Investigating the mechanisms of chemotherapy induced relapse and metastasis in colorectal cancer

Abstract

Colorectal Cancer (CRC) is the second leading cause of cancer related deaths globally. This is largely attributable to relapse and metastatic disease. Currently there are no effective treatment options for recurrence, and scarce understanding of the mechanisms of relapse and consequent metastasis.

In recent publications, phenotypic plasticity governed on the transcriptomic level has been identified as a potential driver force in drug tolerance and resistance besides the classical clonal selection theory. In view of this idea, I set out to find the mechanisms of CRC relapse following chemotherapy, hypothesising that transcriptomic plasticity plays a key role in these processes, and aiming to identify cell states enriched following chemotherapy, providing novel therapy targets.

The absence of clinically relevant models of chemotherapy and relapse contributes to the slow progress within this area of research; therefore, my first goal was to develop in vivo mouse models, that accurately recapitulate primary CRC progression, standard chemotherapy administration and relapse after treatment. Using these models, I found evidence of increased invasive capacity and metastatic potential of primary CRC treated with chemotherapy compared to control tumours.

Transcriptomic and protein level analyses of primary tumours revealed robust immune evasion by chemotherapy treated primary tumours as well as emergence and clonal expansion of a squamous-like cell population marked by the expression of Keratin 5 (Krt5). Squamous-like clones were increasingly prevalent in relapsing and metastatic primary tumours, however mostly absent from metastases. Krt5 positive cells showed lower levels of proliferation and resistance to chemotoxic stress, however did not appear to have a dormant or senescent phenotype.

Induction of the squamous-like cell state in our in vitro organoid models has proven to be a challenge suggesting the tumour microenvironment and extracellular matrix as key contributors to this cell state. Nonetheless, upregulation of Krt5 in response to chemotherapy is robust across various genetic backgrounds. Further clarifications into the exact mechanism of emergence and function of squamous cell states in CRC treatment response and metastasis are still required. However, recent publications and patient survival data suggest a potential prognostic value of Krt5 positive squamous-like cell populations when predicting aggressive metastatic disease and response to therapy.

Overall, this project shows that adjuvant chemotherapy induces cellular plasticity introducing non-canonical cell states in the primary tumours leading to more aggressive disease and worse patient outcomes. These results contribute to our understanding of chemotherapy response in late-stage CRC and can guide further investigations for clinical decision making and novel therapies.