The metabolism of DDT has not been fully described 27
and this gap limits our ability to fully interpret our findings. However, available evidence does establish several features of DDT metabolism relevant to the present study.
First, metabolic studies, 27-28
primate feeding experiments,28
human feeding experiments,9
and human survey studies10-11
support the concept that p,p′
-DDT is more rapidly metabolized and excreted than p,p′
-DDE. The higher DDT/DDE ratio observed in mothers of cases is consistent with recent exposure to commercial DDT or alternatively, slower elimination of p,p′
Second, although there is a paucity of metabolic studies of o,p′
human feeding experiments and human survey studies have also established that o,p′
-DDT is more rapidly metabolized and excreted than p,p′
-DDT is acquired as a low level contaminate of commercial DDT, it would be expected that mothers with recent exposure to DDT would have both a higher DDT/DDE ratio and higher levels of o,p′
In the present study, mothers of sons with testicular cancer had a higher DDT/DDE ratio, but lower o,p′-DDT, presenting an informative paradox. Possible explanations for this paradox include: 1) Case mothers had higher exposures earlier in life at a critical period, possibly even prior to pregnancy. 2) Case mothers have slower p,p′-DDT elimination so that their internal dose of p,p′-DDT is greater even for equivalent or lower total exposure. 3) Case mothers have accelerated elimination of o,p′-DDT associated with exposure to a toxic intermediate metabolite.
Human studies of experimental ingestion of DDT showed “marked differences” in excretion among individuals following ingestion of DDT.9, 28
We suggest that this observation supports the plausibility of individual differences in DDT metabolism for case mothers compared to control mothers in this study.
This picture is certainly further complicated by the finding that DDT itself induces enzymes that facilitate its own metabolism and excretion,27
further supported by human feeding experiments that showed rates of elimination of p,p′
-DDT increased as dose increased.9
Controls, who showed evidence of recent exposure (higher o,p′
-DDT), paradoxically also have a lower ratio of p,p′
-DDT to p,p′
-DDE. We speculate that high and recent DDT exposure could have led to faster p,p′
-DDT elimination in controls. Additionally, the metabolism of p,p′
-DDT may depend on the CYP system27
which can be highly polymorphic. At this time there is not sufficient understanding of DDT metabolism to fully explain study findings, nor can we establish the time course of DDT exposure and elimination.
Our findings suggest that any single DDT–related compound is an inadequate proxy for exposure. High correlations among DDT-related compounds and varying biological activities7
suggest that multiple compounds should be measured and considered where possible. For example, the strong positive correlation of p,p′
-DDE with p,p′
-DDT confounds associations for each compound in this study (See models 1,2 and 4).
To our knowledge there are no other prospective studies of prenatal organochlorine exposure and testicular cancer. However, Hardell and colleagues measured maternal serum levels of organochlorines at the time of the diagnosis of testicular cancer, decades after birth, between 1997 and 2000.29
That study found that case mothers had higher levels of polychlorinated biphenyls, chlordanes and hexachlorobenzene, but not DDT-related compounds. Levels of DDT-related compounds in humans declined even before the ban on the pesticide DDT in 1972.11, 18
For this reason, levels of DDT-related compounds found in mothers' blood between 1997 and 2000, decades after commercial DDT was banned, do not necessarily reflect levels during gestation 30 years earlier. Differences in timing of blood sampling could explain the inconsistency between this study and that of Hardell and colleagues.
In contrast, McGlynn et al. used the United States Department of Defense Serum Repository to conduct a large nested case-control study that reported an association between persistent organochlorines and testicular cancer.30
The average time between sample donation and diagnosis was 5 years. This design has the advantage of good power, and the disadvantage of being unable to assess exposure during prenatal life, as noted by the authors.
The present study was undertaken to test the hypothesis that there is a sizable effect of maternal DDT exposure in pregnancy on risk of testicular cancer in sons. Although the CHDS is one of the largest U.S. pregnancy cohorts with cancer follow-up of parents and children, conclusions drawn from this study are limited to detection of sizable effects due to the rarity of testicular cancer and the small number of cases observed over four decades even among the 9,744 live born sons in the CHDS. To our knowledge there have been no studies that directly measure maternal exposure to DDT-related compounds during pregnancy in relation to subsequent testicular cancer in sons. The current study addresses this gap and has several unique features: 1) Direct measurement of three DDT-related compounds from maternal serum samples which were obtained one to three days after delivery. 2) Maternal serum samples collected at the peak period of active commercial DDT use (1960's). 3) Prospective follow-up for testicular cancer incidence beginning before birth.
The biological effects of commercial DDT have been considered largely with reference to classical steroid hormone activity via binding to estrogen or androgen receptors.7
The validity of this approach has been questioned, as in vitro studies may not represent actions of exposures in the entire organism.31
The balance of exposures, including the DDT/DDE ratio, has been shown to influence effects in vivo and may differ between species and across tissues.31
Moreover, toxicity of DDT-related compounds may be independent of endocrine disruption effects and could occur via changes in gene expression that control metabolism of many exogenous exposures by cells.32
It has also been suggested that DDT may be a tumor promoter through its strong inhibition of intercellular communication.33
Our conclusions are tempered by the small sample size available for study, despite a sizeable source population. This limitation is inherent in prospective testicular cancer studies due to the rarity of this cancer. We were also unable to control for multiple confounding factors due to sample size limitations. However, the ratio of p,p′
-DDT to p,p′
-DDE was also recently reported to be a risk factor for liver cancer,12
and for delayed time to pregnancy in CHDS daughters following in utero
Higher levels of p,p′
-DDT at each level of p,p′
-DDE also predicted greater breast cancer risk in CHDS mothers.18
The similar findings for time to pregnancy in women and testicular cancer in men of the same generation of the CHDS cohort are unlikely to be serendipitous, and support the alternative idea that p,p′
-DDT, an intermediate metabolite, or another exposure that shares its metabolic pathway, may be a reproductive toxin and/or a carcinogen.
The associations we observed in this study are of considerable interest given the low statistical power of this study, and the lack of other strong risk factors for testicular cancer identified to date.1
We suggest that our ability to observe DDT associations is due to timing of the maternal blood collection allowing measurement of exposure relevant to early testicular development, in a time period when DDT was being actively acquired by mothers. A better understanding of individual differences in DDT metabolism and the relationship of DDT to the induction of enzymes that metabolize other endogenous and exogenous exposures will be important for full interpretation of our findings. Such information will be helpful for informing the continuing debate on the cost and benefits of using DDT for malaria control34
and for understanding the human health effects of one of the most universal man-made exposures in history.