Cancers from Novel Pole-Mutant Mouse Models Provide Insights into Polymerase-Mediated Hypermutagenesis and Immune Checkpoint Blockade

Abstract

POLE mutations are a major cause of hypermutant cancers, yet questions remain regarding mechanisms of tumorigenesis, genotype–phenotype correlation, and therapeutic considerations. In this study, we establish mouse models harboring cancer-associated POLE mutations P286R and S459F, which cause rapid albeit distinct time to cancer initiation in vivo, independent of their exonuclease activity. Mouse and human correlates enabled novel stratification of POLE mutations into three groups based on clinical phenotype and mutagenicity. Cancers driven by these mutations displayed striking resemblance to the human ultrahypermutation and specific signatures. Furthermore, Pole-driven cancers exhibited a continuous and stochastic mutagenesis mechanism, resulting in intertumoral and intratumoral heterogeneity. Checkpoint blockade did not prevent Pole lymphomas, but rather likely promoted lymphomagenesis as observed in humans. These observations provide insights into the carcinogenesis of POLE-driven tumors and valuable information for genetic counseling, surveillance, and immunotherapy for patients. Significance: Two mouse models of polymerase exonuclease deficiency shed light on mechanisms of mutation accumulation and considerations for immunotherapy. See related commentary by Wisdom and Kirsch p. 5459

DNA Polymerases at the Eukaryotic Replication Fork Thirty Years after: Connection to Cancer

“…High mutation rates lead to catastrophic accumulation of mutations and cell death [24,28,128]. Consistently, the hypermutator alleles encoding for the analog of P286R change in homozygous state are inviable in mice, while causing cancer when heterozygous [122,135] (Table 2, two last columns of row 3). In the critique of this explanation, we can note that different mutations causing defects of proofreading exo cause different increases of mutation rates (Table 2), and thus it is not clear why tumors do not accumulate "mild" or "leaky" alleles, moderately affecting the exo function of pol δ.…”

“…Defects in MMR for a long time were the only factors in hereditary non-polyposis, endometrial, and other cancers in Lynch syndrome, connecting replication fidelity to cancer [116][117][118][119]. The topic of MMR role in cancer is extensively discussed and reviewed [116,[120][121][122] and is not touched here. Recent studies of cancer genomes discovered mutations in genes of the DNA pols of the B family in many sporadic and hereditary cancers [1,2] (Figure 1).…”

Mouse model and human patient data suggest critical roles for Pten and p53 in suppressing POLE mutant tumor development

“…All POLE mutation signatures (SBS 10a, 28 and 10b) were also present in tumors from Pole +/P286R ;Trp53 +/mice (Fig. 3C) 3 , as (which was not certified by peer review) is the author/funder. All rights reserved.…”

“…We have also shown that p53 is activated in human cancer cell lines engineered to express cancer mutant alleles of POLE 9 . We previously characterized the effects of the Pole-P286R mutation on tumor development and mutagenesis in a mouse model 3 . We noted that Pole P286R/P286R was likely embryonic lethal due to lack of any observed viable offspring.…”

“…Tail gDNA was extracted and PCR amplified. PCR products were directly treated with DdeI restriction enzyme (NEB) per product protocol and run on agarose gel as previously described 3 . The Pole locus was PCR amplified using the following primers:…”