Sickle Cell Anemia: Causes and Genetic Inheritance

• Sickle cell anemia is a global disorder occurring when the sickle(s) gene is inherited from both parents in the homozygous SS state.
• Heterozygous inheritance of the sickle gene (AS) is known as sickle trait.
• It was initially thought that malaria causes sickle cell disease because the disease develops where malaria exists.
• The occurence of sickle cell provides a protective benefit against malaria.

What Causes Sickle Cell Disease

• Sickle cell disease involves the presence and activity of the β gene, but it is a mutant gene where a mutation has occurred at position 17.
• The 17th nucleotide of the gene for the beta chain of hemoglobin has A replaced by T. This changes the codon GAG (for glutamic acid) to GTG (encodes valine).
• The 6th amino acid of the β chain (made of 146 a.a) becomes valine instead of glutamic acid forming an abnormal chain called βº (S from sickle). The resulting hemoglobin is a2 β2º called Sickle Hb (HbS).
• Glutamic acid is highly charged and renders the β chain highly soluble, while valine is neutral. The chain produced by the replacement easily precipitates upon reduced O2.
• When α2 β2º is exposed to reduced O2 tension, its molecule polymerizes into lengthy needles called tactoids, which is the defect itself. If tactoids form in RBCs, they'll be deformed into a sickle cell.
• Reduced O2 tension causes the RBCs to sickle, occurring at the tissue level.
• When oxygenated, HbS is fully soluble and the sickle cells go back to normal (in the lungs).
• Hemoglobin polymers form in the sickle red cells when oxygen is released, causing deformation. The deformed cells interrupt the delivery of oxygen to the tissues by blocking the flow of cells.
• The central patho-physiology of sickle cell disease is the loss of red blood cell elasticity. They become quite elastic allowing for deformation to pass through capillaries, a feature lost in sickle cell.
• Low oxygen promotes red blood cell sickling, and frequent episodes of sickling damage the cell membrane, reducing its elasticity.
• The rigid blood cells cannot deform through narrow capillaries, leading to increased destruction in the spleen, vessel occlusion, and Ischemia.
• Severe symptoms include fatigue, weakness, pallor, and various symptoms associated with sickle cell crises, resulting from vessel occlusions.

Cause of Anemia in Sickle Cell Disease

• The misshape of the red cells causes their destruction inside the spleen, resulting in anemia.
• Intravascular hemolysis may contribute to anemia to a certain degree.
• The bone marrow attempts to compensate by producing new red cells, but its production rate does not match the rate of destruction.
• While healthy red blood cells survive for 90-120 days, sickle cells survive only for 10-20 days.

Sickle Cell Crises

• Sickle cells are rigid and do not easily pass through small vessels, causing them to block these vessels. The tissues supplied by these vessels will die as a result.
• Patients experience sickle crises with local death of tissues, causing multiple damage to the liver, joints, bones, heart, lungs, spleen, and kidneys.
• Splenic infarction and fibrosis occur due to sickle occlusion. Eventually, the spleen is reduced in size and function (auto splenectomy).
• Any situation that causes excessive deoxygenation of the red cells can lead to a crisis like infection, dehydration, violent exercise, high altitude, anesthesia, exposure to cold
• Auto splenectomy predisposes patients to infections, which are a major cause of morbidity and mortality in sickle cell patients.

Diagnosis of the Homozygote

• Homozygous persons can’t produce HbA at all (0%); the main hemoglobin is HbS (90% or more), with remaining HbF (varying amount up to 5%-10%)
• Too little Hb A2 makes its ratio unimportant.
• There is no sickling, no anemia, and these people live entirely normal lives if the HbF is higher.
• In African populations, only 5% of the total amount of HbF exsists, which causes the population to suffer from sickle cell disease.
• In Arabian populations (Saudi Arabia, Kuwait, Bahrain), HbF is elevated up to 40-50% and HbS reaches 50-60%. These people are symptom free, and normal.
• HbF dilutes HbS rendering the picture similar to the heterozygote.
• HbF has a great affinity for O2, so that even under hypoxic conditions, the cell remains oxygenated.
• The cause of elevated HbF in this tribe and newborns is unknown.
• Anemia is usually severe (Hb is 5-9 g/dl) and of the normocytic, slightly hypochromic type.
• The peripheral blood smear shows slight Anisopoikilocytosis.
• The majority of RBC(s) are normocytic round and ~ hypochromic.
• Few sickle cells and target cells.
• Nucleated RBC(s), polychromatic and stippled RBCs

Hb Electrophoresis

• HbS: 90-95%
• HbF: 5-10%
• HbA2: variable
• The diagnosis can be confirmed with high -performance liquid chromatography (HPLC).

Diagnosis of the Heterozygote

• A person who receives the defective gene from both parents develops the disease, while a person who receives one defective and one healthy allele remains healthy and is known as a carrier.
• Carriers have one normal gene and one sickle gene, with ranges of HbA at 60-70% and HbS at 30-40%, with normal values for HbF & HbA2.
• When a cell contains less than 60% HbS, it does not sickle spontaneously.
• Only normal RBCs can be found in the peripheral blood film of a person with sickle cell trait.
• Sickle cell tests require the reduction of O2 tension, since these people are normal and will only sickle when O2 tension is low.

Sickling Test

• A small drop of blood is mixed with a reducing agent like 2% sodium metabisulfite.
• With the reduced O2 tension, the heterozygote person will show a characteristic feature with holly leaf deformity where there is enough HbA to dilute the HbS.

Solubility Test

• This test uses the principle that Hb S is less soluble in the presence of a reducing agent, dithionite, in a concentrated organic buffer than other hemoglobins.
• A positive test shows Hb S precipitating and forming a cloudy solution.
• Electrophoresis at alkaline pH yields a major band of HbA (60-70%), a smaller band of HbS (30-40%) between HbF and Hb A2, a normal band of Hb A2, and a normal band of Hb F.

Treatment

• Occasional transfusions combined with iron chelation therapy
• Management of pain
• Hydroxyurea that increases HbF production
• B.M/Stem Cell transplantation