Project 4: Impact of cardiovascular disease on proliferation and genetic diversity of hematopoietic stem cells

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During aging, descendants of individual hematopoietic stem cell (HSC) clones can begin dominating significant portions of the peripheral blood, a phenomenon called clonal hematopoiesis (CH). CH has recently been shown to correlate with an increased risk for CVD. The mechanisms behind this association are only beginning to be explored. Existing studies have focused on examining how driver genes that are frequently mutated in CH affect the inflammatory phenotype of peripheral immune cells that drive CVD. Here, we propose to investigate the possibility of reverse causality. We will study how CVD influences HSC evolution and CH emergence, using mathematical modeling and novel experimental methods. In aim 1, we will construct a mathematical model of somatic HSC evolution in steady state and CVD. Myocardial infarction has been shown to double the proliferation rate of HSCs. Furthermore, our unpublished preliminary data, collected in collaboration with other PPG projects, show that atherosclerosis can similarly increase the proliferation of HSCs by up to 2.5-fold. We hypothesize that chronically elevated proliferation in the HSC compartment accelerates CH due to neutral drift and/or due to accelerated emergence of clones with a selective advantage. To test these hypotheses, we will construct a stochastic branching process model of genetic diversity in the HSC population over a human lifetime. We will begin by studying neutral drift alone and will then carefully expand our model to include the occurrence of beneficial mutations. Results from this aim will elucidate the role of HSC proliferation in CH emergence and provide a firm quantitative basis for future research into the mechanisms of CH. In aim 2, we will investigate experimentally whether CVD causes clonal hematopoiesis in mice. We will establish murine models of CH and test our hypothesis that increased HSC proliferation rates caused by CVD enhance CH. We will use polyguanine genotyping, a genetic analysis that relies on non-coding, hypermutable DNA repeats which rapidly accumulate insertion/deletion mutations, to measure with high sensitivity whether clonal expansions occur in the blood of aging mice. We will then determine whether the incidence of CH is significantly increased in two CVD models (atherosclerosis and myocardial infarction). In Aim 2B, we will study CVD and CH in a transplantation setting. We will isolate and transduce highly purified LT-HSC populations with a lentivirus carrying a library of genetic barcodes, along with a collection of seven fluorescent proteins that can be used to track clonal dynamics in vivo. We will then transplant tagged HSCs into myeloablated hosts, titrating their numbers such that CH emerges within a relatively short time frame. Finally, we will induce myocardial infarction and atherosclerosis and score the incidence of CH in comparison to age-matched controls. In conjunction with our in silico approach in aim 1, we anticipate that these experiments will provide fundamental insights into how HSC population dynamics are influenced by CVD.