Breast cancer is a significant global health issue, impacting millions of women worldwide. ER-positive breast cancer, the most common subtype, accounts for over 70% of cases. While hormone-based therapies like tamoxifen are effective, resistance develops in many patients, posing clinical challenges. Recent research has focused on the molecular mechanisms behind this resistance, aiming to identify new therapeutic targets.
This thesis investigates the regulatory role of microRNAs (miRNAs) in ER-positive breast cancer, particularly their impact on ER expression, cell proliferation, and tamoxifen sensitivity. Through a genome-wide miRNA high-throughput screen, we analysed regulatory networks involving miRNAs to understand their mechanisms better.
Two miRNAs affected ER expression in MCF7 cells, including miR-130a and miR-18b/miR-106a. MiR-18b was highlighted as a key regulator, influencing ER expression and cell proliferation. Analysis of potential targets of miR-18b identified amongst others target genes like ESR1, HMGCS1, and SON, supporting a role of miR-18b in these processes.
Additionally, we identified five miRNAs associated with tamoxifen resistance and validated their role in response to tamoxifen for miR-130a, let-7g, and miR-15b/miR-16-2. For miR-130a, 20 genes, including ESR1 and AIB1, were identified as predicted targets influencing tamoxifen sensitivity. Downregulation of ESR1 and AIB1 by miR-130a was confirmed via Western blotting.
These findings underscore the importance of miRNA regulation in ER-positive breast cancer and pave the way for potential miRNA-based therapeutic strategies to improve treatment outcomes. The research also highlights the complexity of miRNA-mediated regulation and the need for further studies to validate these findings across different breast cancer models.

Classic Hodgkin lymphoma (cHL) is a B-cell malignancy marked by Hodgkin and Reed/Sternberg (HRS) cells that highly express CD30. We explored the roles of microRNAs (miRNAs) in cHL pathogenesis and CD30-targeting therapy using Brentuximab Vedotin (BV) through high-throughput screening techniques. Review of miRNA-related studies on known and potential miRNA-target gene interactions identified multiple miRNA-gene pairs with potential relevance for cHL pathogenesis. This highlighted the relevance of miRNAs in cHL, warranting further studies to deepen our understanding.
A microRNA overexpression library screen revealed 11 miRNA-target gene pairs implicated in cHL pathogenesis and 66 miRNA constructs that regulate BV treatment response. Among these, miR-590 was found to inhibit CD30 expression and induce BV resistance. Additionally, miR-191-5p not only conferred BV resistance but also promoted cell growth. By using AGO2-immunoprecipitation, we identified a network of involved target genes, including PTEN.
To investigate endogenous miRNAs involved in BV resistance mechanisms, we established two BV-resistant cHL cell lines and observed dynamic changes in CD30 expression. Moreover, we utilized a miRNA-focused CRISPR-Cas9 screen to identify endogenous miRNAs that regulate CD30 expression and/or BV sensitivity. To this end, we generated two inducible Cas9 monoclonal cell lines and optimized the entire screening process. These screens await further data analysis.
In summary, this thesis highlights the functional importance of specific miRNAs in cHL and provides detailed procedures of the miRNA-focused screening approaches, from initial setup to optimization and validation of results.

The goal of this thesis was to investigate the role of circRNAs in B-cell lymphoma and breast cancer. We focused on the characterization of their expression patterns, functional mechanisms, and regulatory roles. In the first part, we conducted RNA sequencing to characterize the circRNA landscape of three main B-cell lymphoma subtypes and normal B-cells. This was followed by more in-depth studies of the roles of circPVT1 and circZDHHC11 in B-cell lymphoma. CircPVT1 and the linear PVT1 transcripts showed opposite differential expression patterns and both supported cell growth. In contrast to other cancers, we showed that the effect on growth was independent of their ability to bind miRNAs. CircZDHHC11 was strongly enriched in the AGO2-IP fraction upon miR-150 overexpression and supported the growth of B-cell lymphoma. However, its growth-supporting effect was independent of its ability to bind miR-150. In the second part, we investigated the role of circ-NOL10 in breast cancer. Circ-NOL10 was selected based on its most pronounced downregulation in triple-negative breast cancer (TNBC) compared to control tissue. Upon binding of MTDH and CASC3, circ-NOL10 levels were strongly reduced, resulting in the release of miR-149-5p, miR-330-3p, and miR-452-5p. These miRNAs subsequently targeted PDCD4 and this promoted progression of breast cancer. Overall, our study extends current knowledge of circRNAs and highlights specific functions of selected circRNAs in B-cell lymphoma and breast cancer.

The study of this thesis is related to the previously established MYC/miR-150/MYB/ZDHHC11 network regulating Burkitt lymphoma (BL) growth. Our first aim was to study the role of this network in Hodgkin lymphoma (HL) and diffuse large B-cell lymphoma (DLBCL). We found a similar role of three of the network components, while miR-150 overexpression showed no or only very mild effects on HL and DLBCL growth. Therefore, we considered miR-150 as a BL-specific component. Our second aim was to study the specific roles of the protein coding and circular ZDHHC11 gene products. ZDHHC11 belongs to a family of 24 ZDHHC proteins that are characterized by a DHHC motif which mediates palmitoylation. Loss of palmitoylation induced apoptosis in three B-cell lymphoma subtypes indicating the overall importance of protein palmitoylation for B-cell lymphoma. Based on the observed expression patterns we pinpointed seven DHHC family members that might be relevant in B-cell lymphoma. Further studies to elucidate the role of the ZDHHC11 protein were hampered due to difficulties in detecting the protein but provided some evidence that this protein is not a critical factor in regulating BL growth. Lastly, we showed that circular ZDHHC11 RNA transcripts do play a critical role in growth of B cell lymphoma, but this effect is independent of binding miR-150, as deletion of the miR-150 binding site region did not alter the observed phenotype. In summary, we extended current knowledge on the role of the MYC/miR-150/MYB/ZDHHC11 network and addressed the function of ZDHHC11 transcripts and protein in B-cell lymphoma.

A large number of aberrantly expressed microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) have been reported in B-cell lymphomas. However, their role in the pathogenesis of B-cell lymphoma remains largely unknown. In this thesis, the functions of non-coding RNAs were explored in two distinct subtypes of germinal center B-cell (GC-B) derived lymphomas, i.e. classical Hodgkin lymphoma (cHL) and Burkitt lymphoma (BL).
In a loss-of-function screen in three cHL cell lines we identified four miRNAs with an effect on cell growth. Follow-up experiments for miR-21-5p showed that this miRNA was upregulated in cHL compared to GC-B cells and protects cHL cells from apoptosis possibly via targeting BTG2 and PELI1. Small RNA sequencing in BL and GC-B cells revealed a clearly aberrant expression profile. In subsequent miRNA loss- and gain-of-function screens we identified 18 miRNAs that affected growth of BL, including some previously reported oncogenic miRNAs. Functional follow-up studies revealed promising target genes for miR-378a-3p and miR-26b-5p. Focusing on MYC-induced lncRNAs revealed 18 candidates that were consistently higher expressed in BL cell lines compared to GC-B cells. Three of these lncRNAs showed effects on BL cell growth in a loss-of-function screen. Further validation experiments of MAFG-AS1 confirmed its effect on growth in BL cell lines.
In summary, we identified several aberrantly expressed and MYC-regulated non-coding RNAs, with clear effects on growth of cHL and BL cells, and unveiled their underlying mechanisms in cell growth regulation. Our findings add to the current knowledge about the roles of non-coding RNAs in B-cell lymphomas.