Variant types of Haemoglobinopathies

What are hemoglobinopathies? A group of inherited disorders characterized by structural variations of the Hb molecule. They are Disorders of globin synthesis rather than heme synthesis. These may result from : Synthesis of abnormal Hb Reduced rate of synthesis of NORMAL α or β globin chains Genetic defects of Hb are the most common genetic disorders worldwide.

People only have these disorders if they inherit two unusual hemoglobin genes – one from their mothers, and one from their fathers. People who inherit just one unusual gene are known as ‘carriers’. (Some people call this having a ‘trait’.) Carriers are healthy and do not have the disorders.

It is estimated that globally at least : 5% of adults are carriers for a hemoglobin disorder, 2.9% for thalassaemia and 2.3% for sickle cell diseases. In KSA*, some studies reported the incidence of Hb S for the studied neonates to be 14.4%, ranged from 0.8% in Najran to 26.4% in Al-Qurayyat In the eastern provinces the disease is generally milder whereas in the western provinces the disease is severe. *Kuwait Medical Journal 2002, 34 (2):156-160

Classification Can be classified into three main groups : 1. Variant Haemoglobins. 2. Thalassaemia. 3. Hereditary Persistence of Foetal Haemoglobin (HPFH)

Variant Hemoglobins Variant Hbs result from substitution of one or more amino acids in the globin portion of the molecule at selected positions in the two alpha or two beta polypeptide chains. Usually caused by single point mutations. Although more than 100 variants have been described, only hemoglobins S, C, and D are commonly seen. Hb S Sickle cell disease.

Many variant Hbs are hematologically and clinically silent because the underlying mutation causes no alteration in the function, solubility, or stability of Hb. Some structural variants are associated with severe clinical phenotypes ; these mutations affect the physical or chemical properties of Hb, resulting in changes in hemoglobin solubility, stability, or oxygen-binding properties.

Common variants Hb S Hb C Hb E Hb D-Punjab Hb O-Arab Hb G-Philadelphia Hb Hasharon Hb Korle-Bu Hb Lepore Hb M Hb AS Hb AC Hb ADPunjab Hb AE Hb AOArab

Thalassaemias Result from reduced rate of synthesis of α or β globin chains. These may be due to point mutations or insertion/deletions.

Hereditary Persistence of Foetal Haemoglobin (HPFH) Benign group of conditions. Synthesis of foetal haemoglobin remains raised throughout life. Molecular causes may be deletional or non deletional

The abnormality in Haemoglobinopathies may occur in the heterozygous or the homozygous form. Zygosity refers to the similarity of genes for a trait (inherited characteristic). If both genes are the same, the person is homozygous for the trait. If both genes are different, the person is heterozygous for that trait

In the heterozygous form, hemoglobin A and the variant both appear in the red cell : ( Hb AS, Hb AC). Little or no clinical manifestation of disease may be present. In the homozygous form, only the variant hemoglobin is present (Hb SS, Hb CC), and the characteristic symptoms of that hemoglobinopathy appear. Mixed heterozygous forms are also known to occur (Hb SC). The normal hemoglobin A may be absent, and two or three hemoglobin variants may be present.

Laboratory methods Initial tests recommended include: Complete blood count (CBC). Electrophoresis at pH 9.2 Tests for solubility and sickling Quantification of Hb A2 and Hb F. If an abnormal Hb is identified on the initial tests, then further techniques are recommended to identify the variants. These techniques include electrophoresis at pH 6.0–6.2, globin chain separation, and isoelectric focusing (IEF).

Although electrophoresis at alkaline and acid pH has been used for many years, cation-exchange HPLC is emerging as the method of choice for quantification of Hb A2 and Hb F and identification of Hb variants. HPLC = High Performance Liquid Chromatography PCR techniques are used in definition of known globin chain mutations/deletions, including those for Hb S, E, D, and O, and several ß-thalassemias. PCR = Polymerase Chain Reaction