Elucidating the line of succession for a predominant DNA repair pathway to tackle cancer’s adapted DNA repair abilities.

July 31, 2024

Researchers at Stanford University have described a back-up DNA repair mechanism which could lead to new treatments for cancer.

The study, published in the journal Science Advances, also sheds light on the mechanism of action of V(D)J recombination and chromosome translocations when this alternative repair pathway takes over.

“This research reveals how DNA double-strand breaks can be repaired by a collaboration with the DNA damage response and components of a single-strand break repair pathway,” said Richard Frock, principal investigator on the study and an assistant professor of radiation oncology at the Stanford School of Medicine.

DNA double-strand breaks arise when cells are exposed to ionizing radiation, chemotherapies, stressed internal processes such as DNA replication, or though physiological processes that constitute our adaptive immunity. They are the most serious type of DNA damage because they can form a dynamic range of repair scars from small insertions and deletions to gross chromosomal rearrangements.

In 2021, Frock and collaborators revealed a robust alternative end joining (A-EJ) pathway in quiescent B cell progentor lines which harbor DNA repair scars with little-to-no microhomology and that mostly resembled the predominant repair pathway, nonhomologous end joining or NHEJ. This contrasted the dogma that back-up repair mechanisms predominantly use microhomologies to complete repair. 

“The repair scar overlap could implicate this alternative repair mechanism in cancer mutation signatures originally thought to be formed by NHEJ,” Frock says.

Later, in 2024, his group reported how DNA damage response kinase inhibitors would promote genome instability using either nonhomologous or alternative end joining, depending on the dose of the inhibitor used. However, they found the kinase responsible for directing NHEJ also promoted the microhomology-mediated end joining (MMEJ) pattern when the DNA ligase for NHEJ is deleted in cells. 

“Eliminating DNA protein kinase or its activity enabled the transition to using alternative end joining,” said Jinglong Wang, a postdoctoral scholar in the Frock lab and lead author of both 2024 studies. 

In the current study, Wang, Frock , and colleagues implicated PARP1 and XRCC1, factors involved in base excision repair, as integral A-EJ components, supported by the DNA damage response kinase, ATM. Curiously, while removing XRCC1 or PARP1 combined with ATM kinase inhibition decreased overall repair, the use of microhomologies predominated, suggesting MMEJ could back-up both NHEJ and A-EJ. 

“Collectively, our data indicate multiple repair paths, whether they are primary or secondary, seem to rely on DNA end microhomologies as a last ditch effort to complete repair, “ Frock says.

The findings have significant implications for some cancer treatments.

Radiotherapy is provided to more than half of all cancer patients and can be used in combination with other treatments.  Although many cancers are radiosensitive, implying a DNA repair defect, some are radioresistant. As radiation damage includes a mix of lesions that are repaired mostly by NHEJ and base excision repair, radiotherapies combining inhibitors that encompass the three DNA end joining pathways could synergize cancer treatments and expand their utility in genome editing.

Furthermore, offsetting the deleterious effects of ionizing radiation to normal tissues by using ultra-high dose rate (FLASH) radiotherapy, which Frock and collaborators reported in 2023 to have no additional impacts to genome instability than conventional radiotherapy, may provide additional tumor specificity but awaits preclinical evaluation for efficacy. 

 

Jinglong Wang (left) with Richard Frock (right)