Researchers are unlocking
the genetic mutations
that cause cancer

Scientists "Fix" Breast Cancer Gene Defect

April 06, 2010
by Brendon Nafziger, DOTmed News Associate Editor
Scientists have figured out a way to protect mice from the consequences of a gene mutation linked to breast and ovarian cancers in women, raising hopes medicine can cut cancer risk from the mutant genes in humans, too.

In the study appearing in the current issue of Cell, the researchers found that by deleting a protein, they could restore a critical DNA repair process, thereby dramatically reducing the risk for cancer in mice with the defective gene.

The gene, BRCA1, is one of two linked to deadly and hard-to-treat breast and ovarian cancers. According to the National Cancer Institute, six out of ten women who carry the mixed-up version of the gene will get breast cancer, making them five times more likely to come down with the disease than other women.

"In BRCA1 cells, the most important defect seems to be the inability to repair DNA breaks," Andre Nussenzweig, lead author of the study and a researcher at the Experimental Immunology Branch, National Cancer Institute, tells DOTmed News. "If you could overcome that defect, you'd have a cure for that risk."

Repairing errors

During DNA replication, the DNA strands sometimes snap. To fix the DNA, the body relies on a process known as homologous recombination. In this, the broken DNA borrows material from a neighboring (or sister) strand to fix the break. But with the BRCA1 mutation, this favored pathway can fail.

Instead, another, less efficient pathway takes over to fix the botched DNA replication. This one, called the non-homologous end-joining (NHEJ), works by gluing broken ends of the DNA back together.

"Normally, it's not toxic," says Nussenzweig. "And it's critical for the immune system," where it helps create immune cells to fight infection.

But it's not very good at fixing DNA replication errors. And its shoddy repairs over time lead to chromosome aberrations, which in turn lead to genomic instability -- fertile grounds for tumor growth.

In their study, Nussenzweig and his colleagues found that could shut off NHEJ and let the more efficient HR pathway resume its work. And they did this by canceling out a protein called 53BP1.

Knocking out the protein

What appears to be happening is that the two repair pathways compete over the same resource, a protein called ATM. In normal mice with functioning BRCA1 genes, the HR pathway is able to somehow deactivate 53BP1 on its own, letting it use ATM to do the repair work. But in mice with the mutant, broken BRCA1 gene, the NHEJ pathway, thanks to the 53BP1 protein, is able to somehow start working first, taking up all the ATM.

So by knocking out the 53BP1 protein, the scientists appeared to be able to restore the HR pathway, and prevent chromosomal aberrations from building up. In the mice that had the 53BP1 protein turned off, the researchers found a 10-fold decrease in chromosomal errors, according to Nussenzweig.

More important, survival rates shot up. Twelve out of 27 mice with the mutant gene developed mammary tumors -- the mouse version of breast cancer -- after about 18 months. But of the mice with the mutant gene who had the 53BP1 protein blocked, only one got mammary tumors after 22 months.

"If you can inhibit 53BP1 early enough, you can restore HR," says Nussenzweig. "When we combine 53BP1 and BRCA1 deficiency, the mice don't get breast cancer."

But there's a catch: even if it works on people, you'll have to introduce it at the right time -- too late, and it might actually do more harm than good.

PARP resistance

For the mutant cells relying on the NHEJ end-gluing pathway, the cells need a protein called poly(ADP-ribose) polymerase, or PARP, to allow the replication repairs to work. Turning off, or inhibiting, PARP, can then kill the cells. That's why now, PARP inhibitors are a promising treatment for breast and ovarian cancers due to BRCA1 mutations. The inhibitors are even the subject of an ongoing phase 2 clinical trial run by AstraZeneca.

Though the results so far are encouraging, Nussenzweig worries that, like all cancers, the BRCA-associated ones can develop resistance to the drug. While many cancer cells would fall victim to the PARP inhibitors at first, others would somehow turn back on the HR process thereby making them, in effect, immune to the drug, as the HR pathway doesn't need PARP to function.

"We propose that one way they get resistant is losing 53BP1," Nussenzweig says. "They would then turn back on the homologous recombination process, just like the mice in the study, allowing them to replicate without PARP. The problem is that they already have the cancer growing, which would then become immune to the PARP inhibitor."

Unanswered questions

Nussenzweig's laboratory is now planning on testing mice with PARP inhibitors, to see if their BRCA-associated cancers acquire resistance to the drugs in this fashion. But he's also eager to explore other unanswered questions about the BRCA1 mutation.

One curious feature of the mutation is that though the BRCA1 gene is found almost everywhere in the body, mutations to it seem to increase risk for cancers only in the breasts and ovaries.

In most cells, if they lose both copies of the BRCA1 gene (as can eventually happen if it's mutated), they don't develop cancer; they die, as cells depend on the HR process for their very survival.

"So there's something about the breast and ovary that they can tolerate the loss of both copies and not die, and it promotes growth and cancer. What it really means, there are secondary mutations that allow for -- probably -- for the growth of the cell," says Nussenzweig. A tumor-suppressant protein, known as p53, widely believed to play a role in preventing cancer in healthy cells, appears to be defective in BRCA tumors, too. But if that's the real culprit, no one knows. "People really don't understand it," admits Dr. Nussenzweig.

Another aspect they don't understand is whether this same HR pathway defect also causes the risk of cancer from BRCA2 gene mutations, the other main breast and ovarian cancer mutation gene.

And to answer that, Dr. Nussenzweig is experimenting with BRCA2 mice in his lab to see if knocking out 53BP1 helps them the way it does for their BRCA1 mutated cousins. But until the study's finished, the jury's out.