A 3-Bp Deletion Between Transcription Factor Binding Motifs In the HBS1L-MYB Intergenic Region on Chromosome 6q23 Is Associated with HbF Expression

Abstract 1013

More than 3% of Chinese in Hong Kong are heterozygous carriers of β-thalassemia. Homozygotes or compound heterozygotes for β-thalassemia are usually severely ill and require monthly transfusions. Increased production of fetal hemoglobin (HbF) can modulate the disease severity by compensating for the shortfall of HbA caused by the β-thalassemia mutations. HbF level in adults varies and is regulated as a multigenic trait. Three major HbF quantitative trait loci (QTL) have been identified: the C/T SNP also known as the Xmn I site at the ^Gγ-globin gene promoter, the BCL11A polymorphism on chromosome 2p16, and the HBS1L-MYB intergenic polymorphism (HMIP) on chromosome 6q23. The functional motif for each of these 3 QTLs responsible for their effects upon HbF is not known. We undertook a genome-wide association study (GWAS), using Illumina Human 610-Quad BeadChip array, on 619 Chinese β-thalassemia heterozygotes from Hong Kong. In this population, the variance in HbF due to HMIP is 13.5%, significantly higher than that due to BCL11A polymorphism (6.4%). We used 1,000 Genomes Project data, SNP imputation, comparisons of association results across populations, predicted binding of transcription factors, and phylogenetic conservation to identify the functional variant in HMIP. Based on these lines of evidence, a hitherto unreported association between HbF expression and a 3-bp deletion on chromosome 6q23 was found. In 335 Chinese β-thalassemia heterozygotes, the 3-bp deletion polymorphism is in complete linkage disequilibrium with rs9399137, the SNP found in multiple GWAS to be most significantly associated with HbF (P=1.4E-24 in the Chinese cohort GWAS). Flanking this deletion are conserved binding sites for TAL1/SCL1, E47, GATA, and RUNX1/AML1, which are essential erythropoiesis-related transcription factors. The 3-bp deletion changes the normal DNA binding configuration of these transcription factors and spatial configuration for DNA-protein binding and/or protein-protein interactions. Furthermore, this 3-bp deletion polymorphism resides within a likely erythroid distal regulatory region manifested by DNase I hypersensitivity and GATA-1 binding (Wahlberg et al, Blood 114:1254, 2009). We hypothesized that a 61-bp fragment of DNA that encompasses the site of the 3-bp deletion polymorphism might have enhancer-like activity. When ligated to the ^Gγ-globin gene 1.4 kb proximal promoter linked to a luciferase reporter gene, the 61-bp fragment of DNA enhances the ^Gγ-globin gene promoter activity by more than 3-fold after transient transfection into K562 cells. A 58-bp fragment of DNA that includes the 3-bp deletion has 60% more enhancer-like activity than the 61-bp fragment without the deletion. These findings suggest that this 3-bp deletion polymorphism is most likely the functional motif accounting for HMIP modulation of HbF. Further studies are needed to identify target genes for this enhancer-like activity mediated by the DNA sequences encompassing the 3-bp deletion polymorphism in HMIP. These studies also suggest that this experimental approach could be used to identify functional motifs in other genotype-phenotype association studies.