A few weeks ago I heard about a 24 year old man who had been living with the sickle cell condition but had never had a crisis, not even a small one. The man always thought he was AA until he was taken to a doctor-friend’s hospital for the first time for routine medical checks as required by his law school. It was discovered through the tests that he was a Sickle Cell Patient.
Before I heard this story, I had always wondered why some individuals living with sickle cell were very sickly, looked thin and had all the symptoms of the condition while some others didn’t. I somehow dismissed it to the fact that perhaps some people took better care of themselves. Even though I was not entirely wrong, there is more to why sickle cell individuals had different levels of symptons and conditions.
Now without going too technical, I will try and break it down (thank you Dr Ope). There is something called Haplotypes which in layman terms mean subtypes of genotypes. So it has been discovered that even though there are SS genotypes, there are many types which depend on geographical locations that they were first identified. There are however four major types which are:
- The Senegal (SEN)
- The Benin (BEN)
- The Central African Republic or Bantu (CAR)
- The Asia
Other types include Cameroon (CAM), Atypical etc.
The SEN haplotype has the best clinical course, which means it has the least symptoms. They also usually have a high hemoglobin F. The higher the hemoglobin F, the lesser the sickle cell manifestation. Therefore SEN is the best.
CAR or Bantu is known as the worst type of Haplotype, with the least hemoglobin F. Individuals with this have multiple organ damage and more clinical manifestation.
BEN haplotype is intermediate. From a study, about 92% of S genotype in Nigeria is the BEN. 2% of people in Nigeria have the SEN while 0.1% is CAR/ Bantu which explains why once in a while, we come across people who are not as symptomatic but have the SS genotype.
To go beyond Nigeria, according to Fields (2000), “the Senegal haplotype is represented most prominently in Senegal and in the most westerly regions of Africa above the Niger River. The Benin haplotype designates those found in Nigeria, Benin, and countries in the Bight of Benin. The Bantu or CAR haplotype encompasses those haplotypes discovered in the Central African Republic and countries in south central Africa. The Cameroon haplotype has been found in only one ethnic group in the Cameroons. The Arabo-Indian haplotype usually refers to those haplotypes found in the Persian gulf and India.” He further went ahead to say that “Sickle cell disease in India has been poorly investigated, relative to that in west Africa and the Middle East. Sickle cell disease is quite prevalent among tribal people in India. Unfortunately, the tribal people continue to have limited access to health care, in part due to their largely rural location.”
Each haplotype has a number. SEN is 3, BEN is 19, CAR is 20. So usually the numbers are portrayed like this 19/19, 3/19, 20/3 etc. So this means the type of S inherited from each parent will determine how a Sickle Cell individual will manifest symptoms. For example, if I inherit a SEN gene from my mother and a BEN gene from my father, I won’t have the worst type of symptoms, as a matter of fact, my manifestations will not be so bad. However, some people are not so lucky and can inherit BEN from both parents, they will be prone to having the worst type of symptoms.
To further explain why different people will bare different or similar haplotypes is based on the ancestors they have. So if you had an ancestor from Senegal, you are likely to inherit the haplotype from that ancestor.
All the regions or countries mentioned above have the sickle trait as a result of a natural mutation which was meant to protect individuals in those regions from Malaria. People with one sickle hemoglobin gene and one normal hemoglobin gene (sickle cell trait or AS) are somewhat more resistant to malaria than people with two normal hemoglobin genes (AA). People with sickle cell trait would have a better chance of surviving an outbreak of malaria and passing their genes (for sickle and normal hemoglobin) to the next generation when they have children.