MMR-proficient Colorectal Cancer Research Articles

Association of class II and III BRAF mutations with EGFR blockade therapy response and representation of molecularly distinct subgroups of BRAF mutations in MMR proficient CRC.

168 Background: BRAF mutations (mts) represent a highly heterogeneous group of molecular alterations seen in colorectal cancer (CRC). Class I BRAF mts (V600) render aggressive biology to CRC and poor response to anti EGFR inhibitors. Currently there are limited data on clinical and molecular features of class II and III BRAF mts. In this study, we investigated the clinical and molecular characteristics of BRAF mutation classes and their impact on clinical outcomes in a large cohort of patients (pts) with mismatch proficient (pMMR) CRC. Methods: A total of 18575 pMMR CRC specimens were profiled by next-generation sequencing (592-gene, NextSeq; WES, WTS NovaSeq) (Caris Life Sciences, Phoenix, AZ). BRAF mutations were detected by NGS and classified using published literature (Sahin et al. JCO OP 2021). Interferon gamma signature and MAPK pathway activity score (MPAS) were calculated using RNA expression data (TPM). Real-world overall survival information was obtained from insurance claims and calculated from tissue collection to last contact, while post-treatment survival from first of treatment to last contact. KM estimates were calculated for molecularly defined cohorts. Significance was determined as p values of <0.05. Results: A total of 930,105, and 262 pts with class I, II, and III BRAF mts were identified. Pts with class III BRAF mts were significantly more common among younger pts (age<45) compared to class I and class II (8.8% vs. 4.8% vs. 1.0 % respectively; P<0.05). Class I BRAF mts were significantly enriched with consensus molecular subtype 1 (CMS1) (Class I, II and III: 44% vs. 17% vs. 18%) while class II and III BRAF mts were more often CMS2 subtype (canonical) compared to class I (2%, 30% and 35%, p<0.05). Class I BRAF and KRAS/ NRAS mts were mutually exclusive, while KRAS mts incidences were 15% and 22% for class II and class III, and NRAS mts incidence were 8% and 12%, respectively. MPAS score was significantly lower for class III (-0.32, arbitrary unit) compared to Class I (p<0.05), but similar MPAS scores were seen in class I and II (1.3 versus 1.38). No difference in interferon gamma signature was noted between classes. Pts with class II and III mts had significantly better overall survival compared to patients with class I mts (HR=0.69 CI: 0.597-0.804 p<0.0001) and slightly worse overall survival compared to wild-type BRAF pts. (HR; 0.85 CI: 0.74-0.96 P=0.011). Among pts treated with anti-EGFR, pts with class II and III BRAF mts had significantly better post-anti-EGFR survival compared to class I BRAF mts (HR 0.498 CI 0.32-0.766 P=0.001 and similar survival compared to those with BRAF wild-type (P=0.21). Conclusions: Class II and III BRAF mts are associated with improved outcomes with EGFR blockade and represent a distinct biological subgroup of pMMR CRC. Class III BRAF mts have lower MAPK activation, consistent with the pattern of kinase-dead mutations.

Non-canonical antigens are the largest fraction of peptides presented by MHC class I in mismatch repair deficient murine colorectal cancer

BackgroundImmunotherapy based on checkpoint inhibitors is highly effective in mismatch repair deficient (MMRd) colorectal cancer (CRC). These tumors carry a high number of mutations, which are predicted to translate into a wide array of neoepitopes; however, a systematic classification of the neoantigen repertoire in MMRd CRC is lacking. Mass spectrometry peptidomics has demonstrated the existence of MHC class I associated peptides (MAPs) originating from non-coding DNA regions. Based on these premises we investigated DNA genomic regions responsible for generating MMRd-induced peptides.MethodsWe exploited mouse CRC models in which the MMR gene Mlh1 was genetically inactivated. Isogenic cell lines CT26 Mlh1+/+ and Mlh1-/- were inoculated in immunocompromised and immunocompetent mice. Whole genome and RNA sequencing data were generated from samples obtained before and after injection in murine hosts. First, peptide databases were built from transcriptomes of isogenic cell lines. We then compiled a database of peptides lost after tumor cells injection in immunocompetent mice, likely due to immune editing. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) and matched next-generation sequencing databases were employed to identify the DNA regions from which the immune-targeted MAPs originated. Finally, we adopted in vitro T cell assays to verify whether MAP-specific T cells were part of the in vivo immune response against Mlh1-/- cells.ResultsWhole genome sequencing analyses revealed an unbalanced distribution of immune edited alterations across the genome in Mlh1-/- cells grown in immunocompetent mice. Specifically, untranslated (UTR) and coding regions exhibited the largest fraction of mutations leading to highly immunogenic peptides. Moreover, the integrated computational and LC-MS/MS analyses revealed that MAPs originate mainly from atypical translational events in both Mlh1+/+ and Mlh1-/- tumor cells. In addition, mutated MAPs—derived from UTRs and out-of-frame translation of coding regions—were highly enriched in Mlh1-/- cells. The MAPs trigger T-cell activation in mice primed with Mlh1-/- cells.ConclusionsOur results suggest that—in comparison to MMR proficient CRC—MMRd tumors generate a significantly higher number of non-canonical mutated peptides able to elicit T cell responses. These results reveal the importance of evaluating the diversity of neoepitope repertoire in MMRd tumors.

IGSF6 is a novel biomarker to evaluate immune infiltration in mismatch repair-proficient colorectal cancer

Immunotherapy has dramatically changed the landscape of treatment for colorectal cancer (CRC), but currently lack of effective predictive biomarker, especially for tumors with mismatch repair (MMR) proficiency. The response of immunotherapy is associated with the cell–cell interactions in tumor microenvironment, encompassing processes such as cell–cell recognition, binding, and adhesion. However, the function of immunoglobulin superfamily (IGSF) genes in tumor immune microenvironment remains uncharacterized. This study quantified the immune landscape by leveraging a gene expression matrix from publicly accessible databases. The associations between IGSF6 gene expression and immune cell infiltration were assessed. The expression levels of IGSF6, CD8+ T cells, CD4+ T cells and CD68+ macrophage cells in cancer tissues from CRC patients and CRC cell lines were evaluated. IGSF6 was more highly expressed in CRC tumor tissues than adjacent normal tissues. And IGSF6 was significantly correlated with immune cell infiltration in MMR-proficient patients. Remarkably, MMR-proficient patients with high IGSF6 expression showed more sensitive to immunotherapy and chemotherapy than those with low IGSF6 expression. In summary, IGSF6 could be a novel biomarker to evaluate immune infiltration and predict therapeutic effect for MMR-proficient CRC.

Veliparib (ABT-888), a PARP inhibitor potentiates the cytotoxic activity of 5-fluorouracil by inhibiting MMR pathway through deregulation of MSH6 in colorectal cancer stem cells

ABSTRACT Objective Sensitization of mismatch repair (MMR)-deficient colorectal cancer (CRC) cells by 5-Fluorouracil (5-FU) is well-documented. But not much is known about the treatment of MMR-proficient CRC cancer stem cells (CRC-CSCs). Here, we investigated whether a PARP inhibitor (ABT-888) can enhance the 5-FU-mediated apoptosis in CRC-CSCs through MMR pathway inhibition. Methods The anti-cancer action of 5-FU+ABT-888 combination in CRC-CSCs has been studied by using in vitro, ex vivo, and in vivo preclinical model systems. Results 5-FU caused DNA damage in CRC-CSCs, and ABT-888 enhanced the accumulation of DNA mismatches by downregulating the MMR pathway, triggering S-phase arrest, and finally apoptosis and cell death in 5-FU-pre-treated MMR-proficient-CRC-CSCs at much lower concentrations than their individual treatments. After 5-FU treatment, PARylated-PARP1 activated MMR pathway by interacting with MSH6. But, upon ABT-888 treatment in 5-FU-pre-exposed CSCs, PARylation was inhibited, as a result of which PARP1 could not interact with MSH6, and other MMR proteins were downregulated. The role of MSH6 in PARP1-mediated MMR activation, was confirmed by silencing MSH6 gene, which resulted in MMR pathway shutdown. Similar results were obtained in ex vivo and in vivo model systems. Conclusions 5-FU+ABT-888 combination enhanced CRC-CSCs death by increasing DNA damage accumulation and simultaneously inhibiting the MMR pathway in MMR-proficient cells. But this study does not discuss whether the combination treatment will increase the sensitivity of MMR-deficient CSCs, for which further research will be performed in the future.

Prevalence and Clinical Implications of Mismatch Repair-Proficient Colorectal Cancer in Patients With Lynch Syndrome.

Lynch syndrome (LS)-associated colorectal cancer (CRC) is characterized by mismatch repair-deficiency (MMR-D) and/or microsatellite instability (MSI). However, with increasing utilization of germline testing, MMR-proficient (MMR-P) and/or microsatellite stable (MSS) CRC has also been observed. We sought to characterize MMR-P/MSS CRC among patients with LS. Patients with solid tumors with germline MMR pathogenic/likely pathogenic (P/LP) variants were identified on a prospective matched tumor-normal next-generation sequencing (NGS) protocol. CRCs were evaluated for MMR-D via immunohistochemical (IHC) staining and/or MSI via NGS. Clinical variables were correlated with MMR status using nonparametric tests. Among 17,617 patients with solid tumors, 1.4% (n = 242) had LS. A total of 36% (86 of 242) of patients with LS had at least one CRC that underwent NGS profiling, amounting to 99 pooled CRCs assessed. A total of 10% (10 of 99) of CRCs were MMR-P, with 100% concordance between MSS status and retained MMR protein staining. A total of 89% (8 of 9) of patients in the MMR-P group had MSH6 or PMS2 variants, compared with 30% (23 of 77) in the MMR-D group (P = .001). A total of 46% (6 of 13) of PMS2+ patients had MMR-P CRC. The median age of onset was 58 and 43 years for MMR-P and MMR-D CRC, respectively (P = .07). Despite the later median age of onset, 40% (4 of 10) of MMR-P CRCs were diagnosed <50. A total of 60% (6 of 10) of MMR-P CRCs were metastatic compared with 13% (12 of 89) of MMR-D CRCs (P = .002). A total of 33% (3 of 9) of patients with MMR-P CRC did not meet LS testing criteria. Patients with LS remained at risk for MMR-P CRC, which was more prevalent among patients with MSH6 and PMS2 variants. MMR-P CRC was later onset and more commonly metastatic compared with MMR-D CRC. Confirmation of tumor MMR/MSI status is critical for patient management and familial risk estimation.

Synthetic enforcement of STING signaling in cancer cells appropriates the immune microenvironment for checkpoint inhibitor therapy

Immune checkpoint inhibitors (ICIs) enhance anticancer immunity by releasing repressive signals into tumor microenvironments (TMEs). To be effective, ICIs require preexisting immunologically "hot" niches for tumor antigen presentation and lymphocyte recruitment. How the mutational landscape of cancer cells shapes these immunological niches remains poorly defined. We found in human and murine colorectal cancer (CRC) models that the superior antitumor immune response of mismatch repair (MMR)-deficient CRC required tumor cell-intrinsic activation of cGAS-STING signaling triggered by genomic instability. Subsequently, we synthetically enforced STING signaling in CRC cells with intact MMR signaling using constitutively active STING variants. Even in MMR-proficient CRC, genetically encoded gain-of-function STING was sufficient to induce cancer cell-intrinsic interferon signaling, local activation of antigen-presenting cells, recruitment of effector lymphocytes, and sensitization of previously "cold" TMEs to ICI therapy in vivo. Thus, our results introduce a rational strategy for modulating cancer cell-intrinsic programs via engineered STING enforcement to sensitize resistant tumors to ICI responsiveness.

PMS2-associated Lynch syndrome: Past, present and future.

Carriers of any pathogenic variant in one of the MMR genes (path_MMR carriers) were traditionally thought to be at comparable risk of developing a range of different malignancies, foremost colorectal cancer (CRC) and endometrial cancer. However, it is now widely accepted that their cancer risk and cancer spectrum range notably depending on which MMR gene is affected. Moreover, there is increasing evidence that the MMR gene affected also influences the molecular pathogenesis of Lynch syndrome CRC. Although substantial progress has been made over the past decade in understanding these differences, many questions remain unanswered, especially pertaining to path_PMS2 carriers. Recent findings show that, while the cancer risk is relatively low, PMS2-deficient CRCs tend to show more aggressive behaviour and have a worse prognosis than other MMR-deficient CRCs. This, together with lower intratumoral immune infiltration, suggests that PMS2-deficient CRCs might have more in common biologically with sporadic MMR-proficient CRCs than with other MMR-deficient CRCs. These findings could have important consequences for surveillance, chemoprevention and therapeutic strategies (e.g. vaccines). In this review we discuss the current knowledge, current (clinical) challenges and knowledge gaps that should be targeted by future studies.

CD103 and CD39 coexpression identifies neoantigen-specific cytotoxic T cells in colorectal cancers with low mutation burden

BackgroundExpression of CD103 and CD39 has been found to pinpoint tumor-reactive CD8+ T cells in a variety of solid cancers. We aimed to investigate whether these markers specifically identify neoantigen-specific...

Association between mutational subgroups, Warburg-subtypes, and survival in patients with colorectal cancer.

Previous research suggests that Warburg-subtypes are related to potentially important survival differences in colorectal cancer (CRC) patients. In the present study, we investigated whether mutational subgroups based on somatic mutations in RAS, BRAF, PIK3CA, and MET, which are known to promote the Warburg-effect, as well as mismatch repair (MMR) status, hold prognostic value in CRC. In addition, we investigated whether Warburg-subtypes provide additional prognostic information, independent of known prognostic factors like TNM stage. CRC patients (n= 2344) from the prospective Netherlands Cohort Study (NLCS) were classified into eight mutually exclusive mutational subgroups, based on observed mutations in RAS, BRAF, PIK3CA, and MET, and MMR status: All-wild-type + MMRproficient , KRASmut + MMRproficient , KRASmut +PIK3CAmut + MMRproficient , PIK3CAmut + MMRproficient , BRAFmut + MMRproficient , BRAFmut + MMRdeficient , other + MMRproficient , and other + MMRdeficient . Kaplan-Meier curves and Cox regression models were used to investigate associations between mutational subgroups and survival, as well as associations between our previously established Warburg-subtypes and survival within these mutational subgroups. Compared to patients with all-wild-type + MMRproficient CRC, patients with KRASmut + MMRproficient , KRASmut +PIK3CAmut + MMRproficient , BRAFmut + MMRproficient , or other + MMRproficient CRC had a statistically significant worse survival (HRCRC-specific ranged from 1.29 to 1.88). In contrast, patients with other + MMRdeficient CRC had the most favorable survival (HRCRC-specific 0.48). No statistically significant survival differences were observed for the Warburg-subtypes within mutational subgroups. Our results highlight the prognostic potential of mutational subgroups in CRC. Warburg-subtypes did not provide additional prognostic information within these mutational subgroups. Future larger-scale prospective studies are necessary to validate our findings and to examine the potential clinical utility of CRC subtyping based on mutational subgroups.

Targeting Metastatic Colorectal Cancer with Immune Oncological Therapies.

Simple SummaryColorectal cancer that has become metastatic continues to have a poor outlook. Many patients will undergo intensive chemotherapy, often with limited effects but many side effects. In other types of cancer, immunotherapy has shown to be effective by targeting the immune cells of the patient and restoring their function to fight off cancer. This review explores the up-to-date evidence for immunotherapy in metastatic colorectal cancer. We have discussed in which patients this treatment is effective, but also why this has not been effective in a large number of patients. By summarizing the key components of the immune cells within the proximity to the tumour and its areas of spread, we discuss how these components can be targeted. Areas of future research are also highlighted, which includes combining immunotherapy with current treatments, such as surgery, radiotherapy and chemotherapy as well as some recent advances from basic and translational studies that promises to improve outcomes in these patients.Metastatic colorectal cancer carries poor prognosis, and current therapeutic regimes convey limited improvements in survival and high rates of detrimental side effects in patients that may not stand to benefit. Immunotherapy has revolutionised cancer treatment by restoring antitumoural mechanisms. However, the efficacy in metastatic colorectal cancer, is limited. A literature search was performed using Pubmed (Medline), Web of Knowledge, and Embase. Search terms included combinations of immunotherapy and metastatic colorectal cancer, primarily focusing on clinical trials in humans. Analysis of these studies included status of MMR/MSS, presence of combination strategies, and disease control rate and median overall survival. Evidence shows that immune checkpoint inhibitors, such as anti-PD1 and anti-PD-L1, show efficacy in less than 10% of patients with microsatellite stable, MMR proficient colorectal cancer. In the small subset of patients with microsatellite unstable, MMR deficient cancers, response rates were 40–50%. Combination strategies with immunotherapy are under investigation but have not yet restored antitumoural mechanisms to permit durable disease regression. Immunotherapy provides the potential to offer additional strategies to established chemotherapeutic regimes in metastatic colorectal cancer. Further research needs to establish which adjuncts to immune checkpoint inhibition can unpick resistance, and better predict which patients are likely to respond to individualised therapies to not just improve response rates but to temper unwarranted side effects.

Reduction in the resident intestinal myelomonocytic cell population occurs during ApcMin/+ mouse intestinal tumorigenesis.

With its significant contribution to cancer mortality globally, advanced colorectal cancer (CRC) requires new treatment strategies. However, despite recent good results for mismatch repair (MMR)-deficient CRC and other malignancies, such as melanoma, the vast majority of MMR-proficient CRCs are resistant to checkpoint inhibitor (CKI) therapy. MMR-proficient CRCs commonly develop from precursor adenomas with enhanced Wnt-signalling due to adenomatous polyposis coli (APC) mutations. In melanomas with enhanced Wnt signalling due to stabilized β-catenin, immune anergy and resistance to CKI therapy has been observed, which is dependent on micro-environmental myelomonocytic (MM) cell depletion in melanoma models. However, MM populations of colorectal adenomas or CRC have not been studied. To characterize resident intestinal MM cell populations during the early stages of tumorigenesis, the present study utilized the ApcMin/+ mouse as a model of MMR-proficient CRC, using enhanced green fluorescent protein (EGFP) expression in the mouse lysozyme (M-lys)lys-EGFP/+ mouse as a pan-myelomonocytic cell marker and a panel of murine macrophage surface markers. Total intestinal lamina propria mononuclear cell (LPMNC) numbers significantly decreased with age (2.32±1.39×107 [n=4] at 33 days of age vs. 1.06±0.24×107 [n=8] at 109 days of age) during intestinal adenoma development in ApcMin/+ mice (P=0.05; unpaired Student's t-test), but not in wild-type littermates (P=0.35). Decreased total LPMNC numbers were associated with atrophy of intestinal lymphoid follicles and the absence of MM/lymphoid cell aggregates in ApcMin/+ mouse intestine, but not spleen, compared with wild-type mice. Furthermore, during the early stage of intestinal adenoma development, there was a two-fold reduction of M-lys expressing cells (P=0.05) and four-fold reduction of ER-HR3 (macrophage sub-set) expressing cells (P=0.05; two tailed Mann-Whitney U test) in mice with reduced total intestinal LPMNCs (n=3). Further studies are necessary to determine the relevance of these findings to immune-surveillance of colorectal adenomas or MMR-proficient CRC CKI therapy resistance.

Evaluating the utility of tumour mutational signatures for identifying hereditary colorectal cancer and polyposis syndrome carriers

ObjectiveGermline pathogenic variants (PVs) in the DNA mismatch repair (MMR) genes and in the base excision repair gene MUTYH underlie hereditary colorectal cancer (CRC) and polyposis syndromes. We evaluated the...

MMR-proficient and MMR-deficient colorectal cancer cells: 5-Fluorouracil treatment response and correlation to CD133 and MGMT expression

Cancer stem cells (CSCs) from colorectal cancer (CRC), characterized by CD133 expression, have been associated with 5-fluorouracile (5-FU) chemoresistance. DNA repair mechanisms, such as O6-alkylguanine DNA alkyltransferase (MGMT) and mismatch repair (MMR) systems, have also been correlated to 5-FU resistance in CRC. The aim of this study was to evaluate the modulation of CD133 and MGMT in MMRproficient and MMR-deficient CRC cells under 5-FU treatment and the effect of this drug in CSCs. CD133 and MGMT methylation status were determined in MMR-proficient (SW480 and HT29) and MMR-deficient (RKO and HCT116) cell lines by methylation-specific PCRs. SW480 and RKO were selected to determine modulation of CD133, MGMT and MMR expression after 5-FU treatment by qPCR. In addition, CD133, MGMT and MMR were analyze in SW480 and RKO CSCs. No association between promoter methylation and MGMT and CD133 expression was found. 5-FU treatment increased CD133 expression independently to MMR status in SW480 and RKO and was able to increase hMLH1 expression in RKO, a MMR-deficient cell line. RKO/ CSCs overexpressed CD133 and MMR (hMSH2 and hMSH6) while SW480/CSCs showed a significant increase in CD133, MMR (hMLH1, hMSH2 and hMSH6) and MGMT, moreover 5-FU resistance than parental cell lines. Thus, although CSCs 5-FU chemoresistance appears to be independently to MMR status, hMLH1 might play a key role in CSC response to 5-FU. New drugs exploding these differences could benefit the prognostic of patients with CRC.

Characteristics of colorectal carcinoma patients with PMS2 defects detected by immunohistochemistry.

Lynch syndrome is the most prevalent form of familial colorectal cancer (CRC) and is caused by pathogenic germline mismatch repair (MMR) gene mutations. MLH1, MSH2 and MSH6 mutations have been well studied, but the rate and characteristics of PMS2 mutations are rare, especially in China. This study enrolled 1706 unselected patients with CRC who underwent colorectal resection from June 2016 to November 2018, the MMR status and clinicopathological features were analysed. A total of 11.8% of patients with CRC had defects in at least one MMR-related protein. Among them, 8.3% were identified with PMS2 defects, and 3.1% of patients had isolated PMS2 defects. Compared with MMR-proficient CRC, PMS2-defect CRC occurred more frequently in the right colon and less frequently in the rectum, had more poorly differentiated and mucinous carcinoma cases, and had fewer perineural invasions and a lower pN stage but a more advanced pT stage and a larger tumour size. In the cases with PMS2 defect, there were fewer tumours in the right colon, fewer poorly differentiated cases and smaller tumour sizes than in the cases with both MLH1 and PMS2 defects. In addition, in cases with isolated PMS2 defects, there were more tumours in the right colon and, more mucinous carcinoma cases than in cases with MMR-proficient CRCs, but had a similar cancer onset age. This study identified the rate, clinicopathological and age characteristics of PMS2 defects in CRCs in China and highlighted the importance of universal screening and germline detection of PMS2 in CRC.

Dominantly Inherited Hereditary Nonpolyposis Colorectal Cancer Not Caused by MMR Genes.

In the past two decades, multiple studies have been undertaken to elucidate the genetic cause of the predisposition to mismatch repair (MMR)-proficient nonpolyposis colorectal cancer (CRC). Here, we present the proposed candidate genes according to their involvement in specific pathways considered relevant in hereditary CRC and/or colorectal carcinogenesis. To date, only pathogenic variants in RPS20 may be convincedly linked to hereditary CRC. Nevertheless, accumulated evidence supports the involvement in the CRC predisposition of other genes, including MRE11, BARD1, POT1, BUB1B, POLE2, BRF1, IL12RB1, PTPN12, or the epigenetic alteration of PTPRJ. The contribution of the identified candidate genes to familial/early onset MMR-proficient nonpolyposis CRC, if any, is extremely small, suggesting that other factors, such as the accumulation of low risk CRC alleles, shared environmental exposures, and/or gene–environmental interactions, may explain the missing heritability in CRC.

Tumor-associated macrophage infiltration and prognosis in colorectal cancer: systematic review and meta-analysis.

Tumor-associated macrophages (TAMs) are key components of colorectal cancer (CRC) microenvironment, but their role in CRC prognosis is not fully defined. This study aimed to evaluate prognostic value of different types and distribution of TAMs in CRC. Total 27 studies with 6115 patients were searched from PubMed and Embase and analyzed to determine the association between TAMs, including distinct TAM subsets and infiltration location, and CRC survival. The prognostic impact of TAMs on CRC was further stratified by tumor type and mismatch repair system (MMR) status. A pooled analysis indicated that high density of TAMs in CRC tissue was significantly associated with favorable 5-year overall survival (OS) but not with disease-free survival (DFS). CD 68+ TAM subset correlated with better 5-year OS, while neither CD68+NOS2+ M1 subset nor CD163+ M2 subset was correlated with 5-year OS. Increased CD68+ TAM infiltration in tumor stroma but not in tumor islet predicted improved 5-year OS. Stratification by tumor type and MMR status showed that in colon cancer or MMR-proficient CRC, elevated TAM density was associated with better 5-year OS. High infiltration of CD68+ TAMs could be a favorable prognostic marker in CRC. Future therapies stimulating CD68+ TAM infiltration may be promising in CRC treatment.

Abstract A08: Tumor microbiome in subtypes of mismatch repair-deficient colorectal cancer

Abstract Background: Colorectal cancer (CRC) comprises different molecular subtypes, including those tumors that develop defective DNA mismatch repair (MMR) evidenced by microsatellite instability (MSI) and/or loss of MMR protein expression (MMR-deficiency). Tumor MMR-deficiency can result from inherited causes, namely germline mutations in the MMR genes (Lynch syndrome) or from somatic inactivation resulting from hypermethylation of the MLH1 gene promoter or from biallelic somatic mutations. Little is known about the etiologies of these cancers or the avenues for risk amelioration. We hypothesized that dysbiosis of the gut microbiome is a risk factor for the development of MMR-deficient subtypes of CRC and that the microbiome composition differs between inherited and somatic MMR-deficient CRC. Materials and Methods: The microbiome composition was determined by 16S rRNA sequencing of the V3-V4 regions in DNA extracted from 94 FFPE-derived tumor specimens comprised of 14 MMR-proficient CRCs and 80 MMR-deficient CRCs from the Australasian Colorectal Cancer Family Registry. The MMR-deficient CRCs comprised three subtypes: 35 people with germline MMR gene mutations (Lynch syndrome), 13 people with sporadic MLH1 hypermethylated CRCs, and 32 people with suspected Lynch syndrome (no germline or MLH1 methylation). LEfse analyses were used to identify differential abundance in taxa between groups. Results: In all tumor samples, Proteobacteria, Firmicutes, Bacteroidetes, Actinobacteria, and Fusobacteria were most abundant. Of these, the Firmicute population was significantly higher in the MMR-proficient than MMR-deficient CRCs (0.33 v 0.19, P &amp;lt; 0.05). When compared with the Lynch syndrome-related CRCs, we found 22 bacterial species, including Corynebacteriaceae, that were significantly more abundant in MLH1 hypermethylated tumors (P &amp;lt; 0.05), whereas we found 4 species significantly more abundant, including Enterobacter, in suspected Lynch syndrome group. Although the microbial diversity was notably smaller compared with the sporadic groups, several species of the family Comamanadaceae including Acidovorax were abundant in Lynch syndrome tumors (P &amp;lt; 0.05). Fusobacterium nucleatum was detected in 11/35 (31.4%) Lynch syndrome-related CRCs, 5/13 (38.5%) MLH1 hypermethylated CRCs, 13/32 (40.6%) suspected Lynch syndrome CRCs, and 4/14 (28.6%) sporadic MMR-proficient CRCs. Conclusions: Inherited and somatic subtypes of MMR-deficient CRC show significant differences in their tumor microbiome composition where Lynch syndrome-related MMR-deficient CRC showed a reduced microbiome diversity compared with somatic MMR-deficient CRC. Further validation of these findings will improve our understanding of MMR-deficient CRC tumorigenesis and aid in CRC prevention through the identification of modifiable microbial risk factors. Citation Format: Jihoon E. Joo, Mark Clendenning, Khalid Mahmood, Christophe Rosty, Ingrid M. Winship, Mark A. Jenkins, Daniel D. Buchanan. Tumor microbiome in subtypes of mismatch repair-deficient colorectal cancer [abstract]. In: Proceedings of the AACR Special Conference on the Microbiome, Viruses, and Cancer; 2020 Feb 21-24; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2020;80(8 Suppl):Abstract nr A08.

Abstract 2793: Mismatch repair proficient colorectal cancer and adaptive immunosuppression of endogenous anti-tumor immune response: Implications for immunotherapy

Abstract The tumor immune microenvironment (TiME) of mismatch-repair deficient (MMRd) colorectal cancer (CRC) is characterized by a cytotoxic T cell immune signature and counter-expression of IFNγ -inducible T cell checkpoints, features underlining intratumoral adaptive immunosuppression and sensitivity to immune checkpoint blockade. Since single-agent checkpoint inhibitors have not demonstrated, so far, similar meaningful clinical activity for MMR proficient (MMRp) CRC, we compared the tumor immune microenvironment (TiME) in CRC patients that responded to anti-PD1 therapy with the TiME of patients that did not (NCT01876511) to identify immune signatures and biomarkers that could help deciding on combinatorial immunotherapies to treat patients with MMRp CRC. With this approach, we detected immunoreactive MMRp (irMMRp) tumors characterized by a dense infiltration with cytotoxic T cells (CTL) as well as high expression of IFNγ and CD274 (PD-L1), despite their low tumor mutation burden. We observed that these irMMRp colon tumors are characterized by a negative association between Indolamine 2,3 dioxygenase 1 (IDO1) expression and Th17 infiltration. Despite the high expression of CD8 and PD-L1 expression, only irMMRp CRC with high expression of IDO1 on tumor cells and low infiltration with Th17 cells have a TiME resembling the TiME of CRC responding to anti-PD1. Since IDO1-derived tryptophan metabolites, kynurenines, are known to repress Th17 differentiation, these results suggested that the use of IDO1 small molecule inhibitors may derepress Th17 infiltration in CRC and blunt the effect of anti-PD1. Combining therapy with IDO1/PD1 inhibitors with drugs inhibiting IL-17 signaling pathway may help to treat irMMRp CRC patients. Citation Format: Nicolas Llosa, Anas H. Awan, Ada J. Tam, Hongni Fan, Kellie N. Smith, Cynthia L. Sears, Hao Wang, Drew M. Pardoll, Robert A. Anders, Franck Housseau. Mismatch repair proficient colorectal cancer and adaptive immunosuppression of endogenous anti-tumor immune response: Implications for immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2793.

Abstract 2087: Mechanistic studies reveal novel molecular insights on anticancer properties of F10 compound in human colon cancer cells

Abstract Fluoropyrimidine (FP) drugs, mainly 5-Fluorouracil (5-FU), constitute the backbone of combination chemotherapy regimens (e.g. FOLFOX and FOLFIRI) for treating colorectal cancer (CRC) and provide a survival benefit for patients with stage II, III, and IV colorectal cancer. However, 5 year survival rate for CRC patients with metastatic disease is less than 10%. This dreadful situation suggests urgent need for novel and more potent FPs to improve the outcomes of these patients. We have developed novel fluoropyrimidine drug polymers (e.g. F10) that show promising antitumor activity in multiple pre-clinical models even while displaying reduced systemic toxicities. F10 is taken up by malignant cells via active transport compared to 5-Fluorouracil, which is taken up by diffusion in malignant and non-malignant cells. Additionally, F10 compound is primarily incorporated into DNA, and produced thymidylate synthase and topoisomerase toxic metabolites at greater levels, whereas 5-Fluorouracil is primarily incorporated into RNA, and only has about 5-10% production of active form of thymidylate synthase toxic metabolites. MMR is involved in repairing the DNA mismatches and FP drugs induced DNA damage. Interestingly, our studies show that F10 acts by different mechanisms compared to 5-FU. Particularly, F10 treated CRC cells exhibit replication stress as evidenced by paused and slow progression of replication forks and R-loops formation at the active transcriptional regions, suggesting replication and transcription dependent generation of DNA lesions in these cells. F10 treatment induced several fold increased DNA double strand breaks (DSB) as measured by COMET assays and about 1000 folds more cytotoxicity to CRC cells compared to 5-FU. Similarly, F10 treatment activated distinctive DNA damage responses in MMR proficient CRC cells compared MMR deficient cells. Additionally, our studies suggest F10 as a promising therapeutic agent for CRC therapy either alone or in combination with CHK1 and PARP inhibitors. Citation Format: Chinnadurai Mani, William Gmeiner, Komaraiah Palle. Mechanistic studies reveal novel molecular insights on anticancer properties of F10 compound in human colon cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2087.

Comparison of phosphoproteomic profiles in left- and right-sided colorectal cancers.

582 Background: Left- and right-sided colorectal cancers (CRCs) are different in terms of prognosis, treatment response and underlying molecular mechanisms. The aim of this study was to unravel the phosphoproteomic profiles of left- and right-sided primary CRCs and to identify potential drug targets for both primary tumor sites. Methods: In total phosphoproteomic profiles of 69 fresh-frozen biopsies, including 34 left-sided and 35 right-sided primary CRCs, were obtained by LC-MS/MS after cell lysis, digestion and phosphopeptide enrichment using anti-phosphotyrosine antibodies. MS/MS spectra were searched against a UniProt human reference proteome FASTA file using MaxQuant software. Fold changes between normalized intensities were calculated. Group comparison was performed using the R package Limma. P values below 0.01 and fold changes &gt; two were considered significant and biologically relevant. Results: In total 2,840 phosphopeptides were detected in at least 5% of the samples. Of these, 36 phosphopeptides were significantly upregulated and 14 were significantly downregulated in right-sided compared to left-sided CRCs using our stringent selection criteria. Since BRAF mutations and deficient DNA mismatch repair (dMMR) were more frequently observed in right-sided CRC (p &lt; 0.01), group comparison was repeated with MMR proficient CRCs only, resulting in 21 phosphopeptides up- and one downregulated in left- versus right-sided CRC. The identified phosphoproteins from the phosphopeptides included three FDA approved drug targets. Conclusions: Differences in phosphoproteomic profiles were detected between left- and right-sided CRCs, besides molecular differences such as BRAF mutation and MMR status. These results indicate that the molecular biology of left-and right-sided CRCs may explain their differences in clinical behavior and response to treatment.