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.
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.