AB045. Molecular markers for disease severity in beta thalassemia/Hb E disease

Abstract: Thalassemia is a hereditary disease affecting hemoglobin synthesis, characterized by microcytic hypochromic anemia. Homozygote or compound heterozygote patients usually manifested as thalassemia major which require regular treatment. There are five functional genes arranged in the order 5' ε-Gγ-Aγ-ψβ-δ-β 3' that are activated during development. Expression of the individual genes within the β-globin cluster is controlled by the complex interactions between local regulatory sequence (promoter regions) within each gene and the β-locus control region (β-LCR), located 6-18 kb upstream of the ε-globin gene. Beta thalassemia (β-thal) is a very heterogeneous disorder due to variations in inactivation mechanism of the β-genes. Point mutations and small deletions or insertions in the nucleotide sequences are the main molecular defects responsible for most β-thalassemia. In spite of seemingly identical genotypes, severity of β-thal patients can vary greatly. This heterogeneity in the clinical severity may occur from the nature of β-globin gene mutation, α-thalassemia (α-thal) gene interaction and difference in the amount of Hb F production that is partly associated with a specific β-globin haplotype. Co-inheritance of α-thal may ameliorate the severity of β-thal disease in those cases with mild β-thal genotypes. However, many patients who are β°/β+ thal or β°-thal/Hb E do not have a detectable α-thal haplotype but still have a mild clinical symptom suggests that there are other additional factors responsible for the mildness of the disease. Inheritance of a β-thal chromosome with the Xmn I+ haplotype at the position -158 of the Gγ-globin gene was found to be associated with increased Hb F production and milder anemia in patients with thalassemia intermedia and Xmn I +/+ haplotype is necessary to produce a significant clinical effect. Homozygosity for the Xmn I + haplotype, +/+, was also found in the mild cases of β-thal/Hb E. However, there is no severity difference among homozygous β-thal patients with Xmn I +/+, −/+ or −/−. The GWAS study of the whole genome with more than 6000,000 SNPs of 1,100 β-thal/Hb E patients with mild and severe diseases revealed SNPs in three independent genes that show significant association with the disease severity. The strongest SNPs associated with the disease severity located in three regions; the β-globin gene cluster on chromosome 11, the HBS1L-MYB intergenic region on chromosome 6q23 and the BCL11A gene on chromosome 2p15. Further analysis of Hb F level showed that Hb F level was significantly higher in mild patients than moderate and severe patients (%Hb F; mild =42.6±11.5, moderate =35.7±11.1, severe =32.4±12.1; P−5). Thirteen tagging SNPs were selected from three Hb F-QTLs. Of the common haplotypes CA haplotypes of BCL11A; CG haplotypes of HMIP; TTCTGTTAA and TTCTGTTAG haplotypes of β-globin gene cluster showed association with high HbF level. Our data indicated that several genetic loci act in concert to influence Hb F levels and disease severity of β-thal/HbE patients. Understanding the genetic modifier in β-thalassemia is important for the management of β-thalassemia patients from PND to prognosis and decision for difficult treatment such as stem cell transplantation. Moreover, this may lead to future alternative treatment of β-thalassemia patients as well.

Keywords: Thalassemia; functional genes; β-thal/HbE; HbF level