Most Common Childhood Cancer Is Likely Caused by Lack of Exposure to Infections, Major Study Finds

Babies who aren't exposed to common microbes during their first year of life could be more likely to develop the most common form of childhood cancer, according to a landmark paper.

Professor Mel Greaves of the Institute of Cancer Research in London, U.K., studied over three decades worth of research to conclude that acute lymphoblastic leukemia (ALL) is most likely caused by infections. Leukemia is overall a rare disease, but accounts for one third of all cases of childhood cancer in developed societies. Currently, around 90 percent of children recover from the disease thanks to chemotherapy.

In what Professor Greaves has dubbed the "delayed infection" theory, he suggested ALL can be triggered in two steps. First, a child is born with a genetic mutation which could cause leukemia in less than 1 percent of those carrying it. Then, exposure to one or more common viruses and bacteria prompts ALL to develop in the body, according to Professor Greaves. If a baby does not come into contact with common infections in the first year of their life, their immune system may not be primed to fight ALL, he argued. Other forms of childhood cancer, however, are caused in different ways.

Children who have the genetic mutation and spend their first year in clean, modern homes away from other infants and older children are most at risk. That could explain why ALL is most common in wealthier societies, and is growing at a rate of 1 percent a year. It is therefore likely a preventable cancer, according to Professor Greaves.

Everyday activities such as breastfeeding and sending children to daycare centers where they can come into close contact with others could be enough to prime a child's immune system, he suggested.

Making sure the body is confronted with and fights off infections could also stave off the development of autoimmune disorders such as Type 1 diabetes, as well as allergies.

The study, published in the journal Nature Reviews Cancer, also rejected the idea that ALL is caused by environmental factors such as electromagnetic waves, ionizing radiation, or manmade chemicals. There is insufficient evidence to support such claims which Professor Greaves described as "damaging but unsubstantiated."

Professor Greaves analyzed research he and colleagues from around the world compiled over the past 30 years. The papers drilled down into the various causes of cancer, from genetics to epidemiology.

In one study, mice who were engineered to have the gene associated with leukemia developed ALL when they were transferred from an ultra-clean environment to one featuring common microbes.

Now, Professor Greaves is continuing his research to try to understand whether being exposed to benign microbes could prevent leukemia in mice. He hopes his findings could later be related to human children.

He told Newsweek his study is significant because it makes strides towards settling a debate around ALL which has been ongoing for around a century. He said that many incorrect explanations during this time—such as parental smoking and electromagnetic waves—have been both "plausible" and "wacky."

"There is confusion for parents of children with leukemia about what the cause might be. This is probably the first credible explanation for the cause based on very robust evidence," he said.

"Our conclusion is that this is probably a preventable cancer." Professor Greaves also rejected the suggestion that clean home environments, where disinfectants and antibacterial wipes are used, could cause ALL.

"It's not a dirt or hygiene problem, it's to do with the social contact of infants. That's something one can encourage parents to do."

Professor Greaves also urged mothers who can't breastfeed not to be concerned, as ensuring a child mixes with other infants will likely provide the same levels of immune system priming. In 10 years, he hopes his research will have be used to create an oral, preventative treatment which in theory could cut ALL cases by 75 percent.

Dr Alasdair Rankin, director of research at the blood cancer charity Bloodwise who was not involved in the study, urged parents not to be alarmed by the findings.

"Childhood leukemia is very rare and only around one in 2,000 children will develop it," he said. "While developing a strong immune system early in life may slightly further reduce risk, there is nothing that can be currently done to definitively prevent childhood leukemia. As noted by this study, other factors influence its development—including pure chance."

This piece has been updated with comment from Professor Greaves.