Intergenic Variants of HBS1L-MYB Are Responsible for a Major Quantitative Trait Locus on Chromosome 6q23 Influencing HbF Levels in Adults.

Sickle cell disease and β-thalassemia demonstrate a remarkable diversity in phenotypic severity, and the innate ability to produce fetal hemoglobin (HbF, α 2γ 2) is a major ameliorating factor in these β-hemoglobinopathies. HbF and F cells (denoted FC; subset of erythrocytes that contain measurable HbF) are highly heritable quantitative traits; cis-acting variants in the β globin gene locus explain some of the variability but over 50% of the variance is unlinked to the β globin gene locus. We have previously mapped a quantitative trait locus (QTL) controlling FC in an Asian-Indian kindred with β-thalassemia to a 1.5 Mb interval on chromosome 6q23 that encompasses five known protein-coding genes (ALDH8A1, HBS1L, MYB, AHI1 and PDE7B), none of which harbored mutations. From the known genes within this interval, we selected HBS1L, a GTP-binding protein family member, and MYB, a transcription factor central to hematopoiesis, as candidates for detailed study. Two panels of twin-pairs (824 and 1217 individuals, respectively) of North European origin were recruited through the UK Adult Twin Registry. Age and sex, and XmnI-Gγ (−158C/T variant upstream of the Gγ globin gene which is associated with FC levels) show similar associations with FC levels in both panels. Altogether, 121 markers identified by re-sequencing and from public databases were genotyped in the HBS1L-MYB intergenic region. The high-resolution association study in Europeans identified genetic variants within and 5′ to HBS1L that are highly associated with F-cell levels (p=10⁻⁷⁵). Additional HBS1L-MYB variants contribute independently to the QTL (p<10⁻³). Our study provides unequivocal evidence for the mapping of the chromosome 6q HbF / F-cell QTL to a 79 kb region flanked by HBS1L and MYB; the intergenic variants account for 16% of the trait variance. The biological effects of these intergenic variants appear to be complex as they may influence several genes or transcribed sequences in this genomic region. Our study illustrates the power of QTL mapping for positional identification of trans-acting genetic variants influencing fetal hemoglobin levels, which could lead to new diagnostic and therapeutic strategies.