Research shows for the first time that increased levels of organochlorine chemicals PCBs and a DDT breakdown product in men are associated with an extra sex chromosome in sperm that can contribute to reproductive problems.
Long-banned chemicals that still persist in people and the environment are linked to an excessive number of sex chromosomes in sperm, according to a study of men from Massachusetts.
The men with higher levels of persistent organochlorine chemicals PCBs and p,p'-DDE in their blood were more likely than those with lower levels to have a higher percentage - sometime 60 percent more - of sperm with too many sex chromosomes. An abnormal number of chromosomes in the embryo or fetus is the largest known cause of failed pregnancies in people. It can also lead to birth defects.
This is the first study to examine the relationship between exposure to these chemicals and sex chromosomal abnormalities in men's sperm. The results are published in the journal Environmental Health Perspectives.
Sex chromosomes are part of the genetic material donated by the mother in the egg - always the X chromosome - and the father in the sperm - either an X or a Y. At fertilization, the chromosomes normally come together in one of two ways: XX for a girl and XY for a boy.
The abnormal number of sex chromosomes or aneuploides - in this case sperm with an extra sex chromosome - occur in 5 percent of clinical births. Overwhelmingly, the extra sex chromosome originates from the father. Studies show infertile men have higher than normal frequencies of these types of abnormal sperm.
Several triggers may cause chromosome errors during sperm production in the testes. Some research suggests environmental exposures may play a role. Prior studies show benzene and some pesticides are associated with sperm having more than one sex chromosome. Human studies find a link between decreased semen quality and exposure to the organochlorine chemicals PCBs and DDT.
There are more than 200 different types of PCBs. The chemicals were once widely used as coolants in electrical transformers and motors. Some types of PCBs chemically resemble the insecticide DDT. DDT was banned in the 1970s in the United States yet is still used in some tropical countries to combat malaria-carrying mosquitoes.
The United States and other countries have banned PCBs and DDT because of their health effects, which include toxicity to developing nervous and endocrine systems. PCBs and DDT's main breakdown form - p,p'-DDE - are incredibly stable, persist in the environment and accumulate in fatty tissue. Despite the bans, they are found in most people.
In the study, researchers examined whether exposure to PCBs or p,p'-DDE was associated with heightened rates of extra sex chromosomes. They measured blood levels of 57 different types of PCBs and p,p'-DDE in 341 men that were part of a couple seeking help for infertility. Researchers examined subject's semen to look for the abnormal presence of an extra sex chromosome - a condition called disomy. Normal sperm should carry a single sex chromosome - either X or Y. With disomy, sperm carries two - XX, XY or YY.
To understand the association between chemical exposure and abnormal sex chromosome number, researchers separated the subjects into four groups. Group 1 contained 25 percent of men enrolled in the study that had the lowest levels of the chemicals in their blood whereas Group 4 contained 25 percent of subjects with the highest levels.
In comparison to the men in Group 1 with the lowest levels of p,p'-DDE, men in Groups 2, 3 and 4 with higher levels of the chemicals were more likely to produce sperm with XX or XY disomy. For example, men in the highest exposure group had a 60 percent increase in the rate of XX-carrying sperm than the men in the lowest exposure group. p,p'-DDE exposure was not associated with elevated rates of YY disomy.
Researchers also looked for an association between sex chromosome number and levels of the four most commonly found PCBs. Again, when compared to the men in Group 1 who had the lowest chemical levels, elevated PCB exposure was associated with increased rates of sperm with YY or XY disomy. Interestingly, increased PCB levels were associated with a decreased rate of XX disomy.
Researchers also categorized the PCBs into groups of estrogenic and dioxin-like PCBs. Increased exposure to dioxin-like PCBs in groups 2, 3 and 4 was associated with higher rates of XY containing sperm while the estrogenic PCBs were associated with both XY and YY disomy. Again, higher levels of both estrogenic and dioxin-like PCBs were associated with lower rates of XX disomy.
More research is needed to confirm these findings and understand whether organochlorine exposure either drives - or is just associated with - disomy formation.
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