24/05/2015
Sickle cell disease (SCD)
Sickle-cell disease (SCD), also known as sickle-cell anaemia (SCA) and drepanocytosis, is a hereditary blood disorder, characterized by an abnormality in the oxygen-carrying haemoglobin molecule in red blood cells. This leads to a propensity for the cells to assume an abnormal, rigid, sickle-like shape (sickle cell) under certain circumstances.
The first modern report of sickle-cell disease may have been in 1846 but the abnormal characteristics of the red blood cells, which later lent their name to the condition, was first described in 1910 by Ernest Edward Irons (1877–1959), an intern to the Chicago cardiologist and professor of medicine James B. Herrick (1861–1954). Herrick made the first description in the medical literature. In the 1940s and 1950s contributions by Nobel prize-winner Linus Pauling made it the first disease where the exact genetic and molecular defect was elucidated.
In 1954, the introduction of haemoglobin electrophoresis allowed the discovery of a particular subtypes "SC" disease different from the original "SS" disease.
GENETICs
Normally, humans have haemoglobin A, which consists of two alpha and two beta chains, haemoglobin A2, which consists of two alpha and two delta chains, and haemoglobin F, consisting of two alpha and two gamma chains in their bodies. Of these, haemoglobin F dominates until about 6 weeks of age then A dominates throughout life.
The sickle-cell hemoglobin occurs when the sixth amino acid (glutamic acid) in the A chain, is replaced by valine to change its structure and function; as such, sickle-cell anemia is also known as E6V. Valine is hydrophobic, causing the haemoglobin to collapse on itself occasionally.
Sickle-cell disease occurs when a person inherits two abnormal copies of the haemoglobin gene (SS or SC), one from each parent. Several subtypes exist, depending on the exact mutation in each haemoglobin gene. A person with a single abnormal copy (AS or AC) does not experience symptoms and is said to have sickle-cell trait. Such people are also referred to as carriers.
Sickle-cell conditions have an autosomal recessive pattern of inheritance from parents. The types of haemoglobin a person makes in the red blood cells depend on what haemoglobin genes are inherited from her or his parents. If one parent has sickle-cell anaemia (SS or SC) and the other has sickle-cell trait (AS or AC), then the child has a 50% chance of having sickle-cell disease (SS or SC) and a 50% chance of having sickle-cell trait (AS or AC). When both parents have sickle-cell trait, a child has a 25% chance of sickle-cell disease, 25% do not carry any sickle-cell alleles, and 50% have the heterozygous condition.
Sickle-cell gene mutation probably arose spontaneously in different geographic areas, as suggested by restriction endonuclease a**lysis. These variants are known as Cameroon, Senegal, Benin, Bantu, and Saudi-Asian. They are important because some are associated with higher HbF levels, e.g., Senegal and Saudi-Asian variants, and tend to have milder disease (less crisis).
DATA
Almost 300,000 children are born with a form of sickle-cell disease every year, mostly in sub-Saharan Africa, but also in other parts of the world such as the West Indies and in people of African origin elsewhere in the world. In 2013 it resulted in 176,000 deaths which is an increase from 113,000 deaths in 1990.
PROBLEMS
Sickle-cell disease is associated with a number of acute and chronic health problems:
Increased risk of severe bacterial infections due to loss of functioning spleen tissue (autosplenectomy) caused by encapsulated organisms such as Streptococcus pneumoniae and Haemophilus influenza.
Clogging of the blood vessels by sickled cells leading to occlusion (blockage) of the vessels and hence malfunctioning of the organs such vessel is serving. This is vaso-occlusive crisis (VOC). These includes, bone pain crisis, acute chest syndrome, mesenteric infarction, priapism, stroke etc. Further complications of this include: kidney failure, chronic leg ulcer, and necrosis of the bones (most commonly avascular necrosis ‘AVN’ of femoral head), osteomyelitis, arthritis and fracture, blindness and pregnancy loss
Other crisis include hyper hemolytic crisis (excessive blood destruction) leading to anemia and heart failure. This can also cause cholelithiasis (gall stones) and cholecystitis.
There is also Sequestration crisis in which blood pools into the spleen and Aplastic crisis in which the bone marrow suddenly fails to produce any blood cell. Severe anemia, heart failure and sepsis may result.
All these can lead to death.
Carriers (AS and AC) have symptoms only if they are deprived of oxygen (for example, while climbing a mountain) or while severely dehydrated.
TREATMENT
Children born with sickle-cell disease undergo close observation by the pediatrician and require management by a haematologist to assure they remain healthy.
They’re given FOLIC ACID daily for life.
From birth to five years of age, they may also have to take PENICILLIN daily due to the immature immune system that makes them more prone to early childhood illnesses.
They are more vulnerable to malaria hence they take malaria prophylaxis, PROGUANIL tablet daily. The malaria vaccine undergoing trials in recent time will be very helpful if found successful.
HYDROXYUREA is the first approved drug for the causative treatment of sickle-cell anaemia. It has been found to decrease the number and severity of attacks.
Other medications that generally improve health may be beneficial to them such as MULTIVITAMINS, B-COMPLEX and VIT C.
They should take WATER (can be flavored) liberally everyday to prevent dehydration.
They should avoid hash conditions such as extreme cold, sunlight, stress and maintain generally a good level of personal hygiene.
Bone marrow transplants have proven effective in children. Bone marrow transplants are the only known cure for SCD. However, bone marrow transplants are difficult to obtain because of the specific HLA typing necessary. Ideally, a twin family member (syngeneic) or close relative (allogeneic) should donate the bone marrow necessary for transplantation.
About 90% of patients survive to age 20, and close to 50% survive beyond the fifth decade.
QUESTION: Should a prospective couple each with genotypes AS or AC marry each other?