DNA Polymerase Delta 2 Activates Cell Cycle in Lung Adenocarcinoma, Leading to High Malignancy and Poor Prognosis

Background: DNA polymerase delta 2 (POLD2) plays a crucial role in DNA repair and replication. POLD2 is upregulated and related to poor prognosis in several types of cancer. However, the biological and clinical importance of POLD2 in lung adenocarcinoma remains unclear. Patients and Methods: We investigated the relationship between POLD2 expression and tumor malignancy in lung adenocarcinoma cell lines. Immunohistochemistry (IHC) was performed on 373 patients with completely resected lung adenocarcinoma, and the association between POLD2 expression, clinicopathological features, and prognosis was examined. Results: POLD2 knockdown decreases cell proliferation and migration, resulting in apoptosis and G1 arrest in the cell cycle in lung adenocarcinoma cells. Additionally, POLD2 knockdown attenuates MYC expression, which may decrease the expression of cyclin-dependent kinases 4 and 6, pRb, Rb, and E2F1. Furthermore, among 373 patients with completely resected lung adenocarcinoma, smoking history, advanced pathological stage, and vascular invasion were significantly more prevalent in patients with high POLD2 expression (n = 146, 39.3%) than in those with low POLD2 expression (n = 227, 60.7%). Patients with high POLD2 expression also had a significantly worse recurrence-free survival (RFS) and overall survival. In the multivariable analysis, high POLD2 expression was an independent poor prognostic factor of RFS. Conclusions: We provide the possibility of POLD2 as a potential new therapeutic target because high POLD2 expression is associated with high malignancy and poor prognosis in specimens from patients with lung adenocarcinoma and POLD2 depletion triggers the suppression of cell migration, cell cycle progression, and cell proliferation.