Deciphering Effects of Thalidomide on Red Blood Cells in Transfusion Dependents Beta Thalassemia Patients: A Pharmacodynamics and Pharmacogenetics Analysis

Thalassemias are a group of genetic disorder of hemoglobin synthesis characterized by a decreased synthesis of globin chain. Thalassemias are classified as α-,β-, γ-,or δ-thalassemias depending upon the chain whose synthesis is reduced. Among these, β-thalassemia is more common. In β-thalassemia there is a decreased or absent hemoglobin A (HbA). Normal adult hemoglobin is primarily HbA, which represents approximately 98% of circulating hemoglobin. HbA is formed from 2 α and 2 β chains (α2β2). Combination of 2 α and 2 δ (α2δ2) form hemoglobin A2 (HbA2). Combination of 2 α and 2 γ (α2γ2) form hemoglobin F (HbF) which is a major form of hemoglobin in fetal life but comprises less than 1% of adult hemoglobin. Human beta-globin protein is encoded by HBB gene on chromosome 11, which contributes two β polypeptide chains. Therefore, one chain is inherited from the mother and one inherited from the father. In normal (homozygote) both genes are normal and produce normal β polypeptide chains to normal quantity. In heterozygote one gene is normal and other is abnormal which does not produce β polypeptide chain. Such condition is called β-thalassemia minor. In such a person normal gene produces enough β chain to maintain hemoglobin level. In abnormal homozygote both beta chain genes are abnormal and do not produce β polypeptide chain. Such condition is called β-thalassemia major. Reduced or absent production of β-globin chains with relative excess of α-chains cause imbalance and free α chains precipitate within the red blood cells. This result in extensive premature destruction of red cell precursors in the bone marrow, referred to as "ineffective erythropoiesis". The imbalance causes peripheral hemolysis as well. The two phenomena culminate in clinically significant anemia.

Severity of anemia due to β-thalassemia depends upon the type of mutations in β-globin gene. Mostly point mutations and frame shift mutations, in intronic as well exonic region, are reported in Pakistani β-thalassemia patients. Some mutations completely block the β-gene and thus no β globin chain is produced (βo) while in other mutations there is production of some β globin chains (β+). In β-thalassemia major both β globin alleles undergo mutations (βo/βo) and disease is severe. In β-thalassemia minor only one of β globin alleles are mutated (β+/β or βo/β) and patient is asymptomatic. β-thalassemia intermedia (β+/β+ or βo/β+) is the form of disease which is milder than thalassemia major but more severe than thalassemia minor. Besides less severe β+ mutations, inheritance of HbF inducing genetic variations and co-inheritance of alpha globin mutations can also ameliorate the clinical severity of the disease. In regards to HbF induction, genetic association studies have shown that there are at least three major loci that play a major role in increasing HbF and are associated with severity of β-thalassemia. These include -158 C > T in the promoter gene Gamma 2 (locus XmnI), intergenic regions HBS1L-MYB in the 6q23.3 chromosomal region, and the BCL11A gene on chromosome 2p16.1.

Annually about 60000 babies are born with thalassemia all over the world. Global annual incidence of β-thalassemia is estimated at a rate of 1/100,000 . Carrier rate for β-thalassemia in Pakistan ranges between 5-8%, and around 5000 children are diagnosed each year with the disease .

Severe anemia in β-thalassemia patients necessitates frequent transfusions, which lead to iron overload. Chronic anemia and iron overload lead to several complications including skeletal deformities, splenomegaly, osteoporosis, endocrinopathies, growth retardation and cardiac complications. Iron overload is treated with iron chelating agents. Definitive treatment is bone marrow transplantation (9).Thalassemia patients who do not respond well to blood transfusions can take hydroxyurea or thalidomide, and sometimes a combination of both.

Thalidomide, an immuno-modulatory drug is currently approved by FDA for the treatment of multiple myeloma and Leprosy. Thalidomide is used off-label for the treatment of β-thalassemia patients worldwide. Thalidomide has been reported to reduce or even eliminate the need for red blood cell transfusions in patients with transfusion dependent β-thalassemia . Although association of β globin gene mutations and genetic modifiers with severity of β-thalassemia have been well studied but association of these genetic factors with response to thalidomide therapy still needs to be explored. A study conducted on 48 Chinese patients showed association of Xmn1 HBG2, BCL11A & HBSIL-MYB single nucleotide polymorphisms (SNPs) with response to thalidomide therapy. .