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Last updated:
June 01, 2022
Years published: 2013, 2016, 2020
NORD gratefully acknowledges Maureen Poh-Fitzpatrick, MD, Professor Emerita and Special Lecturer (Dermatology), Columbia University College of Physicians and Surgeons, for assistance in the preparation of this report.
Summary
X-linked protoporphyria is an extremely rare genetic disorder characterized by an abnormal sensitivity to the sun (photosensitivity) that can cause severe pain, burning, and itching of sun-exposed skin. Symptoms may occur immediately or shortly after exposure to the sun, including direct exposure or indirect exposure such as sunlight that passes through window glass or that is reflected off water or sand. Redness and swelling of affected areas can also occur. Blistering and severe scarring occur infrequently. Chronic episodes of photosensitivity may lead to changes in the skin of sun-exposed areas. Some individuals eventually develop potentially severe liver disease. X-linked protoporphyria is caused by mutations of the ALAS2 gene and is inherited in an X-linked dominant pattern. Males often develop a severe form of the disorder while females may not develop any symptoms (asymptomatic) or can develop a form as severe as that seen in males.
Introduction
X-linked protoporphyria belongs to a group of disorders known as the porphyrias. This group of at least eight disorders is characterized by abnormally high levels of porphyrins and porphyrin precursors due to deficiency of certain enzymes essential to the creation (synthesis) of heme, a part of hemoglobin and other hemoproteins. There are eight enzymes in the pathway for making heme and at least eight different forms of porphyria. The symptoms associated with the various forms of porphyria differ. It is important to note that people who have one type of porphyria do not develop any of the other types. Porphyrias are generally classified into two groups: the “hepatic” and “erythropoietic” types. Porphyrins and porphyrin precursors and related substances originate in excess amounts chiefly from the liver in the hepatic types and mostly from the bone marrow in the erythropoietic types. Porphyrias with skin manifestations are sometimes referred to as “cutaneous porphyrias.” The term “acute porphyria” is used to describe porphyrias that can be associated with sudden attacks of pain and other neurological symptoms.
X-linked protoporphyria is an erythropoietic form of porphyria and is extremely similar clinically to erythropoietic protoporphyria (EPP). X-linked protoporphyria was first described in the medical literature in 2008.
Hypersensitivity of the skin to sunlight is the characteristic finding of X-linked protoporphyria. Affected individuals develop pain, itching, and burning of the skin after exposure to sunlight. Sometimes these symptoms are accompanied by swelling and redness (erythema) of the affected areas. Large blisters and severe scarring, which are common to other forms of cutaneous porphyria, usually do not occur in individuals with X-linked protoporphyria. Symptoms may be noticed as quickly as a few minutes after exposure to the sun. Although most symptoms usually subside within 24-48 hours, pain and a red or purple discoloration of the skin may persist for several days after the initial incident. Pain is disproportionately severe in relation to the visible skin lesions. Pain associated with X-linked protoporphyria can be excruciating and is often resistant to pain medications, even narcotics.
Repeated episodes of photosensitivity may eventually cause changes in the skin of affected individuals. Such changes include thickening and hardening of the skin, development of a rough or leathery texture, small facial pock-like pits, and grooving around the lips.
Some individuals with X-linked protoporphyria develop liver disease, which can range from mild liver abnormalities to liver failure. Information on liver disease is limited, but the risk of liver disease is believed to be higher in X-linked protoporphyria than in EPP. Affected individuals may experience back pain and severe abdominal pain especially in the upper right area of the abdomen. In some affected individuals, the flow of bile through the gallbladder and bile ducts may be interrupted (cholestasis) leading to gallstones. These stones can cause obstruction and inflammation of the gallbladder (cholecystitis). Scarring of the liver (cirrhosis) may also develop and some individuals may eventually develop end stage liver failure.
Additional symptoms have been reported in individuals with X-linked protoporphyria including mild anemia (low levels of circulating red blood cells) and iron deficiency.
X-linked protoporphyria is caused by gain-of-function mutations to the ALAS2 gene located on the X chromosome and is inherited as an X-linked dominant disorder. In contrast to most X-linked disorders, which are recessive, X-linked dominant disorders are evident in a female with one normal X chromosome and one affected X chromosome.
The ALAS2 gene encodes a protein known as erythroid specific 5-aminolevulinate synthase 2. Mutations of the ALAS2 gene lead to the overproduction of this enzyme, which, in turn, results in elevated levels of a chemical called protoporphyrin. Protoporphyrin abnormally accumulates in certain tissues of the body, especially the blood, liver, and skin. The symptoms of X-linked protoporphyria develop because of this abnormal accumulation of protoporphyrin. For example, when protoporphyrin molecules absorb energy from sunlight, they enter an excited state (photoactivation) and this abnormal activation results in the characteristic damage to the skin. Accumulation of protoporphyrin in the liver causes toxic damage to the liver and may contribute to the formation of gallstones. Protoporphyrin is formed within red blood cells in the bone marrow and then enters the blood plasma, which carries it to the skin where it can be photoactivated by sunlight and cause damage. The liver removes protoporphyrin from the blood plasma and secretes it into the bile.
X-linked protoporphyria affects males and females. However, males usually develop a severe form of the disorder while females with an ALAS2 mutation may range from having no symptoms (asymptomatic) to developing a severe form of the disorder. The exact incidence or prevalence of X-linked protoporphyria is unknown. The disorder has only been reported in the medical literature in a handful of families in Europe, South Africa and Japan.
A diagnosis of X-linked protoporphyria is based upon identification of characteristic symptoms (e.g., non-blistering photosensitivity), a detailed patient history, a thorough clinical evaluation, and a variety of specialized tests.
Clinical Testing and Workup
A diagnosis of X-linked protoporphyria may be made through blood tests that can detect markedly increased levels of metal-free and zinc-bound protoporphyrins within red blood cells (erythrocytes). A higher ratio of zinc-bound protoporphyrin to metal-free protoporphyrin can differentiate X-linked protoporphyria from EPP.
Molecular genetic testing can confirm a diagnosis of X-linked protoporphyria by detecting mutations in the ALAS2 gene (the only gene known to cause this disorder).
Additional tests may be performed such as blood tests to evaluate anemia and iron stores in the body and vitamin D levels, or an abdominal sonogram to detect and evaluate liver disease potentially associated with X-linked protoporphyria.
The treatment of X-linked protoporphyria is directed toward the specific symptoms that are apparent in each individual. Treatment may require the coordinated efforts of a team of specialists. Pediatricians, hematologists, dermatologists, hepatologists, and other healthcare professionals may need to systematically and comprehensively plan an affected child’s treatment.
Genetic counseling is recommended affected individuals and their families.
There is no specific, FDA-approved therapy for individuals with X-linked protoporphyria. Because the disorder is so rare, most treatment information is based on EPP, which is clinically similar to X-linked protoporphyria.
Avoidance of sunlight will benefit affected individuals and can include the use of clothing styles with long sleeves and pant legs, made with double layers of fabric or of light-exclusive fabrics, wide brimmed hats, gloves, and sunglasses. Topical sunscreens are generally ineffective, unless they contain light-reflective ingredients. Certain tanning products with ingredients that increase pigmentation may be helpful. Affected individuals may also benefit from window tinting and the use of light-filtering vinyl or films to cover the windows of their homes and cars.
Avoidance of sunlight can potentially cause vitamin D deficiency and some individuals may require supplemental vitamin D.
A high potency form of oral beta-carotene (Lumitene) may be given to improve an affected individual’s tolerance of sunlight. This drug causes skin discoloration and may improve tolerance to sunlight. For more information on this treatment, contact the organizations listed at the end of this report (i.e. American Porphyria Foundation and the EPPREF). Another drug sometimes used to improve tolerance to sunlight is cysteine.
In some patients, the drug cholestyramine may be given. Cholestyramine absorbs porphyrin. The drug may interrupt the recirculation of protoporphyrin secreted into the bile back into the liver and promote its excretion through the feces. Other drugs that absorb porphyrins such as activated charcoal have also been used to treat affected individuals. These drugs may lead to improvement of liver disease.
Individuals with any form of protoporphyria should avoid substances associated with cholestasis including alcohol and certain drugs such as estrogens. Immunizations for hepatitis A and B are recommended as well.
Afamelanotide, an alpha-melanocyte-stimulating hormone analogue, increases the production of melanin in the skin. Afamelanotide has been available for adults with EPP in the European Union since 2014. It was approved for adults with EPP by the US Food and Drug Administration (FDA) in October 2019. The long-term safety and effectiveness of this drug and its role in treating individuals with X-linked protoporphyria remain under investigation.
Plasmapheresis and red blood cell transfusions have been used to treat people with EPP. In individuals with severe liver disease, a liver transplantation may be required. Extreme caution should be used by physicians considering these treatment options for individuals with X-linked protoporphyria (or EPP). Each individual case should be evaluated on its own merits.
Iron supplementation may be considered in individuals with anemia and abnormal iron metabolism. Such therapy requires strict monitoring by physicians. In EPP, iron supplementation has resulted in clinical improvement, but also carries a risk of increased photosensitivity.
Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.
For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:
Toll-free: (800) 411-1222
TTY: (866) 411-1010
Email: prpl@cc.nih.gov
Some current clinical trials also are posted on the following page on the NORD website:
https://rarediseases.org/living-with-a-rare-disease/find-clinical-trials/
For information about clinical trials sponsored by private sources, in the main, contact:
www.centerwatch.com
For more information about clinical trials conducted in Europe, contact:
https://www.clinicaltrialsregister.eu/
The Porphyrias Consortium is a joint endeavor including five of the leading porphyria centers in the United States. Staff includes physicians, researchers, research coordinators, and technical laboratory staff. The Consortium aims to expand the knowledge about porphyrias to benefit patients and families. Study information regarding porphyrias is also posted at the Porphyrias Consortium website: https://www.rarediseasesnetwork.org/cms/porphyrias
JOURNAL ARTICLES
Balwani M, Naik H, Anderson KE et al. Clinical, biochemical, and genetic characterization of North American patients with erythropoietic protoporphyria and X-linked protoporphyria. JAMA Dermatol. 2017;153(8):789-796. https://www.ncbi.nlm.nih.gov/pubmed/2861458
Landefeld C, Kentouche K, Gruhn B, et al. X-linked protoporphyria: Iron supplementation improves protoporphyrin overload, liver damage and anaemia. Br J Haematol. 2016;173(3):482-4. https://www.ncbi.nlm.nih.gov/pubmed/26193873
Ninomiya Y, Kokunai Y, Tanizaki H, et al. X-linked dominant protoporphyria: The first reported Japanese case. J Dermatol. 2016 Apr;43(4):414-8. Epub 2015 Sept 21. https://www.ncbi.nlm.nih.gov/pubmed/26387792
Balwani M, Doheny D, Bishop DF, et al. Loss-of-funtion ferrochelatase and gain-of-function erythroid 5-aminolevulinate synthase mutations causing erythropoietic protoporphyria and X-linked protoporphyria in North American patients reveal novel mutations and a high prevalence of X-linked protoporphyria. Mol Med. 2013;19(1): 26–35; [Epub ahead of print]: https://www.ncbi.nlm.nih.gov/pubmed/23364466
Bishop DF, Tchaikovskii V, Nazarenko I, Desnick RJ. Molecular expression and characterization of erythroid-specific 5-aminolevulinate synthase gain-of-function mutations causing X-linked protoporphyria. Mol Med. 2013;19:18-25. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3592931/
Livideanu CB, Ducamp S, Lamant L,e al. Late-onset of X-linked dominant protoporphyria: an etiology of photosensitivity in the elderly. J Invest Dermatol. 2013;133:1688-90. Epub 2012 Dec. 28. https://www.ncbi.nlm.nih.gov/pubmed/23223129
Balwani M, Desnick RJ. The porphyrias: advances in diagnosis and treatment. Blood. 2012;120:4496-4504. https://www.ncbi.nlm.nih.gov/pubmed/22791288
Whatley SD, Ducamp S, Gouya L, et al. C-Terminal deletions in the ALAS2 gene lead to gain of function and cause X-linked dominant protoporphyria without anemia or iron overload. Am J Hum Genet. 2008;83:408-414. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2556430/
INTERNET
Balwani M, Bloomer J, Desnick R; Porphyrias Consortium of the NIH-Sponsored Rare Diseases Clinical Research Network. X-Linked Protoporphyria. 2013 Feb 14. In: Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2019. Available from: https://www.ncbi.nlm.nih.gov/books/NBK121284/ Accessed October 30, 2019.
Poh-Fitzpatrick MB. Protoporphyria. Medscape. Updated: Oct 10, 2019. Available at: https://emedicine.medscape.com/article/1104061. Accessed October 30, 2019.
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