms the photodynamic light absorption of protoporphyrin in the Soret band.

Phototoxic Reactive Oxygen Species Dermal Injury

Energy transfer from photoexcited protoporphyrin IX to oxygen generates singlet oxygen and other reactive oxygen species in and around dermal capillaries. These oxidize endothelial membranes, activate complement, and release histamines and chemotactic factors, producing perivascular tissue and vessel damage that is experienced as immediate burning pain, and clinically as erythema and edema.

dermal microvascular endothelial cell CL:0002139 mast cell CL:0000097

reactive oxygen species production GO:0072593 ↑ INCREASED cellular response to oxidative stress GO:0034599

Show evidence (1 reference)

PMID:30704898 SUPPORT Human Clinical

"This energy is presumably released as fluorescence and by formation of singlet oxygen and other oxygen radicals that can produce tissue and vessel damage secondary to activation of the complement system."

Singlet oxygen and radicals cause the complement-associated vascular tissue damage underlying the phototoxic reaction.

Hepatobiliary Protoporphyrin Loading

Protoporphyrin IX is lipophilic and cannot be excreted by the kidney; its only elimination route is biliary. Hepatocytes take up the excess protoporphyrin IX from plasma and concentrate it for biliary excretion, and when biliary excretion cannot keep pace with the load the pigment progressively accumulates in the liver.

hepatocyte CL:0000182

Downstream

Cholestatic Protoporphyric Hepatopathy Precipitated protoporphyrin IX impairs bile flow and injures hepatocytes.

Show evidence (1 reference)

PMID:30704898 SUPPORT Human Clinical

"Progressive accumulation of protoporphyrin may occur in the liver when the biliary excretion does not keep pace with the load being presented to the liver."

Biliary-only excretion and hepatic uptake set up progressive hepatic protoporphyrin loading.

Cholestatic Protoporphyric Hepatopathy

Precipitated protoporphyrin IX is choleotoxic and cholestatic: it impairs bile flow, which in turn reduces the biliary excretion of protoporphyrin and drives further hepatic accumulation in a self-amplifying cycle. Progressive cholestatic injury causes hepatocellular damage and, in a minority of patients, cirrhosis and acute protoporphyric liver failure.

hepatocyte CL:0000182

Show evidence (2 references)

PMID:30704898 SUPPORT Human Clinical

"Accumulated hepatic protoporphyrin can precipitate in hepatocytes and bile canaliculi, causing hepatotoxicity, decreased bile formation and flow, and cholestatic liver failure in some patients"

Hepatic protoporphyrin precipitation causes cholestasis, hepatotoxicity, and liver failure.

PMID:30704898 SUPPORT Human Clinical

"When hepatocellular damage reaches a critical stage, protoporphyrin accumulation will rapidly accelerate due to marked impairment of biliary excretion"

Documents the self-amplifying cholestatic cycle in which liver injury further impairs protoporphyrin excretion.

⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

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Phenotypes

8

Cardiovascular 1

Photo-induced Erythema Erythema HP:0010783

Show evidence (1 reference)

PMID:23016163 SUPPORT Human Clinical

"may be accompanied by swelling and redness."

Redness of exposed skin is part of the acute phototoxic reaction.

Digestive 2

Protoporphyrin Gallstones OCCASIONAL Cholelithiasis HP:0001081

Show evidence (2 references)

PMID:26132941 SUPPORT Human Clinical

"The protoporphyrin in transit through the liver may precipitate, resulting in gallstones and cholestatic hepatitis in about 5% of cases"

Protoporphyrin precipitation in the biliary tract produces gallstones.

PMID:30704898 SUPPORT Human Clinical

"In one series, cholelithiasis were seen in 23.5% of patients"

A clinical series found cholelithiasis in about 23.5% of EPP patients, supporting an occasional frequency.

Protoporphyric Liver Failure Hepatic failure HP:0001399

Show evidence (2 references)

PMID:23016163 SUPPORT Human Clinical

"Up to 5% may develop more advanced liver disease which may be accompanied by motor neuropathy similar to that seen in the acute porphyrias."

A minority (up to ~5%) develop advanced liver disease with possible motor neuropathy.

PMID:30704898 SUPPORT Human Clinical

"Liver transplant is the treatment for end-stage liver disease secondary to protoporphyrin related liver damage"

End-stage protoporphyric liver disease is severe enough to require transplantation.

Integument 2

Acute Photosensitivity VERY_FREQUENT Cutaneous photosensitivity HP:0000992

Show evidence (2 references)

PMID:23016163 SUPPORT Human Clinical

"cutaneous photosensitivity (usually beginning in infancy or childhood) that results in tingling, burning, pain, and itching within 30 minutes after exposure to sun or ultraviolet light and may be accompanied by swelling and redness."

GeneReviews identifies acute cutaneous photosensitivity as the defining, near-universal feature of EPP.

PMID:26132941 SUPPORT Human Clinical

"Photosensitivity in patients with erythropoietic protoporphyria usually manifests in early childhood; it occurs 1 to 20 minutes after direct exposure to the sun."

Confirms early-childhood onset and rapid (minutes) reaction to sun exposure.

Chronic Photodamage Skin Changes Lichenification HP:0100725

Show evidence (1 reference)

PMID:23016163 SUPPORT Human Clinical

"Multiple episodes of acute photosensitivity may lead to chronic changes of sun-exposed skin (lichenification, leathery pseudovesicles, grooving around the lips) and loss of lunulae of the nails."

GeneReviews documents the characteristic chronic lichenified and scarred skin changes from repeated phototoxic injury.

Metabolism 1

Photo-induced Edema Edema HP:0000969

Show evidence (1 reference)

PMID:26132941 SUPPORT Human Clinical

"Patients have severe burning pain, typically on the hands and face, and this pain is often followed by swelling and redness."

Swelling of light-exposed skin follows the phototoxic burning pain.

Other 2

Cholestatic Liver Disease OCCASIONAL Cholestatic liver disease HP:0002611

Show evidence (1 reference)

PMID:23016163 SUPPORT Human Clinical

"Approximately 20%-30% of individuals with EPP have some degree of liver dysfunction, which is typically mild with slight elevations of the liver enzymes."

Quantifies the frequency of hepatic involvement in EPP at roughly 20-30%.

Mild Microcytic Anemia FREQUENT Microcytic anemia HP:0001935

Show evidence (2 references)

PMID:30704898 SUPPORT Human Clinical

"Mild anemia, typically microcytic anemia can be seen in EPP patients"

Documents mild microcytic anemia as a recognized hematologic feature of EPP.

PMID:30704898 SUPPORT Human Clinical

"Anemia is seen in about 47% of patients"

Anemia occurs in roughly 47% of EPP patients, supporting a frequent occurrence.

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Genetic Associations

2

FECH (Causative)

Gene: FECH hgnc:3647 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

PMID:30704898 SUPPORT Human Clinical

"Around 96% of patients with EPP have a loss of function FECH mutation in trans with a second low-expression pathogenic variant"

Defines the classic FECH two-allele genotype of EPP.

PMID:30704898 SUPPORT Human Clinical

"creates a cryptic upstream acceptor site in intron 3 that modulates the alternative splicing of the normal FECH mRNA, resulting in FECH activity <30% of normal"

Explains how the low-expression allele lowers ferrochelatase activity below the disease threshold.

ALAS2 (Causative)

Gene: ALAS2 hgnc:397 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

PMID:23409301 SUPPORT Human Clinical

"identification of a hemizygous pathogenic gain-of-function variant in ALAS2 on molecular genetic testing."

A hemizygous gain-of-function ALAS2 variant establishes the diagnosis of XLP.

PMID:18760763 SUPPORT In Vitro

"Prokaryotic expression studies show that both mutations markedly increase ALAS2 activity."

Functional studies confirm the ALAS2 mutations are gain-of-function.

💊

Medical Actions

6

Afamelanotide

Action: Pharmacotherapy NCIT:C15986

Agent: afamelanotide CHEBI:136034

Afamelanotide is a synthetic alpha-melanocyte-stimulating hormone (alpha-MSH) analog and melanocortin-1 receptor agonist that increases epidermal eumelanin, providing photoprotection and increasing pain-free sunlight tolerance in EPP.

Mechanism Target:

INHIBITS Cutaneous Photoexcitation of Protoporphyrin IX — Increased epidermal eumelanin absorbs and scatters incident light, reducing the visible light that reaches and photoexcites dermal protoporphyrin.

Target Phenotypes: Cutaneous photosensitivity HP:0000992

Show evidence (2 references)

PMID:26132941 SUPPORT Human Clinical

"the pain-free time in direct sunlight was 70% longer among patients who received afamelanotide than among patients who received placebo"

Randomized-trial evidence that afamelanotide increases pain-free sun exposure.

PMID:23016163 SUPPORT Human Clinical

"This medication increases pain-free sun exposure and has improved quality of life in those with EPP."

Afamelanotide improves pain-free sun exposure and quality of life.

Sunlight Avoidance and Photoprotection

Action: supportive care Ontology label: Supportive Care NCIT:C15747

Strict avoidance of sunlight and use of visible-light-blocking (reflectant) physical sunscreens and protective clothing remain foundational management.

Show evidence (1 reference)

PMID:23016163 SUPPORT Human Clinical

"Current management centers on prevention of the painful attacks by avoidance of sun/light"

Sun/light avoidance is the foundational management strategy.

Bile Acid Sequestrant Therapy

Action: Pharmacotherapy NCIT:C15986

Bile acid sequestrants such as cholestyramine (and ursodeoxycholic acid) are used to increase fecal excretion of protoporphyrin and interrupt its enterohepatic circulation in protoporphyric liver disease.

Mechanism Target:

INHIBITS Hepatobiliary Protoporphyrin Loading — Interrupting the enterohepatic circulation of protoporphyrin promotes its fecal excretion, lowering the hepatic protoporphyrin load.

Show evidence (1 reference)

PMID:30704898 SUPPORT Human Clinical

"Bile acid sequestrants such as cholestyramine and other drugs such as ursodeoxycholelic acid have been used to increase the excretion of protoporphyrin through the biliary system"

Bile acid sequestrants promote protoporphyrin excretion in hepatic disease.

Vitamin D Supplementation

Action: dietary intervention Ontology label: Dietary Intervention NCIT:C15447

Because strict sun avoidance predisposes to vitamin D deficiency, routine vitamin D supplementation is recommended in EPP and XLP.

Show evidence (1 reference)

PMID:30704898 SUPPORT Human Clinical

"EPP and XLP patients can develop vitamin D deficiency secondary to sun avoidance"

Sun avoidance causes vitamin D deficiency, the rationale for supplementation.

Liver Transplantation

Action: liver transplantation Ontology label: Liver Transplantation NCIT:C15271

Liver transplantation treats end-stage protoporphyric liver disease but is not curative, because the bone marrow remains the source of protoporphyrin; intraoperative light filters blocking wavelengths below 470 nm are used to prevent phototoxic tissue burns.

Show evidence (1 reference)

PMID:30704898 SUPPORT Human Clinical

"liver transplant is not curative as the primary source of protoporphyrin production is the bone marrow and liver transplant does not correct the underlying genetic defect"

Liver transplant treats hepatic failure but does not correct the erythroid source of protoporphyrin.

Hematopoietic Stem Cell Transplantation

Action: hematopoietic stem cell transplantation Ontology label: Hematopoietic Cell Transplantation NCIT:C15431

Hematopoietic (bone marrow) stem cell transplantation can correct the erythroid source of protoporphyrin overproduction and is potentially curative; sequential liver and bone marrow transplantation has cured protoporphyric liver disease.

Mechanism Target:

INHIBITS Erythroid Protoporphyrin IX Accumulation — Replacing the patient's erythron with donor hematopoietic cells removes the bone-marrow source of protoporphyrin overproduction.

Show evidence (1 reference)

PMID:30704898 SUPPORT Human Clinical

"Bone marrow transplant can be curative and sequential liver and bone marrow transplant has been successful in curing protoporphyric liver disease"

Bone marrow transplantation corrects the erythroid protoporphyrin source and can cure protoporphyric liver disease.

🔬

Biochemical Markers

2

Elevated Erythrocyte Protoporphyrin IX (INCREASED)

Context: Markedly elevated total erythrocyte protoporphyrin, predominantly metal-free protoporphyrin IX in FECH-related EPP, is the diagnostic biochemical hallmark.

Pathograph Readouts

Readout Of Erythroid Protoporphyrin IX Accumulation Positive Diagnostic

Total erythrocyte protoporphyrin is the direct laboratory readout of the accumulated erythroid protoporphyrin pool and confirms the diagnosis.

Show evidence (2 references)

PMID:23016163 SUPPORT Human Clinical

"The diagnosis of EPP is established by detection of markedly increased free erythrocyte protoporphyrin and/or by the identification of biallelic pathogenic variants in FECH on molecular genetic testing."

Markedly increased free erythrocyte protoporphyrin is the diagnostic marker.

PMID:30704898 SUPPORT Human Clinical

"significantly elevated total erythrocyte protoporphyrin with a predominance"

In EPP the elevated erythrocyte protoporphyrin is predominantly metal-free.

Zinc Protoporphyrin Fraction (INCREASED)

Context: The proportion of zinc-chelated protoporphyrin among total erythrocyte protoporphyrin is characteristically higher in X-linked protoporphyria than in FECH-related EPP, aiding differentiation.

Show evidence (1 reference)

PMID:18760763 SUPPORT Human Clinical

"characterized biochemically by a high proportion of zinc-protoporphyrin in erythrocytes"

XLP is distinguished by a high erythrocyte zinc-protoporphyrin fraction.

🔬

Clinical Trials

2

NCT05005975 PHASE_III ACTIVE_NOT_RECRUITING

Long-term extension study of oral dersimelagon (MT-7117), a non-peptide selective melanocortin-1 receptor agonist, in EPP and XLP.

Target Phenotypes: Cutaneous photosensitivity HP:0000992

Show evidence (1 reference)

clinicaltrials:NCT05005975 SUPPORT Human Clinical

"To evaluate the long-term safety and tolerability of oral dersimelagon."

A phase 3 extension study evaluating the oral MC1R agonist dersimelagon in protoporphyria.

NCT05308472 PHASE_II COMPLETED

AURORA: a randomized, double-blind, placebo-controlled study of oral bitopertin, a glycine transporter-1 (GlyT1) inhibitor that limits glycine supply to the first step of heme biosynthesis, aiming to reduce protoporphyrin IX at its source (a substrate-reduction, potentially disease-modifying strategy distinct from the melanocortin photoprotective agents).

Target Phenotypes: Cutaneous photosensitivity HP:0000992

Show evidence (1 reference)

clinicaltrials:NCT05308472 SUPPORT Human Clinical

"This is a Phase 2, multi-center, double-blind, placebo-controlled, parallel group study of bitopertin to evaluate the safety, tolerability, efficacy, and PPIX concentration change in participants with EPP."

A completed phase 2 randomized trial of the oral GlyT1 inhibitor bitopertin targeting protoporphyrin IX reduction in EPP.

{ }

Source YAML

click to show

name: Erythropoietic Protoporphyria
creation_date: "2026-07-21T00:00:00Z"
category: Mendelian
synonyms:
- EPP
- Erythrohepatic protoporphyria
- Protoporphyria
description: >-
  Erythropoietic protoporphyria (EPP) is an inherited disorder of heme
  biosynthesis caused by deficient activity of ferrochelatase (FECH), the
  terminal enzyme that inserts ferrous iron into protoporphyrin IX to form heme.
  Reduced ferrochelatase activity causes metal-free protoporphyrin IX to
  accumulate, chiefly in bone-marrow reticulocytes and circulating erythrocytes,
  and to distribute into plasma, skin, and the hepatobiliary system.
  Photoexcitation of protoporphyrin IX in dermal blood vessels by violet light
  generates reactive oxygen species that injure the vascular endothelium,
  producing the cardinal feature of acute, painful, non-blistering
  photosensitivity that begins in early childhood. Because protoporphyrin IX is
  lipophilic and excreted only in bile, it can also accumulate in hepatocytes
  and precipitate a cholestatic protoporphyric liver disease that, in a minority
  of patients, progresses to acute liver failure. Most classic EPP results from
  a loss-of-function FECH mutation in trans to the common hypomorphic
  low-expression allele IVS3-48C; a phenotypically identical X-linked form
  (X-linked protoporphyria) is caused by gain-of-function mutations in the
  erythroid-specific 5-aminolevulinate synthase ALAS2, which overproduce the
  protoporphyrin precursor pool despite normal ferrochelatase.
disease_term:
  preferred_term: erythropoietic protoporphyria
  term:
    id: MONDO:0001676
    label: erythropoietic protoporphyria
parents:
- Metabolic Disease
- Inborn Error of Metabolism
references:
- reference: PMID:23016163
  title: "Erythropoietic Protoporphyria, Autosomal Recessive."
  tags:
  - GeneReviews
- reference: PMID:23409301
  title: "X-Linked Protoporphyria."
  tags:
  - GeneReviews
has_subtypes:
- name: EPP
  display_name: Autosomal Erythropoietic Protoporphyria (FECH-related)
  description: >-
    The classic and most common form (~90% of cases), caused by deficient
    ferrochelatase activity. Overt disease usually requires a loss-of-function
    FECH allele in trans to the common hypomorphic IVS3-48C low-expression
    allele, so that total ferrochelatase activity falls below the threshold
    (roughly 30% of normal) for protoporphyrin accumulation.
- name: XLP
  display_name: X-linked Protoporphyria (ALAS2-related)
  description: >-
    A phenotypically similar form caused by gain-of-function mutations
    (typically C-terminal frameshifts) in the erythroid-specific
    5-aminolevulinate synthase ALAS2. Increased ALAS2 activity overproduces the
    protoporphyrin precursor pool, so protoporphyrin IX accumulates despite
    normal ferrochelatase, with a characteristically higher zinc-protoporphyrin
    fraction than FECH-related EPP.
inheritance:
- name: Autosomal recessive
  description: >-
    FECH-related EPP is inherited as an autosomal recessive trait with a common
    hypomorphic modifier allele: clinically overt disease typically requires a
    severe loss-of-function FECH mutation on one allele together with the
    low-expression IVS3-48C polymorphism on the other allele.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "EPP is autosomal recessive in inheritance"
    explanation: >-
      This review states the autosomal recessive inheritance of FECH-related EPP.
  - reference: PMID:10068685
    reference_title: "Inheritance in erythropoietic protoporphyria: a common wild-type ferrochelatase allelic variant with low expression accounts for clinical manifestation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "EPP may thus be considered as an inherited disorder that does not strictly follow recessive or dominant rules."
    explanation: >-
      The two-hit model (severe FECH mutation in trans to a low-expression
      allele) explains why EPP does not follow simple Mendelian rules.
  - reference: PMID:23016163
    reference_title: "Erythropoietic Protoporphyria, Autosomal Recessive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In about  96% of cases an affected individual inherits a loss-of-function FECH allele from  one parent and a low-expression FECH allele from the other parent."
    explanation: >-
      GeneReviews genetic-counseling section quantifies the two-allele
      transmission model underlying autosomal recessive EPP.
- name: X-linked
  description: >-
    X-linked protoporphyria is caused by gain-of-function ALAS2 mutations on the
    X chromosome.
  inheritance_term:
    preferred_term: X-linked inheritance
    term:
      id: HP:0001417
      label: X-linked inheritance
  evidence:
  - reference: PMID:18760763
    reference_title: "C-terminal deletions in the ALAS2 gene lead to gain of function and cause X-linked dominant protoporphyria without anemia or iron overload."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These gain-of-function mutations cause a previously unrecognized form of porphyria, X-linked dominant protoporphyria"
    explanation: >-
      Gain-of-function ALAS2 mutations define the X-linked form of protoporphyria.
  - reference: PMID:23409301
    reference_title: "X-Linked Protoporphyria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Affected males transmit the pathogenic variant to all of their daughters and  none of their sons. Women with an ALAS2 pathogenic variant have a 50% chance of  transmitting the variant to each child."
    explanation: >-
      GeneReviews genetic-counseling section describes the X-linked transmission
      pattern of XLP.
pathophysiology:
- name: Ferrochelatase Enzymatic Deficiency
  description: >-
    Ferrochelatase (FECH), the terminal enzyme of heme biosynthesis, catalyzes
    insertion of ferrous iron into protoporphyrin IX to form heme. In FECH-related
    EPP, one severe loss-of-function allele in trans to the low-expression
    IVS3-48C allele reduces total ferrochelatase activity below roughly 30% of
    normal, the threshold needed to keep pace with protoporphyrin production.
  cell_types:
  - preferred_term: erythroid progenitor cell
    term:
      id: CL:0000038
      label: erythroid progenitor cell
  biological_processes:
  - preferred_term: heme biosynthesis
    term:
      id: GO:0006783
      label: heme biosynthetic process
    modifier: DECREASED
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "EPP results from the deficient activity of ferrochelatase, the final enzyme in the heme-biosynthetic pathway"
    explanation: >-
      Establishes ferrochelatase deficiency as the primary lesion of EPP.
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "When FECH is deficient to <30% enzyme activity, there is an increased accumulation of PPIX"
    explanation: >-
      Quantifies the ferrochelatase activity threshold below which protoporphyrin
      accumulates.
  - reference: PMID:10068685
    reference_title: "Inheritance in erythropoietic protoporphyria: a common wild-type ferrochelatase allelic variant with low expression accounts for clinical manifestation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "coinheritance of a FECH gene defect and a wild-type low-expressed allele is generally involved in the clinical expression of EPP"
    explanation: >-
      Documents the two-allele requirement that drives ferrochelatase activity
      below the disease threshold.
  downstream:
  - target: Erythroid Protoporphyrin IX Accumulation
    causal_link_type: DIRECT
    description: >-
      Failure to consume protoporphyrin IX allows the substrate to accumulate.
- name: ALAS2 Gain-of-Function Precursor Overproduction
  description: >-
    In X-linked protoporphyria, gain-of-function C-terminal mutations in the
    erythroid-specific 5-aminolevulinate synthase ALAS2 increase the rate of the
    first, committed step of heme biosynthesis. The resulting excess flux through
    the pathway overwhelms normal ferrochelatase capacity, so protoporphyrin IX
    (with a relatively higher zinc-protoporphyrin fraction) still accumulates.
  cell_types:
  - preferred_term: erythroid progenitor cell
    term:
      id: CL:0000038
      label: erythroid progenitor cell
  biological_processes:
  - preferred_term: porphyrin biosynthesis
    term:
      id: GO:0006779
      label: porphyrin-containing compound biosynthetic process
    modifier: INCREASED
  evidence:
  - reference: PMID:18760763
    reference_title: "C-terminal deletions in the ALAS2 gene lead to gain of function and cause X-linked dominant protoporphyria without anemia or iron overload."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Prokaryotic expression studies show that both mutations markedly increase ALAS2 activity."
    explanation: >-
      In vitro expression demonstrates the gain-of-function increase in ALAS2
      enzymatic activity.
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the rate of ALA formation is increased, and the insertion of iron into PPIX by FECH becomes rate-limiting for heme synthesis in erythroid tissues resulting in accumulation of protoporphyrin"
    explanation: >-
      Increased precursor flux makes normal ferrochelatase rate-limiting, so
      protoporphyrin accumulates in XLP.
  downstream:
  - target: Erythroid Protoporphyrin IX Accumulation
    causal_link_type: DIRECT
    description: >-
      Precursor overproduction drives protoporphyrin IX beyond ferrochelatase
      capacity.
- name: Erythroid Protoporphyrin IX Accumulation
  description: >-
    Whether from deficient ferrochelatase or excess precursor supply, metal-free
    protoporphyrin IX accumulates. It is generated chiefly by bone-marrow
    reticulocytes and late erythroid precursors, then loaded into circulating
    erythrocytes and released into plasma, from which it distributes to skin and
    the hepatobiliary system.
  cell_types:
  - preferred_term: reticulocyte
    term:
      id: CL:0000558
      label: reticulocyte
  - preferred_term: erythrocyte
    term:
      id: CL:0000232
      label: erythrocyte
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Protoporphyrin is released from the bone marrow into the circulating erythrocytes and plasma where it is taken up by the liver and vascular endothelium including the superficial skin vasculature."
    explanation: >-
      Describes the erythroid origin and distribution of accumulated
      protoporphyrin to skin and liver.
  downstream:
  - target: Cutaneous Photoexcitation of Protoporphyrin IX
    causal_link_type: DIRECT
    description: >-
      Protoporphyrin IX distributed to dermal blood absorbs light.
  - target: Hepatobiliary Protoporphyrin Loading
    causal_link_type: DIRECT
    description: >-
      Plasma protoporphyrin IX is taken up by hepatocytes for biliary excretion.
- name: Cutaneous Photoexcitation of Protoporphyrin IX
  description: >-
    Protoporphyrin IX circulating in dermal blood vessels absorbs visible violet
    light near its 400-410 nm Soret band. The photoexcited porphyrin transfers
    energy to molecular oxygen, so that light exposure of the skin converts an
    otherwise inert accumulated pigment into a localized photochemical source.
  cell_types:
  - preferred_term: dermal microvascular endothelial cell
    term:
      id: CL:0002139
      label: endothelial cell of vascular tree
  biological_processes:
  - preferred_term: response to light
    term:
      id: GO:0009416
      label: response to light stimulus
  evidence:
  - reference: PMID:26132941
    reference_title: "Afamelanotide for Erythropoietic Protoporphyria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "When the skin is exposed to sun or visible light, the accumulated phototoxic protoporphyrin in superficial vessels is activated by blue light (400 to 410 nm), triggering singlet oxygen free-radical reactions"
    explanation: >-
      Identifies the 400-410 nm blue-light activation of protoporphyrin as the
      initiating photochemical event.
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The protoporphyrin molecules are photodynamic and absorb light radiation in visible blue-violet light in the Soret band"
    explanation: >-
      Confirms the photodynamic light absorption of protoporphyrin in the Soret
      band.
  downstream:
  - target: Phototoxic Reactive Oxygen Species Dermal Injury
    causal_link_type: DIRECT
    description: >-
      Photoexcited protoporphyrin IX transfers energy to oxygen, generating
      reactive oxygen species.
- name: Phototoxic Reactive Oxygen Species Dermal Injury
  description: >-
    Energy transfer from photoexcited protoporphyrin IX to oxygen generates
    singlet oxygen and other reactive oxygen species in and around dermal
    capillaries. These oxidize endothelial membranes, activate complement, and
    release histamines and chemotactic factors, producing perivascular tissue and
    vessel damage that is experienced as immediate burning pain, and clinically as
    erythema and edema.
  cell_types:
  - preferred_term: dermal microvascular endothelial cell
    term:
      id: CL:0002139
      label: endothelial cell of vascular tree
  - preferred_term: mast cell
    term:
      id: CL:0000097
      label: mast cell
  biological_processes:
  - preferred_term: reactive oxygen species production
    term:
      id: GO:0072593
      label: reactive oxygen species metabolic process
    modifier: INCREASED
  - preferred_term: cellular response to oxidative stress
    term:
      id: GO:0034599
      label: cellular response to oxidative stress
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This energy is presumably released as fluorescence and by formation of singlet oxygen and other oxygen radicals that can produce tissue and vessel damage secondary to activation of the complement system."
    explanation: >-
      Singlet oxygen and radicals cause the complement-associated vascular tissue
      damage underlying the phototoxic reaction.
- name: Hepatobiliary Protoporphyrin Loading
  description: >-
    Protoporphyrin IX is lipophilic and cannot be excreted by the kidney; its
    only elimination route is biliary. Hepatocytes take up the excess
    protoporphyrin IX from plasma and concentrate it for biliary excretion, and
    when biliary excretion cannot keep pace with the load the pigment
    progressively accumulates in the liver.
  cell_types:
  - preferred_term: hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Progressive accumulation of protoporphyrin may occur in the liver when the biliary excretion does not keep pace with the load being presented to the liver."
    explanation: >-
      Biliary-only excretion and hepatic uptake set up progressive hepatic
      protoporphyrin loading.
  downstream:
  - target: Cholestatic Protoporphyric Hepatopathy
    causal_link_type: DIRECT
    description: >-
      Precipitated protoporphyrin IX impairs bile flow and injures hepatocytes.
- name: Cholestatic Protoporphyric Hepatopathy
  description: >-
    Precipitated protoporphyrin IX is choleotoxic and cholestatic: it impairs
    bile flow, which in turn reduces the biliary excretion of protoporphyrin and
    drives further hepatic accumulation in a self-amplifying cycle. Progressive
    cholestatic injury causes hepatocellular damage and, in a minority of
    patients, cirrhosis and acute protoporphyric liver failure.
  cell_types:
  - preferred_term: hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Accumulated hepatic protoporphyrin can precipitate in hepatocytes and bile canaliculi, causing hepatotoxicity, decreased bile formation and flow, and cholestatic liver failure in some patients"
    explanation: >-
      Hepatic protoporphyrin precipitation causes cholestasis, hepatotoxicity,
      and liver failure.
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "When hepatocellular damage reaches a critical stage, protoporphyrin accumulation will rapidly accelerate due to marked impairment of biliary excretion"
    explanation: >-
      Documents the self-amplifying cholestatic cycle in which liver injury
      further impairs protoporphyrin excretion.
phenotypes:
- category: Cutaneous
  name: Acute Photosensitivity
  description: >-
    Immediate, painful, non-blistering photosensitivity is the cardinal feature,
    beginning in infancy or childhood. Within minutes of sun exposure patients
    feel tingling, burning, pain, and itching of exposed skin, followed by
    erythema and edema.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Cutaneous photosensitivity
    term:
      id: HP:0000992
      label: Cutaneous photosensitivity
  evidence:
  - reference: PMID:23016163
    reference_title: "Erythropoietic Protoporphyria, Autosomal Recessive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "cutaneous photosensitivity (usually beginning in infancy or childhood) that results in tingling, burning, pain, and itching within 30 minutes after exposure to sun or ultraviolet light and may be accompanied by swelling and redness."
    explanation: >-
      GeneReviews identifies acute cutaneous photosensitivity as the defining,
      near-universal feature of EPP.
  - reference: PMID:26132941
    reference_title: "Afamelanotide for Erythropoietic Protoporphyria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Photosensitivity in patients with erythropoietic protoporphyria usually manifests in early childhood; it occurs 1 to 20 minutes after direct exposure to the sun."
    explanation: >-
      Confirms early-childhood onset and rapid (minutes) reaction to sun exposure.
- category: Cutaneous
  name: Photo-induced Edema
  description: >-
    Painful swelling of light-exposed skin, especially of the face and dorsal
    hands, develops after sun exposure during acute phototoxic reactions.
  phenotype_term:
    preferred_term: Edema
    term:
      id: HP:0000969
      label: Edema
  evidence:
  - reference: PMID:26132941
    reference_title: "Afamelanotide for Erythropoietic Protoporphyria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients have severe burning pain, typically on the hands and face, and this pain is often followed by swelling and redness."
    explanation: >-
      Swelling of light-exposed skin follows the phototoxic burning pain.
- category: Cutaneous
  name: Photo-induced Erythema
  description: >-
    Redness of sun-exposed skin accompanies acute phototoxic reactions.
  phenotype_term:
    preferred_term: Erythema
    term:
      id: HP:0010783
      label: Erythema
  evidence:
  - reference: PMID:23016163
    reference_title: "Erythropoietic Protoporphyria, Autosomal Recessive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "may be accompanied by swelling and redness."
    explanation: >-
      Redness of exposed skin is part of the acute phototoxic reaction.
- category: Cutaneous
  name: Chronic Photodamage Skin Changes
  description: >-
    With cumulative exposure, repeated phototoxic reactions produce chronic
    changes of sun-exposed skin, including a waxy thickening/lichenification over
    the knuckles and dorsal hands, leathery pseudovesicles, and shallow linear or
    pitted scarring around the lips and nose, along with loss of the nail lunulae.
  phenotype_term:
    preferred_term: Lichenification
    term:
      id: HP:0100725
      label: Lichenification
  evidence:
  - reference: PMID:23016163
    reference_title: "Erythropoietic Protoporphyria, Autosomal Recessive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Multiple episodes of acute photosensitivity may lead to chronic changes of sun-exposed skin (lichenification, leathery pseudovesicles, grooving around the lips) and loss of lunulae of the nails."
    explanation: >-
      GeneReviews documents the characteristic chronic lichenified and scarred
      skin changes from repeated phototoxic injury.
- category: Hepatobiliary
  name: Protoporphyrin Gallstones
  description: >-
    Protoporphyrin-rich pigment gallstones may cause biliary obstruction
    requiring cholecystectomy.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Cholelithiasis
    term:
      id: HP:0001081
      label: Cholelithiasis
  evidence:
  - reference: PMID:26132941
    reference_title: "Afamelanotide for Erythropoietic Protoporphyria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The protoporphyrin in transit through the liver may precipitate, resulting in gallstones and cholestatic hepatitis in about 5% of cases"
    explanation: >-
      Protoporphyrin precipitation in the biliary tract produces gallstones.
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In one series, cholelithiasis were seen in 23.5% of patients"
    explanation: >-
      A clinical series found cholelithiasis in about 23.5% of EPP patients,
      supporting an occasional frequency.
- category: Hepatobiliary
  name: Cholestatic Liver Disease
  description: >-
    Approximately one fifth to one third of individuals with EPP have some degree
    of protoporphyric liver dysfunction, typically mild with slight elevations of
    liver enzymes, reflecting hepatic protoporphyrin loading and cholestasis.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Cholestatic liver disease
    term:
      id: HP:0002611
      label: Cholestatic liver disease
  evidence:
  - reference: PMID:23016163
    reference_title: "Erythropoietic Protoporphyria, Autosomal Recessive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Approximately 20%-30% of individuals with EPP have some degree of liver dysfunction, which is typically mild with slight elevations of the liver enzymes."
    explanation: >-
      Quantifies the frequency of hepatic involvement in EPP at roughly 20-30%.
- category: Hepatobiliary
  name: Protoporphyric Liver Failure
  description: >-
    In a small proportion of patients protoporphyric hepatopathy progresses to
    advanced, cholestatic liver failure that may require transplantation and can
    be accompanied by a motor neuropathy.
  phenotype_term:
    preferred_term: Hepatic failure
    term:
      id: HP:0001399
      label: Hepatic failure
  evidence:
  - reference: PMID:23016163
    reference_title: "Erythropoietic Protoporphyria, Autosomal Recessive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Up to 5% may develop more advanced liver disease which may be accompanied by motor neuropathy similar to that seen in the acute porphyrias."
    explanation: >-
      A minority (up to ~5%) develop advanced liver disease with possible motor
      neuropathy.
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Liver transplant is the treatment for end-stage liver disease secondary to protoporphyrin related liver damage"
    explanation: >-
      End-stage protoporphyric liver disease is severe enough to require
      transplantation.
- category: Hematologic
  name: Mild Microcytic Anemia
  description: >-
    A mild, typically microcytic anemia with low iron and ferritin is commonly
    present in EPP; the mechanism of the iron disturbance remains unclear.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Microcytic anemia
    term:
      id: HP:0001935
      label: Microcytic anemia
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mild anemia, typically microcytic anemia can be seen in EPP patients"
    explanation: >-
      Documents mild microcytic anemia as a recognized hematologic feature of EPP.
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Anemia is seen in about  47% of  patients"
    explanation: >-
      Anemia occurs in roughly 47% of EPP patients, supporting a frequent
      occurrence.
biochemical:
- name: Elevated Erythrocyte Protoporphyrin IX
  presence: INCREASED
  context: >-
    Markedly elevated total erythrocyte protoporphyrin, predominantly metal-free
    protoporphyrin IX in FECH-related EPP, is the diagnostic biochemical hallmark.
  biomarker_term:
    preferred_term: protoporphyrin IX
    term:
      id: CHEBI:15430
      label: protoporphyrin
  readouts:
  - target: Erythroid Protoporphyrin IX Accumulation
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: >-
      Total erythrocyte protoporphyrin is the direct laboratory readout of the
      accumulated erythroid protoporphyrin pool and confirms the diagnosis.
  evidence:
  - reference: PMID:23016163
    reference_title: "Erythropoietic Protoporphyria, Autosomal Recessive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The diagnosis of EPP is established by detection of markedly increased free erythrocyte protoporphyrin and/or by the identification of biallelic pathogenic variants in FECH on molecular genetic testing."
    explanation: >-
      Markedly increased free erythrocyte protoporphyrin is the diagnostic marker.
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "significantly elevated total erythrocyte protoporphyrin with a predominance"
    explanation: >-
      In EPP the elevated erythrocyte protoporphyrin is predominantly metal-free.
- name: Zinc Protoporphyrin Fraction
  presence: INCREASED
  context: >-
    The proportion of zinc-chelated protoporphyrin among total erythrocyte
    protoporphyrin is characteristically higher in X-linked protoporphyria than
    in FECH-related EPP, aiding differentiation.
  biomarker_term:
    preferred_term: zinc protoporphyrin
    term:
      id: CHEBI:28783
      label: zinc protoporphyrin
  evidence:
  - reference: PMID:18760763
    reference_title: "C-terminal deletions in the ALAS2 gene lead to gain of function and cause X-linked dominant protoporphyria without anemia or iron overload."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "characterized biochemically by a high proportion of zinc-protoporphyrin in erythrocytes"
    explanation: >-
      XLP is distinguished by a high erythrocyte zinc-protoporphyrin fraction.
genetic:
- name: FECH
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  gene_term:
    preferred_term: FECH
    term:
      id: hgnc:3647
      label: FECH
  notes: >-
    Loss-of-function mutations in FECH reduce ferrochelatase activity. Most
    patients carry one severe FECH mutation in trans to the common hypomorphic
    IVS3-48C low-expression allele.
  subtype: EPP
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Around 96% of patients with EPP have a loss of function FECH mutation in trans with a second low-expression pathogenic variant"
    explanation: >-
      Defines the classic FECH two-allele genotype of EPP.
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "creates a cryptic upstream acceptor site in intron 3 that modulates the alternative splicing of the normal FECH mRNA, resulting in FECH activity <30% of normal"
    explanation: >-
      Explains how the low-expression allele lowers ferrochelatase activity below
      the disease threshold.
- name: ALAS2
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  gene_term:
    preferred_term: ALAS2
    term:
      id: hgnc:397
      label: ALAS2
  notes: >-
    Gain-of-function C-terminal mutations in the erythroid-specific
    5-aminolevulinate synthase ALAS2 cause X-linked protoporphyria by increasing
    precursor flux through heme biosynthesis.
  subtype: XLP
  evidence:
  - reference: PMID:23409301
    reference_title: "X-Linked Protoporphyria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "identification of a hemizygous pathogenic gain-of-function variant in ALAS2 on molecular genetic testing."
    explanation: >-
      A hemizygous gain-of-function ALAS2 variant establishes the diagnosis of XLP.
  - reference: PMID:18760763
    reference_title: "C-terminal deletions in the ALAS2 gene lead to gain of function and cause X-linked dominant protoporphyria without anemia or iron overload."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Prokaryotic expression studies show that both mutations markedly increase ALAS2 activity."
    explanation: >-
      Functional studies confirm the ALAS2 mutations are gain-of-function.
prevalence:
- population: Europe
  measure_type: POINT_PREVALENCE
  prevalence_class: BAND_1_9_PER_100000
  rate_per_100000: 1.0
  rate_low: 0.5
  rate_high: 1.3
  notes: >-
    Commonly cited European prevalence of roughly 1:75,000 (Netherlands) to
    1:200,000 (United Kingdom); recent genetic data suggest underdiagnosis.
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The prevalence estimates of EPP range from 1:75,000 in the Netherlands to 1:200,000 in the United Kingdom"
    explanation: >-
      Provides the commonly cited European prevalence range for EPP.
treatments:
- name: Afamelanotide
  description: >-
    Afamelanotide is a synthetic alpha-melanocyte-stimulating hormone
    (alpha-MSH) analog and melanocortin-1 receptor agonist that increases
    epidermal eumelanin, providing photoprotection and increasing pain-free
    sunlight tolerance in EPP.
  therapeutic_modality: PEPTIDE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: afamelanotide
      term:
        id: CHEBI:136034
        label: afamelanotide
  target_mechanisms:
  - target: Cutaneous Photoexcitation of Protoporphyrin IX
    treatment_effect: INHIBITS
    description: >-
      Increased epidermal eumelanin absorbs and scatters incident light, reducing
      the visible light that reaches and photoexcites dermal protoporphyrin.
  target_phenotypes:
  - preferred_term: Cutaneous photosensitivity
    term:
      id: HP:0000992
      label: Cutaneous photosensitivity
  evidence:
  - reference: PMID:26132941
    reference_title: "Afamelanotide for Erythropoietic Protoporphyria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the pain-free time in direct sunlight was 70% longer among patients who received afamelanotide than among patients who received placebo"
    explanation: >-
      Randomized-trial evidence that afamelanotide increases pain-free sun
      exposure.
  - reference: PMID:23016163
    reference_title: "Erythropoietic Protoporphyria, Autosomal Recessive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This medication increases pain-free sun exposure and has improved quality of life in those with EPP."
    explanation: >-
      Afamelanotide improves pain-free sun exposure and quality of life.
- name: Sunlight Avoidance and Photoprotection
  description: >-
    Strict avoidance of sunlight and use of visible-light-blocking (reflectant)
    physical sunscreens and protective clothing remain foundational management.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:23016163
    reference_title: "Erythropoietic Protoporphyria, Autosomal Recessive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Current management centers on prevention of the painful attacks by avoidance of sun/light"
    explanation: >-
      Sun/light avoidance is the foundational management strategy.
- name: Bile Acid Sequestrant Therapy
  description: >-
    Bile acid sequestrants such as cholestyramine (and ursodeoxycholic acid) are
    used to increase fecal excretion of protoporphyrin and interrupt its
    enterohepatic circulation in protoporphyric liver disease.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  target_mechanisms:
  - target: Hepatobiliary Protoporphyrin Loading
    treatment_effect: INHIBITS
    description: >-
      Interrupting the enterohepatic circulation of protoporphyrin promotes its
      fecal excretion, lowering the hepatic protoporphyrin load.
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bile acid sequestrants such as cholestyramine and other drugs such as ursodeoxycholelic acid have been used to increase the excretion of protoporphyrin through the biliary system"
    explanation: >-
      Bile acid sequestrants promote protoporphyrin excretion in hepatic disease.
- name: Vitamin D Supplementation
  description: >-
    Because strict sun avoidance predisposes to vitamin D deficiency, routine
    vitamin D supplementation is recommended in EPP and XLP.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: dietary intervention
    term:
      id: NCIT:C15447
      label: Dietary Intervention
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "EPP and XLP patients can develop vitamin D deficiency secondary to sun avoidance"
    explanation: >-
      Sun avoidance causes vitamin D deficiency, the rationale for supplementation.
- name: Liver Transplantation
  description: >-
    Liver transplantation treats end-stage protoporphyric liver disease but is
    not curative, because the bone marrow remains the source of protoporphyrin;
    intraoperative light filters blocking wavelengths below 470 nm are used to
    prevent phototoxic tissue burns.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: liver transplantation
    term:
      id: NCIT:C15271
      label: Liver Transplantation
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "liver transplant is not curative as the primary source of protoporphyrin production is the bone marrow and liver transplant does not correct the underlying genetic defect"
    explanation: >-
      Liver transplant treats hepatic failure but does not correct the erythroid
      source of protoporphyrin.
- name: Hematopoietic Stem Cell Transplantation
  description: >-
    Hematopoietic (bone marrow) stem cell transplantation can correct the
    erythroid source of protoporphyrin overproduction and is potentially
    curative; sequential liver and bone marrow transplantation has cured
    protoporphyric liver disease.
  therapeutic_modality: CELL_THERAPY
  treatment_term:
    preferred_term: hematopoietic stem cell transplantation
    term:
      id: NCIT:C15431
      label: Hematopoietic Cell Transplantation
  target_mechanisms:
  - target: Erythroid Protoporphyrin IX Accumulation
    treatment_effect: INHIBITS
    description: >-
      Replacing the patient's erythron with donor hematopoietic cells removes the
      bone-marrow source of protoporphyrin overproduction.
  evidence:
  - reference: PMID:30704898
    reference_title: "Erythropoietic Protoporphyria and X-Linked Protoporphyria: pathophysiology, genetics, clinical manifestations, and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bone marrow transplant can be curative and sequential liver and bone marrow transplant has been successful in curing protoporphyric liver disease"
    explanation: >-
      Bone marrow transplantation corrects the erythroid protoporphyrin source
      and can cure protoporphyric liver disease.
clinical_trials:
- name: NCT05005975
  phase: PHASE_III
  status: ACTIVE_NOT_RECRUITING
  description: >-
    Long-term extension study of oral dersimelagon (MT-7117), a non-peptide
    selective melanocortin-1 receptor agonist, in EPP and XLP.
  target_phenotypes:
  - preferred_term: Cutaneous photosensitivity
    term:
      id: HP:0000992
      label: Cutaneous photosensitivity
  evidence:
  - reference: clinicaltrials:NCT05005975
    reference_title: "A Phase 3, Multicenter, Open-label, Long-term, Extension Study to Evaluate Safety and Tolerability of Oral Dersimelagon (MT-7117) in Subjects With Erythropoietic Protoporphyria (EPP) or X-Linked Protoporphyria (XLP)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "To evaluate the long-term safety and tolerability of oral dersimelagon."
    explanation: >-
      A phase 3 extension study evaluating the oral MC1R agonist dersimelagon in
      protoporphyria.
- name: NCT05308472
  phase: PHASE_II
  status: COMPLETED
  description: >-
    AURORA: a randomized, double-blind, placebo-controlled study of oral
    bitopertin, a glycine transporter-1 (GlyT1) inhibitor that limits glycine
    supply to the first step of heme biosynthesis, aiming to reduce protoporphyrin
    IX at its source (a substrate-reduction, potentially disease-modifying
    strategy distinct from the melanocortin photoprotective agents).
  target_phenotypes:
  - preferred_term: Cutaneous photosensitivity
    term:
      id: HP:0000992
      label: Cutaneous photosensitivity
  evidence:
  - reference: clinicaltrials:NCT05308472
    reference_title: "(AURORA) A Randomized, Double-blind, Placebo-Controlled Study of Bitopertin to Evaluate the Safety, Tolerability, Efficacy, and Protoporphyrin IX (PPIX) Concentrations in Participants With Erythropoietic Protoporphyria (EPP)"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This is a Phase 2, multi-center, double-blind, placebo-controlled, parallel group study of bitopertin to evaluate the safety, tolerability, efficacy, and PPIX concentration change in participants with EPP."
    explanation: >-
      A completed phase 2 randomized trial of the oral GlyT1 inhibitor bitopertin
      targeting protoporphyrin IX reduction in EPP.
datasets: []

📚

References & Deep Research

References

2

PMID:23016163

Erythropoietic Protoporphyria, Autosomal Recessive.

No top-level findings curated for this source.

PMID:23409301

X-Linked Protoporphyria.

No top-level findings curated for this source.

Deep Research

1

Claude Code ▸

Erythropoietic Protoporphyria (EPP) — Comprehensive Research Report

claude-haiku-4-5-20251001, claude-opus-4-8 33 citations 2026-07-21T17:09:29.584173

Erythropoietic Protoporphyria (EPP) — Comprehensive Research Report

sup — quick orientation before the deep dive. EPP is the one where sunlight isn't a slow tan-and-burn thing but an immediate "my skin is on fire" alarm, sometimes within minutes and with almost nothing to see on the surface. The whole disease is basically one broken step at the very end of the heme assembly line: the enzyme that's supposed to tuck an iron atom into a porphyrin ring quits early, so the ring — protoporphyrin IX — piles up. That ring happens to be a tiny solar panel that turns light into cellular shrapnel. Everything else downstream flows from that one traffic jam.

1. Disease Information

What it is. Erythropoietic protoporphyria is an inherited disorder of heme biosynthesis caused by partial deficiency of ferrochelatase (FECH; EC 4.98.1.1), the terminal enzyme of the heme pathway, which inserts ferrous iron (Fe²⁺) into protoporphyrin IX (PPIX) to make heme. Reduced FECH activity → accumulation of metal-free, photoreactive PPIX in erythroid cells, plasma, skin, and liver. It is the most common cutaneous porphyria of childhood and the third most common porphyria overall. A phenotypically identical variant, X-linked protoporphyria (XLP / XLPP), arises from gain-of-function mutations in erythroid ALAS2 that overdrive substrate flux into the same pathway.

Key identifiers (MONDO verified locally via sqlite:obo:mondo): - MONDO: MONDO:0001676 (erythropoietic protoporphyria); MONDO:0008319 (protoporphyria, erythropoietic, 1) — both confirmed present in local MONDO. - OMIM: #177000 EPP1 (FECH); #618015 EPP2 (CLPX); #300752 X-linked protoporphyria (ALAS2). Gene entries: FECH *612386, ALAS2 *301300, CLPX *615611. - Orphanet: ORPHA:79278 (autosomal EPP); a distinct X-linked entry exists for XLP. ⚠verify exact XLP ORPHA code via just fetch-reference ORPHA:79278 and Orphanet lookup. - ICD-10: E80.0 (hereditary erythropoietic porphyria). ICD-11: 5C58.10. - MeSH: D046351 "Protoporphyria, Erythropoietic." - HGNC genes: FECH hgnc:3647, ALAS2 hgnc:397, CLPX hgnc:17820. (Confirm the numeric HGNC IDs with OAK before committing — repo uses lowercase hgnc: prefix.)

Synonyms / alternative names: EPP; protoporphyria; erythrohepatic protoporphyria (older term reflecting liver involvement); ferrochelatase deficiency. For the variant: X-linked dominant protoporphyria, XLPP, XLP.

Data provenance. Almost everything here is from aggregated disease-level resources — OMIM, Orphanet, GeneReviews, consensus guidelines, and cohort studies from specialist porphyria centers (Mass General, Erasmus MC Rotterdam, UCSF, Mount Sinai). The recent UK Biobank analysis is the notable population-genomic (EHR-adjacent) dataset showing systematic underdiagnosis.

2. Etiology

Disease causal factors (genetic — three molecular routes to one phenotype)

Route 1 — FECH loss-of-function (EPP1, ~90% of cases). The dominant paradigm is genetically counterintuitive and worth stating precisely: most clinically overt patients are compound heterozygous for a rare pathogenic FECH null/missense allele on one chromosome and a common hypomorphic low-expression allele, c.315-48T>C (IVS3-48C), on the other. The IVS3-48C polymorphism strengthens use of a cryptic aberrant splice acceptor, so ~40% of transcripts are aberrantly spliced and degraded by nonsense-mediated decay, lowering steady-state FECH mRNA.

"For 96% of patients, EPP results from coinheriting a rare pathogenic variant in trans of a common hypomorphic variant c.315-48T>C" (minor allele frequency ~0.05). — UK Biobank underdiagnosis study, Genetics in Medicine 2021 (PMC7796935; PMID likely 33257847 ⚠verify).

"the IVS3-48C minigene gave rise to 40% aberrantly spliced mRNA, and the IVS3-48T minigene to only 20%." — PMID:21132468, low-expression allele leads to low enzyme activity.

Residual FECH activity in symptomatic patients is roughly 10–35% of normal. A minority carry two loss-of-function FECH alleles (true autosomal recessive) — this subgroup carries a higher risk of severe liver disease and palmar keratoderma.

Route 2 — ALAS2 gain-of-function (X-linked protoporphyria, ~2–10% overall, up to ~40% in some North American cohorts). C-terminal frameshift deletions in exon 11 of ALAS2 — canonically c.1699_1700delAT (p.Met567GlufsTer2) and c.1706_1709delAGTG (p.Glu569GlyfsTer24) — truncate an autoinhibitory C-terminal region, increasing ALAS2 activity/stability. This pushes more 5-aminolevulinic acid into the pathway, so PPIX accumulates despite normal FECH.

"deletions in ALAS2 cause a … X-linked protoporphyria that, in contrast to autosomal dominant porphyrias, has close to 100% penetrance." — Whatley et al., Am J Hum Genet 2008 (PMID:18760763 ⚠verify).

Route 3 — CLPX dominant mutation (EPP2, very rare). A heterozygous CLPX p.Gly298Asp mutation in the mitochondrial AAA+ unfoldase CLPX impairs its normal turnover of ALAS, stabilizing ALAS and raising PPIX — an indirect gain-of-function on the same node.

"Cells with the mutant protein showed … increased posttranslational stability of ALAS and pathologic accumulation of PPIX." — Yien et al., PNAS 2017 (PMID:28874591).

Risk / modifying factors

Iron status: iron deficiency tends to worsen EPP (limits FECH substrate/activity and de-represses ALAS2); iron repletion may help XLP but effects are variable and sometimes worsen classic EPP — a genuinely unresolved clinical knob.
Sunlight is the phenotype trigger, not a cause. Even visible violet light (~400–410 nm) transmitted through window glass provokes symptoms.
Hepatotoxic stressors (alcohol, fasting, drugs that induce ALAS/CYP, intercurrent illness) can precipitate hepatic decompensation once liver disease exists.
Genetic ancestry: IVS3-48C allele frequency is markedly higher in East/Southeast Asian and Japanese populations than in Northern Europeans, shaping regional penetrance patterns.
Consanguinity matters for the rare biallelic-LOF autosomal recessive form.

Protective factors

Homozygosity for the high-expression IVS3-48T allele in trans to a pathogenic FECH mutation → subclinical/latent carriers (higher residual FECH). This is the single strongest protective genetic factor.
Constitutive skin melanin (darker pigmentation) is modestly photoprotective — the mechanistic basis for the afamelanotide/dersimelagon therapeutic strategy.

Gene–environment interaction

The defining GxE story is genotype (FECH residual activity) × light dose × iron availability. The trans-allele expression level sets a PPIX ceiling; ambient light converts that latent chemistry into phenotype; iron status tunes flux at both FECH and ALAS2.

3. Phenotypes

Suggested HPO anchors in brackets.

Acute cutaneous phototoxicity (near-universal, ~99%; onset infancy–early childhood). - Painful, burning/stinging/prickling photosensitivity within minutes of sun exposure, often with little visible sign at first — a hallmark that causes years of diagnostic delay. [HP:0000992 Cutaneous photosensitivity], pain [HP:0012531]. - Erythema [HP:0000988 skin rash / HP:0010783 erythema], edema, pruritus. Blistering is uncommon (distinguishes EPP from other cutaneous porphyrias like PCT). - Severity moderate–severe and highly disabling; course episodic/acute-on-chronic (each light exposure is a discrete event), lifelong.

Chronic skin changes (frequent with cumulative exposure). - Waxy thickening/lichenification over the knuckles and dorsal hands, shallow linear/pitted scars on the nose and cheeks, leathery hyperkeratosis. [HP:0000962 Hyperkeratosis], scarring [HP:0100699].

Hepatobiliary. - Cholelithiasis — PPIX-rich pigment gallstones, often at a young age [HP:0001081 Cholelithiasis]. Common. - Protoporphyric hepatopathy — mild transaminase elevation in up to ~20% of LOF-FECH patients; severe cholestatic liver disease / acute liver failure in ~2–5%. [HP:0001394 Cirrhosis], cholestasis [HP:0001396], hepatic failure [HP:0001399].

"Liver involvement is observed in 5%–20% of patients harbouring loss-of-function FECH variants and its manifestations are heterogeneous, ranging from mildly elevated liver transaminases, cholelithiasis to severe acute cholestatic hepatitis/liver failure." — Erythropoietic protoporphyrias: Pathogenesis, diagnosis and management (PMC11669082).

Hematologic. - Mild microcytic, hypochromic anemia with low ferritin/iron stores in a substantial fraction [HP:0001935 Microcytic anemia]. Usually mild and non-hemolytic.

Secondary / systemic. - Vitamin D deficiency and reduced bone mineral density from lifelong sun avoidance [HP:0100512 Decreased vitamin D level], osteopenia [HP:0000938]. - Motor polyneuropathy — a rare, dramatic complication seen in acute protoporphyric liver failure.

Quality of life. Impact is large and under-appreciated: patients organize life around darkness, curtail schooling/work/socializing, and report anxiety and depression. QoL instruments respond strongly to treatment — in an Austrian afamelanotide cohort the EPP-QoL score rose from a median of 11.11 to 79.17 and phototoxic burn-tolerance time from 15 to 250 minutes (JDDG 2023, Seidl-Philipp et al.).

4. Genetic / Molecular Information

Causal genes. | Gene | Locus | Mechanism | Disorder | OMIM | |---|---|---|---|---| | FECH | 18q21.31 | Loss of function | EPP1 (~90%) | #177000 | | ALAS2 | Xp11.21 | Gain of function | XLP | #300752 | | CLPX | 15q22.31 | Dominant, stabilizes ALAS | EPP2 (rare) | #618015 |

Pathogenic variants. - FECH: >180 reported alleles — missense, nonsense, splice-site, small indels, and large multi-exon/whole-gene deletions. Most are private/family-specific null alleles. The recurring functional partner is the common IVS3-48C hypomorph (gnomAD MAF ~0.05 in Europeans; considerably higher in East Asians). ClinVar RCV000000592 covers the c.315-48T>C allele. - ALAS2: recurrent C-terminal exon-11 frameshifts (c.1699_1700delAT, c.1706_1709delAGTG); additional deletions reported (e.g., a four-base ALAS2 deletion in a Chinese pedigree, PMC7186625). - CLPX: single dominant p.Gly298Asp to date.

Variant classification / origin. Germline. FECH nulls → loss of function; ALAS2 C-terminal deletions → gain of function; CLPX → dominant with a gain-of-function-like effect on ALAS stability. No somatic involvement (this is not a neoplastic process).

Modifier genes. The FECH IVS3-48C allele is the canonical modifier/permissive locus. Iron-regulatory genes and ALAS2 iron-responsive-element biology modulate flux. Zygosity for LOF FECH modifies liver-disease risk.

Epigenetics / chromosomal abnormalities. No established disease-driving epigenetic marks; the IVS3-48C effect is a splicing phenomenon, not methylation. Large FECH deletions are the relevant structural lesions (detectable by MLPA/CMA when sequencing is negative). No aneuploidy association.

5. Environmental Information

Environmental factor (defining): visible light, ~400–410 nm (violet/Soret band) — the phenotype's obligatory trigger. Passes through window glass; even fluorescent/operating-room lighting can burn.
Lifestyle: alcohol and fasting stress the liver; smoking and hepatotoxins accelerate hepatopathy in susceptible patients. Sun-avoidance lifestyle itself causes secondary vitamin D deficiency.
Occupational: outdoor work is often untenable; even phototherapy lamps and surgical lights are hazards (surgical-light phototoxic burns are a documented intra-operative risk in EPP patients).
Infectious agents: none causal. (Hepatitis A/B are relevant only as avoidable liver insults — hence vaccination is recommended.)

6. Mechanism / Pathophysiology

Core causal chain (upstream → downstream):

Enzymatic block at the terminal heme step. FECH deficiency (or ALAS2/CLPX overdrive) → the metal-insertion step fails or is outrun. [GO:0004325 ferrochelatase activity; GO:0006783 heme biosynthetic process]. FECH is an inner-mitochondrial-membrane enzyme [GO:0005743].
PPIX accumulation in erythroid cells. The bone-marrow reticulocyte/erythroblast is the dominant source of excess metal-free protoporphyrin IX [CHEBI:15430 protoporphyrin IX]; in XLP, zinc-protoporphyrin rises too. Cell types: [CL:0000765 erythroblast; CL:0000558 reticulocyte].
Systemic distribution. Lipophilic PPIX loads into erythrocytes and plasma, deposits in skin, and is excreted into bile — the only elimination route (not renal).
Cutaneous phototoxicity (the acute arm). PPIX absorbs violet light → excited triplet state → type I/II photochemistry generating singlet oxygen and other reactive oxygen species [GO:0006979 response to oxidative stress] → oxidative injury to dermal microvascular endothelium [CL:0000115], mast-cell degranulation [CL:0000097], and complement activation → immediate neurogenic/inflammatory pain and edema. [GO:0009416 response to light stimulus].
Protoporphyric hepatopathy (the chronic/severe arm). PPIX is cholestatic and directly hepatotoxic: it precipitates as crystalline deposits in hepatocytes [CL:0000182] and bile canaliculi, injuring cholangiocytes [CL:0002326] and Kupffer cells → cholestasis → reduced biliary PPIX clearance → further hepatic PPIX retention. This feed-forward vicious cycle is what converts stable disease into fulminant liver failure.

"Protoporphyric hepatopathy results from the accumulation of protoporphyrin in hepatocytes and bile canaliculi, with toxic effects on cholangiocytes and Kupffer cells leading to cholestasis." — liver-management consensus guidelines (PMC10818013).

Protein dysfunction. FECH is a homodimeric [2Fe-2S]-cluster mitochondrial enzyme; pathogenic variants reduce catalytic activity or destabilize the protein. ALAS2 C-terminal deletions remove an autoinhibitory element (structural gain of function). CLPX G298D disrupts unfoldase-mediated ALAS turnover.

Metabolic changes. The lesion is confined to the heme biosynthetic pathway (KEGG map00860); the phenotype is a substrate-accumulation disease — no broad energy-metabolism derangement, though secondary anemia reflects constrained heme output.

Immune involvement. Innate/inflammatory rather than autoimmune — ROS-driven mast cell and complement activation in skin; sterile inflammatory hepatic injury.

Molecular profiling. The bitopertin mechanistic work used CD34⁺-derived and iPSC-derived erythroid cultures to show that limiting glycine (via GlyT1) reduces PPIX — an in vitro substrate-limitation demonstration (PMC12435829 / PMC12435834). Erythroid-specific transcriptional control of ALAS2 (GATA1, iron-responsive element) is the relevant expression biology.

7. Anatomical Structures Affected

Skin [UBERON:0002097 skin of body], specifically sun-exposed sites — face, dorsal hands, ears, nose (bilateral, symmetric, light-distribution).
Liver [UBERON:0002107] and biliary tract / gallbladder [UBERON:0002110 gallbladder; UBERON:0002394 bile duct].
Bone marrow [UBERON:0002371] — the erythroid production source of excess PPIX.
Blood / erythrocytes [UBERON:0000178 blood].
Subcellular: mitochondrion [GO:0005739] (site of FECH, ALAS2, and terminal heme synthesis); mitochondrial inner membrane [GO:0005743].
Secondary: bone [UBERON:0002481] (low BMD from vitamin D deficiency); peripheral nerves (post-liver-failure neuropathy).

8. Temporal Development

Onset: typically infancy to early childhood — often the first prolonged sun exposures. Screaming/crying with sun in a nonverbal infant is a classic (and easily missed) presentation.
Onset pattern of episodes: acute (minutes) with each light exposure; underlying disease is chronic and lifelong.
Progression: skin phenotype is generally stable in severity across life (not progressive neurodegeneration-style). The dangerous variable is hepatic: usually absent/mild, but can convert to rapid, life-threatening acute liver failure, sometimes precipitated by an intercurrent stressor.
Course: non-remitting baseline; phototoxic events are episodic/provoked. Symptom-free periods require darkness, not spontaneous remission.
Critical windows: liver-function surveillance is the key intervention window — catching rising PPIX/LFTs before decompensation is what changes outcomes.

9. Inheritance and Population

Inheritance patterns. - EPP1: operationally behaves as autosomal recessive / pseudodominant — clinical disease usually requires a rare LOF FECH allele in trans to the common IVS3-48C hypomorph (a "one severe hit + one weak hit" model). Rare biallelic-LOF families are frankly recessive. HPO inheritance: [HP:0000007 Autosomal recessive] for the two-hit model; some pedigrees historically labeled autosomal dominant with low penetrance. - XLP: X-linked [HP:0001417]; near-100% penetrance in hemizygous males, variable in heterozygous females depending on X-inactivation (skewed lyonization can make carrier females symptomatic — PMID:25615817). - EPP2 (CLPX): autosomal dominant [HP:0000006].

Penetrance / expressivity. Penetrance of the FECH mutation is gated by the trans-allele expression level — the reason many obligate carriers are asymptomatic. Expressivity is variable (mild latent to severe with hepatopathy).

Epidemiology. - Prevalence: commonly cited 1:75,000 (Netherlands) to 1:200,000 (Wales); worldwide range ~1:17,000–1:100,000. Europe-wide diagnosed prevalence ≈ 0.00092%. - Underdiagnosis: UK Biobank genetics suggest true prevalence is ~2.3× higher than clinically estimated (corrected ≈ 0.0059%); diagnosis is frequently delayed >10 years.

"the prevalence of erythropoietic protoporphyria is 2.3 times higher than previously estimated in Europe" — UK Biobank underdiagnosis analysis (Genetics in Medicine 2021).

Sex ratio: roughly equal for EPP1; male predominance among symptomatic XLP.
Ancestry: higher IVS3-48C frequency in East/Southeast Asian populations; XLP proportion notably higher in North American cohorts (~40% in one Mount Sinai series, PMC3646094).
Age distribution: overwhelmingly diagnosed in children once symptomatic; carriers span all ages.

10. Diagnostics

The decisive biochemical test: markedly elevated total erythrocyte protoporphyrin with a predominance of metal-free (non-zinc) PPIX. [LOINC candidates for erythrocyte protoporphyrin; confirm exact LOINC codes].

Fractionation is critical and distinguishes entities:
EPP1: mostly metal-free PPIX (zinc-PP fraction typically <15%).
XLP: substantially higher zinc-protoporphyrin fraction (~15–50%).
Iron deficiency / lead poisoning: predominantly zinc-protoporphyrin (helps exclude mimics).
Plasma fluorescence scan: emission peak at ~634 nm — a rapid confirmatory screen.
Biochemistry pattern: urinary porphyrins are typically normal (PPIX is not water-soluble) — a useful negative that separates EPP from acute hepatic porphyrias.

Genetic testing. - FECH: sequencing for the rare pathogenic allele plus targeted IVS3-48C genotyping (essential — the common allele won't be flagged as pathogenic on its own). MLPA/deletion analysis when sequencing finds only one or no variant (large FECH deletions). - ALAS2: targeted exon 11 analysis for C-terminal frameshifts when biochemistry suggests XLP. - CLPX: consider in FECH/ALAS2-negative dominant pedigrees. - Gene panels (porphyria panels) and WES are reasonable when the phenotype is atypical.

Monitoring labs. LFTs (surveillance for hepatopathy), ferritin/iron studies, 25-OH vitamin D, CBC.

Imaging / pathology. Not required for diagnosis. Liver biopsy in hepatopathy shows birefringent, Maltese-cross PPIX deposits under polarized light; abdominal imaging for gallstones.

Differential diagnosis. Other cutaneous porphyrias (PCT, variegate, hereditary coproporphyria — but those blister and have abnormal urinary porphyrins), solar urticaria, polymorphous light eruption, phototoxic drug reactions, hydroa vacciniforme. The immediate painful, largely non-blistering photosensitivity with normal urinary porphyrins is the discriminating fingerprint.

Screening. No newborn screening. Cascade genetic testing of first-degree relatives (for both the LOF allele and IVS3-48C) is appropriate for counseling.

11. Outcome / Prognosis

Life expectancy: normal for the great majority; the disease's weight is on quality of life, not survival — except for the small subset with progressive liver disease.
Principal mortality driver: protoporphyric liver failure (~2–5%). Rare but potentially fatal, and can be abrupt.
Morbidity: chronic pain, profound activity restriction, social/occupational limitation, secondary vitamin D deficiency/low BMD, and cholelithiasis. QoL scores are low at baseline and highly treatment-responsive.
Prognostic factors: biallelic LOF FECH genotype, very high/rising erythrocyte PPIX, and abnormal LFTs flag higher hepatic risk. Erythrocyte PPIX level tracks disease burden and is the practical biomarker.
Recovery: phototoxic symptoms are fully reversible with light avoidance/therapy; liver failure is not reversible without transplant ± marrow replacement.

12. Treatment

The therapeutic logic splits cleanly: (a) shield the solar panel or darken the skin over it, (b) turn down PPIX production at the source, and (c) rescue the liver when the feed-forward cycle runs away.

Photoprotection & supportive care (foundation). - Strict sunlight avoidance, protective clothing, and opaque physical sunscreens with visible-light reflectants (zinc oxide, titanium dioxide, iron-oxide-tinted formulations — chemical UV filters alone are useless here because the culprit is visible light). [MAXO:0000950 supportive care]. - Vitamin D supplementation; hepatitis A/B vaccination to protect the vulnerable liver.

Melanocortin-1-receptor agonists (approved / advanced pipeline). - Afamelanotide (Scenesse) — α-MSH analog MC1R agonist, subcutaneous controlled-release implant every ~60 days; stimulates eumelanin to raise the phototoxic threshold. EMA-approved 2014, FDA-approved 2019. Randomized and long-term real-world data show longer pain-free sun exposure and large QoL gains. [MAXO/NCIT:C15986 Pharmacotherapy; therapeutic_agent: afamelanotide].

"Afamelanotide … increased duration of sun exposure without pain and improved quality of life." — Langendonk et al., N Engl J Med 2015 (PMID:26132941).

Dersimelagon (MT-7117) — oral, non-peptide selective MC1R agonist; phase 2 (ENDEAVOR) met its primary endpoint (increased pain-free sun-exposure time), with phase 3 in EPP/XLP (NCT extension NCT05005975). A genuinely convenient oral alternative to implants if approved. [therapeutic_modality: SMALL_MOLECULE].

Substrate-limiting / disease-modifying (investigational). - Bitopertin — oral glycine transporter-1 (GlyT1) inhibitor that starves the very first heme step of glycine, lowering PPIX at the source (potentially disease-modifying rather than just photoprotective). The randomized AURORA phase 2 (75 patients, 20/60 mg vs placebo, 17 weeks) showed dose-dependent whole-blood PPIX reductions of −21.6% (20 mg) and −40.7% (60 mg); sunlight-tolerance improvements did not reach significance against a strong placebo response.

"Bitopertin shows efficacy in patients with erythropoietic protoporphyria: Results from the randomized, double-blind, placebo-controlled AURORA trial." — PMID:41390126 ⚠verify (recent; fetch before quoting).

Older adjuncts (limited evidence): oral β-carotene (historical, modest at best), cysteine, N-acetylcysteine, antioxidants, narrowband UVB skin-hardening.

Liver disease management (escalating). - Suppress erythroid PPIX output and promote elimination: cholestyramine / activated charcoal (interrupt enterohepatic PPIX recycling), ursodeoxycholic acid, IV hemin/heme arginate (represses erythroid ALAS), RBC exchange transfusion / plasmapheresis, iron optimization. - Liver transplantation for acute protoporphyric liver failure — life-saving but NOT curative, because the marrow keeps overproducing PPIX and can damage the graft. [MAXO:0010039 organ transplantation]. - Allogeneic hematopoietic stem cell transplantation (HSCT) — the only curative therapy, as it replaces the erythroid PPIX source; performed after liver transplant in the combined strategy for severe cases.

"The strategy of hematopoietic stem cell transplantation after liver transplantation cures erythropoietic protoporphyria and prevents recurrent erythropoietic protoporphyria from damaging the allograft." — liver-disease consensus guidelines (PMC10818013).

Pharmacogenomics. No routine PGx gating, but genotype is the therapeutic map: XLP (ALAS2) is the strongest rationale for source-reduction approaches (bitopertin, hemin), while FECH-EPP with hepatopathy anchors the transplant/HSCT pathway.

13. Prevention

Primary prevention: none (inherited) — but genetic counseling and cascade family testing identify at-risk relatives, and prenatal testing / preimplantation genetic diagnosis are available for known familial variants. [MAXO:0000079 genetic counseling].
Secondary prevention: photoprotection to prevent phototoxic injury; annual LFT/PPIX surveillance to catch hepatopathy early; vitamin D repletion; hepatitis A/B vaccination; avoidance of alcohol and hepatotoxins.
Tertiary prevention: aggressive early management of rising PPIX/abnormal LFTs to head off liver failure; timely referral to a specialist porphyria center with transplant capability.
Public-health angle: raising clinician awareness is itself a prevention lever — the >10-year diagnostic delay is the biggest modifiable failure point.

14. Other Species / Natural Disease

Taxonomy of natural/model disease: mouse [NCBITaxon:10090], cattle [NCBITaxon:9913], zebrafish [NCBITaxon:7955].
Bovine protoporphyria — a naturally occurring FECH-deficiency disease documented in cattle (notably Limousin), with photosensitivity paralleling human EPP; catalogued in OMIA. [VBO breed term for Limousin — verify].
Orthologous genes: Fech, Alas2, Clpx are conserved across mammals and in zebrafish; the pathway is deeply conserved (heme synthesis is ancient), making cross-species mechanism transfer strong.
Comparative biology: natural bovine and induced rodent/fish models reproduce both the photosensitivity and, in FECH-null rodents, the hepatopathy — a nice illustration of evolutionary conservation of the PPIX-phototoxicity mechanism.
Zoonosis: not applicable (non-infectious, non-transmissible).

15. Model Organisms

Fech^m1Pas mouse (BALB/c background; a chemically induced Fech point mutation, Tutois et al.) — the workhorse model; homozygotes recapitulate cutaneous photosensitivity + cholestatic liver disease + biliary PPIX deposits, capturing both arms of human disease. [evidence_source: MODEL_ORGANISM]. ⚠verify PMID (Tutois 1991, J Clin Invest).
Ferrochelatase c.315-48C modifier mouse — engineered to model the human low-expression splice modifier (PMC5374324), directly testing the IVS3-48C mechanism in vivo.
CLPX mouse models — probe the ALAS-stabilization mechanism of EPP2.
Zebrafish dracula (fech mutant) — classic vertebrate heme-synthesis model showing porphyrin accumulation and light-dependent hemolysis/phenotype.
Chemically induced protoporphyria — griseofulvin or DDC (3,5-diethoxycarbonyl-1,4-dihydrocollidine) feeding induces PPIX accumulation and protoporphyric hepatopathy in rodents; a standard tool for studying the liver arm.
Cellular / in vitro models — CD34⁺-derived and iPSC-derived erythroid cultures used to demonstrate GlyT1/glycine-limitation reduction of PPIX (the bitopertin mechanism; PMC12435829/PMC12435834). [evidence_source: IN_VITRO].
Model resources: MGI (Fech, Alas2, Clpx), ZFIN (fech), OMIA (bovine protoporphyria), IMPC/IMSR for allele availability.
Recapitulation vs limitations: rodent Fech models capture skin + liver disease well; the compound-heterozygous IVS3-48C human genetics (a splicing hypomorph) is not naturally reproduced by simple null alleles — the engineered c.315-48C mouse exists specifically to close that human-model gap. Consider a HUMAN_MODEL_MISMATCH discussion note if you're modeling the splice-modifier arm.

Priority citation set for the KB entry

Verified via PubMed/PMC search this session (safe to fetch and quote): - PMID:26132941 — Langendonk et al., afamelanotide RCT, NEJM 2015. - PMID:28874591 — Yien et al., CLPX/EPP2, PNAS 2017. - PMID:21132468 — FECH IVS3-48C low-expression allele → low enzyme activity. - PMID:30704898 — Balwani, EPP & XLP pathophysiology/genetics/management review. - PMID:25615817 — X-inactivation and XLP phenotype in females. - Blood 2023;141:2921–2931 — Karp Leaf & Dickey, "How I treat EPP and XLP" (fetch for PMID). - PMC10818013 — consensus guidelines, protoporphyria-related liver dysfunction. - PMC7796935 — UK Biobank underdiagnosis (Genet Med 2021).

⚠verify before quoting (cited from memory or secondary mention): PMID:18760763 (Whatley ALAS2 2008), the AURORA bitopertin primary paper (PMID:41390126), the dersimelagon phase 2 primary paper, and the Fech^m1Pas mouse origin paper. Run just fetch-reference PMID:XXXX and confirm the snippet is an exact abstract substring before any of these lands in an evidence block — standard DR-hallucination hygiene per the repo SOP.

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Bottom line for curation: EPP is a clean, well-understood substrate-accumulation disease that maps neatly onto the dismech pathograph — one enzymatic block (FECH↓, or ALAS2↑ / CLPX for the variants), PPIX buildup, then a forking downstream into an acute photochemical skin arm (light → singlet oxygen → dermal injury) and a chronic hepatic arm (PPIX cholestasis → feed-forward liver failure). The genetics deserve special care in the entry: model the two-hit FECH-null + IVS3-48C hypomorph genotype explicitly (it's the whole reason penetrance is weird), and give X-linked protoporphyria its own subtype block with the ALAS2 gain-of-function mechanism rather than folding it in silently.

Erythropoietic Protoporphyria

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Erythropoietic Protoporphyria (EPP) — Comprehensive Research Report

1. Disease Information

2. Etiology

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