For more than two decades, scientists have known that mutations in a gene called adenomatous polyposis coli (Apc) are responsible for close to 90% of all cases of colon cancer—and more than 600,000 deaths each year. Tumor growth occurs when Apc is turned off, leaving cells vulnerable to a cascade of unchecked signals leading to rapid cell proliferation. Although researchers have long recognized that process as the initiating event in tumor development, whether Apc is needed for tumor maintenance has been less clear.
A study published in June (Cell 2015;161:1539–52; doi:.10.1016//j.cell.2015.05.033) has highlighted the importance of Apc in both tumor growth and maintenance. Lukas E. Dow, Ph.D., is an assistant professor of biochemistry in the Department of Medicine at Weill Cornell Medical College in New York. Scott Lowe, Ph.D., is a senior scientist and chair of the Cancer Biology and Genetics Program at Memorial Sloan–Kettering Cancer Center in New York. They and their colleagues reactivated Apc in genetically engineered mice. That accomplishment represents the first time that scientists have restored the gene to normal function.
“Until now, we’ve been really good at removing genes and proteins, but not so good at putting them back into the cells,” said Owen Sansom, Ph.D., professor at the Beatson Institute for Cancer Research at the University of Glasgow in Scotland and a longtime researcher in this field. “These researchers found a way to overcome this technical challenge.”
The results are intriguing: With Apc restored, cells began differentiating and functioning normally within days and showed no signs of reverting to cancerous cells after a full year of monitoring.
“This was by far the most surprising result,” Lowe said. “We wouldn’t have imagined that cells with multiple mutations could retain the ability to function normally. Equally interesting, however, is that by turning Apc back on, we were able to eradicate malignant tumors with multiple cancer-predisposing lesions.”
Now researchers face another difficult challenge: how to put these findings to work in the clinic.
Homing In on Apc and the Wnt Pathway
Apc acts as a tumor suppressor, protecting cells from unchecked growth. Along with two proteins, AXIN1 and GSK3β, Apc also controls a second molecule, called β-catenin. With Apc inactivated, β-catenin floods the cells, driving cell division, often in combination with two other mutations that often occur in colon cancer—Kras and p53. Those signals are part of the Wnt pathway, and they set off a ripple effect of rapid cell proliferation and tumor growth.
Although restoring a gene to a person is “probably impossible,” Lowe noted, the findings suggest different ways to achieve similar results. Dow concurs, pointing out that “while we haven’t formally shown that suppression of the Wnt pathway is responsible for tumor regression once Apc is restored, it seems like the most likely candidate.”
Therefore, in his follow-up research, Dow said that he plans to focus on this pathway by using the mouse models developed for this study, as well as a three-dimensional culture system. These tools will enable his team to manipulate other genes along the pathway. One of the prime targets are tankyrase inhibitors, which have shown promise by acting indirectly on the pathway. “These inhibitors stabilize AXIN1, which is bound to Apc,” Dow said. “They work by reducing outputs from the Wnt pathway, mimicking Apc restoration.”
The biggest concern with tankyrase inhibitors is toxic effects. In experiments with healthy mice conducted at the University of Oslo in conjunction with Roche’s Genentech, researchers found that regeneration of healthy intestinal cells was affected by treatment with tankyrase inhibitors (SciBX April 18, 2013; doi:10.1038/scibx.2013.353). Dow noted that many laboratories in academia and the commercial sector are working on problems related to adverse effects of these inhibitors on normal cells.
Although tankyrase inhibitors are not yet in clinical trials, other drugs targeting the Wnt pathway are being investigated. One such drug is PRI-724, whose efficacy is being evaluated in combination with chemotherapy (modified fluorouracil, leucovorin calcium, and oxaliplatin 6) and a monoclonal antibody (bevacizumab). That trial is in its early stages, led by the University of Southern California in Los Angeles in collaboration with the National Cancer Institute and Prism Pharma. Designed for patients newly diagnosed with metastatic colorectal cancer, the trial compares overall survival in patients receiving all three drugs with those receiving just chemotherapy and bevacizumab.
Other Research Agendas
Building on this research, Lowe is considering moving in other directions. “The powerful technology we developed enables us to turn genes on and off, something we weren’t able to do before,” he said. “We can apply this method to any gene, including other genes that contribute to the growth of colon cancer, to learn more about the impact of different combinations of genetic alterations.”
Owen Sansom, Ph.D.
Lowe also pointed out that this work didn’t address what happens to cells when they metastasize to other sites, notably the liver. He is interested in studying how restoring Apc function affects these tumors and whether they could develop resistance to the pathway. Cancer cells that reverted to normal could also reacquire the ability to spur tumor growth. That is another issue that warrants a second look.
“Our lab is uniquely positioned to study these kinds of questions,” Lowe said. “We draw on the expertise of clinicians like oncologists and pathologists, as well as bench scientists. Their input helps stimulate new ideas.”
In particular, clinicians can help researchers determine the most important clinical questions to ask. “The big unanswered question surrounding the Apc gene is where to target therapy,” Lowe said. “Further research will be needed to determine if activation of the Wnt pathway produces the results we’re looking for or whether another solution leads to more effective long-term outcomes.”
