We found a 70% increased risk of TGCT associated with current marijuana use, and the risk was particularly elevated for current use that was at least weekly or that began in adolescence. These associations were independent of known TGCT risk factors. In addition, all of the associations we observed appeared to be limited to nonseminoma/mixed histologies.
Our results must be interpreted in light of several limitations of our study. First, we only interviewed 67.5% and 52.2% of apparently eligible cases and controls, respectively. Our results may be biased if, among the cases and controls we were unable to interview, the association between marijuana use and TGCT was different than among those men that we did interview. In order to have produced a spurious positive association, there would need to be an inverse association among the non-participating subjects. Second, we had to rely on self-report of marijuana use, an illicit drug. Cancer cases might be expected to more accurately admit to the use of an illegal substance than controls. However, our finding of an increased risk of TGCT associated with marijuana use that was confined to nonseminoma or mixed histologies indicates that it is unlikely that over-reporting occurred, since there would be no reason to expect that recall bias would occur preferentially according to tumor type. Furthermore, after adjusting for age and race, the marijuana use characteristics (ever, current and frequency of use) of our controls were essentially the same as would be predicted from national data. Finally, we did not conduct centralized pathologic review but relied on the histologic description provided by community pathologists and coded by the CSS. Any resulting misclassification, however, would be expected to obscure differences in associations between pure seminomas and non-seminomas/mixed seminomas.
Our original hypothesis sought an increasing exposure that would be associated with the risk of all histologic types of TGCT, since the incidence of seminomas, non-seminomas and mixed histologies has been increasing. We found, however, that the excess risk of TGCT associated with marijuana use was essentially confined to the nonseminomas and mixed histology tumors. In fact, the increase in the incidence of seminoma from 1973 to 1998 in the United States was 64% compared to an increase of only 24% for nonseminoma (2
). However, the opposite is true in the Netherlands and Norway with the largest increase occurring in the non-seminoma histologic groups (30
). If the increase in nonseminomas is in part due to an increase in the use of marijuana, some other increasing exposures must account for the higher incidence of seminomas over time. Akre et al. (8
) suggest that increased maternal age, increased placental weight, and decreased parity are factors that are more closely associated with seminoma compared with nonseminoma. These exposures have also been increasing over the past decades (31
) and thus could explain differential increases in incidence according to histology.
We can only speculate whereby marijuana use may be associated with TGCT. Moller (34
) showed a significant association between male subfertility and subsequent risk of TGCT, and it has been suggested that both TGCT and male subfertility may be caused by one or more common exposures. Could one of these common exposures be the use of marijuana? Marijuana use is known to adversely affect male fertility including sperm output, motility and fertilizing capacity in various species including humans (17
). In addition, chronic marijuana exposure adversely affects both the endocrine and reproductive systems in humans (17
). It has been suggested that puberty is a “window of vulnerability” during which environmental factors increase the risk of TGCT (37
). This is consistent with our finding that the elevated risk of nonseminomatous TGCTs was particularly associated with the use of marijuana starting prior to the age of 18 years. It is also speculated that primitive germ cells persisting into the pubertal period multiply under the stimulation of gonadotropins and other hormones (38
). It is then possible that altered levels of gonadotropins and other hormones during this “window of vulnerability” due to exposure of marijuana increase the risk of TGCTs. However, none of these explanations would likely be specific to nonseminomas. If indeed the association is true, new avenues of research will be needed to address the specificity of the association to nonseminomas.
The mechanism by which marijuana exerts its effects on various biological processes remained unknown until cannabinoid receptors were identified in the 1990s. Cannabinoid receptors are part of the G-protein coupled receptor family and comprise two major subtypes, brain-type receptors (CB1) and spleen-type receptors (CB2) (20
). They are G-protein coupled seven transmembrane spanning receptors and influence a variety of biological responses. CB1 and CB2 are expressed in the testes and sperm as well as in the brain, heart, uterus, embryo, spleen and immune cells (17
There are two major endogenous cannabinoid-like (endocannabinoid) lipid mediators, N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol (2-AG) that are produced from arachidonic acid. They mimic many of the effects of THC and activate both CB1 and CB2 (18
). The endocannabinoid system is operative in the male reproductive organs (14
). Endocannabinoid signaling is associated with anti-tumor effects on a variety of human tumor cells in vitro and in xenograft models in vivo (44
), findings that would appear to be inconsistent with our observation of an association between marijuana use and TGCTs. Endocannabinoids are rapidly degraded by fatty acid amid hydrolase and monoacylglycerol lipase, whereas marijuana derivatives are mainly metabolized by cytochrome P450
enzymes with a half life of about 4 days in chronic marijuana users (46
). Thus, relatively prolonged activation of CB1 and CB2 in marijuana users may disrupt normal anti-tumorigenic endocannabinoid signaling. Alternatively, the effects of cannabinoid/endocannabinoid signaling on tumorigenesis may be organ specific and age dependent. Although both the main psychoactive component of marijuana, THC, and anandamide have higher affinity for CB1, cannabinol, an oxidation product of THC, has 10- fold higher affinity for CB2 compared to CB1(21
). Therefore, the activation of CB receptors coupled to different effectors may lead to distinct biological functions. In addition, other biologically active components of marijuana may function through pathways other than endocannabinoid system. Future epidemiologic and model system studies are needed to confirm or refute our findings. Such studies should include assessment of the role of CB receptors and endocannabinoid signaling in TGCTs.