INTERACTION BETWEEN HAPLOTYPES OF BCL11A AND HBS1L-MYB, MAJOR QUANTITATIVE TRAIT LOCI FOR FETAL HEMOGLOBIN (HBF) LEVELS, AND INCIDENCE OF CLINICAL PHENOTYPES IN CHILDREN WITH SICKLE CELL ANEMIA

Abstract

Sickle cell anemia (SCA) is a monogenic disease characterized by HbSS genotype with a complex pathophysiology. Fetal hemoglobin(HbF) is the major modifier of clinical phenotypes, and increased HbF levels inhibit polymerization of HbS and ameliorate clinical severity of SCA. GWAS have identified the BCL11A gene and HBS1L-MYB intergenic region as major quantitative trait loci for regulation of HbF expression. We identified functional SNPs of BCL11A and HBS1L-MYB associated with increased HbF concentration and milder clinical phenotypes in children with SCA. Moreover, we found HbF-boosting haplotypes formed by these SNPs to be associated with reduced rate of clinical outcomes in children with SCA: BCL11A haplotype with the minor alleles of rs1427407, rs766432, and rs4671393 (“TCA”) was associated with higher HbF, total hemoglobin, and lower reticulocyte count compared to reference haplotype “GAG”; the HBS1L-MYBhaplotype with the minor alleles of rs9399137, rs4895441, and rs9494145 (“CGC”) was associated with higher HbF and total hemoglobin compared to reference haplotype “TAT”. We further found that these HbF-boosting haplotypes may interact to regulate HbF levels. However, no previous study has examined their interaction and association with risks of vaso-occlusive and hemolytic complications of SCA. Objective: We examined whether the interaction of HbF-boosting haplotypes of BCL11A and HBS1L-MYB modifies their individual effect on the rate of transfusion, acute chest syndrome (ACS), pain crisis, infection and acute splenic sequestration (ASS). Methods: We analyzed a retrospective cohort of 220 unrelated children with SCA recruited from the Center of Hematology and Hemotherapy of Minas Gerais State. Haplotype frequencies were estimated using Haplo.stats. Reference and minor haplotypes combined wild-type and minor SNP alleles for BCL11A (GAG or TCA) and HBS1L-MYB (TAT or CGC), respectively. We grouped children who carry (“TCA+”or “CGC+”) or not carry (“TCA-”or “CGC-“) the “TCA”and “CGC”minor haplotypes, respectively. Incidence of clinical outcomes was reported by relative rates to 10 patient-years, with 95% confidence intervals. Incidence Rate Ratio of clinical outcomes was compared between haplotype combinations using OpenEpi online software. Cumulative incidence of ASS was estimated using Kaplan–Meier and logrank test to compare the incidence between haplotypes using SPSS. Results: Non-carriers of both HbF-boosting haplotypes (TCA-/CGC-) had a significantly higher need for transfusion than children carrying only the HbF-boosting haplotype of BCL11A (TCA+) or those carrying both HbF-boosting haplotypes (TCA+/CGC+). Regarding ACS and infection, non-carriers of both HbF-boosting haplotypes (TCA-/CGC-) had a significantly higher risk of events than children carrying only the BCL11A (TCA+) or HBS1L-MYB (CGC+) haplotypes. Carriers of both HbF-boosting haplotypes (TCA+/CGC+) did not show significant lower rate of ACS and infection. Cumulative incidence of ASS was marginally significant lower (P = 0.051) in children carrying only the HbF-boosting haplotype of BCL11A (TCA+/CGC-) than the HBS1L-MYB (TCA-/CGC+) or non-carriers of both haplotypes (TCA-/CGC-). Discussion: Our findings showed that the presence of the HbF-boosting haplotypes don't make a difference in rate of ACS, ASS and infection. On the other hand, our data support that the presence of, at least, one of the HbF-boosting haplotypes are related to lower rates of transfusion. Conclusion: Therefore, our novel findings contribute to understand multilocus interactions underlying molecular basis of major clinical events of SCA.