Tumor mutation burden in colorectal cancers with POLE exonuclease and nonexonuclease domain variants.

224 Background: Driver mutations in the exonuclease domain (ExoD) of DNA polymerase epsilon ( POLE) have been associated with high tumor mutation burden (TMB). High TMB is an important marker for immunotherapy, and thus understanding the mechanisms that lead to high TMB is an area of active investigation. Many tumors with POLE ExoD driver mutations have additional POLE variants of uncertain significance (VUS). Some of these VUS lie within the ExoD, while most lie outside the ExoD. This study investigates the role of such POLE VUS on TMB and POLE functionality of ExoD-mutated tumors. Methods: Retrospective colorectal cancer (CRC) genomic profiles from Caris Life Sciences (n=1,870) were separated into groups based on the presence of POLE ExoD mutations and the TMB status. Groups were defined as: ‘TMB-high (TMB-H) POLE ExoD driver without additional POLE VUS’, ‘TMB-H POLE ExoD driver plus POLE VUS’, and ‘TMB-low (TMB-L) POLE variant(s) without ExoD driver’. An additional group with ‘TMB-H POLE variant(s) without ExoD driver’ was identified as potentially novel drivers for future study. The microsatellite instability status (MSI or MSS) was also considered. The level of TMB, the presence of polymerase proofreading-associated mutational signatures and POLE stability/functionality analyzed using AlphaFold2 models and Rosetta Molecular Modeling Suite, were evaluated. Results: POLE variants were identified in 4.9% of the CRCs studied (92/1870). In all, 52.2% (48/92) of those tumors were TMB-H, and 38.0% (35/92) of them contained POLE ExoD drivers. The highest median TMB (mTMB) was found in the ‘TMB-H POLE ExoD driver plus POLE VUS’ group, and this was significantly different from the ‘TMB-H POLE ExoD driver without additional POLE VUS’ and the ‘TMB-L POLE variant without ExoD driver’ group, even when MSI tumors were excluded (p<0.001, Mann-Whitney). These results were validated in TCGA CRC tumors and in other tumor types. Within the ‘TMB-H POLE ExoD driver plus POLE VUS’ group, the mTMB increased with the number of accumulated POLE VUS (p<0.001, Mann Whitney). The nucleotide sequence context of 77.8% of the POLE VUS matched mutational spectra associated with POLE ExoD defects, suggesting that most of the POLE VUS are secondary to the ExoD driver mutation. Finally, the majority of POLE VUS within ‘TMB-H POLE ExoD driver plus POLE VUS’ group have a predicted effect on POLE stability and/or function. Conclusions: Our results suggest that the presence of secondary POLE non-driver variants increase TMB in POLE ExoD-mutated tumors. Although still preliminary, these data could impact prognosis and response to immune checkpoint inhibitors in patients. [Table: see text]