Variation and impact of polygenic hematological traits in monogenic sickle cell disease

Several complications observed in sickle cell disease (SCD) are influenced by variation in hematological traits (HT), such as fetal hemoglobin (HbF) level and neutrophil count. Previous large-scale genome-wide association studies carried out in largely healthy individuals have identified 1000s of variants associated with HT, which have then been used to develop multi-ancestry polygenic trait scores (PTS). Here, we tested if these PTS associate with HT in SCD patients and can improve the prediction of SCD-related complications. In 2,056 SCD patients, we found that the PTS predicted less HT variance than in non-SCD African-ancestry individuals. This was particularly striking at the Duffy/DARC locus, where we observed an epistatic interaction between the SCD genotype and the Duffy null variant (rs2814778) that led to a two-fold weaker effect on neutrophil count. PTS for these routinely measured HT were not associated with complications in SCD. In contrast, we found that a simple PTS for HbF that includes only six variants explained a large fraction of the phenotypic variation (17.1-26.4%), associated with acute chest syndrome and stroke risk, and improved the prediction of vaso-occlusive crises. Using Mendelian randomization, we found that increasing HbF by 4.8% reduces stroke risk by 36% (P = 0.0008). Taken together, our results highlight the importance of validating PTS in large diseased populations before proposing their implementation in the context of precision medicine initiatives.