Genetic and immune landscape evolution in MMR-deficient colorectal cancer.

Mismatch repair-deficient (MMRd) colorectal cancers (CRCs) have high mutation burdens, which make these tumours immunogenic and many respond to immune checkpoint inhibitors. The MMRd hypermutator phenotype may also promote intratumour heterogeneity (ITH) and cancer evolution. We applied multiregion sequencing and CD8 and programmed death ligand 1 (PD-L1) immunostaining to systematically investigate ITH and how genetic and immune landscapes coevolve. All cases had high truncal mutation burdens. Despite pervasive ITH, driver aberrations showed a clear hierarchy. Those in WNT/ß-catenin, mitogen-activated protein kinase, and TGF-ß receptor family genes were almost always truncal. Immune evasion (IE) drivers, such as inactivation of genes involved in antigen presentation or IFN-γ signalling, were predominantly subclonal and showed parallel evolution. These IE drivers have been implicated in immune checkpoint inhibitor resistance or sensitivity. Clonality assessments are therefore important for the development of predictive immunotherapy biomarkers in MMRd CRCs. Phylogenetic analysis identified three distinct patterns of IE driver evolution: pan-tumour evolution, subclonal evolution, and evolutionary stasis. These, but neither mutation burdens nor heterogeneity metrics, significantly correlated with T-cell densities, which were used as a surrogate marker of tumour immunogenicity. Furthermore, this revealed that genetic and T-cell infiltrates coevolve in MMRd CRCs. Low T-cell densities in the subgroup without any known IE drivers may indicate an, as yet unknown, IE mechanism. PD-L1 was expressed in the tumour microenvironment in most samples and correlated with T-cell densities. However, PD-L1 expression in cancer cells was independent of T-cell densities but strongly associated with loss of the intestinal homeobox transcription factor CDX2. This explains infrequent PD-L1 expression by cancer cells and may contribute to a higher recurrence risk of MMRd CRCs with impaired CDX2 expression. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Rationale and design of the POLEM trial: avelumab plus fluoropyrimidine-based chemotherapy as adjuvant treatment for stage III mismatch repair deficient or POLE exonuclease domain mutant colon cancer: a phase III randomised study.

BACKGROUND: 10%-15% of early-stage colon cancers harbour either deficient mismatch repair (dMMR), microsatellite instability high (MSI-H) or POLE exonuclease domain mutations, and are characterised by high tumour mutational burden and increased lymphocytic infiltrate. Metastatic dMMR colon cancers are highly sensitive to immune checkpoint inhibition, and recent data show POLE-mutant tumours are similarly responsive. We are conducting a phase III randomised trial to determine if the addition of the anti-PD-L1 antibody avelumab following adjuvant chemotherapy improves disease-free survival (DFS) in patients with stage III dMMR/MSI-H or POLE mutant colon cancer and is a cost-effective approach for the UK National Health Service (NHS). METHODS: We are recruiting patients with completely resected, stage III colon cancer confirmed to have dMMR/MSI-H, locally or POLE exonuclease domain mutation on central testing. Eligible patients are randomised in a 1:1 ratio to standard fluoropyrimidine-based chemotherapy (capecitabine, oxaliplatin for 12 weeks or capecitabine for 24 weeks) or chemotherapy, followed by avelumab (10 mg/kg, 2 weekly for 24 weeks). Stratification is by chemotherapy received and MMR/MSI-H status. The primary endpoint is DFS. Secondary endpoints include overall survival, toxicity, quality of life and health resource use. The 3-year DFS rate in the control arm is expected to be ~75%. Avelumab is expected to improve the 3-year DFS rate by 12% (ie, 87%). Target accrual is 402 patients, which provides 80% power to detect an HR of 0.48 for DFS at a two-sided alpha of 0.05. This national, multicentre phase III trial is sponsored by the Royal Marsden NHS Foundation Trust and it is anticipated that approximately 40 centres in the UK will participate. This study opened to recruitment in August 2018. TRIAL REGISTRATION NUMBER: NCT03827044.