Sickle cell malaria survival secret found, according to a report in Nature News 10 November 2011 and ScienceDaily 18 November 2011.
It has been known for many years that people who carry one gene for a disease named sickle cell anaemia have some increased resistance to malaria, and the sickle cell gene is more common in populations in malaria prone regions. Scientists at Heidelberg University in Germany and the Biomedical Research Center Pietro Annigoni in Ouagadougou, Burkina Faso have studied red blood cells infected with malaria parasites, and compared how one type of malaria parasite, named Plasmodium falciparum, functions in normal cells and in cells from people who carry one sickle cell gene.
After the parasites invade a red blood cell they take a protein in the red cell named actin and use it to transport a protein, named adhesin, made by the parasite, to the cell surface. The parasite protein makes the cells sticky and causes them to stick to each other and to the blood vessel walls, causing widespread inflammation of small blood vessels. The sickle cell mutation causes a small change in the haemoglobin molecules (molecules that carry oxygen in the blood) so the haemoglobin molecules become unstable and form a degraded molecule named ferryl haemoglobin inside the cell. This hinders the parasite from using the actin to send adhesin to the cell surface.
According to Nature News this is “evolution to the rescue” for people who live in regions where malaria is endemic. According to Michael Lanzer, one of the researchers, the “take home message” is “that the parasite, in order to survive within the red blood cell, has to remodel the host actin — and that evolutionary pressure has resulted in mutations in human haemoglobin that prevent this remodelling.”
Editorial Comment: The sickle cell mutation has been used as an example of a good mutation produced by evolution, as evidenced by the large number of people who have the gene in malaria prone areas. However, as this new research shows, people with both the sickle cell mutation and malaria parasites are simply experiencing the result of two bad effects colliding with one another. This does not make either effect good.
Sickle cell resistance to malaria is not complete so the parasites still cause illness in anyone they invade. The reason the gene persists in populations exposed to malaria is that people with one sickle cell mutation are less likely to experience the worst aspects of falciparum malaria, and are more likely to survive to pass the sickle cell gene to the next generation.
People with two sickle cell genes have a devastating blood disease. The ferryl haemoglobin does not carry oxygen, and causes red blood cells to become severely distorted and damage small blood vessels. People with one sickle cell gene and one normal gene can make enough normal haemoglobin to survive, but it is not good.
Evidence News 28 March 2012
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