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Intra-tumoural heterogeneity and cancer cell plasticity in colorectal cancer (CRC) have been key challenges to effective treatment for patients. It has been suggested that a subpopulation of LGR5-expressing cancer stem cells (CSCs) is responsible for driving tumour relapse and therapy resistance in CRC. However, studies have revealed that the LGR5+ve CSC population is highly sensitive to chemotherapy. It has been hypothesised that another subset of tumour cells can phenotypically revert to a stem-like state in response to chemotherapy treatment which replenishes the LGR5+ve CSC population and maintains tumour growth. Recently, a unique stem cell population marked by enriched clusterin (CLU) expression and termed the revival stem cell (RevSC) was identified in the regenerating murine intestine. This CLU-expressing cell population is quiescent during homeostasis but has the ability to survive and regenerate other stem cells upon injury. More recently, the CLU+ve signature has been implicated in several adverse outcomes in CRC, including chemotherapy resistance and poor patient survival; however, the mechanism behind this remains undetermined. In this review, we discuss recent insights on CLU in CRC and its roles in enhancing the plasticity of cells and further consider the implications of CLU as a prospective target for therapeutic intervention.

Colorectal cancer (CRC) is stratified by heterogeneity between disease sites, with proximal right-sided CRC (RCRC) multifactorial in its distinction from distal left-sided CRC (LCRC). Notably, right-sided tumors are associated with aggressive disease characteristics which culminate in poor clinical outcomes for these patients. While factors such as mutational profile and patterns of metastasis have been suggested to contribute to differences in therapy response, the exact mechanisms through which RCRC resists effective treatment have yet to be elucidated. In response, recent analyzes, including those utilizing whole genome sequencing, transcriptional profiling, and single-cell analyses, have demonstrated that key molecular differences exist between disease sites, with differentially expressed genes spanning a diverse range of cellular functions. Here, we review and contextualize the most recent data on molecular biomarkers found to exhibit discordance between RCRC and LCRC, and highlight candidates for further investigation, including those which present promise for future clinical application. Given the present disparity in survival outcomes for RCRC patients, we expect the prognostic biomarkers presented in our review to be useful in establishing future directions for the side-specific treatment of CRC.

Colorectal cancer stem cells have been proposed to drive disease progression, tumour recurrence and chemoresistance. However, studies ablating leucine rich repeat containing G protein-coupled receptor 5 (LGR5)-positive stem cells have shown that they are rapidly replenished in primary tumours. Following injury in normal tissue, LGR5+ stem cells are replaced by a newly defined, transient population of revival stem cells. We investigated whether markers of the revival stem cell population are present in colorectal tumours and how this signature relates to chemoresistance. We examined the expression of different stem cell markers in a cohort of patient-derived colorectal cancer organoids and correlated expression with sensitivity to 5-fluorouracil (5-FU) treatment. Our findings revealed that there was inter-tumour variability in the expression of stem cell markers. Clusterin (CLU), a marker of the revival stem cell population, was significantly enriched following 5-FU treatment and expression correlated with the level of drug resistance. Patient outcome data revealed that CLU expression is associated with both lower patient survival and an increase in disease recurrence. This suggests that CLU is a marker of drug resistance and may identify cells that drive colorectal cancer progression.

The impact of aging on intestinal stem cells (ISCs) has not been fully elucidated. In this study, we identified widespread epigenetic and transcriptional alterations in old ISCs. Using a reprogramming algorithm, we identified a set of key transcription factors (Egr1, Irf1, FosB) that drives molecular and functional differences between old and young states. Overall, by dissecting the molecular signature of aged ISCs, our study identified transcription factors that enhance the regenerative capacity of ISCs.

Inflammatory bowel disease (IBD) is secondary to an abnormal immune response to the microbiota. To study this, models of host-microbe interactions that represent mucosal bacterial communities and inter-patient diversity are required. Human intestinal organoids (HIOs) are an established model to investigate epithelial responses. Here, we describe a technique of culturing bacteria directly from the sites of inflammation in IBD, while simultaneously sampling host tissue. We generated HIOs from a cohort of newly diagnosed paediatric IBD patients, without confounding treatments or comorbidities, and explored their response to site-specific bacteria. A unique biobank of matched HIOs and cultured mucosa-attached bacteria was established from 27 paediatric patients. Transcriptional profiling revealed differential gene expression between control and IBD-derived organoids. We used microinjection to introduce bacteria to the apical surface of the epithelium, to determine the effect of bacteria on host epithelial cells. We measured survival and growth of bacteria within the HIOs and tested several related bacterial isolates for their impact on the epithelium. An isolate from a control patient stimulated inflammatory signalling pathways but this was not observed in response to a closely related isolate originating from an IBD patient. This study demonstrates the feasibility of isolating bacteria and generating organoids from the same biopsy tissue, to explore personalised host-microbe interactions. The microinjections, while labour-intensive, demonstrate that closely related bacteria can induce very different epithelial responses, with downstream implications for immune response. This highlights the importance of understanding host-microbe interactions in a strain- and site-specific manner and developing techniques for personalised microbiome-based therapeutics.

Purpose: Regorafenib is a multi-kinase inhibitor approved for refractory metastatic colorectal cancer. Previous studies have suggested that combining kinase inhibitors with aspirin may improve patient outcomes. We aimed to determine the effects of aspirin and regorafenib combination treatment in preclinical models of colorectal cancer. Experimental Design: SW480, RKO and LIM1215 colorectal cancer cell lines were treated with aspirin and regorafenib to determine effects on proliferation and cytotoxicity. RNA sequencing and Western blotting were performed to explore underlying molecular effects. Aspirin and regorafenib combination treatment was also tested using organoids derived from three human colorectal cancer tissue specimens. For the in vivo study, SW480-derived tumors were established in athymic mice. Tumor volume was measured during treatment with aspirin and regorafenib, followed by immunohistochemical staining for markers of proliferation and apoptosis. Results: Aspirin and regorafenib synergistically inhibited proliferation of colorectal cancer cell lines and patient-derived organoids, irrespective of KRAS or BRAF mutation status. This was associated with inhibition of the PI3K-Akt-mTOR pathway and activation of the AMPK pathway. Aspirin and regorafenib effectively inhibited growth of microsatellite stable KRAS-mutant SW480-derived tumors in vivo. Immunohistochemical staining for Ki67 and cleaved caspase 3 showed that combination treatment elicited a synergistic anti-proliferative effect, in addition to a pro-apoptotic effect that was driven by regorafenib. Conclusions: Aspirin and regorafenib demonstrate synergistic anti-proliferative effects in preclinical models of colorectal cancer. This suggests that combining regorafenib with aspirin may be an improved treatment strategy for patients with refractory metastatic colorectal cancer. Competing Interest Statement The authors have declared no competing interest.