HDAC Inhibitors Target Replication Forks to Take a Stab at Cancer

SAHA reduces replication fork speed by influencing the architecture of the chromatin (DNA and its associated proteins) in the area near the fork.

SAHA reduces replication fork speed by influencing the architecture of the chromatin (DNA and its associated proteins) in the area near the fork.

The stability and function of many proteins within the cell can be altered with the addition or removal of certain chemical groups, including acetyl groups. Therefore, the enzymes that regulate these modifications have an important impact on the cell. One class of such enzymes—histone deacetylases, or HDACs—has been implicated in cancer and has become a target for cancer therapy. One HDAC inhibitor, called SAHA, has been approved for use against cutaneous T-cell lymphoma, and more than 60 ongoing clinical trials are continuing to test this class of drugs in various forms of cancer. However, the mechanisms by which SAHA and other HDAC inhibitors undermine the viability of tumor cells are not completely understood.

DNA replication is essential for the duplication of the genome during cell division. Abnormalities in this process are a major cause of genomic instability, which is a hallmark of cancer cells. Chiara Conti, Ph.D., and Elisabetta Leo, Ph.D., researchers in the CCR Laboratory of Molecular Pharmacology headed by Yves Pommier, M.D., Ph.D., , were part of a research team that carried out a series of experiments to determine whether SAHA influences DNA replication. The results were published in a recent issue of Cancer Research. Previous studies showed that SAHA induces programmed cell death, or apoptosis, in cancer cells within about one day, but Drs. Conti and Leo and their colleagues decided to investigate earlier cellular responses to SAHA. They found that SAHA induces DNA double-strand breaks in breast cancer cells within four hours. A closer look at the sites of the DNA damage revealed that most were at so-called replication forks, which are the numerous points along DNA at which replication begins. The link between DNA replication and SAHA-mediated DNA damage was strengthened by the observation that damage in response to SAHA was significantly reduced if DNA replication was blocked using chemical inhibitors. Together, these experiments revealed that SAHA causes replication-associated DNA damage.

The researchers then used fluorescent labels to visualize the molecular events that transpire at replication forks following exposure to SAHA. In order to determine how quickly the replication machinery was progressing, two different fluorescent labels were applied at fixed intervals. The distance between the two labels represented the progress in replication that took place between application of the first and second label and was thus an indicator of replication fork speed. These experiments and others showed that SAHA reduces replication fork speed by influencing the architecture of the chromatin (DNA and its associated proteins) in the area near the fork. In addition, it was discovered that SAHA treatment resulted in the activation of replication forks that are not normally activated. Similar results were observed upon depletion of HDAC3, one of several HDACs that may be targeted by SAHA, suggesting that HDAC3 may mediate the effects of SAHA on DNA replication and DNA damage.

These studies demonstrate that the HDAC inhibitor SAHA causes profound alterations in DNA replication that lead to DNA damage. These insights should inform future development and clinical use of HDAC inhibitors. In addition, DNA damage may provide an early indicator of whether HDAC inhibitors are hitting their targets and having an effect.

Summary Posted: 06/2010

Reference

Cancer Res. 2010 Jun 1;70(11):4470-80 PubMed Link