Intrinsic and extrinsic mechanisms of clonal expansion in Del(5q) MDS

The studies presented in this thesis focus on finding intrinsic and extrinsic mechanisms of clonal advantage in HSCs carrying common mutations in myeloid malignancies, beginning with studying the intrinsic clonal advantage provided by haploinsufficiency of genes commonly deleted in del(5q) MDS in murine models.
We systematically tested the effect of haploinsufficieny of candidate genes commonly deleted in del(5q) MDS (Apc, Csnk1a1 and Egr1) in chapter 2. We found that Csnk1a1 provides a consistent clonal advantage that is driven by cell cycle activation and preservation of functional HSCs in serial transplantation. We hypothesized that this pro-proliferative drive predisposes HSCs to malignant transformation, which is guarded by p53 unless it loses its function in a second-hit event. In chapter 3 we indeed observed that Csnk1a1 haploinsufficiency together with p53 deficiency has the potential to cause malignant transformation. We investigated the derived leukemia further and found that it is transplantable and can be cultured ex-vivo, thereby serving as a disease model for malignant transformation of myeloid disease with 5q-.
In chapter 4, we explored the effect of compound deficiency for Csnk1a1, Rps14 and miR145/miR146, all commonly deleted in del(5q) MDS. We observed that Csnk1a1, Rps14 and miR145/miR146 compound haploinsufficiency more truly recapitulates the full del(5q) phenotype than each of them alone. We demonstrated that increased expression of the alarmin S100A8 in Csnk1a1/Rps14/miR deficient macrophages induces S100A8 expression in neighboring mesenchymal stromal cells and creates a general pro-inflammatory environment, which leads to loss of their hematopoiesis-supporting capacity.
We studied these HSC-extrinsic effects further in other myeloid malignancies: In chapter 5 we applied single-cell RNA sequencing to investigate in detail the changes of the bone marrow microenvironment during fibrotic injury induced by myeloproliferative neoplasms (MPN).
We investigated in more depth the non-hematopoeietic bone marrow niche in mouse and human as well as the in-vitro models for niche cells commonly used by our lab and others in chapter 6. Here we systematically investigated on single-cell level the heterogeneous bone marrow stromal cell types and the role of TGF-b in fibroblast differentiation to place cultured primary bone marrow stromal cells (cBMSC) into the context of the bone marrow niche in vivo.