Genetic Modifiers of HbF and Phenotypic Severity in Malaysian β-Thalassemia Patients
By: Siti Aisyah bt. Abdul Razak (aisyahrazak2391@gmail.com)
β-thalassemia is one of the most common genetic disorders in the world. It is caused by the imbalance of β-²µ±ô´Ç²ú¾±²Ô chain synthesis in the haemoglobin tetramer due to mutations in the β-²µ±ô´Ç²ú¾±²Ô gene. The excess unbound α-globin chain will then precipitates in the red blood cells which lead to haemolysis of the red blood cells.  β-thalassemia is divided into 3 major types, which are: major, intermedia and carrier. In Malaysia, it was estimated that nearly 1 million of Malaysians are thalassemia carriers, while 1-5 babies born in 1000 live births are diagnosed with β-thalassemia. With a total number of 4,768 transfusion-dependent thalassemia patients from the National Thalassemia Registry in May 2010, the clinical management of thalassemia major in our country impose a remarkable economic burden for our health care system, due to the expensive cost of RM1104-3313 (US $294-882) for iron chelation per patient per month. Hence, the current treatment needs to be revolutionized, in order to reduce the morbidity and mortality of β-thalassemia in Malaysia.
The phenotypic severity of β-thalassemia is highly dependent on several types of genetic modifiers. The primary genetic modifier is responsible in determining the disease severity based on the mutation of β-²µ±ô´Ç²ú¾±²Ô gene carried by the patients. On the other hand, secondary genetic modifiers have been detected from Genome Wide Association Study (GWAS) in several populations. These genetic modifiers are responsible for the modulation of fetal haemoglobin (HbF) production in adults. HbF is highly expressed throughout the fetal life. The γ-²µ±ô´Ç²ú¾±²Ô gene in the HbF has high affinity towards oxygen, which in turn facilitates the delivery of oxygen to the fetus through placenta circulation. Hence, re-activation of the HbF for β-thalassemia patients during adult could reduce the disease severity.
There are 3 genes involved in the modulation of HbF: BCL11A, HBS1L-MYB intergenic region and XmnI-HBG2. BCL11A is a repressor for γ-²µ±ô´Ç²ú¾±²Ô gene during the haemoglobin maturational switching; HBS1L-MYB intergenic region act as a regulatory element to control the expression of MYB (activator for BCL11A repressor), while XmnI-HBG2 is a restriction site located at the promoter region of γ-²µ±ô´Ç²ú¾±²Ô gene. The polymorphisms found in these 3 genes act as genetic biomarkers for detecting clinical severity in β-thalassemia patients.
To date, local studies have only emphasized on the β-²µ±ô´Ç²ú¾±²Ô gene mutations that cause β-thalassemia, and have yet to focus on the genes modulating the phenotypic severity of β-thalassemia among the Malaysian population. Hence, in our study, genotyping is performed using Agena Mass Spectrometry platform in order to identify the polymorphisms that are highly associated with clinical severity of the disease in Malaysia.
A total of 37 SNPs from 4 different genes, eg: BCL11A, HBS1L-MYB, olfactory receptor and HBB gene are selected for further studies. In order to retain the specificity, the SNPs are divided into 2 different assays of genotyping. As of now, we have genotyped 81 samples using these 37 SNPs. Most of the SNPs were able to be genotyped, except for 2 SNPs, eg: rs6924609 and rs1320693 from HBS1L-MYB intergenic region. The study is currently still on-going.