Youth may increase vulnerability to a carcinogen found in contaminated water and some drugs

A new study from MIT suggests that a carcinogen that has been found in medications and in drinking water contaminated by chemical plants may have a much more severe impact on children than adults.

In a study of mice, the researchers found that juveniles exposed to drinking water containing this compound, known as NDMA, showed dramatically higher rates of DNA damage and cancer than adults.

The findings may help to explain an epidemiological association between childhood cancer and prenatal exposure to NDMA in people living near a contaminated site in Wilmington, Massachusetts, the researchers say. The study also suggests that it is critical to evaluate the impact of potential carcinogens across all ages.

"We really hope that groups that do safety testing will change their paradigm and start looking at young animals, so that we can catch potential carcinogens before people are exposed," says Bevin Engelward, an MIT professor of biological engineering. "As a solution to cancer, cancer prevention is clearly much better than cancer treatment, so we hope we can spot dangerous chemicals before people are exposed, and therefore prevent extensive cancer risk."

MIT postdoc Lindsay Volk is the lead author of the paper. Engelward is the senior author of the study, which appears in Nature Communications.

From DNA damage to cancer

NDMA (N-Nitrosodimethylamine) can be generated as a byproduct of many industrial chemical processes, and it is also found in cigarette smoke and processed meats. In recent years, NDMA has been detected in some formulations of the drugs valsartan, ranitidine, and metformin. It was also found in drinking water in Wilmington, Massachusetts, in the 1990s, as a result of contamination from the Olin Chemical site.

In 2021, a study from the Massachusetts Department of Health suggested a link between that water contamination and an elevated incidence of childhood cancer in Wilmington. Between 1990 and 2000, 22 Wilmington children were diagnosed with cancer. The contaminated wells were closed in 2003.

Also in 2021, Engelward and others at MIT published a study on the mechanism of how NDMA can lead to cancer. In the new Nature Communications paper, Engelward and her colleagues set out to see if they could determine why the compound appears to affect children more than adults.

Most studies that evaluate potential carcinogens are performed in mice that are at least 4 to 6 weeks old, and often older. For this study, the researchers studied two groups of mice - one 3 weeks old (juvenile), and one 6 months old (adult). Each group was given drinking water with low levels of NDMA, about five parts per million, for two weeks.

Inside the body, NDMA is metabolized by a liver enzyme called CYP2E1. This produces toxic metabolites that can damage DNA by adding a small chemical group known as a methyl group to DNA bases, creating lesions known as adducts.

When the researchers examined the livers of the mice, they found that juveniles and adults showed similar levels of DNA adducts. However, there were dramatic differences in what happened after that initial damage. In juvenile mice, DNA adducts led to significant accumulation of double-stranded DNA breaks, which occur when cells try to repair adducts. These breaks produce mutations that eventually lead to the development of liver cancer.

In the adult mice, the researchers saw essentially no double-stranded breaks and significantly fewer mutations compared to juveniles. Furthermore, the livers did not develop severe pathology, including tumors, even though they experienced the same initial level of DNA adducts.

"The initial structural changes to the DNA had very different consequences depending on age," Engelward says. "The double-stranded breaks were exclusively observed in the young."

Further experiments revealed that these differences stem from differences in the rates of cell proliferation. Cells in the juvenile liver divide rapidly, giving them more opportunity to turn DNA adducts into mutations, while cells of the adult liver rarely divide.

"This really emphasizes the overall problem that we're trying to highlight in the paper," Volk says. "With toxicological studies, oftentimes the standard is to use fully grown mice. At that point, they're already slowing down cell division, so if we are testing the harmful effects of NDMA in adult mice, then we're completely missing how vulnerable particular groups are, such as younger animals."

While most of these effects were seen in the liver, because that is where NDMA is metabolized, a few of the mice developed other types of cancer, including lung cancer and lymphoma.

Adult risk is not zero

For most of these studies, the researchers used mice that had two of their DNA repair systems knocked out. This speeds up the mutation process, allowing the researchers to see the effects of NDMA exposure more easily, without needing to study a large population of mice.

However, a small study in mice with normal DNA repair showed that juveniles experienced NDMA-induced double-strand breaks, regenerative proliferation, and large-scale mutations that were completely absent in adults. This occurs because the fast-growing juveniles possess highly active DNA replication machinery that encounters the DNA adducts before the cell has time to repair them.

The researchers also found that if they treated adult mice with thyroid hormone, which stimulates proliferation of liver cells, those cells began accumulating mutations as quickly as the juvenile liver cells. Previous work done in the Engelward laboratory has shown that inflammation can also stimulate cell proliferation-driven vulnerability to DNA damage, so the findings of this study suggest that anything that causes liver inflammation could make the adult liver more vulnerable to damage caused by agents such as NDMA.

"We certainly don't want to say that adults are completely resistant to NDMA," Volk says. "Everything impacts your susceptibility to a carcinogen, whether that's your genetics, your age, your diet, and so forth. In adults, if they have a viral infection, or a high fat diet, or chronic binge alcohol drinking, this can impact proliferation within the liver and potentially make them susceptible to NDMA."

The researchers are now investigating how a high-fat diet might influence cancer development in mice that also have exposure to NDMA.

This collaborative effort across several MIT labs was funded by the National Institutes of Environmental and Health Sciences (NIEHS) Superfund Research Program, a NIEHS Core Center Grant, a National Institutes of Health Training Grant, and the Anonymous Fund for Climate Action.