Tag: DNA repair

Categories : CancerTags :

Scientists Upend the Current Understanding of How PARP Inhibitors Kill Cancer

On By ModernMedia
Breast cancer cells. Image by National Cancer Institute

Research by UMass Chan Medical School scientists poses a new explanation for how PARP inhibitor drugs attack and destroy BRCA1 and BRCA2 tumour cells. Published in Nature Cancer, this study illustrates how a small DNA nick – a break in one strand of the DNA – can expand into a large single-stranded DNA gap, killing BRCA mutant cancer cells, including drug-resistant breast cancer cells. These findings identify a novel vulnerability that may be a potential target for new therapeutics. 

Mutations in BRCA1 and BRCA2, tumour suppressor genes that play a crucial role in DNA repair, substantially increase the likelihood of cancer. These cancers are, however, quite sensitive to anticancer drugs such as poly (ADP-ribose) polymerase inhibitors (PARPi). When successful, these cancer treatments cause enough DNA damage to trigger cancer cell death. However, the array of different damages potentially induced by these drugs makes it difficult to pinpoint the exact cause of cell death. Additionally, PARPi resistance does occur, complicating treatment and leading to recurrent cancer.

“The conventional thinking has been that single-stranded DNA breaks from PARPi ultimately generated DNA double-strand breaks, and that was what was killing the BRCA mutant cancer cells,” said Sharon Cantor, PhD, professor of molecular, cell and cancer biology. “Yet, there wasn’t much in the literature that experimentally confirmed this belief. We decided to go back to the beginning and use genome engineering tools to see how these cells dealt with single-strand nicks to their DNA.” 

Using CRISPR technology, Cantor and Jenna M. Whalen, PhD, a postdoctoral researcher in the Cantor lab, introduced small, single strand breaks into several breast cancer cell lines, such as those with the BRCA1 and BRCA2 mutation, as well BRCA-proficient cells. They found that cells with BRCA1 or BRCA2 deficiency were uniquely sensitive to nicks. They also found that breast cancer cells that lose components of the complex that protects DNA from unnecessary DNA end cuts become resistant to chemotherapy drugs such as PARP inhibitors. However, restoring double strand DNA repair functions in breast cancer cells did not save the cells from dying, thus demonstrating that these repair functions are not critical for breast cancer cell survival. Instead, the cells become even more sensitive to single strand nicks, which then accumulate and form large gaps.  

“Our findings reveal that it is the resection of a nick into a single-stranded DNA gap that drives this cellular lethality,” said Whalen. “This highlights a distinct mechanism of cytotoxicity, where excessive resection, rather than failed DNA repair by homologous recombination, underpins the vulnerability of BRCA-deficient cells to nick-induced damage.” 

The findings suggest that PARPi may also work by generating nicks in BRCA1 and BRCA2 cancer cells, exploiting their inability to effectively process these lesions. For cancers that have developed PARPi-resistance, nick-inducing therapies provide a promising mechanism to bypass resistance and selectively target resection-dependent vulnerabilities.  

“Importantly, our findings suggest a path forward for treating PARPi-resistant cells that regained homologous recombination repair: to kill these cells, nicks could be induced such as through ionizing radiation,” said Cantor. “By targeting nicks in this way, therapies could effectively exploit the persistent vulnerabilities of these resistant cancer cells.”

Source: UMass Chan Medical School

Categories : Cancer GeneticsTags :

How Cancer Cells Repair their DNA so Quickly

On By ModernMedia
DNA repair
Source: Pixabay/CC0

Research into how the body’s DNA repair process works has made a discovery into how the process works, and by understanding how cancer cells repair their DNA so rapidly may lead to potent new chemotherapy treatments.

One of the great mysteries of medical science is the ability of DNA to be repaired after damage, but complicating the study of this is how different pathways are involved in the repair process over the cell’s life cycle. In one of the repair pathways known as base excision repair (BER), the damaged material is removed, and proteins and enzymes work together to create DNA to fill in and then seal the gaps.

In a study appearing in Proceedings of the National Academy of Sciences, Eminent Professor Zucai Suo led a team that discovered that BER has a built-in mechanism to increase its effectiveness: it just needs to be captured at a very precise point in the cell life cycle.

In BER, an enzyme called polymerase beta (PolyB) fulfils two functions: It creates DNA, and it initiates a reaction to clean up the leftover ‘chemical junk’. Through five years of study, Prof Suo’s team learned that by capturing PolyB when it is naturally cross-linked with DNA, the enzyme will produce new genetic material 17 times faster than when the two are not cross-linked. This suggests that the two functions of PolyB are interlocked, not independent, during BER.

The research improves the understanding of cellular genomic stability, drug efficacy and resistance associated with chemotherapy.

“Cancer cells replicate at high speed, and their DNA endures a lot of damage,” Prof Suo said. “When a doctor uses certain drugs to attack cancer cells’ DNA, the cancer cells must cope with additional DNA damage. If the cancer cells cannot rapidly fix DNA damage, they will die. Otherwise, the cancer cells survive, and drug resistance appears.”

This research examined naturally cross-linked PolyB and DNA, unlike previous research that mimicked the process. Studies had previously identified the enzymes involved in BER but did not fully grasp how they work together.

“When we have nicks in DNA, bad things can happen, like the double strand breaking in DNA,” said Thomas Spratt, a professor of biochemistry and molecular biology at Penn State University College of Medicine who was not a part of the research team. “What Zucai found provides us with something we didn’t understand before, and he used many different methods to reach his findings.”

Source: Florida State University