X-Linked Protoporphyria

CLINICAL CHARACTERISTICS

X-linked protoporphyria (XLP) is characterized in affected males by cutaneous photosensitivity (usually beginning in infancy or childhood) that results in tingling, burning, pain, and itching within minutes of sun/light exposure and may be accompanied by swelling and redness. Blistering lesions are uncommon. Pain, which may seem out of proportion to the visible skin lesions, may persist for hours or days after the initial phototoxic reaction. Photosensitivity is lifelong. 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. An unknown proportion of individuals with XLP develop liver disease. Except for those with advanced liver disease, life expectancy is not reduced. The phenotype in heterozygous females ranges from asymptomatic to as severe as in affected males.

DIAGNOSIS/TESTING: The diagnosis of XLP is established in a male proband with markedly increased free erythrocyte protoporphyrin and zinc-chelated erythrocyte protoporphyrin by identification of a hemizygous pathogenic gain-of-function variant in on molecular genetic testing. The diagnosis of XLP is established in a female proband with increased free erythrocyte protoporphyrin and zinc-chelated erythrocyte protoporphyrin by identification of a heterozygous pathogenic gain-of-function variant in on molecular genetic testing.

MANAGEMENT

The phototoxicity and subsequent pain can be reduced by the administration of afamelanotide, an α-melanocyte-stimulating hormone analog. Otherwise, the only effective treatment is prevention of the painful attacks by avoidance of sun/light (including the long-wave ultraviolet light that passes through window glass) through use of protective clothing (e.g., long sleeves, gloves, wide-brimmed hats, protective tinted glass for cars and windows). Although topical sunscreens are typically not useful, some tanning products containing creams that cause increased pigmentation may be helpful. Oral Lumitene™ (β-carotene) has been used to improve tolerance to sunlight by causing mild skin discoloration due to carotenemia; however, a systematic review of treatment options showed no evidence of efficacy. Vitamin D supplementation is recommended to prevent vitamin D insufficiency resulting from sun avoidance. Severe liver complications are difficult to treat: cholestyramine and other porphyrin absorbents (to interrupt the enterohepatic circulation of protoporphyrin and promote its fecal excretion) and plasmapheresis and intravenous hemin are sometimes beneficial. Liver transplantation can be a lifesaving measure in individuals with severe protoporphyric liver disease; combined bone marrow and liver transplantation is indicated in those with liver failure to prevent future damage to the allografts. Monitoring of: hepatic function every 6-12 months and hepatic imaging if cholelithiasis is suspected; erythrocyte protoporphyrin levels (free and zinc-chelated), hematologic indices, and iron profile annually; vitamin D 25-OH levels. Sunlight and UV light; for those with hepatic dysfunction, drugs that may induce cholestasis (e.g., estrogens). For those with cholestatic liver failure, protective filters should be used for the operating room lights for liver transplant surgery to avoid phototoxic damage. If the pathogenic variant has been identified in an affected family member, at-risk relatives can be tested as newborns or infants so that those with the pathogenic variant can benefit from early intervention (sun protection) and future monitoring for signs of liver dysfunction.

GENETIC COUNSELING

By definition, XLP is inherited in an X-linked manner. Affected males transmit the pathogenic variant to all of their daughters and none of their sons. Women with an pathogenic variant have a 50% chance of transmitting the variant to each child. Once the pathogenic variant has been identified in an affected family member, heterozygote testing for at-risk female relatives, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible.