Sickle cell anemia
Sickle cell anaemia is the most common of the haemoglobinopathies, a family of disorders caused by structurally abnormal haemoglobin or insufficient quantities of normal haemoglobin. Sickle cell occurs primarily in individuals of African descent, affecting one in every five hundred newborn infants in the United States alone. Sickle cell is a homozygous recessive disorder and occurs in those who have inherited two mutant genes from their parents (one from each parent). An individual can inherit a single gene from one parent, but will not develop sickle cell anaemia. The child must inherit one sickle cell gene from each parent in order to develop sickle cell disease. In the United States, the proportion of people affected correlates with the probability of two individuals carrying the sickle cell gene to meet and have a child. With genetic counselling, detection of the trait in each parent is possible. This helps parents in the decision making process of having children considering the existing possibility of passing two defective genes to a child. This is important considering one in ten African-Americans have one normal and one sickle cell gene. In Africa, the proportions are much higher. There are more carriers of the sickle cell gene and without the supportive measures of genetic counselling, there is a higher risk of having a child with sickle cell disease.
When a child inherits a sickle cell gene from each parent, the haemoglobin in the erythrocyte is slightly altered. The defect is caused by the mere substitution of a glutamate residue (amino acid) by a valine residue. These amino acids along with others comprise proteins. Since the chains of globin molecules composing haemoglobin are proteins, they are now structurally different. The alteration is minute, yet devastating for the child. The difference in substitution for a single amino acid significantly alters the shape of haemoglobin. The molecules now aggregate into fibres that deform the red blood cells into crescent, or sickle-shape, characteristic of this disease. Normal red blood cells flow smoothly through vessels because of their characteristic rounded shape. Sickle cells, on the other hand, frequently get caught in small diameter capillaries due to their crescent shape. This blocks normal blood flow and leads to local oxygen deprivation of the blood deficient area. The resulting anoxia also causes intense pain and death of the local cells. The irregular-shaped sickle cell also contacts the wall of blood vessels with more force causing it to rupture more easily. Sickle shaped cells also have a decreased life span compared with the one-hundred twenty day average life span of the normal erythrocyte.
Sickle cell anaemia is an example of not only a genetic disorder, but also natural selection in action and its ability to combat an infectious disease. Although the effects of sickle cell are both unfortunate and devastating, the high incidence of the sickle cell gene amongst ethnic Africans suggests that this defect is a selective advantage for heterozygotes (carriers of a single sickle cell gene) and homozygotes (carriers of two sickle cell genes). Both heterozygotes and homozygotes for the sickle cell gene are less susceptible to certain forms of malaria.
Malaria is a disease caused by one of four parasites from the genus Plasmodium. Plasmodium are parasites of blood cells and require mosquitoes and humans as hosts for different phases of reproduction. A bite from an Anopheles mosquito introduces the infectious plasmodia sporozoites through its saliva, into the circulatory system of its human host. From the blood stream, the sporozoites reach the liver where asexual reproduction occurs. Eventually, hepatocytes (cells in the liver) rupture, releasing the plasmodia, which ultimately reach and enter the red blood cells. Asexual reproduction of the parasite occurs in the red blood cell producing different stages of the parasite. The red blood cell eventually ruptures, releasing the parasites to infect other red blood cells.
Many red blood cells are damaged by malaria. Depending on the species of malaria the individual is infected with, they can survive for years without treatment. Without treatment, however, malaria infections can lead to organ (e.g., brain, liver) damage from malarial products, cellular debris of the damaged cells, and capillary blockage of these organs by masses of adherent erythrocytes.
The most dangerous species of Plasmodia is Plasmodium falciparum. However, the red blood cells in both the heterozygote and homozygote for sickle cell anaemia have a shorter life span than normal red blood cells. Since sickle cells have a shorter life span, the parasite cannot complete its development. This may offer a selective advantage to heterozygotes living in parts of the world where malaria is a major cause of death.