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
Phthalates linked to poor semen quality
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
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.
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,
sperm samples were then subjected to a classic assay used
by cell biologists to quantify DNA damage, called the comet
assay. This assay takes its name from the way the DNA looks
during the final analysis when examined under flourescent
light: a large bright spot of undamaged DNA with a comet's
tail extending outward. Damaged DNA strands are in the tail.
from the Comet
Assay Interest Group
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.
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
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.
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.
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.