Duty, SM, NP Singh, MJ Silva, DB Barr, JW Brock, L Ryan, RF Herrick, DC Christiani and R Hauser 2003. The relationship between environmental exposures to phthalates and DNA damage in human sperm using the neutral comet assay. Environmental Health Perspectives: doi:10.1289/ehp.5756 About
phthalates Duty et al. report a link between phthalate exposure and DNA damage in human sperm. Their finding is important because the damaged sperm were obtained from men living in the Boston area who had not been exposed to unusually high levels of phthalates. Indeed, two prior studies of patterns of phthalate exposure of Americans (one of adults, one of children) conducted by the US Centers for Disease Control had found comparable levels of phthalates to be common in the US population. The current study, in light of those prior two, suggests that DNA sperm damage due to phthalates may be widespread in American men. Whether this damage is linked to infertility or to reproductive outcomes is unknown. What did they do? Duty et al. obtained sperm from male participants in an ongoing study of subfertile couples being conducted by the Massachusetts General Hospital Andrology Laboratory in Boston, MA.
The comet assay takes advantage of the fact that when DNA strands break they will move away from undamaged DNA when placed in agar and exposed to a small electric field, a technique known as electrophoreisis. The larger the tail, the more DNA damage has occurred. Duty et al. also obtained urine specimens from the sperm donors and assessed the mens' exposure level to different phthalates using a technique developed by by Blount et al., in 2000. This technique measures the concentrations of metabolites of phthalates present in the urine following exposure to the parent phthalate compound (when ingested, the original ("parent") compound is often converted chemically to other forms, called metabolites). Duty et al. then carried out a series of statistical analyses looking at relationships between metabolite level and amount of DNA damage. What did they find? Duty et al. measured DNA damage and phthalate level in 168 subjects. Phthalates were detected in all of them. MEP (monoethyl phthalate was by far the most abundant urinary metabolite (its parent compound is diethyl phthalate), ranging from 9.8 to 5396 ppb(ng/ml). The geometric mean for MEP was 186.8 ppb. This level of MEP is comparable to the CDC's assessment from 2000. Average values for standard semen variables (sperm count, motility, % normal morphology) were all above WHO reference values, but a more detailed analysis showed that 52% of the men had values of at least one of these parameters beneath WHO norms. Twenty-four subjects had fewer than 20 million sperm/ml; 68 subjects had less than 50% motile sperm; 38 subjects (22.6%) had less than 4% normally shaped sperm. The key finding presented by Duty et al. is that MEP (monoethyl phthalate) was strongly associated with DNA damage as indicated by "comet extent." As noted above, this was also the most abundant of the phthalate metabolites, indeed median MEP values were 32x higher than the second most abundant phthalate. None of the other phthalates demonstrated a statistically significant association. What does it mean? This study raises the possibility of widespread DNA damage as a result of exposure to diethyl phthalate, the parent compound of MEP. In their cautious discussion of the results, Duty et al. note that their result is based upon a single urinary sample from a relatively small number of men, and hence the results require confirmation. They also comment on the absence of a relationship between DNA damage and the other phthalate metabolites, suggesting it could be due either to true toxicological differences between the compounds or, alternatively, to the fact that the other metabolites were at significantly lower levels than MEP.
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