Polymerase theta inhibition impairs tumor growth and amplifies melphalan-induced DNA damage in multiple myeloma

The dysregulation of the DNA damage response (DDR) plays a vital role in cancer development and inadequate response to anti-cancer therapy. We investigated the involvement of DNA polymerase theta (Polθ), the principal component of the microhomology-mediated end joining (MMEJ) pathway, in the pathogenesis of multiple myeloma (MM).

We explored the expression levels of Polθ and assessed its prognostic relevance in a cohort of 124 MM patients at our institution. We demonstrated the impacts of Polθ inhibition on genomic stability and cell viability in MM using shRNA and a pharmacological inhibitor. To elucidate the role of Polθ-mediated MMEJ in repairing melphalan-induced DNA damage, we employed Western blot and EJ2-GFP reporter assay to evaluate the activation of the MMEJ pathway following melphalan exposure. We analyzed the synergistic cytotoxicity between the Polθ inhibitor and melphalan in MM, both in vitro and in vivo. The impacts of combination therapy on genomic damage and mitotic catastrophe were examined by comet assay, Western blot, and immunofluorescence.

We revealed that Polθ was markedly upregulated in MM. The enhanced expression of Polθ was found to be significantly associated with advanced disease stages and adverse clinical outcomes. Polθ inhibition promoted genomic instability and impaired MM growth, underscoring the polymerase's function in sustaining myeloma survival under ongoing genomic stress. Furthermore, we observed a further increase in Polθ expression and MMEJ repair efficiency following melphalan treatment. Polθ inhibitors sensitized MM cells to melphalan by increasing unrepaired DNA damage and inducing mitotic catastrophe, both in vitro and in vivo.

Collectively, our results confirm Polθ as a promising prognostic biomarker and highlight its potential to serve as a synthetic lethal target that can be used in combination with melphalan for the treatment of MM.