Researchers discover key aspects involved in DNA repair mechanism

Researchers from Tokyo Metropolitan University have identified key aspects within the mechanism behind DNA repair in our bodies. For the primary time, they showed that the “proofreading” portion of the DNA replicating enzyme polymerase epsilon ensured secure termination of replication at damaged portions of the DNA strand, ultimately saving DNA from severe damage. This recent knowledge arms scientists with ways to make anti-cancer drugs simpler, and recent diagnostic methods.

Our DNA is under attack. Daily, around 55,000 single-strand breaks (SSBs) appear within the strands making up DNA helices in individual cells. When polymerases, molecules that replicate DNA strands, attempt to make recent helices from strands with breaks in them, they’ll break the helix, creating what’s often known as a single-ended double-stranded break (seDSB). Thankfully, cells have their very own ways of coping with strand damage. One is homology directed repair (HDR), where double stranded breaks are fixed. One other is “fork reversal”, where the replication process is reversed, stopping the single-strand nicks turning into DSBs in the primary place.

The precise mechanism behind fork reversal stays unknown. Understanding how DNA damage is prevented is paramount not only to forestall cancers, but additionally make sure the effectiveness of cancer drugs which depend on DNA damage. Take camptothecin (CPT), an anti-cancer drug that introduces a number of single-strand breaks; since cancer cells are likely to replicate quicker, they create a number of seDSBs and die out, leaving normal cells less harmed.

Now, a world team led by Professor Kouji Hirata of Tokyo Metropolitan University have shed recent light on how fork reversal works. They focused on polymerase epsilon, an enzyme chargeable for making recent DNA from a portion of the DNA which has unzipped. They found that the exonuclease, the “proofreading” portion of the polymerase that ensures copy accuracy, played a key role, a brand new, rare insight into the largely unknown molecular mechanism behind fork reversal.

Firstly, they found that cells that are deficient within the exonuclease part showed strong susceptibility to exposure to CPT. Suppression of an element often known as PARP, the one other player known to affect fork reversal, also led to increased cell death. Nonetheless, when each were suppressed, there was no further increase in cell death beyond what was seen with PARP. This implies that PARP and the polymerase epsilon exonuclease work together to trigger fork reversal. Moreover, the team studied cells with the gene coding for BRCA1 (the breast cancer susceptibility protein) disrupted; additional deficiency of the exonuclease caused drastically increased sensitivity to CPT, excess of expected from either defect. Since BRCA1 deficiency is linked to a high risk of breast cancer, the exonuclease is perhaps targeted to make drug treatments simpler.

The importance of this work is manyfold. They’ve shown that drugs targeting the polymerase epsilon exonuclease can amplify the effect of anti-cancer drugs. Equally importantly, defects to the exonuclease have also already been seen in a wide selection of cancers, including intestinal cancer; this makes it likely that such cells have impaired fork reversal capability, a promising goal for future diagnostics in addition to treatments.

This work was supported by the Kanae Foundation, Shenzhen University, the Pearl River Talent Plan to introduce high-level talents [2021JC02Y089], the Takeda Science Foundation, a Tokyo Metropolitan Government Advanced Research Grant [R3-2], the Network-type Joint Usage/Research Center for Radiation Disaster Medical Science of Hiroshima University, Nagasaki University, Fukushima Medical University, the Yamada Science Foundation; JSPS KAKENHI Grant Numbers 16H06306, 16H12595, 20K06760, 22K15040, JP19KK0210, JP20H04337, JP21K19235, the National Natural Science Foundation of China [32250710138], the Uehara Memorial Foundation, Advanced Research Networks, the national key research and development program [2022YFA1302800], and the JSPS Core-to-Core Program. Funding to make the work open-access was provided through Tokyo Metropolitan Government Advanced Research Grant Number [R3-2].