SICKLE CELL ANAEMIA
Sickle- cell anaemia is the most prevalent genetic disease in the world. It is caused by a mutation (change in base sequence) of the gene which codes for the production of the alpha-globin polypeptide in haemoglobin. Haemoglobin is a protein found in red blood cells which carries oxygen.
The symbol for a normal haemoglobin allele is Hb. HbA is the common allele found in humans which codes for normal haemoglobin. However, in certain cases, a base substitution mutation converts the sixth codon of the gene from GAG to GTG. This results in the formation of a new allele- HbS which causes sickle-cell anaemia. This allele can be inherited by offspring if their parent(s) carry it, but an individual will only show symptoms if he/she inherits it from both their parents. This allele is more common amongst people of African, Caribbean, Middle Eastern and Asain descent.
During the transcription process in the nucleus, the mRNA produced has GUG in place of GAG as the sixth codon. This codes for the production of the amino acid valine instead of glutamic acid. This replacement in the haemoglobin molecules causes them to stick together and form stiff fibers. These pull the red blood cells into a sickle shape.
This not only reduces the surface-area to volume ratio of the cells thus reducing their oxygen- carrying capacity, but also causes severe joint pain since these cells get stuck in joints and prevent blood flow. The fragile nature of the sickle cell reduces its lifespan to 4 days. The body cannot produce and replace red blood cells at the same rate at which the sickle- cells degenerate, hence causing anaemia. In serious cases, the accumulation of sickle cells in joints can completely block blood from flowing to organs, resulting in a “sickle cell crisis” leading to severe pain and organ failure. Blood flow to the brain being blocked can result in a stroke.
Hence, sickle cell anaemia is a disease that results in an overall low quality of life. It can be managed by medication, blood transfusions or stem cell transplants.