From OMIMPorphyrias are inherited defects in the biosynthesis of heme. Acute intermittent porphyria (AIP), the most common form of porphyria, is an autosomal dominant disorder characterized by recurrent attacks of abdominal pain, gastrointestinal dysfunction, and neurologic disturbances. The disorder is notably incompletely penetrant: fewer than 10% of heterozygous mutation carriers actually develop acute porphyric attacks. Acute attacks rarely occur before puberty; they may be precipitated by porphyrinogenic drugs such as barbiturates and sulfonamides (for list, see Tschudy et al., 1975), some of which are known to induce the earlier rate-controlling step in heme synthesis, delta-aminolevulinic acid (ALA) synthesis. Other known precipitants are alcohol, infection, starvation, and hormonal changes; attacks are more common in women. The precipitants cause increased ALAS1 (125290) activity and increased levels of the porphyrin precursors ALA and PBG. The half-normal hepatic HMBS activity in heterozygous AIP patients is insufficient to prevent pathologic accumulation of these precursors, which are most likely responsible for the symptoms (Petrides, 1998; Solis et al., 2004; Kevelam et al., 2016).
In the classic form of AIP, both the ubiquitous 'nonerythroid' housekeeping HMBS isoform and the 'erythroid' HMBS isoform are deficient. However, about 5% of families have the 'nonerythroid variant' of AIP, with a defect only in the ubiquitous nonerythroid HMBS isoform and normal levels of the erythroid HMBS isoform. Clinical characteristics in the 2 forms are identical; diagnostic methods based on the level of enzyme in erythrocytes are ineffective (Puy et al., 1998; Petrides, 1998; Whatley et al., 2000).
There are several other forms of porphyria: see porphyria cutanea tarda (176100), variegata porphyria (176200), coproporphyria (121300), acute hepatic porphyria (125270), and congenital erythropoietic porphyria (263700).
http://www.omim.org/entry/176000 From MedlinePlus GeneticsPorphyria is a group of disorders caused by abnormalities in the chemical steps that lead to heme production. Heme is a vital molecule for all of the body's organs, although it is most abundant in the blood, bone marrow, and liver. Heme is a component of several iron-containing proteins called hemoproteins, including hemoglobin (the protein that carries oxygen in the blood).
The porphyrias can also be split into erythropoietic and hepatic types, depending on where damaging compounds called porphyrins and porphyrin precursors first build up in the body. In erythropoietic porphyrias, these compounds originate in the bone marrow. Erythropoietic porphyrias include erythropoietic protoporphyria and congenital erythropoietic porphyria. Health problems associated with erythropoietic porphyrias include a low number of red blood cells (anemia) and enlargement of the spleen (splenomegaly). The other types of porphyrias are considered hepatic porphyrias. In these disorders, porphyrins and porphyrin precursors originate primarily in the liver, leading to abnormal liver function and an increased risk of developing liver cancer.
Researchers have identified several types of porphyria, which are distinguished by their genetic cause and their signs and symptoms. Some types of porphyria, called cutaneous porphyrias, primarily affect the skin. Areas of skin exposed to the sun become fragile and blistered, which can lead to infection, scarring, changes in skin coloring (pigmentation), and increased hair growth. Cutaneous porphyrias include congenital erythropoietic porphyria, erythropoietic protoporphyria, hepatoerythropoietic porphyria, and porphyria cutanea tarda.
Other types of porphyria, called acute porphyrias, primarily affect the nervous system. These disorders are described as "acute" because their signs and symptoms appear quickly and usually last a short time. Episodes of acute porphyria can cause abdominal pain, vomiting, constipation, and diarrhea. During an episode, a person may also experience muscle weakness, seizures, fever, and mental changes such as anxiety and hallucinations. These signs and symptoms can be life-threatening, especially if the muscles that control breathing become paralyzed. Acute porphyrias include acute intermittent porphyria and ALAD deficiency porphyria. Two other forms of porphyria, hereditary coproporphyria and variegate porphyria, can have both acute and cutaneous symptoms.
Environmental factors can strongly influence the occurrence and severity of signs and symptoms of porphyria. Alcohol, smoking, certain drugs, hormones, other illnesses, stress, and dieting or periods without food (fasting) can all trigger the signs and symptoms of some forms of the disorder. Additionally, exposure to sunlight worsens the skin damage in people with cutaneous porphyrias.
https://medlineplus.gov/genetics/condition/porphyria