The researchers knew from their previous studies that the protein MAD2L2 plays a vital role in the repair of DNA double strand breaks, one of the most toxic lesions in a cell. A few years ago, Jacobs’ team discovered, in collaboration with several other labs, that MAD2L2 works with three allies. They named these four proteins together the shieldin complex because it shields the ends of the DNA from the attack of enzymes that can degrade DNA.
They have now found that MAD2L2 is recruited to reversed replication forks. These are unique DNA structures that form when the forward movement of the DNA replication machinery is interrupted due to a problem. The reversed movement of the replication fork puts it into a favorable position for repair. Importantly, such reversed forks contain exposed DNA ends that very much resemble a DNA double strand break, the preferred substrate of MAD2L2.
In a series of genetic and single-molecule analyses, the researchers discovered that MAD2L2 is indeed required for protecting reversed replication forks against degradation. But surprisingly, it does so independently of the shieldin complex. Instead, MAD2L2 was found to work with two ‘old’ allies: the DNA polymerases REV3L and REV1.
Thus, MAD2L2 protects DNA from degradation both at DNA breaks and at reversed replication forks by differentially engaging shieldin and REV3L/REV1, respectively.
The team believes that the new results open the possibility of targeting MAD2L2 to enhance the therapeutic effects of drugs that induce replication stress. Moreover, their results could be especially relevant in the clinical setting, because the expression levels or mutations of MAD2L2 in tumors could be an indicator of how patients will respond to chemotherapy.
This work was supported by KWF Dutch Cancer Society.