Glioma is a rare cancer, occurring in approximately 6 in every 100,000 individuals each year. (For reference, about 442 in 100,000 people are diagnosed with any type of cancer each year in the U.S.) But it's a menacing one. The 5-year survival rate is estimated to be just 5 percent. And researchers currently know very little about how to prevent the deadly tumor. All of these facts make new research linking glioma incidence with previous infection with the common foodborne parasite Toxoplasma gondii (T gondii) noteworthy, despite the small sample size in the study.
For the new study, funded by the American Cancer Society (ACS) and the H. Lee Moffitt Cancer Center and Research Institute, researchers found that people who had T gondii antibodies (evidence of a previous infection) were more likely to develop glioma compared with matched controls. This association was consistent in two cohorts of participants from two different cohort studies. The study was published in the International Journal of Cancer in January online ahead of print (2021; https://doi.org/10.1002/ijc.33443).
"The etiology of glioma remains largely unknown and there are no currently understood modifiable risk factors," said the study's lead author, James Hodge, JD, MPH, an epidemiologist at the ACS. "While this research is preliminary, if our results can be replicated in a larger population, it would provide a potentially modifiable risk factor where people could be screened and treated for T gondii infections."
Hodge explained that earlier work investigating whether infections, particularly those that cross the blood-brain barrier, play a role in the development of brain cancers had some interesting early results that led him and his colleagues to look more closely at the link between T gondii and glioma. The earlier work had tested for antibodies in blood samples of patients who had already been diagnosed with cancer.
For the new study, Hodge's group was able to investigate the association between T gondii and glioma in a prospective study where blood samples were taken prior to cancer incidence.
T Gondii-Glioma Link Found
The researchers looked at data from two separate cohort studies: the ACS Cancer Prevention Study II Nutrition Cohort and the Norwegian Cancer Registry's Janus Serum Bank. From the ACS cohort, they included 37 patients with glioma and 74 matched controls. From the Janus registry, they analyzed data for 323 patients with glioma and 323 controls. Notably, the average age for the people in the ACS cohort was 70, while the average age for the people in the Janus cohort was 40.
For the analysis, the researchers measured two different types of T gondii antibodies from blood samples drawn prior to glioma diagnosis. For both cohorts, the people with T gondii antibodies were more likely to have a subsequent glioma diagnosis. The association was strongest for those with glioblastoma and those who were diagnosed within the first 3 or 5 to 13 years after the blood draw.
Hodge noted that the sample size for each cohort was relatively small. And there was little racial or ethnic diversity within the cohorts. And further data are needed before these findings will have implications for clinical practice. But, he added that the group hopes to investigate this association in larger studies with more diversity.
Byram Ozer, PhD, MD, Assistant Professor of Oncology and Neurology at Johns Hopkins Kimmel Cancer Center, cautions that this study reports very preliminary data.
"With frustratingly few ideas about modifiable risk factors in glioma and glioblastoma development, it's always welcomed to see research thinking outside the box about tumor etiology so that we can hopefully develop better strategies of glioma treatment and prevention," noted Ozer, who specializes in the treatment of neurological cancers. "However, the key takeaway is that there is weak correlation between one out of two markers of previous Toxoplasmosis [T gondii infection] exposure with a slightly higher risk of developing glioma and glioblastoma."
It's worth noting that T gondii infection- mostly commonly passed via food, including undercooked or contaminated meat or shellfish-is very common. In the U.S., it's estimated that 11 percent of people over age 6 have had the infection; in other parts of the world, infection rates are estimated to be as high as 60 percent.
In addition to studies in larger populations, more mechanistic data is needed to understand whether this infection can indeed affect glioma risk, Ozer said. "The authors cite the ability of T gondii to cross the blood-brain barrier and cause neurological inflammation as a possible mechanism of glioma development, but those citations are related to non-oncological disease and are difficult to extrapolate to oncological disease."
Follow-up research showing evidence of T gondii infection in resected brain tumor tissue could also provide more compelling evidence to support this hypothesis, Ozer added.
Sarah DiGiulio is a contributing writer.