Diagnostic Use
This test is used to identify pathogenic variants in the HBB gene associated with β-thalassaemia and related β-globin disorders. The results may assist in diagnosis, carrier detection, reproductive risk assessment, and prenatal or preconception evaluation, particularly when both partners are known or suspected carriers of an inherited haemoglobin disorder.
Prior to requesting HBB gene analysis, patients should undergo appropriate first-line investigations, including a full blood count (FBC), haemoglobinopathy screen (including thalassaemia screening as locally available), and iron studies. These investigations help determine whether molecular testing is indicated and guide selection of the most appropriate genetic test, including HBB (β-globin) and/or HBA1/HBA2 (α-globin) gene analysis.
The indication for genetic testing should be reviewed by a haematologist, clinical geneticist, or genetic counsellor prior to testing. The request should include relevant clinical and family history, the indication for testing, and details of affected family members where available (including relationship and unique patient identifiers, if applicable). The specific gene(s) to be analysed should also be clearly indicated.
Interpretation
Information on β-Globin (HBB) Genetic Analysis
Pathogenic variants in the HBB gene, which encodes for the β-globin chain of haemoglobin, are among the most common inherited genetic disorders worldwide. Haemoglobinopathies and thalassaemias are particularly prevalent in populations originating from the Mediterranean region, Africa, the Middle East, the Indian subcontinent, Southeast Asia, and East Asia.
The spectrum and frequency of HBB pathogenic variants vary substantially between populations. In individuals of Chinese ancestry, several recurrent β-thalassaemia variants account for the majority of affected cases, including:
- Codons 41/42 (-TTCT)
- IVS-II-654 (C>T)
- Promoter variant -28 (A>G)
- Codon 17 (A>T)
Collectively, these variants account for a large proportion of β-thalassaemia alleles in many Chinese populations. Similarly, a limited number of recurrent variants account for most β-thalassaemia cases in populations from the Indian subcontinent, although the distribution of variants varies geographically within the region.
Haemoglobin E (HbE) is one of the most common structural haemoglobin variants worldwide and is particularly prevalent in Southeast Asia and parts of the Indian subcontinent. HbE results from the HBB c.79G>A (p.Glu27Lys; historically designated Glu26Lys) variant. This change produces a structurally altered haemoglobin and also affects normal RNA splicing, resulting in reduced β-globin production. Individuals who are heterozygous or homozygous for HbE are typically asymptomatic or have mild microcytic anaemia. However, compound heterozygosity for HbE and a β-thalassaemia pathogenic variant (HbE/β-thalassaemia) can result in a clinically significant disorder with variable severity.
Sickle cell disease results from the HBB c.20A>T (p.Glu7Val; historically designated Glu6Val) pathogenic variant, which causes production of haemoglobin S (HbS). This disorder represents one of the earliest molecular diseases to be characterised at the genetic level and remains an important example of a monogenic disorder affecting haemoglobin structure and function.
More than 1,000 HBB variants have now been described, including single-nucleotide variants, small insertions and deletions, and larger structural alterations. Because a targeted approach may not identify all disease-causing variants, comprehensive molecular analysis is often required. Depending on the clinical indication and laboratory methodology, testing may include full HBB gene sequencing, copy-number analysis, and evaluation for other clinically relevant globin gene variants.
Results should be interpreted in conjunction with haematological findings, haemoglobin studies, family history, and, where appropriate, genetic counselling.
References
- Chang JG, et al. Blood. 1992;80:2092-2096.
- Varawalla NY, et al. British Journal of Haematology. 1991;78:242-247.
Test Method
Beta-globin (HBB) variant analysis was performed by direct sequencing of PCR amplicons spanning the coding regions and relevant exon-intron boundaries of the HBB gene. Results and interpretations are based on the assumption that the submitted specimen was correctly identified and corresponds to the patient named on the request form.
Sequence analysis was performed using the reference transcript NM_000518.5 (HBB). Variant nomenclature follows the recommendations of the Human Genome Variation Society (HGVS). cDNA numbering uses the A of the translation initiation codon (ATG) as nucleotide c.1, with the initiator methionine designated as codon 1.