Investigating POLG-driven modulation of cancer stemness: a pilot study in breast cancer cells

Mitochondria have emerged as key regulators of breast cancer stem cell (CSC) biology. Mitochondrial metabolic pathways, including oxidative phosphorylation (OXPHOS) and mitochondrial biogenesis, are frequently altered during tumorigenesis, highlighting their role in breast cancer pathogenesis. Since breast CSCs are highly dependent on mitochondrial metabolism, targeting mitochondrial DNA (mtDNA) replication may represent a strategy to impair CSC maintenance. Mitochondrial DNA polymerase-γ (POLG), composed of a catalytic subunit encoded by POLG1 and an accessory subunit encoded by POLG2, is essential for mtDNA replication and repair. In this pilot study, we investigated whether targeting POLG could modulate breast CSC activity. Genetic knockdown of POLG1 and POLG2 in MCF-7 cells resulted in mtDNA depletion, reduced mitochondrial protein expression, impaired energy production, and loss of stemness-related features. To pharmacologically target POLG, we tested Alovudine, a nucleoside reverse transcriptase inhibitor known to act as an off-target POLG inhibitor. In MCF-7 cells, Alovudine reduced clonogenic potential, decreased mitochondrial DNA-encoded protein levels, and lowered oxygen consumption. To further validate these findings, we employed the independent POLG inhibitor Zalcitabine (ddC) in additional breast cancer models. ddC impaired mitochondrial respiration, reduced mammosphere formation, and modulated SOX2 and NANOG expression. Preliminary Kaplan-Meier analyses in a cohort of 458 breast cancer patients showed that high POLG1 expression correlates with worse clinical outcomes, including relapse-free and overall survival. Taken together, these findings suggest that POLG supports mitochondrial function and CSC maintenance, highlighting its potential as a therapeutic target and prognostic biomarker in breast cancer.