Variable Expression of Fetal Hemoglobin In Sickle Cell Disease Patients From Oman

Abstract 4808

Although sickle cell disease (SCD) is a recessively inherited monogenic disorder, its clinical expression is variable from patient to patient, even within the same family; and, both heritable and environmental factors significantly contribute to this inter-patient difference in disease severity. The most prominent genetic modifiers include co-inherited alpha thalassaemia, over expression of fetal hemoglobin (HbF) in adulthood, and the excessive production of insoluble bilirubin. The purpose of this study was to ascertain the emerging role of HbF by studying several known SNP's in Chromosome 2, 6 and 11 that are involved in modulating the fetal hemoglobin levels in patients from Oman. This case-control study was conducted at Sultan Qaboos University Hospital from February to May 2010. Eighty eight SCD patients were consecutively studied along with forty four Omani blood donors who were selected randomly to complete a 2:1 case-control enrollment after an informed consent. Whole blood samples were collected in EDTA tubes from SCD patients and donors. DNA was extracted from the whole blood after Complete Blood Count and High Performance Liquid Chromatography were performed on the patient and donor samples. The molecular analysis included genotyping of BCL11A, HBSIL-MYB, HBG2 SNP's by direct DNA sequencing with 3100 genetic analyzer (ABI systems, Illinois, USA) using Tag SNP's in each category namely rs11886868, rs4671393 (BCL11A-Ch.2), rs7776054, rs9399137, rs4895441 (HBSIL-MYB-Ch.6), & rs7482144 (HBG2-Ch.11) to correlate these SNP's with the HbF expression in the SCD patients.

A stepwise regression analysis result of the 6 SNP's genotyped at the BCL11A, HBSIL-MYB, and HBG2 gene locus were correlated to the HbF levels in our SCD study cohort and is shown below. At each step, the least significant SNP was removed, until all SNP's left in the multivariate model were significant. The model at step 4 has the best goodness of fit value, as assessed by the likelihood ratio test, than models 1, 2 & 3. [Table]

Step 1: HbF levels~rs7482144 + rs7776054 + rs9399137 + rs4895441 + rs11886868 + rs4671393

Step 2: HbF levels~rs7482144 + rs7776054 + rs9399137 + rs11886868 + rs4671393

Step 3: HbF levels~rs7482144 + rs9399137 + rs11886868+ rs4671393

Step 4: HbF levels~rs7482144 + rs9399137 + rs4671393

Our study has demonstrated a significant association between the BCL11A, HBSIL-MYB, HBG2 SNP's and HbF levels. Together these 6 SNP's accounted for ~46% variation in the HbF levels in our SCD study subjects and in part explains the clinical heterogeneity seen in these patients. Three SNP's one each in Chromosome 2, 6 and 11 namely rs4671393, rs9399137 and rs7482144 respectively demonstrated the strongest effect on HbF levels.