Mapping the genetic code of Africa
Figure A shows normal red blood cells flowing freely in a blood vessel. The inset image shows a cross-section of a normal red blood cell with normal haemoglobin. Figure B shows abnormal, sickled red blood cells blocking blood flow in a blood vessel. The inset image shows a cross-section of a sickle cell with abnormal (sickle) haemoglobin forming abnormal strands. Image courtesy Wikipedia Commons, The National Heart, Lung, and Blood Institute (NHLBI)
African populations have the greatest genetic diversity in the world, yet very little is known about their DNA variations. As scientists begin to dig deeper into this genetic landscape, they are unravelling vital clues on how to combat deadly disease, and gathering fascinating historical insight into hominids that trudged from the Cradle of Humankind.
“Our genes carry the signatures of where we have been and what we have endured, but it is a history as yet unwritten on the African continent,” says geneticist in the Division of Human Genetics at the National Health Laboratory Service and Wits University, Michèle Ramsay.
Studies investigating complex genetic traits have been applied extensively to Europeans, but rarely to Africans due to a lack of funding, poor infrastructure and public health systems, and a small pool of trained scientists on the continent.
Ramsay explains it is vital to map these genetic variations in order to better understand why diseases have a greater impact on some populations than others, and in turn design ways to counter these illnesses.
One interesting African case study surrounds the sickle-cell mutation. During the 1940s, doctors noticed that family members of patients who had sickle-cell anaemia, a serious hereditary blood disease, were more likely to survive malaria. Malaria, according to the World Health Organisation, killed more than 655 000 people worldwide in 2010 in documented cases.
Although patients with sickle-cell anaemia are seriously ill, it turns out that people carrying the sickle-cell haemoglobin (oxygen-carrying molecule in our blood) in high-risk malaria regions have a greater chance of survival (sometimes referred to as natural selection) because the trait confers some resistance to malaria.
Says sickle-cell specialist and professor at Albert Einstein College of Medicine Dr Ronald Nagel, “Their red blood cells, containing some abnormal haemoglobin, tend to sickle when they are infected by the malaria parasite. Those infected cells flow through the spleen, which culls them out because of their sickle shape, and the parasite is eliminated along with them.”
Because some patients with sickle-cell anaemia have a milder variation of the disease, researchers are looking at ways to translate this phenomenon into potential solutions to treat sickle cell anaemia.
Human diversity and evolution
A recent genomic study of Africa’s hunter-gatherer populations analysed the fully sequenced genomes of 15 Africans belonging to three different hunter-gatherer groups from Cameroon and Tanzania, which, according to ScienceDaily, “deciphers some of what these genetic codes have to say about human diversity and evolution”.
“[The study] identifies several million previously unknown genetic mutations in humans. It suggests that different groups evolved distinctly in order to reap nutrition from local foods and defend against infectious disease, and it identifies new candidate genes that likely play a major role in making Pygmies short in stature,” says ScienceDaily.
Project lead scientist Professor Sarah Tishkoff and her group plan to sequence the genomes from more African populations, a task that new technology has made faster and cheaper than ever before, to increase their sample size.
South African scientists have also begun a study under the Southern African Human Genome Programme, funded by the Department of Science and Technology, to study the genomes of our populations.
Will studies based on these populations lead to a new wave of discovery of genetic contributors to disease? We’re drawing closer to more conclusive answers.