Cancer Treatment Genetic Effects in Healthy Cells May Reveal Clues to Side Effects and Resistance

The results of a study by scientists at Wellcome Sanger Institute, University of Cambridge, University College London, and collaborators indicate that cancer treatments, including chemotherapy and radiotherapy, give a growth advantage to cells with particular genetic changes in healthy tissue. The team used DNA sequencing to map mutations in normal esophagus tissue from esophageal cancer patients who had received either chemotherapy, chemotherapy and radiotherapy, or no treatment before surgery.

The results showed that different cancer treatments changed the landscape of mutations in normal tissue. In particular, combined treatment using chemotherapy and radiotherapy led to significantly more normal cells with cancer-related mutations in these patients. The team suggests that sequencing normal tissue from cancer patients receiving treatment could show how our genes regulate our tissue’s response to drugs, including side effects.

Research co-lead Phil Jones, FRS, a professor of cancer development at the University of Cambridge and a senior group leader at the Wellcome Sanger Institute, said: “Our bodies are a Darwinian battleground, where cells are constantly evolving, expanding and fighting for space in our normal tissues. If you change the rules of this competition by introducing a drug, different genetic mutations are going to enable cells to win or lose. We were surprised to find that only a few weeks of cancer treatment can drastically change decades of evolution in our cells. By looking at normal tissues, we can begin to uncover how drugs work in the body, in order to make more effective treatments with fewer side effects in the future.”

Jones is co-senior and co-corresponding author of the researchers’ published paper in Nature Genetics, titled “Cancer treatment alters mutant selection in normal esophagus,” in which they stated, “Sequencing normal epithelia reveals treatment-specific selection of mutations and may identify genes implicated in cellular responses to therapy.”

Over time, all cells in the body acquire genetic changes, known as somatic mutations. While the majority of these do not affect how the cell functions, some make cells fitter, so they outcompete their neighbors. Sometimes, combinations of these mutations cause uncontrollable growth leading to cancer and the formation of tumors.

By middle age, the human esophagus has evolved into a patchwork of mutated cells. “Aging epithelial tissues, including the esophagus, are colonized by somatic mutant clones under strong competitive selection,” the authors wrote. “Mutant clones with increased fitness expand, collide and compete for space in the tissue, with only the fittest mutations surviving.” However, as the authors also pointed out, “The effect of cancer treatment on mutant selection in normal epithelium is unknown.”

By age 60 to 70 years almost all of the cells in the esophagus will be mutated. While the majority of these mutations do not lead to cancer, if tumors do form they can be hard to treat, as often symptoms appear when the cancer has started to spread.

Around 9,500 people are diagnosed with esophageal cancer in the U.K. each year, with almost half of new cases in people aged 75 and over. It is treated with surgery, chemotherapy, radiotherapy, a combination of the two—chemoradiotherapy—and immunotherapy. “We hypothesized that anticancer treatment may alter the selection of mutant clones in the already densely mutated normal esophagus,” the team commented.

For their newly reported study the Sanger Institute researchers and their collaborators set out to understand the effects of cancer treatments on normal cells, and whether chemotherapy and radiotherapy treatments give some mutant cells an advantage. The team used DNA sequencing to analyze normal cells from the esophageal lining—esophageal epithelium—that had been removed from 70 patients after treatment for esophageal cancer. The patients had either received combination chemotherapy, chemoradiotherapy, or no treatment before surgery. “The presence of normal esophagus within the surgically excised tissue gave us the opportunity to test if the mutational landscape of the normal esophageal epithelia was altered by cancer treatment using duplex, whole-genome (WGS) and targeted DNA sequencing,” they further explained.

The investigators found significant differences in genetic mutations in the cells from the patients, depending on the treatment they had received. In patients who received chemoradiotherapy, there were significantly more clones, with mutations in TP53—a vital tumor suppressor gene known as the “guardian of the genome”—and in PPM1D, a gene that makes an enzyme that manages cell stress and TP53 function.

Among patients who had received combination chemotherapy there was an increase in normal cells carrying mutations associated with resistance to the chemotherapy drug 5-fluorouracil (5-FU). The increased resilience to 5-FU in a patient’s healthy cells during cancer treatment leads to protection from life-threatening toxicities.

“These findings demonstrate that the mutational landscape of a normal epithelium that has evolved over decades may be altered dramatically in just a few weeks under the selective pressure of anticancer treatment,” they noted. “This leads to the selective expansion of preexisting mutant clones in certain treatment groups.”

Chemotherapy drugs usually leave tell-tale patterns of mutations, known as mutational signatures, in the genomes of normal tissues. Despite seeing changes in mutant cell fitness following treatments, the team found no mutational signatures associated with the chemotherapies. “The absence of a chemotherapy mutational signature in both NanoSeq and whole-genome samples of normal epithelium is notable,” they stated.

Cancer treatments can cause severe side effects in normal tissues, which may result in reducing treatment dose. The researchers suggest that their findings begin to uncover the genes and protein domains that make normal cells sensitive or resistant to treatment. The results could help shape cancer treatment in the future to help reduce damage to normal tissues. The study findings might also help inform the development of targeted treatments that destroy cancer cells while leaving normal tissue unharmed.

By identifying the mutant cells in normal tissue that are selected for by cancer treatment, the study may also provide a catalogue of potential genetic targets that modify how our cells respond to treatment, and lead to further research into how tumors become drug resistant. “… mutants in normal tissues provide in vivo evidence of actionable targets for mitigating normal tissue toxicity and may inform strategies for overcoming drug resistance in tumors,” the scientists stated.

For the next steps, the team is conducting a pilot study to investigate these effects in other tissues, taking cheek swabs, blood and urine samples from patients before and after having treatment for skin, head and neck cancers. The researchers are investigating on a larger scale, whether there is further evidence of genetic mutations in normal cells that are being selected for by cancer treatment.

Commenting on the study Hayley Brown, research information manager at Cancer Research UK, said, “People with esophageal cancer often need intensive treatment, but we still have much to learn about how these therapies affect the rest of the body. Cancer treatments can be incredibly effective, but they can also affect healthy tissues. This study gives us an unusual opportunity to see how healthy cells change during treatment, helping us understand what happens elsewhere in the body, not just in the tumor. The more we learn about these changes, the better chance researchers have of finding ways to reduce the impact of treatment on patients without making it less effective against cancer.”