14 DAY TRIAL // There's no doubt that smoking induces cancer.
In the end, cancer is a mutation of the cell's DNA.
But tests on mice have shown that smoking-induced mutations have been proved to occur also in the sperm cells and in this case, the mutations can be transmitted to the offspring. "Here we are looking at male germline mutations, which are mutations in the DNA of sperm. If inherited, these mutations persist as irreversible changes in the genetic composition of offspring." said lead author Dr. Carole Yauk, from the Mutagenesis Section of Health Canada's Environmental and Occupational Toxicology Division. "We have known that mothers who smoke can harm their fetuses, and here we show evidence that fathers can potentially damage offspring long before they may even meet their future mate."
Mammal males - mice or men - produce a constant number of new sperm from continuously dividing spermatogonial stem cells.
The mixed team from Health Canada and McMaster University investigated the spermatogonial cells of mice that had inhaled cigarette smoke for either 6 or 12 weeks focusing for DNA changes on a repeated portion called Ms6-hm, which does not harbor any known genes.
The mice were exposed to two cigarettes daily, which means, measured on blood levels of tobacco chemicals, the average amount smoked by a human smoker.
The speed of Ms6-hm mutations in the smoking mice was 1.4 times greater than that of non-smoking mice on the 6 weeks case, and 1.7 times higher for the 12 weeks case. "This suggests that damage is related to the duration of exposure, so the longer you smoke the more mutations accumulate and the more likely a potential effect may arise in the offspring," Yauk said.
"Previous studies have shown that Ms6-hm and similar locations of non-coding DNA are sensitive to damage from radiation, mutagenic chemicals and intense industrial air particulate pollution. While the researchers did not specifically study the protein-coding regions of DNA where genes reside, previous studies correlate mutations in non-coding regions with those in coding regions, and some repetitive regions of DNA are associated with genes." said Yauk.
"It stands to reason that mutations could also interfere with genes, but our ongoing research looks to clarify the severity of DNA damage throughout the genome. So, while some men say they'll quit smoking after their child is born, this represents a good reason to quit well in advance of trying to conceive." said Yauk.
The team is going to investigate how the mutated DNA acts in the lineage of male mice exposed to firsthand smoke; but future research will also study the effects of secondhand smoke on male mice and eggs of female mice exposed to it.