Pol η–dependent DNA synthesis at stalled replication forks during S phase suppresses chronic fragile site instability by preventing checkpoint-blind under-replicated DNA in mitosis.

"Replicative stress" is one of the main factors underlying neoplasia from its early stages. Genes involved in DNA synthesis may therefore represent an underexplored source of potential prognostic markers for cancer. To this aim, we generated gene expression profiles from two independent cohorts (France, n = 206; United Kingdom, n = 117) of patients with previously untreated primary breast cancers. We report here that among the 13 human nuclear DNA polymerase genes, DNA Polymerase θ (POLQ) is the only one significantly up-regulated in breast cancer compared with normal breast tissues. Importantly, POLQ up-regulation significantly correlates with poor clinical outcome (4.3-fold increased risk of death in patients with high POLQ expression), and this correlation is independent of Cyclin E expression or the number of positive nodes, which are currently considered as markers for poor outcome. POLQ expression provides thus an additional indicator for the survival outcome of patients with high Cyclin E tumor expression or high number of positive lymph nodes. Furthermore, to decipher the molecular consequences of POLQ up-regulation in breast cancer, we generated human MRC5-SV cell lines that stably overexpress POLQ. Strong POLQ expression was directly associated with defective DNA replication fork progression and chromosomal damage. Therefore, POLQ overexpression may be a promising genetic instability and prognostic marker for breast cancer.specialized DNA replication | prognosis marker | S-phase checkpoint B esides the "replicative" DNA polymerases POLA, POLD, and POLE, which are involved in conventional DNA replication of the undamaged genome, mammalian nuclei contain 10 additional specialized DNA polymerases that play a role in replication, repair, and recombination of damaged DNA (1, 2) and thus may be of paramount importance to preserve the integrity of the genome.Specialized DNA polymerases are frequently deregulated in neoplasia (3-10). Indeed, the intracellular balance between the error-free, replicative polymerases POLA, POLD, and POLE and the error-prone, specialized DNA polymerases (POLH, POLL, POLM, POLN, POLK, POLB, POLI, POLQ, POLZ/REV3L, and REV1) appears to be of great importance for the maintenance of genome stability (11)(12)(13)(14). Here, we wondered whether misregulation of DNA polymerases could be a signature of breast cancer progression. Indeed, beside the standard classification used by pathologists, there is a clear lack of tools to accurately predict the clinical outcome of many patients.We specifically measured the expression levels of the 13 human replicative and specialized DNA polymerases in 206 breast carcinomas. We report that, differently from the replicative and the other specialized DNA polymerases, POLQ was significantly upregulated in most of the breast tumors analyzed. Such up-regulation was associated with poor clinical outcome.POLQ is an error-prone, specialized DNA polymerase that might operate during "normal" genomic replication because it bypasses some endogenous DNA lesions an...