Thalassemia: Understanding the Disease

Thalassemia refers to a group of inherited disorders caused due to abnormal or inadequate production of the hemoglobin molecule, present in our red blood cells. Thalassemia is derived from two words – Thalassa means ‘the sea’ in Greek, which refers to the Mediterranean Sea; Haema is ‘blood’ in Greek, indicating the obvious prevalence of the disorder among people of the Mediterranean region. However thalassemia is not restricted to this region alone and a high prevalence of this disease is seen China, the Philippines, Thailand, and Indonesia, also from Persia, Iran, Iraq and Turkey, Tunisia, Belgian Congo, and South Africa. Worldwide, approximately 15 million people are estimated to suffer from thalassemic disorders. In India too the burden of these disorders is very high with nearly 12,000 infants being born every year with a severe disorder. These numbers imply that every hour 1 child is born who will suffer with this genetic disorder. Few of the ethnic groups like Sindhis, Gujaratis, Punjabis, Jains, Marwadis, etc are at high-risk communities for this disease.

Let us understand a little more in depth about the disease.

The Hemoglobin (Hb) molecule which is present in the RBCs is responsible for carrying the oxygen that we breathe from the air and transporting it to the tissues for energy. The Hb molecule is made up of two types of proteins called as alpha-globin (a-globin) and beta-globin (b-globin). Genetic defect in any of these proteins results in an inadequate production of the Hb-molecule resulting in thalassemia. If the a-globin production is abnormal, then the resulting condition is called a-thalassemia, whereas if b-globin production is abnormal it is called b-thalassemia.

The reason for the abnormal or inadequate production of these components is due to mutations in the genes (alterations in the DNA which is the genetic code) of these proteins. If an individual receives the mutated DNA from both the parents (we get 50% of our DNA from our mother and 50% from our father called alleles) then this situation is homozygous thalassemia or thalassemia major. On the other hand if the individual gets only one defective gene from one parent and the other gene is normal then it results in a heterozygous condition also known as the carrier state. In fact these diseases were among the first to be analyzed with the use of molecular biology. Their detailed characterization has established many of the general principles supporting our understanding of human molecular genetics. Till date more than 200 β-thalassemia alleles have been described in the database of human haemaglobin variants and thalassaemias, which involve mutations in any of the stages of β-globin production These mutations are detectable by DNA analysis using molecular biology techniques and provide the basis for genetic counseling.

Clinical Phenotypes

Beta-Thalassemia exists in 3 forms:  

Thalassemia trait or the asymptomatic carrier stage – The carrier does not exhibit any symptoms and leads an absolutely normal life.

Thalassemia major – In β thalassemia major, the production of β-globin chains is severely impaired, because both β-globin genes are mutated.Clinical presentation of thalassemia major occurs at 6 months of age. Affected infants fail to thrive and become progressively pale. Feeding problems, diarrhea, irritability, recurrent bouts of fever, and progressive enlargement of the abdomen due to splenomegaly may occur. Children with untreated or partially treated thalassaemia major die in the first or second decade of life.

Thalassemia intermedia – Genotypically these individuals are similar to thalassemia major, meaning that they too are homozygous or are compound heterozygotes (have two different mutations) in the beta-globin gene, but are phenotypically different in that they do not require regular transfusions.

Alpha-thalassemia (α-thalassemia) has two clinically significant forms

Hb Bart syndrome (–/–): All the alleles of the alpha-globin (there are 4 copies of this gene) are inactivated. This is a very severe condition characterized by fetal onset of generalized edema, pleural and pericardial effusions, and severe hypochromic anemia. Death usually occurs in the neonatal period.

HbH disease(–/-a)- Deletion or dysfunction of three alleles of the a-globin gene results in HbH disease. It is characterized by microcytic hypochromic hemolytic anemia, hepatosplenomegaly, mild jaundice, and sometimes thalassemia-like bone changes.

Alphaº (–/aa) -thalassemia results from deletion or dysfunction of two alleles, and α+-thalassemia results from deletion or dysfunction of one allele (-a/aa) or (-a/-a). Carriers of αº-thalassemia (α-thalassemia trait) may show mild changes in red blood cell parameters, whereas Carriers of α+-thalassemia have either a silent hematologic phenotype or present with a moderate thalassemia-like hematologic picture.

Diagnosis & Screening

The first step in the screening and diagnosis of thalassemias would be the blood film with the classical phenotype being hypochromic microcytic red blood cells and reduced RBC indices such as MCH and MCV.

 

 

The next logical step that follows is a qualitative and quantitative analysis of the hemoglobin fractions by Hb electrophoresis and/or HPLC (High Pressure Liquid Chromatography) since there is abnormal production of the hemoglobin chains.

Basically there are four major types of hemoglobin:

► embryonic hemoglobin, ( from 3rd to 10th week of gestation )

► fetal hemoglobin HbF (during pregnancy)

► adult hemoglobin HbA (which replaces HbF after birth)

► minor adult component HbA2

Under normal conditions the red cell of an adult human contains approximately 98%HbA, 2% HbA2 and traces of HbF. The ratio of the various Hemoglobin components deviates from the normal range depending on the type of thalassemia and this is detected by Hb analysis.

Molecular Diagnosis

However the confirmation of the disease can only be done by Molecular Genetic testing. Molecular genetic testing detects the specific mutations and other defects in the beta-globin gene or the alpha-globin gene depending on the clinical suspicion.

More importantly molecular diagnosis using techniques such as Polymerase Chain reaction (PCR) allows in correctly highlighting the specific typology of thalassemias in individuals with uncertain hematological picture.

It also helps characterize individuals who are carriers for the thalassemia trait and could be at risk for having a child with thalassemia major.

However the highest contribution of molecular diagnostic testing in the context of thalassemias is in the pre-natal diagnostic testing arena. Prenatal testing is based on performing the molecular genetic test on the fetal material which is obtained through an invasive procedure, either amniocentesis or chorionic villus sampling. Using PCR the fetal genetic material is then tested for specific genetic mutations for which the parents may be carriers or an extended panel where the entire spectrum of mutations in the alpha-globin or beta-globin gene may be examined.

With tremendous advances in technology and molecular biology techniques researchers and scientists worldwide are looking towards developing non-invasive methods for testing fetal material. Success has already been achieved for certain genetic disorders like Down’s syndrome and hopefully will soon be extended to other disorders.

International guidelines indicate that Prenatal diagnosis should be offered to all carrier couples at risk of having a child with a clinically significant thalassemia and should be performed with the informed consent of the woman or couple. As with many of the blood disorders, no cure exists for thalassemia. Therefore, prevention efforts focused on early identification and the promotion of health behaviors that prevent or lessen complications of this disease are emphasized.

Author-

       

       Dr. Aparna Bhanushali,     

       

       Research Scientist,     

       

        SRL R&D

 

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