X-linked erythropoietic protoporphyria

X-linked erythropoietic protoporphyria (XLEPP), also known as X-linked dominant protoporphyria or X-linked protoporphyria, is a rare inherited disorder of heme biosynthesis caused by gain-of-function mutations in the ALAS2 gene located on the X chromosome. This gene encodes the erythroid-specific form of delta-aminolevulinic acid synthase, the first enzyme in the heme biosynthetic pathway in red blood cell precursors. Overactivity of this enzyme leads to excessive production and accumulation of free (metal-free) protoporphyrin IX in red blood cells, plasma, and tissues, particularly the skin and liver. The condition primarily affects the skin and the hepatobiliary system. Patients typically present in early childhood with acute, painful photosensitivity — burning, stinging, and swelling of sun-exposed skin within minutes of sunlight exposure. Unlike many other photosensitivity disorders, the skin changes may be subtle, with minimal visible blistering or scarring, which can lead to delayed diagnosis. Prolonged sun exposure can cause edema, redness, and in some cases waxy scarring of the skin over time. A serious complication is protoporphyrin-related liver disease, which can progress to liver failure in a subset of patients due to accumulation of protoporphyrin in hepatocytes and bile ducts, leading to cholestatic liver damage. Gallstones containing protoporphyrin may also develop. The clinical presentation of XLEPP closely resembles that of autosomal dominant erythropoietic protoporphyria (EPP) caused by FECH gene mutations, and the two conditions are distinguished primarily through genetic testing and biochemical analysis showing markedly elevated zinc-protoporphyrin in addition to free protoporphyrin in XLEPP. Management focuses on strict sun avoidance and photoprotection. Afamelanotide, a synthetic alpha-melanocyte-stimulating hormone analogue administered as a subcutaneous implant, has been approved in some regions to increase sun tolerance. Patients with severe liver involvement may require liver transplantation, though disease can recur in the transplanted organ. Oral beta-carotene and other antioxidants have been used with variable benefit. Regular monitoring of liver function and protoporphyrin levels is essential for long-term management.

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