More than 14.1 million common cancer cases are estimated around the world in 2012, and this number is anticipated to increase to 24.0 million by 2035 [1]. About 256,000 central nervous system tumors (1.8% of all tumors) are diagnosed each year worldwide [1]. Previous studies have found that risk factors for brain tumors include diverse chemical products, family history, and ionizing radiation from therapeutic and diagnostic devices in the head [2]. However, the exact causes of these malignancies are yet unclear. Approximately 20% of various malignancies worldwide are due to infectious agents, including viruses, bacteria, and parasites [3,4]. Infectious agents can interfere with the host cell genetic machinery, such as DNA repair and cell cycle, and can lead to chronic inflammation and immune system impairments [5].
The infectious agents associated with human cancers are most commonly viral pathogens, including human papilloma virus and hepatitis B and C viruses [6]. However, there are only a few studies on the association between parasites and human cancers. It has been suggested that several parasite species, namely, Paragonimus westermani, Plasmodium sp., Opisthorchis viverrini, Clonorchis sinensis, Schistosoma haematobium, and Hymenolepis nana are related to development of various types of human cancers [7,8]. Many other species of parasites may also have potential roles in development of human cancers.
Toxoplasma gondii is an intracellular protozoan that can modulate the microenvironment of the infected host [9]. T. gondii can invade vital organs, including the central nervous system (CNS); however, its infection in humans is usually mild and asymptomatic in immunocompetent individuals [9]. In immunocompromised patients, T. gondii infection may cause severe diseases in the brain, including fatal meningitis and encephalitis [9]. In addition, a possible correlation was suggested between T. gondii infection and brain tumor development [10]. This suggestion has been supported by other authors [11-18]. The mechanisms of the brain tumor induction by T. gondii need to be further studied. In this study, in order to estimate the possible relationship between T. gondii infection and brain tumor development, we investigated the seroprevalence of T. gondii among brain tumor patients diagnosed in Korea.
We used sera of 93 patients (44 men and 49 women) diagnosed with various types of brain tumors supplied by the Biobank of Chonnam National University Hwasun Hospital (Hwasun, Jeollanam-do Province, Korea), a member of the Korea Biobank Network. As the control group, sera of 93 randomly selected healthy volunteers (45 men and 48 women) who visited the Korea Association of Health Promotion for health check-up were included for the assay. The ages of the brain tumor patients were 18-82 years (52.9±14.8 years) and those of the healthy controls were 10-86 years (51.9±12.2 years). The sera and blood were stored at -80˚C until analyzed. This study protocol was approved by the Institutional Review Board of Seoul National University Hospital, Seoul, Korea (IRB no. E-1507-065-687). The purpose and procedures of this study were explained to all participants, and a written informed consent was obtained from each of them.
The process of preparation for Toxoplasma lysate antigens (TLA) [19] and the procedure of ELISA [14] followed previous studies with slight modifications. Briefly, 96-well microtiter plates (Costar, Cambridge, Massachusetts, USA) coated with TLA were incubated at 4˚C overnight. After washing, each well was reacted with the test serum samples (1:100) at 37˚C for 1 hr, and horseradish peroxidase-conjugated goat anti-human IgG (1:10,000; Bethyl Laboratories, Montgomery, Texas, USA) was applied at 37˚C for 1 hr. After several washes, freshly prepared o-phenylenediamine dihydrochloride (Sigma-Aldrich, St. Louis, Illinois, USA) was added, and the reaction was stopped by adding 8 N H2SO4. IgG antibody titers were determined at the optical density of 490 nm. To analyze the risk factors for toxoplasmosis, the chi-square test was applied. P<0.05 was considered statistically significant. Pearson’s chi-square and Fisher’s exact tests were used to investigate associations among qualitative categorical variables using SPSS (SPSS Inc., Chicago, Illinois, USA). All tests were 2-sided, and the level of significant difference was defined as P<0.05.
The IgG seropositive rate for T. gondii, as analyzed by ELISA, was 18.3% (17/93) among patients with variable types of brain tumors (Table 1). The seropositivity among brain tumor patients was significantly (P<0.05) higher than the one among healthy persons, 8.6% (6/93 sera). In particular, patients with meningioma (41.7%; P<0.05), metastatic carcinoma (30.0%; P<0.05), astrocytoma (21.4%; P>0.05), and glioblastoma (16.1%; P>0.05) showed remarkably higher seropositivity for T. gondii than healthy controls (Table 1). The seropositivity was significantly (P<0.05) higher in men than in women both in the brain tumor patients and healthy controls (Fig. 1A). The age-seropositivity curve in healthy controls generally showed a steady increasing pattern according to increase of the age (Fig. 1B), whereas in brain tumor patients the younger age group, for example, 30-49 years, revealed a significantly (P<0.05) higher seropositivity compared with other age groups (Fig. 1B).
Previous studies have shown that meningioma and astrocytoma were positively linked to high serum T. gondii IgG antibody levels [10,13]. Similarly, our study showed higher prevalence of Toxoplasma specific IgG in patients with meningioma (41.7%; P<0.05) and astrocytoma (21.4%; P>0.05) (Table 1). Furthermore, in healthy controls, the seropositivity of T. gondii was increased with age [19,20]. However, in our study, the seropositivity in brain tumor patients showed a different tendency; a significantly higher rate was observed in younger aged patients (30-49 years) with brain tumors, compared with age-matched healthy controls. These results support strongly the close relationship between Toxoplasma infection and brain tumor incidence.
Since the 1960s, quite a number of studies [10-18] reported correlations between T. gondii infection and brain tumor incidence (Table 2). Various types of brain and multiple organ tumors, including glioma, acoustic neuroma, meningioma, Hodgkin’s lymphoma, multiple myeloma, and leukemia, have been reported to be positively associated with chronic T. gondii infection (Table 2). In this study, we also observed a high prevalence of T. gondii infection in sera of various brain tumor patients in Korea. Especially, our study supports the previous finding that IgG seropositivity to T. gondii is a risk factor for meningioma [13]. These findings provide a strong need for a further study to establish the precise correlation between meningioma incidence and T. gondii infection. However, results in this study have limitation to precisely explain the relationship between T. gondii infection and brain tumor.
To evade host immune responses, T. gondii parasites transform themselves into tissue cysts, and they are parasitic on various tissues, including the brain, heart, and skeletal muscle for lifetime of the host modulating the host immune responses [21]. Furthermore, they can also modulate the cell cycle and apoptosis of the host cells for their proliferation [17,22]. Such behavioral pathogenesis of T. gondii infection may be linked to the related hallmarks of tumor development. The unfavorable environment modulated by T. gondii may be similar to appropriate precancerous conditions. Thus, we can presume that T. gondii infection should help to develop brain tumors. Hosts with chronic T. gondii infections are also vulnerable to the attack by other pathogens such as viral pathogens. If such viral pathogens are associated with brain tumor development, it can be another oncogenic effect of T. gondii and may synergistically act as a carcinogen. In this respect, it is noteworthy that Plasmodium falciparum and Epstein-Barr virus contributed synergistically to the formation of Burkitt’s lymphoma [23].
Despite the studies on the association of T. gondii and brain tumors, we cannot conclude whether chronic T. gondii infection, with seropositivity, is involved in causing these brain tumors, or reversely brain tumor patients are at risk of recrudescence and dissemination of T. gondii to undergo a chronic infection [14]. Researches are also needed to determine whether T. gondii itself acts as a carcinogen or takes the role of a creator of a precancerous environment to develop into brain tumors.
