Diagnostic Use

This test is used to detect the genetic basis of beta globin disorders. This is particularly useful for prenatal diagnosis for couples where both partners carry an inherited globin disorder.

Before any thalassaemia gene testing is requested, patients should have a full blood count, thalassaemia / haemoglobinopathy screen and iron studies completed. These results help determine if thalassaemia gene testing is appropriate and whether alpha or beta globin gene tested is required. The need of gene testing should be discussed and approved by a haematologist or a geneticist / genetic counsellor prior to ordering the test. The request form should include information on the indications for testing, the relationship and NHI of affected family members and which genes (alpha or beta) should be tested.

Interpretation

Information on Beta Globin Genetic Analysis:

Mutations within the beta-globin gene resulting in disorders of haemoglobin represent some of the most common inherited diseases worldwide. Higher frequencies of these diseases (haemoglobinopathies and thalassaemias) have been found in the Mediterranean, Africa, the Middle East, India, Southeast Asia and Southern China.

The four most common mutations detected in the Chinese population are (in descending order of frequency):

These four mutations are present in >86% of most beta-thal Chinese populations.  Each of these defects can be detected using PCR with primers which artificially create a new restriction enzyme site if the mutation is present.

The five most common mutations detected in the Indian population are (in descending order of frequency):

These five mutations should account for >90% of most beta-thal Indian populations.  There are regional differences in the Indian subcontinent and some mutations are more (or less) frequent in some areas.  The percentages shown are an average frequency over the whole subcontinent.

Haemoglobin E (HbE) is the most common haemoglobin variant and is particularly prevalent throughout the eastern Indian sub-continent and Southeast Asia.  The mutation is a G to A in exon one which has a twofold effect.  Firstly this base change alters codon 26 from a glutamic acid to a lysine which results in a less stable haemoglobin and secondly the mutation introduces a cryptic splice site within exon 1 thus resulting in a lowered expression.  Both HbE heterozygotes and homozygotes are asymptomatic, minimally anaemic and have microcytic and hypochromic red blood cells. However, when the HbE allele is paired up with a beta thalassaemia mutation in the compound heterozygous state a variable and often severe anaemia is produced.

The molecular abnormality in Sickle Cell anaemia was first postulated by Linus Pauling in 1949 and represented the first true example of a molecular disease. Vernon Ingram later in 1956 was able to demonstrate that the sickle cell beta-globin differed by a single amino acid at position 6. This glutamic acid to valine change is caused by an A to T mutation.

Although a relatively small number of mutations are common within certain ethnic races, a targeted approach to mutational analysis will never identify a defect in all cases.  So far more than 200 different molecular defects have been identified throughout the human beta-globin locus and therefore a strategy of screening the whole of the beta-globin gene by direct sequencing has been devised. The beta-globin gene can be screened using 7 overlapping PCR sequences.  Seven PCR products are produced for each patient and direct sequencing on the ABI3730 is used to identify the underlying defect.

References:
Chang et al. Blood  80; 2092-2096: 1992
Varawalla NY et al. Brit J Haemat. 78; 242-247: 1991