Diagnostic Use

This test is used to detect the genetic basis of alpha 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 Alpha Thalassaemia Genetic Analysis:

The alpha thalassaemias are common genetic disorders that result from reduced synthesis of the alpha globin chains of fetal (alpha2gamma2, HbF) and adult (alpha2beta2, HbA) haemoglobin. The alpha-globin gene cluster region is located on the short arm of chromosome 16 and consists of four genes and three pseudogenes arranged in the order 5′ to 3′ zeta2, psizeta1, psialpha2, psialpha1, alpha2, alpha1and theta.

Alpha thalassaemia is common throughout Southeast Asia and the Mediterranean area, and deletions resulting in the loss of one (- alpha) or both (- -) alpha genes from the normal chromosome (alpha alpha) account for the majority of cases. Two gene deletions in cis are the most clinically significant although single gene deletions as a compound heterozygote with a two gene deletion also are clinically relevant.

Individuals who inherit two or three functional a genes (- alpha / alpha alpha, -alpha / -alpha, or — / alpha alpha) have a thalassaemia trait with a mild hypochromic microcytic anaemia.  Patients who inherit a single a globin gene (– / -alpha) have HbH (beta4) disease, a moderately severe haemolytic anaemia.  Those who inherit no alpha genes (– /–) develop severe intra-uterine anaemia which , in the absence of intensive neonatal care and life-long transfusion, results in death at or around the time of birth.  This condition is known as Hb Barts ( gamma4 ) hydrops fetalis syndrome.

Analysis for alpha thalassaemia deletions involves a multiplex PCR which looks at three two-gene deletions: —^SEA, —^THAI, and —^FIL as well as the two most common single gene deletions; -alpha^3.7 and -alpha^4.2 for the Southeast Asia area, and also the —^MED and -(alpha)^20.5 two-gene deletions for the Mediterranean area.  As a control for amplification success a large segment (2.3kb) of the 3’UTR of the LIS1 gene is included.

In New Zealand the most common (severe) form of a thalassaemia is within the immigrant population from South East Asia (SEA) and this severe type occurs as a result of no functional a globin genes (– / –).  This two gene deletion form is sometimes written as  — SEA.

Whilst the most common defects resulting in alpha thalassaemia are due to deletions of either one or both alpha-globin genes, a small number of point mutations, usually within the alpha2 gene, may give rise to clinically significant alpha thalassaemia.  The alpha2 and alpha1-globin genes are highly homologous which makes it difficult to distinguish between the two by PCR.  There are however sufficient sequence differences in their 3’UTRs to permit the design of primers for differential amplification and subsequent sequencing of the individual genes.

References:
Bowden DK et al., Brit J Haematology  81 (1992) 104-108
Tan ASC et al Blood 2001; 98, 250-251