MMEJ Repair Maintains Extrachromosomal DNA Stability in Cancer

Microhomology-mediated end joining (MMEJ) plays a critical role in maintaining the stability of extrachromosomal DNA (ecDNA) within cancer cells, according to research published online on September 23, 2026, in the journal Nature. The study, identified by its DOI 10.1038/s41586-026-11048-8, elucidates a fundamental mechanism that underpins the survival and proliferation of cancers driven by ecDNA. Extrachromosomal DNA elements are circular pieces of DNA that exist outside the main chromosomes and are frequently found in cancer cells, where they often carry amplified oncogenes that promote tumor growth and resistance to therapy. The stability of these ecDNA molecules is paramount for their oncogenic function, and this research pinpoints a key pathway responsible for their integrity.

The research highlights that the stability of ecDNA is dependent on MMEJ occurring at specific fragile sites characterized by TA-rich sequences. These TA-rich regions are particularly prone to DNA breaks. The MMEJ pathway is a form of DNA repair that utilizes short, identical sequences of DNA (microhomologies) to bridge broken DNA ends. This process is distinct from other major repair pathways like non-homologous end joining (NHEJ) and homologous recombination (HR), and it is particularly efficient at repairing double-strand breaks that occur at repetitive DNA sequences, such as the TA repeats identified in this study. The study's findings suggest that the precise repair of breaks at these TA repeats by MMEJ is essential for preventing the degradation or aberrant rearrangement of ecDNA, thereby preserving the genetic information that drives cancer.

Furthermore, the study identifies the protein FANCM as a key regulator in this process. FANCM is shown to suppress the formation of DNA breaks at these fragile TA-rich sites. By preventing breaks, FANCM contributes indirectly to the stability of ecDNA. The research proposes that if the MMEJ pathway is disrupted, for instance, by targeting key components like Polθ (also known as POLQ), it could lead to the destabilization of ecDNA. This destabilization, in turn, could render ecDNA-driven tumors more vulnerable to therapeutic interventions. The implication is that therapies designed to inhibit MMEJ or Polθ could be effective in treating cancers that rely on ecDNA for their survival and progression, potentially offering a new avenue for cancer treatment by exploiting the inherent fragility of these extrachromosomal elements.

The significance of this discovery lies in its potential to inform the development of novel cancer therapies. By understanding the intricate mechanisms that cancer cells use to maintain their genetic advantage through ecDNA, researchers can devise strategies to dismantle this advantage. The identification of MMEJ and FANCM as critical players in ecDNA stability opens up possibilities for targeted drug development. Inhibiting MMEJ could lead to the accumulation of unrepaired DNA breaks within ecDNA, causing its loss or dysfunction, and ultimately leading to cancer cell death or reduced proliferation. This research provides a foundational understanding of ecDNA maintenance, paving the way for future investigations into the therapeutic targeting of MMEJ and related pathways in various oncogenic contexts.