Tumor mutational load, microsatellite instability, BRCAness, and actionable alterations in metastatic colorectal cancer: Results from the TRIBE2 study

Background In the TRIBE2 study, molecularly unselected and untreated mCRC patients were randomized to receive FOLFOXIRI/bevacizumab (bev) followed by the same agents after disease progression (PD) or FOLFOX/bev followed by FOLFIRI/bev after PD. We performed a comprehensive NGS analysis of samples from randomized patients in order to investigate the prognostic impact of tumor mutational load (TML), its additional value with respect to the assessment of microsatellite instability (MSI), and the overall prevalence of potentially actionable alterations. We also investigated the prevalence and the prognostic impact of the so-called “BRCAness” alterations, whose prevalence and prognostic impact in mCRC are currently unknown. Methods Tumor DNA was obtained from formalin-fixed, paraffin-embedded blocks from primary tumors of 296 (44%) of 679 randomized patients, and underwent NGS analysis using the Caris MI TumorSeek panel, assessing 592 genes. TML was defined as low, intermediate, or high based on finding 16 mutations/Mb. MSI status was determined both by NGS and by IHC. BRCAness group was defined by the presence of pathogenic alterations in at least one of the following genes: BRCA1/2, PALB2, RAD50, RAD51, ATM, ATR, FANC(A-C- D2-E-F-G-L), EMSY, BARD1, BRIP1, CHEK1, CHEK2, MRE11, BLM, NBN, or WNR. Results TML and MSI were determined by NGS in 224 (76%) cases. NGS and IHC results were concordant in 221 (99%) cases. TML was low, intermediate, or high in 56 (25%), 157 (70%), and 11 (5%) cases, respectively. TML-high tumors were MSI-high or MSS in 8 (73%) and 3 (27%) cases, respectively. Two of 3 TML-high and MSS tumors showed a pathogenic POLE mutation (p.S459F and p.P286R). The other TML-high, MSS, and POLE wt tumor was dMMR at IHC (loss of MSH6 expression) and showed a pathogenic MSH6 mutation (p.F1040fs). As compared with low and intermediate TML, high TML was associated with longer PFS (median PFS: 17.3 vs 10.6; HR: 0.54 [95% CI: 0.35- 1.09], P = .098) and OS (median OS: not reached vs 23.7; HR :0.45 [95% CI: 0-28-1.13], P = .106). No interaction effect between TML and treatment arm was observed, and no difference between TML-low and -intermediate tumors was reported in terms of baseline characteristics and prognosis. Potentially actionable alterations (BRAF V600E and KRAS G12C mutations, HER2 mutations/amplification) were found in 55 (19%) of 296 cases. No NTRK/ROS/ALK or MET amplifications were found. At least one BRCAness alteration was reported in 40 (14%) of 296 patients. In all, 56 different BRCAness alterations were found, with ATM (21/56, 38%) and BRCA1/2 (13/56, 23%) being the most frequently mutated genes. Longer PFS (13.4 vs 10.6 months; HR: 0.67 [95% CI: 0.48-0.93], P = .032) and OS (30.1 vs 23.9, HR: 0.66 [95% CI: 0.43-1.02], P = .062) were observed in the BRCAness group. No interaction effect between BRCAness and treatment arm was reported. Conclusion TML-high tumors are not limited to MSI-high, but may also present POLE or MSH6 somatic mutation and show improved outcomes. No differences are reported between TML-low and -intermediate tumors. BRCAness alterations are associated with better prognosis. Molecular alterations predictive of benefit from targeted strategies are detectable in a small percentage of mCRCs. Legal entity responsible for the study The author. Funding GONO Foundation. Disclosure The presenting author has declared no conflicts of interest.