sickle cell trait

Sickle cell trait (or sicklemia) describes the way a person can inherit one of the genes of sickle cell disease, but not develop recurrent symptoms. Sickle cell disease is a blood disorder in which the body produces an abnormal type of the oxygen-carrying substance hemoglobin in the red blood cells. Sickling and sickle cell disease is an adaptive evolutionary response to reduce malaria parasitization of red blood cells.

Sickle hemoglobin

Normal hemoglobin is called hemoglobin A, but people with sickle cell disease have only hemoglobin S, which turns normal, round red blood cells into abnormally curved (sickle) shapes.

Normally, a person inherits two genes (one from each parent) that produce beta-globin, a protein needed to produce normal hemoglobin (hemoglobin A). A person with sickle cell trait inherits one normal beta-globin gene (hemoglobin A) and one defective gene (hemoglobin S).

Prevalence

Sickle cell trait prevalence is highest in West Africa (25% of the population). However, it also very infrequently appears in Mediterranean countries such as Italy, Greece, and Spain, where it most likely expanded via the selective pressure of malaria, a disease that is endemic to the region.^[1] It has been described in Turks, North Africans, Iranians, Arabs, Middle Eastern nations including Iran, and in Indians.^{[citation needed]}

One out of every ten African-Americans has sickle cell trait.^{[citation needed]}

Symptoms

Sickle cell trait (hemoglobin genotype AS) is generally regarded as a benign condition.

However, individuals with sickle cell trait may have rare complications. There have been reports of pulmonary venous thromboembolism in pregnant women with sickle cell trait,^[2] or men during prolonged airflight, mild strokes and abnormalities on PET scans in children with the trait, complicated migraine headaches,^[3] poor urinary concentrating ability, increased incidence of renal carcinoma, and sudden deaths during physical exertion in US black army recruits,^[4][5] splenic infarcts at high altitude, even in white patients with the trait. However, they can pass the sickle cell trait to their children, and in rare cases, exercise-induced dehydration or exhaustion can cause healthy red blood cells to turn sickle-shaped, which can cause death during sporting activities.^[6] There have been calls to reclassify sickle cell trait as a disease state, based on its malignant clinical presentations.^[7] Sickle cell trait appears to worsen the complications seen in diabetes mellitus type 2 (retinopathy, nephropathy and proteinuria)^[8] and provoke hyperosmolar diabetic coma nephropathy) especially in male patients. In some cases, athletes with sickle cell trait do not achieve the same level of performance as elite athletes with normal hemoglobin AA. Athletes with sickle cell trait and their instructors must be aware of the dangers of the condition during anaerobic exertion especially in hot and dehydrated conditions.

Renal Medullary Carcinoma, a cancer affecting the kidney, is a very rare complication seen in patients with sickle cell trait^[9].

A person in whom both beta-globin genes are abnormal (they produce hemoglobin S) has sickle cell disease, which can cause serious problems. Both parents must have either the sickle cell trait or the disease itself for a child to have sickle cell disease. Genetic/prenatal counselling is available to reduce the incidence of Hemoglobin SS conceptus.

People who have the sickle cell trait have reduced susceptibility to cerebral malaria, a situation which may be referred to as heterozygote advantage. This is called "balanced polymorphism" where a deleterious gene confers a survival advantage. However, people with the sickle cell trait can still contract severe cases of malaria.

References