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Introduction: Hydroxyurea is a potent therapeutic agent for sickle cell anemia (SCA), and treatment at maximum tolerated dose (MTD) is becoming the standard of care. Hydroxyurea exerts its disease-modifying effects primarily through induction of fetal hemoglobin (HbF), although the cellular and molecular mechanisms by which hydroxyurea increases HbF expression remain unclear. Children with SCA treated with hydroxyurea at MTD have substantial phenotypic variation, however, as some have higher HbF responses than others. We hypothesized that unknown quantitative trait loci modulate the pharmacological induction of HbF, so we performed a large genome wide association study (GWAS) of hydroxyurea-associated HbF responses for children with SCA treated prospectively with dose escalation to MTD. Methods: We analyzed genomic DNA from 831 children with SCA enrolled in pediatric research trials from the US (HUSTLE, SWiTCH, TWiTCH), the Caribbean (EXTEND, SACRED) and sub-Saharan Africa (REACH, NOHARM); all of these trials reported robust treatment responses with average HbF >20%. Study participants received hydroxyurea with dose escalation to MTD based on mild myelosuppression. Whole blood DNA was genotyped using the H3Africa SNP array (Illumina) with whole exome sequencing (WES) using NimbleGen VCRome 2.1 capture reagents and the Illumina HiSeq2500 platform. A transformed z-score for each study cohort gave a standardized measure of HbF induction relative to their steady-state level and their treatment HbF level at MTD. These standardized z-score HbF values were then used as a continuous variable for association testing using single-locus mixed model (EMMAX) adjusted for population stratification, using age, hydroxyurea dose at MTD, and sex as co-variates. We first performed an initial GWAS discovery using hydroxyurea response data from four distinct African populations (n=377). Single nucleotide variants (SNVs) with nominal significance (p Results: In the discovery GWAS step, no variant passed genome wide significance (p Conclusions: This large GWAS using global cohorts of children with SCA and robust prospective HbF phenotype data has identified genetic predictors of HbF hydroxyurea treatment responses. Three novel genetic loci, PTPRD, RPH3AL, and ELL2 have SNVs associated with lower HbF responses. PTPRD is a protein tyrosine phosphatase receptor involved in cellular processes such as cell growth and differentiation, while RPH3AL, a rabphilin 3A like protein, is known to be involved in calcium-ion-dependent exocytosis. ELL2 is an elongation factor for RNA polymerase II and could modify RNA processing under the cytostatic effects of hydroxyurea. These genes and variants will be investigated to determine how they impact individual HbF responses to hydroxyurea treatment. Disclosures Aygun: National Heart, Lung, and Blood Institute: Research Funding; bluebird bio: Membership on an entity's Board of Directors or advisory committees, Research Funding; National Institute of Nursing Research: Research Funding; Patient-Centered Outsomes Research Institute: Research Funding.

Introduction: Elevated levels of fetal hemoglobin (HbF) are known to ameliorate both the morbidity and mortality of sickle cell anemia (SCA). Sustained post-natal HbF expression is heritable and regulated by multiple quantitative trait loci. Previous genomic studies have identified three major gene loci (BCL11A, HBS1L-MYB, and HBG2) that account for ~40% of HbF variation in SCA, but additional genetic modifiers remain to be discovered. We performed a genome wide association study (GWAS) using DNA collected from multiple cohorts of children with SCA, to identify novel genes and variants involved in HbF expression. Methods: We analyzed genomic DNA from 1009 children with SCA and pre-treatment steady-state HbF levels who enrolled in prospective research trials from the United States (HUSTLE, SWiTCH, TWiTCH), the Caribbean (SACRED) or sub-Saharan Africa (REACH, NOHARM). Whole blood DNA was first genotyped using the H3Africa SNP array (Illumina) that identifies over 2.2 million single nucleotide variants (SNVs) across the genome. Most samples also underwent whole exome sequencing (WES) using NimbleGen VCRome 2.1 capture reagents and the Illumina HiSeq2500 platform analysis, which identifies coding variants in all known exons. Square root transformed HbF values were the continuous variable for association testing using single-locus mixed model (EMMAX) adjusted for population stratification, with both age and sex as co-variates. The GWAS approach included 3 distinct steps. First, we performed two independent GWAS discovery steps using distinct African populations; these were designated Discovery I (N=211) and Discovery II (223). Second, only SNVs that were significant (p Results: From the combined SNP and WES dataset, 8 BCL11A variants passed genome wide significance (p Conclusions: Our large GWAS of HbF with diverse global cohorts of children with SCA from Africa, the United States, and the Caribbean validated the strong associations of HbF with common genetic variants near the BCL11A and HBS1L-MYB gene loci. We also identified two novel gene loci, ITGA1 and RUNX1T1, that have statistical associations with HbF expression. The RUNX1T1 gene is a broad transcriptional corepressor known to impact myeloid differentiation in hematopoiesis, while ITGA1 encodes the integrin alpha subunit of a cell-surface receptor involved in cell-cell adhesion and inflammation. Both of these genes represent novel loci that may be involved in the regulation of HbF expression in children with SCA and should be investigated further using cellular and animal models. Disclosures No relevant conflicts of interest to declare.

Background: Five to ten percent of patients with sickle cell disease (SCD) have a high risk of suffering an ischemic stroke before they are 10 years of age. The clinical consequences of stroke are severe yet our current mechanistic understanding of the causes of stroke in SCD is limited. There is strong evidence that there is genomic contribution to the risk of stroke, other than just the SCD gene mutation. By whole exome sequencing, we have shown that a specific coding variant in the GOLGB1 gene, Y1212C, is significantly associated with decreased risk of stroke. GOLGB1 encodes a conserved Golgi membrane protein that is essential for maintaining Golgi structure and associated protein transport. To elucidate how the GOLGB1 Y1212C variant is protective against stroke risk in SCD, we performed morphological analysis of Golgi in cultured primary macrophages and whole proteome analyses of peripheral immune cells isolated from SCD patients. Methods: For primary macrophage preparations, we collected whole blood from non-SCD controls (n=11) and pediatric SCD patients (n=12). Monocytic CD14 positive cells were isolated and cultured for nine days with granulocyte monocyte stimulating colony factor, fixed and sequentially stained with antibodies against GOLGB1, TGN-46 and GM130. DAPI was used as a nuclear stain. High resolution images were acquired on Zeiss LSM 780 laser scanning confocal microscope with a 63X oil objective. Golgi morphological analysis was performed using a MATLAB script for cell segmentation, estimation of individual cell area, quantification of Golgi fragments per cell, and Golgi volume and organelle spread index. For the proteomic analysis, we isolated peripheral blood mononuclear cells from 7 patients with SCD. These cells were immediately flash-frozen and stored at -80oC until proteomic processing. We used a label-free protocol using dual-pH reverse phase fractionation and peptide sequencing of digested samples on a Thermo Scientific Orbitrap mass spectrometer. Protein abundances in each sample were estimated by using the intensity-based absolute quantification algorithm and total protein normalization was done with a fraction of total score (iFOT). Data were evaluated using GraphPad Prism and statistical analyses for differences between SCD and non-SCD samples or between samples with and without the GOLGB1 variant were performed with unpaired Student's t-test or Kruskal-Wallis test; analyses for multiple group comparisons were performed with the one-way ANOVA method. Statistical significance was set at P < 0.05. Results: In cultured primary macrophages, SCD individuals with the GOLGB1 Y1212C variant had altered Golgi morphologies compared to individuals with wild-type GOLGB1. Overall, macrophages derived from patients with SCD (n=12) were significantly larger (average cell size 11,728 vs. 7,598; p Conclusions: Our study has shown that a coding variant in GOLGB1, identified as protective against risk of stroke in patients with SCD, has significant effects on Golgi function in SCD samples. We observed that having the GOLGB1 Y1212C variant resulted in more compact and less fragmented Golgi apparatus. Proteomic analysis showed that SCD patients with the GOLGB1 variant also had significantly lower levels of proteins involved in platelet activation and Golgi trafficking. Our findings suggest a novel role for the Golgi apparatus in controlling protein flux that modulates risk of stroke in SCD. Disclosures No relevant conflicts of interest to declare.

Abundant and hydrophilic nonmembrane proteins with isoelectric points below pH 8 are the predominant proteins identified in most proteomics projects. In yeast, however, low-abundance proteins make up 80% of the predicted proteome, approximately 50% have pl's above pH 8 and 30% of the yeast ORFs are predicted to encode membrane proteins with at least 1 trans-membrane span. By applying highly solubilizing reagents and isoelectric fractionation to a membrane fraction of yeast we have a purified and identified 780 protein isoforms, representing 323 gene products, including 28% low abundance proteins and 49% membrane or membrane associated proteins. More importantly, considering the frequency and importance of co- and post-translational modifications, the separation of protein isoforms is essential and two-dimensional electrophoresis remains the only technique which offers sufficient resolution to address this at a proteomic level.