Mirtrons are alternative precursors for microRNA biogenesis that were recently described in invertebrates. These short hairpin introns use splicing to bypass Drosha cleavage, which is otherwise essential for the generation of canonical animal microRNAs. Using computational and experimental strategies, we now establish that mammals have mirtrons as well. We identified 3 mirtrons that are well conserved and expressed in diverse mammals, 16 primate-specific mirtrons, and 46 candidates supported by limited cloning evidence in primates. As with some fly and worm mirtrons, the existence of well-conserved mammalian mirtrons indicates their relatively ancient incorporation into endogenous regulatory pathways. However, as worms, flies, and mammals each have different sets of mirtrons, we hypothesize that different animals may have independently evolved the capacity for this hybrid small RNA pathway. This notion is supported by our observation of several clade-specific features of mammalian and invertebrate mirtrons.

Purpose: To analytically and clinically validate microsatellite instability (MSI) detection using cell-free DNA (cfDNA) sequencing. Experimental Design: Pan-cancer MSI detection using Guardant360 was analytically validated according to established guidelines and clinically validated using 1,145 cfDNA samples for which tissue MSI status based on standard-of-care tissue testing was available. The landscape of cfDNA-based MSI across solid tumor types was investigated in a cohort of 28,459 clinical plasma samples. Clinical outcomes for 16 patients with cfDNA MSI-H gastric cancer treated with immunotherapy were evaluated. Results: cfDNA MSI evaluation was shown to have high specificity, precision, and sensitivity, with a limit of detection of 0.1% tumor content. In evaluable patients, cfDNA testing accurately detected 87% (71/82) of tissue MSI-H and 99.5% of tissue microsatellite stable (863/867) for an overall accuracy of 98.4% (934/949) and a positive predictive value of 95% (71/75). Concordance of cfDNA MSI with tissue PCR and next-generation sequencing was significantly higher than IHC. Prevalence of cfDNA MSI for major cancer types was consistent with those reported for tissue. Finally, robust clinical activity of immunotherapy treatment was seen in patients with advanced gastric cancer positive for MSI by cfDNA, with 63% (10/16) of patients achieving complete or partial remission with sustained clinical benefit. Conclusions: cfDNA-based MSI detection using Guar-dant360 is highly concordant with tissue-based testing, enabling highly accurate detection of MSI status concurrent with comprehensive genomic profiling and expanding access to immunotherapy for patients with advanced cancer for whom current testing practices are inadequate. See related commentary by Wang and Ajani, p. 6887

PURPOSE To develop recommendations for treatment of patients with metastatic colorectal cancer (mCRC). METHODS ASCO convened an Expert Panel to conduct a systematic review of relevant studies and develop recommendations for clinical practice. RESULTS Five systematic reviews and 10 randomized controlled trials met the systematic review inclusion criteria. RECOMMENDATIONS Doublet chemotherapy should be offered, or triplet therapy may be offered to patients with previously untreated, initially unresectable mCRC, on the basis of included studies of chemotherapy in combination with anti–vascular endothelial growth factor antibodies. In the first-line setting, pembrolizumab is recommended for patients with mCRC and microsatellite instability-high or deficient mismatch repair tumors; chemotherapy and anti–epidermal growth factor receptor therapy is recommended for microsatellite stable or proficient mismatch repair left-sided treatment-naive RAS wild-type mCRC; chemotherapy and anti–vascular endothelial growth factor therapy is recommended for microsatellite stable or proficient mismatch repair RAS wild-type right-sided mCRC. Encorafenib plus cetuximab is recommended for patients with previously treated BRAF V600E–mutant mCRC that has progressed after at least one previous line of therapy. Cytoreductive surgery plus systemic chemotherapy may be recommended for selected patients with colorectal peritoneal metastases; however, the addition of hyperthermic intraperitoneal chemotherapy is not recommended. Stereotactic body radiation therapy may be recommended following systemic therapy for patients with oligometastases of the liver who are not considered candidates for resection. Selective internal radiation therapy is not routinely recommended for patients with unilobar or bilobar metastases of the liver. Perioperative chemotherapy or surgery alone should be offered to patients with mCRC who are candidates for potentially curative resection of liver metastases. Multidisciplinary team management and shared decision making are recommended. Qualifying statements with further details related to implementation of guideline recommendations are also included. Additional information is available at www.asco.org/gastrointestinal-cancer-guidelines .

◥Mismatch repair (MMR)-deficient colorectal cancers (dMMR colorectal cancer) are characterized by the expression of highly immunogenic neoantigen peptides, which stimulate lymphocytic infiltration as well as upregulation of inflammatory cytokines. These features are key to understanding why immunotherapy (specifically PD-1 and/or CTLA-4 checkpoint blockade) has proved to be highly effective for the treatment of patients with advanced dMMR colorectal cancer. Importantly, preclinical studies also suggest that this correlation between potent tumor neoantigens and the immune microenvironment is present in early (premalignant) stages of dMMR colorectal tumorigenesis as well, even in the absence of a high somatic mutation burden. Here, we discuss recent efforts to characterize how neoantigens and the tumor immune microenvironment coevolve throughout the dMMR adenoma-to-carcinoma pathway. We further highlight how this preclinical evidence forms the rational basis for developing novel immunotherapy-based colorectal cancer prevention strategies for patients with Lynch syndrome.

Purpose To explore the effects of single nucleotide polymorphisms (SNPs) on pancreatic cancer risk and overall survival. Experimental Design The germline DNA of 531 pancreatic cancer cases and 305 healthy controls from a hospital-based study was genotyped at SNPs previously reported to be associated with pancreatic cancer risk or clinical outcome. We analyzed putative risk SNPs for replication of their reported effects on risk and tested for novel effects on overall survival (OS). Similarly, we analyzed putative survival-associated SNPs for replication of their reported effects on OS and tested for novel effects on risk. Lastly, we performed a genome-wide association study of OS using a subset of 252 cases, with two subsequent validation sets of 261 and 572 patients, respectively. Results Among seven risk SNPs analyzed, two (rs505922, rs9543325) were associated with risk (p<0.05). Among 24 survival-associated SNPs analyzed, one (rs9350) was associated with OS (p<0.05). No putative risk SNPs or putative survival-associated SNPs were found to be associated with OS or risk, respectively. Further, our GWAS identified a novel SNP (rs1482426, combined stage 1 and 2 p = 1.7 ×10−6, per-allele HR = 1.74, 95% CI 1.38–2.18) to be putatively associated with OS. Conclusions The effects of SNPs on pancreatic cancer risk and overall survival were replicated in our study, though further work is necessary to understand the functional mechanisms underlying these effects. More importantly, the putative association with OS identified by GWAS suggests that GWAS may be useful in identifying SNPs associated with clinical outcome in pancreatic cancer.