Polimorfismos e Haplótipos de genes envolvidos na regulação da Hemoglobina Fetal basal e induzida pela Hidroxiureia em pacientes pediátricos com Anemia Falciforme

Abstract

Sickle cell anemia (SCA) is an autosomal recessive disease caused by a point mutation in the beta globin gene. It produces the HbS variant of hemoglobin. The polymerization of HbS triggers intravascular hemolysis and vaso-occlusion, the systemic manifestations of the disease. Fetal hemoglobin (HbF) inhibits the HbS polymerization, thereby reducing the SCA severity. Hydroxyurea (HU) is used in the treatment of SCA and its clinical efficacy is related to the induction of HbF. However, there is variability in the increase of HbF levels or toxicity in response to HU among patients with SCA. We investigated the effect of single nucleotide polymorphism (SNPs) on baseline and HU-induced HbF concentration, and their association with hematological and clinical outcomes in a cohort of 250 pediatric patients with SCA. The patients were diagnosed by the Newborn Screening Program of Minas Gerais from the Blood Center of Mina Gerais state, the HEMOMINAS Foundation. There is a retrospective cohort with 8 years of followup at least. In Chapter 1, we examined the effect of SNPs of candidate genes on baseline HbF concentration, hematologic parameters, and in the incidence of clinical complications of SCA in a cohort of 250 pediatric patients with SCA. The SNPs rs4671393 (BCL11A gene), rs9399137 and rs4895441 (HBS1L-MYB intergenic region, HMIP-2) were found to be independently associated with HbF concentration. SNPs associated with HbF were also found to be associated with reduced reticulocyte count and lower incidence of acute chest syndrome and blood transfusion. In Chapter 2, we estimated haplotypes composed by functional alleles of BCL11A (TAT; rs1427407, rs766432 and rs4671393) SNPs and HMIP-2 (CGC; rs9399137, rs4895441 and rs9494145). These haplotypes were found to be associated with less severe hematological parameters and reduced rate of clinical complications when compared to their respective wild-type haplotypes. Additionally, we observed that the BCL11A and HMIP-2 loci seems to interact in the regulation of HbF levels. In Chapter 3 we conducted a systematic review of the literature to answer whether genetic polymorphisms influence the HbF changes in patients with SCA treated with HU. We found evidence that SNPs of the intron 2 of BCL11A influences the HbF response in patients with SCA. A pathway enrichment analysis was conducted from the genes with associated SNPs found in the systematic review and we identified biological pathways potentially involved in the mechanisms of action of HU. In Chapter 4, we selected from the cohort of 250 pediatric patients with SCA, those who started treatment with HU during the retrospective follow-up (n = 110). We tested the effect of SNPs of the BCL11A gene and HMIP-2 intergenic region on the HbF changes in response to HU. In a multiple linear regression model adjusted for age, gender, follow-up time, HU dosage, and baseline HbF, we identified BCL11A SNPs (rs766432, rs4671393, and rs7599488) as predictors of ΔHbF (final HbF – baseline HbF) in pediatric patients with SCA treated with HU. The rs4671393 SNP was also found to be associated with a higher risk of blood transfusion incidence in these AF patients treated with HU. In conclusion, SNPs of the BCL11A gene and the HMIP-2 intergenic region influence baseline HbF and were found to be associated with a less severe clinical outcomes in patients with SCA. SNPs of the BCL11A gene seems to influence the HbF response in patients with SCA treated with HU, but their clinical useful remains to be evaluated.