An episomal DNA vector platform for the persistent genetic modification of pluripotent stem cells and their differentiated progeny

The genetic modification of stem cells (SCs) is typically achieved using integrating vectors, whose potential integrative genotoxicity and propensity for epigenetic silencing during differentiation limit their application. The genetic modification of cells should provide sustainable levels of transgene expression, without compromising the viability of a cell or its progeny. We developed nonviral, nonintegrating, and autonomously replicating minimally sized DNA nanovectors to persistently genetically modify SCs and their differentiated progeny without causing any molecular or genetic damage. These DNA vectors are capable of efficiently modifying murine and human pluripotent SCs with minimal impact and without differentiation-mediated transgene silencing or vector loss. We demonstrate that these vectors remain episomal and provide robust and sustained transgene expression during self-renewal and targeted differentiation of SCs both in vitro and in vivo through embryogenesis and differentiation into adult tissues, without damaging their phenotypic characteristics

Investigation into hematopoietic stem cell function in the context of developmental specification and stress hematopoiesis

Hematopoietic stem cells (HSCs) derive from an endothelial precursor during embryonic development and undergo endothelial to hematopoietic transition (EHT). EHT is a highly dynamic process that is challenging to study in vivo and problematic to model in vitro, partly due to an inability to identify biologically relevant intermediate cell states. However, a previous study conducted in our group identified Evi2a as a crucial protein in hematopoietic specification. In this PhD thesis we aimed to investigate the role of Evi2a during hematopoietic specification. We performed differential gene expression analysis on differentiated wildtype and Evi2a KO cells to decipher differences in signaling processes and identify the downstream pathways of Evi2a. Furthermore, we developed an Evi2a KO mouse model to identify hematopoietic defects in vivo. Our results suggested that Evi2a KO delayed hematopoietic differentiation and a homozygous Evi2a KO might be embryonically lethal. Adult hematopoietic stem cells are at the peak of the hematopoietic hierarchy and responsible for the lifelong production of all differentiated peripheral blood cells to ensure a healthy and balanced blood composition. In order to avoid DNA damages induced by e.g., stress, it is crucial for HSCs to remain in a quiescent state. Only when desired HSCs proliferate to replenish the need for blood cells. However, during severe infections, chronic inflammation, injury trauma as well as other challenging events HSCs are pushed to proliferate and to actively contribute to the increased cellular demand of differentiated blood cells. Repetitive activation of HSCs, as it happens during a lifetime, has been shown to lead to HSC attrition and in the worst case to aberrant blood production often coming along with a myeloid bias. In this study we investigated the changes in HSC function and their ability to produce all downstream cells after repeated stimulation. HSC attrition has been extensively studied before using inflammatory agents. However, we were interested in HSC activation beyond inflammatory stimuli and used Thrombopoietin (TPO) and the clinically used TPO mimetic Romiplostim to induce HSC cycling. TPO stimulates megakaryocyte and platelet production and has been shown to be required for HSC maintenance and self-renewal divisions. However, we hypothesize that any cell division can induce DNA damages and consequently lead to loss of HSC function indicated by a decrease in reconstitution potential. After repetitive TPO and Romiplostim treatment we observed an increase in the phenotypic long-term HSC pool. Performing competitive repopulation assays showed that no change in donor chimerism between control and treated group was detected. We could neither note loss of HSC function nor a myeloid bias that usually arises from exhausted HSCs. By using a label-retention mouse model we were able to track the proliferative history of HSCs and again, did not observe any reduced repopulation capacity between cells that cycled and cells that remained dormant during TPO treatment. Concluding, repetitive activation of HSCs using TPO did not lead to HSC exhaustion. However, transcriptome and methylome analysis will be carried out next to examine potential changes on molecular level