Many of the clinically used cancer chemotherapies have considerable short-term side effects, including fatigue, bone marrow suppression, nausea, diarrhea, and hair loss. Long-term side effects include cognitive impairment, organ dysfunction, and even secondary cancers. A body of evidence indicates that senescent cells induced by chemotherapy in non-cancerous tissues play a major part in these toxicities of chemotherapy. As part of the SENCAN grant, we have developed a cocktail of two drugs that is efficient in killing senescent cancer cells. This so-called senolytic cocktail synergizes strongly with chemotherapy in eradicating tumors in experimental animals. The senolytic cocktail acts independently of the genetic context of the cancer, thus demonstrating superior efficacy compared to other senolytic agents, which are context-dependent. We will test in this proof-of-concept grant whether this cocktail can also reduce the side effects of cancer chemotherapy by eliminating the senescent cells from non-cancerous tissues in mice. We use transgenic mice that carry a reporter gene that is activated in senescent cells and hence, tissues become luminescent when non-tumor-bearing animals are treated with chemotherapy. In pilot experiments, we have seen suppression of the luminescent signal when mice are treated with our senolytic cocktail post-chemotherapy. In this application, we plan to investigate the effect of the senolytic cocktail on vital organs that are often affected by chemotherapy: the bone marrow, the heart, and muscle (fatigue). If successful, the case for use of the senolytic cocktail in combination with cancer chemotherapy would become even stronger: not only have we already shown that the cocktail synergizes with chemotherapy in eradicating tumors in vivo in SENCAN, but the present work may also support the notion that the senolytic cocktail can suppress the toxicities that are invariably associated with cancer chemotherapy treatment.

The human genome carries genetic information in two distinct forms: Transcribed genes and regulatory DNA elements (rDEs). rDEs control the magnitude and pattern of gene expression, and are indispensable for organismal development and cellular homeostasis. Nevertheless, while large-scale functional genetic screens greatly advanced our knowledge in studying mammalian genes, such tools to study rDEs were lacking, impeding scientific progress. Interestingly, recent advance in genome editing technologies has not only expanded the available screening toolbox to examine genes, but also opened up novel opportunities in studying rDEs. We distinguish two types of rDEs: Transcriptional rDEs that recruit transcription factors to enhancers, and structural rDEs that maintain chromatin 3D structure to insulate transcriptional activities, a feature postulated to be essential for gene expression regulation by enhancers. Recently, we developed a CRISPR strategy to target enhancers. We showed its scalability and effectivity in identifying potential oncogenic and tumour-suppressive enhancers. Here, we will exploit this line of research and develop novel strategies to target structural rDEs (e.g. insulators). By setting up functional genetic screens, we will identify key players in cell proliferation, differentiation, and survival, which are related to cancer development, metastasis induction, and acquired therapy resistance. We will validate key insulators and decipher underlying mechanisms of action that control phenotypes. In a parallel approach, we will analyse whole genome sequencing datasets of cancer to identify and characterize genetic aberrations occurring in the identified regions. Altogether, the outlined research plan forms a natural extension of our successful functional approaches to study gene regulation. Our results will setup the foundation to better understand principles of chromatin architecture in gene expression regulation in development and cancer.

The 3D organization of chromosomes within the nucleus is of great importance to control gene expression. The cohesin complex plays a key role in such higher-order chromosome organization by looping together regulatory elements in cis. How these often megabase-sized looped structures are formed is one of the main open questions in chromosome biology. Cohesin is a ring-shaped complex that can entrap DNA inside its lumen. However, cohesin’s default behaviour is that it only transiently entraps and then releases DNA. Our recent findings indicate that chromosomes are structured through the processive enlargement of chromatin loops, and that the duration with which cohesin embraces DNA determines the degree to which loops are enlarged. The goal of this proposal is two-fold. First, we plan to investigate the mechanism by which chromatin loops are formed, and secondly we wish to dissect how looped structures are maintained. We will use a multi-disciplinary approach that includes refined genetic screens in haploid human cells, chromosome conformation capture techniques, the tracing in vivo of cohesin on individual DNA molecules, and visualization of chromosome organization by super-resolution imaging. With unbiased genetic screens, we have identified chromatin regulators involved in the formation of chromosomal loops. We will investigate how they drive loop formation, and also whether cohesin’s own enzymatic activity plays a role in the enlargement of loops. We will study whether and how these factors control the movement of cohesin along individual DNA molecules, and whether chromatin loops pass through cohesin rings during their formation. Ultimately, we plan to couple cohesin’s linear trajectory along chromatin to the 3D consequences for chromosomal architecture. Together our experiments will provide vital insight into how cohesin structures chromosomes.

When DNA is transcribed or replicated, torsional stress accumulates on the double helix. This tension must be dissipated by spreading it along the DNA fiber, or it must be removed altogether. One of the main factors responsible for the removal of such stress is DNA Topoisomerase I (Top1), an important target of cancer chemotherapy. When Top1 activity is lost, torsional stress accumulates on transcriptionally active genes and can lead to the formation of non-canonical DNA/RNA hybrid structures called R loops. These structures are emerging as important regulators of genome function and stability. By genome-wide mapping of R loops in human cells, I recently found that depletion of Top1 leads to a marked R loop stabilization, specifically on genes that are anchored to the nuclear lamina. This strongly suggests that attachment of DNA to the nuclear lamina may prevent dissipation of torsional stress, but how this works is still largely unclear. I propose to investigate the causal relationships between torsonal stress, Top1, R-loops and nuclear lamina attachment, taking advantage of a suite of unique genomics techniques developed in the host lab. Specifically, I will: 1) Develop two novel reporter assays to probe the effects of chromatin context (in particular lamina associated chromatin) and Top1 on torsional stress and R loop formation, at thousands of locations in the human genome. 2) Investigate if and how Top1 regulates DNA/nuclear lamina contacts by by means of a novel version of the powerful genome-wide DamID mapping method with much-improved time resolution. My expertise in Top1, R-loops and DNA topology combined with the unique genomics methodologies in the host lab, as well as their expertise in lamina-associated DNA, will lead to a unique synergy that should result in new insights into the relationship between nuclear organization, torsional stress and R loop formation. Moreover, it will yield new methods that will boost scientific progress in this field.