Abstract Thesis Title: Application and Development of Advanced Genetic Tools to Study Adult Stem Cells By: Amanda Andersson Rolf In adult mammals, the gastrointestinal (GI) epithelium exhibits the highest turnover rate among the endodermal tissues. The harsh luminal environment of the GI tract necessitates replenishment of epithelial cells to maintain organ structure and function during routine turnover and injury repair. This delicate balance between gain and loss of cells is called tissue homeostasis, and multipotent tissue specific adult stem cells serve as the continuous source of self-renewal. Due to their important contribution to homeostatic maintenance the proliferative capacity of the stem cells needs to be tightly controlled, as an imbalance can result in diverse pathologies such as cancer or insufficient injury repair. Despite the crucial role for regulatory processes the molecular mechanisms and the genes governing these processes remain poorly understood. Rnf43 and its paralogue Znrf3 (RZ) act as tumour suppressors in the intestine, but their role in the gastric epithelium has not been previously investigated. Using a novel unpublished stomach specific CreERT2 expressing mouse line I found that simultaneous knockout of RZ (RZ DKO) result in gastric hyperplasia of the corpus epithelium. Gastric RZ DKO organoids show independence from the essential growth factor Rspondin-1 but require exogenous Wnt. A similar exogenous Wnt dependence was identified in a human gastric cancer cell line harbouring homozygous Rnf43 inactivating mutations. Thus, Wnt secretion inhibition might provide a new treatment paradigm for a subset of patients carrying Rnf43 mutations. The prominent role of the E3s Rnf43 and Znrf3 in the intestinal and gastric epithelial led to the question of whether other E3s either closely related to RZ or specifically expressed in stem or niche cells could play a role in homeostatic regulation, specifically in the small intestine. Using a retroviral overexpression screen I identified Rnf24 and Rnf122, two E3s that rendered intestinal organoids insensitive to withdrawal of the BMP inhibitor Noggin. Moreover, potential substrate candidates located at the cell surface membrane were identified and the generation of in vivo models initiated to provide a basis for further studies investigating the role of these E3s. In trying to address the function of the abovementioned genes using in vitro functional genetics I identified gaps in the current technology for organoid genetic engineering. I therefore developed two gene editing methods; a gRNA concatemer system allowing simultaneous knockout of multiple genes and CRISPR-FLIP enabling generation of conditional gene knockouts In summary, this thesis describes the first stomach specific knockout of Rnf43 and Znrf3 in the gastric epithelium, showing that it results in gastric hyperplasia located to the corpus epithelium. The dependence of the Rnf43 and Znrf3 knockout epithelium on exogenous Wnt signalling provides a potential treatment strategy for a subset of patients harbouring Rnf43 mutations. Next, it identifies Rnf24 and Rnf122 as E3 ubiquitin ligases involved in intestinal stem cell regulation and provide preliminary data and a basis for future studies. Finally, it describes the establishment of two advanced genetic engineering approaches which can be applied to various in vitro culture systems such as 3D organoids, mouse embryonic stem cells and conventional cell lines. Collectively this work and the developed methods will contribute to our understanding of the mechanisms regulating adult stem cell homeostasis