314. Mathematical Modeling of Erythrocyte Chimerism Informs Clinical Strategies for Sickle Cell Disease

Abstract

Recent advances in gene therapy and genome-engineering technologies offer the opportunity to correct sickle cell disease (SCD), which originates from a point mutation in the gamma-globin gene. The developmental switch from fetal gamma-globin to adult beta-globin is governed in part by the transcription factor (TF) BCL11A. This TF can be a therapeutic target for reactivation of corrected gamma-globin and concomitant reduction of sickling beta-globin. Genetic alteration of a portion of the hematopoietic stem cell (HSC) compartment would lead to a mixture of sickling and corrected peripheral red blood cell (RBC) populations. The degree of HSC alteration required to achieve a desired stable fraction of corrected RBCs with high gamma-gobin remained unclear. We developed a mathematical model that describes aging and survival of normal and sickling RBCs: a survival advantage of non-sickled RBCs over sickled RBCs leads to their overrepresentation in periphery. The math model considers HSC and age-structured RBC populations, and we aimed to validate it in an experimental mouse model by transplanting mixtures of Berkeley SCD mouse model and normal murine bone marrow cells to establish chimeric grafts in murine hosts. The math model identified the level of bone marrow chimerism required for successful stem cell-based gene therapies in SCD. We found an equation for the stable fraction of normal RBCs in periphery as a function of the fraction of genetically altered HSCs, predicting that gene therapy leading to 40% altered HSCs results in 55% non-sickling RBCs in periphery (Figure 1A, lineFigure 1A, line). These findings were confirmed in the experimental mouse setting, where we achieved altered HSC mixtures between 0.5 and 38%, leading to RBC fractions between 7 and 62% (Figure 1A, data pointsFigure 1A, data points). We found that a mathematical model incorporating a constant RBC selection approach was sufficient to predict pheripheral chimerism. This approach was then used to make SCD patient-predictions: 10% HSC alteration leads to 40% normal RBCs in periphery, 30% leads to over 70% normal RBCs (medians, assuming variable S cell life span, Figure 1BFigure 1B), which depend on the mean life spans of sickling (median 20 days) and normal (110 days) RBCs. Our integrative approach informs the target frequency of HSC genetic alterations likely required for effective treatment of sickling syndromes in humans.View Large Image | Download PowerPoint Slide