Cholesterol synthesized by the human body plays an important role as part of the cell membrane. It is a type of sterol. However, plants do not synthesize cholesterol. Instead they have phytosterols of which sitosterol is a comparatively abundant phytosterol. When we eat plant derived foods, phytosterols are ingested into the body but they are not absorbed into the blood. Sitosterolemia is a rare recessive autosomal disease in which individuals absorb large quantities of plant sterols and this is stored in their blood and tissues. It is also referred to as phytosterolemia or plant sterol storage disease.
The clinical signs may include small yellowish outgrowths on various parts of the body, called xanthomas. These may also occur within the body such as in the tendons. These are made up of accumulated lipids. Joint stiffness and haemolytic anaemia may also be present.
The underlying cause of this genetic disorder is mutations in the genes ABCG5 or ABCG8. These genes code for a transport protein sterolin (sterolin 1 and sterolin 2 respectively) The sterolin 1 and sterolin 2 then form a heterodimer which acts as a transporter protein. Sterolin is the protein involved in transport of sterols out of the apical cells of the intestines. When there exists a mutation in any one of these genes, this transport protein malfunctions resulting in sitosterolemia. An interesting observation is that, mutations have always been seen in both the alleles of any 1 of the genes never in both of them together.
People diagnosed with sitosterolemia need to consume foods lower in plant and shellfish sterols. Patients may be given ezitimibe which acts as a sterol absorbtion inhibitor. If the individual is not responding to this therapy then use of cholestryramine and/or partial ileal bypass surgery may be recommended.
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