Understanding sickle cell Anemia
World sickle cell day is observed today
Rayees Ahmad Kumar
Each year on June 19, countries all over the world celebrate world sickle cell day which was initially presented as a resolution by the United Nations in 2008. The goal of this day is to recognize sickle cell anemia as a public health problem and promote education about this genetic condition.
Sickle Cell disease is one of the most common inherited diseases worldwide with between 300000 to 400000 babies affected each year. This disease can occur in any race or ethnic group but is most common in people of Africa.
In recent years a large number of human disorders, deformities and diseases have been found to be hereditary. Some of these are caused by gene mutations, others by gross chromosomal abnormalities. These genetic disorders are mainly caused by alteration and mutation in the single gene. They are transmitted to the off springs following the principle of inheritance.
Understanding what the sickle cell anemia is
Sickle cell anemia is an autosomal hereditary disease found widely in tropical Africa and also in American blacks whose ancestors came from that part of Africa. It is characterized by sickle shaped red blood cells formed under low oxygen conditions. Change in the shape of red blood cells is due to the presence in them of a defective type of haemoglobin called sickle cell haemoglobin or haemoglobin S.
This is the major affect of the pleiotropic gene and formation of sickle shaped corpuscles by haemoglobin S is one of the secondary effects. The molecule of haemoglobin S has a different shape from that of the normal haemoglobin called haemoglobin A molecule. When the red blood cells containing haemoglobin S face oxygen shortage, their haemoglobin molecules aggregate and form stiff fibers.
These fibres distort the cells into odd shapes such as sickles. Because of their inflexibility, the sickle shaped cells cannot easily squeeze through narrow capillaries unlike the normal flexible corpuscles which change their shape to pass through narrow capillaries. The sickled cells get stuck in the small capillaries and reduce circulation to the regions supplied by these vessels. The sickled cells also break down more rapidly, decreasing the number of red blood corpuscles leading to anaemia. These effects deprive the tissues of oxygen. This also produces a variety of other symptoms such as tiredness, headache, fever , jaundice , poor growth etc.
The gene involved in sickle cell anaemia codes for the beta polypeptide chain of haemoglobin ( the oxygen carrying protein present in the RBCs imparting them red clour). Sickle Cell anaemia develops in the individuals who have a sickle allele formed by a change in just one nucleotide pair in its DNA.
This alteration results in the substitution of one amino acid Valine for Glutamic acid which is the sixth amino acid in the beta polypeptide chain of haemoglobin. This small change turns the haemoglobin A into haemoglobin S. The disease is menifested only in homozygous recessive state ie when an individual has both the alleles for sickle cell haemoglobin (Hbs,Hbs).
The individuals homozygous for sickle cell anaemia are called affected individuals. About half of the affected individuals die by the age of 20.Moreover affected women have fewer babies than the heterozygous or homozygous normal women. Natural selection keeps the lethal sickle allele at a low frequency in the population because many affected persons die without producing off springs.
About 20 to 40% of people in tropical Africa have the sickle allele in homozygous condition .This shows that the heterozygous have some selective advantage. Presence of a single sickle cell allele lowers the heterozygotes chances of developing Malaria Thus the sickle allele in heterozygous condition protects the person against Malaria.
A person may have one gene for normal haemoglobin and one gene for sickle cell haemoglobin (Hbs, Hba).This heterozygous condition is called sickle cell trait.The heterozygous individuals are carriers of sickle cell gene. The single amino acid substitution from Glutamic acid to Valine because of gene mutation results a change in the shape of RBC from biconcave disc to elongated sickle like structure. Marriage between two carriers produces diseased and normal children in the ratio of 1:2:1( 1 normal, 2 carrier and 1 affected). The affected individual dies when adult.
Birth of the children with sickle cell anaemia can be avoided by checking marriage among the heterozygous or carriers. The heterozygotes are identified through blood examination.
(The writer is a teacher)
KV Network
