New evidence for this relationship is found in the normal variation of two genes, the scientists say.
"Variations in certain genes may make a person more prone to develop asthma or allergies and those same variations may protect adults against the most common kind of brain cancer," said Judith Schwartzbaum, the study's lead author and an associate professor of public health at Ohio State University.
Glioblastoma multiforme (GBM) affects three out of 100,000 people, a rate that quadruples to 13 out 100,000 among people who are 65 and older. The average five-year survival rate from the time of diagnosis for GBM is only 3.3 percent, and is lower for people 65 and older.
The current study supports several years' worth of research by other scientists who have suggested an inverse relationship between asthma, allergies and GBM. But those studies were based only on information that participants gave about their history of asthma and allergies, not on information from DNA testing.
The study is the first to include a genetic component in addition to participant self-reports of asthma and allergy. The findings appear in the current issue of the journal Cancer Research.
The kind of genetic variant Schwartzbaum is talking about is called a polymorphism. While a mutation consists of a rare and abnormal DNA pattern, a polymorphism consists of common patterns, each considered normal.
"We needed an objective way to measure the accuracy of allergy self reports, one that isn't affected by the presence of a brain tumor" Schwartzbaum said. "Looking at genetic variation is one way to do this."
Polymorphisms can offer protection against certain diseases or render a person more vulnerable to particular conditions. For example, researchers suspect that several polymorphic forms of key genes may increase susceptibility to Alzheimer's disease.
"People who have polymorphisms in the two genes that we examined may be susceptible to allergic conditions and may also have a lower risk of GBM," Schwartzbaum said.
She and her colleagues analyzed DNA samples from 533 people, 111 of whom had been diagnosed with GBM. The other 422 randomly selected participants served as controls. All of the subjects were asked if they had ever been diagnosed with asthma, hay fever or eczema and, if so, how long each of these conditions had lasted.
The researchers looked for polymorphisms on two genes associated with asthma and allergies, IL-4RA and IL-13. In this study, individuals with one or two specific polymorphisms on the IL-4RA gene that increase asthma susceptibility seemed to have a lower GBM risk. The same was true for two polymorphisms on the IL-13 gene.
"Our results suggest that self-reports of asthma and allergy are a pretty accurate way to determine someone's susceptibility to this particular type of cancer," Schwartzbaum said. "It's also important to realize that someone could have these asthma-susceptibility polymorphisms and never experience asthma or allergies."
Schwartzbaum's next goal is to figure out the relationship between these allergy-inducing polymorphisms and GBM.
IL-4RA and IL-13 genes code for chemical messengers called cytokines, which control how immune system cells communicate and behave. Ironically, these cytokines may calm the immune system in the brain by helping to inhibit inflammation, even though they also eventually lead to increased inflammation in the lungs, which is a primary symptom of asthma.
It's possible, Schwartzbaum said, that the anti-inflammatory role of these cytokines may hinder tumor growth.
"I'm not sure if these cytokines play independent roles in both allergies and the development of brain tumors, if allergies and GBM share a common pathway in the immune system, or if it is allergies themselves that reduce GBM risk," she said.
At any rate, having asthma or other allergic conditions may be somewhat beneficial.
osu/