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Evolution of sickle cell malaria

Evolution of sickle cell malaria

Genmod11
Sickle cell anemia is caused by a mutation in the beta globin gene resulting in abnormal hemoglobin, called hemoglobin S.  The abnormal hemoglobin is the result of a single nucleotide polymorphism (SNP) which means just one nucleotide is different from "wild type".  This single nucleotide difference results in a change in the sixth amino acid from glutamic acid to valine.  This mutation causes the hemoglobin molecules in red blood cells to aggregate.  Aggregation results in a sickle cell shaped red blood cell which has difficulty moving through tight capillaries.  The resultant inability of red blood cells to deliver oxygen to different tissues is the clinical manifestation of the disease.  1

Clinical SymptomsEdit

Simple physical exertion is enough to cause a patient with sickle cell anemia to have severe physical pain and could be as serious as a stroke or heart attack due to lack of oxygen delivery.  Most sickle cell anemia patients also experience a crisis which is characterized by severe pain in the lower back, legs, joints and chest.  Numerous other symptoms are caused by sickle cell anemia (see reference) and there is unfortunately no cure for the disease. 

Current treatments are to alleviate symptoms.  Blood transfusions can be given during a crisis to prevent stroke and antibiotics are prescribed to fight off infection, which are one most common causes of death in sickle cell patients.  The second most likely cause of death is organ failure.  2

Malaria Resistance Edit

People with only copy of the sickle cell gene and a second, normal copy of the beta-globin gene have some sickling of red blood cells, but also enough normal red blood cells to compensate for this.  These individuals also experience less susceptibility to infection with the protozoan parasite, Plasmodium spp., the causative agent of malaria.  This is due to the fact that the parasite infects red blood cells and with some cells being sickled they are unable to infect as efficiently.  This is thought to be a product of evolution as this is most commonly seen in regions endemic with malaria.  3

Lessons from the 23andme WebsiteEdit

According to John Burke's information on the 23andme website, he does not have the S variant in either copy of the HBB gene.  Therefore, he does not have any degree of intrinsic resistance to malaria nor does he experience illness due to two copies of the S variant resulting in sickle cell anemia.  

ReferencesEdit

1.  Clancy, Suzanne.  Genetic Mutation.  Nature Education.  2008.  1(1):187. 

2.  Sayre, Carolyn.  Sickle Cell Anemia.  New York Times. 3 March 2013.    http://www.nytimes.com/health/guides/disease/sickle-cell-anemia/

3.  Beutler, Ernest et. al.  Effect of Sickle Cell Trait on Resistance to Malaria. British Medical Journal.  1955 May 14.  1(4923):  1189-1191.