By Tan Ee Lyn
HONG KONG (Reuters) - A genetic mutation that removes a coating of carbohydrates around sperm reduces their mobility and may explain why some men are less fertile than others, researchers said on Thursday.
The study, published in the journal Science Translational Medicine, found that couples who had the most trouble conceiving were those where the men inherited both copies of this mutant gene, one from their father and one from their mother.
The loss of this coating makes it more difficult for sperm to travel through fluids in the female reproductive tract, which in turn reduces the rate of conception, lead author Theodore Tollner said.
Using semen donated from 19 participants, Tollner and colleagues observed that sperm from donors who had both copies of the mutant DEFB126 gene exhibited most mobility problems.
"We found that sperm from donors lacking the normal gene have difficulty penetrating or swimming in the mucus surrogate (on laboratory dishes)," wrote Tollner, assistant adjunct professor at the Center for Health and the Environment, University of California.
"The rate at which they are able to penetrate the mucus-like gel is only 15 to 20 percent of the rate observed for sperm from donors with the normal gene," Tollner said in an email, replying to questions from Reuters.
The World Health Organization defines infertility as the inability of a couple to conceive after a year of unprotected sex, and the problem occurs to around 13 to 14 percent of couples in many countries across the world.
In about half of infertile couples, the cause lies with the men and experts have traditionally blamed low sperm count. This paper by Tollner and colleagues is the first time that experts are pointing to the loss of a coating around sperm.
Steven Rozen of Duke University-NUS Graduate Medical School Singapore, who was not related to the study, said this genetic mutation is "quite common".
"That means that a large proportion of men would be affected. Depending on the population, 20 percent to 30 percent of men have two copies of the low fertility variant, which means their sperm lack the coating," Rozen told Reuters.
To test their hypothesis, the scientists recruited 509 young couples in China and tracked them for nearly two years. The average age of the men was 25.8 and the women 23.4.
The couples were put into three groups depending on the DNA of the men: men without the gene mutation, men with one copy of the mutant gene and men with both copies of the mutation.
By the end of the study, wives of 71 percent of men with both copies of the mutant gene had conceived, compared to 81 percent of wives of men with either one or none of the mutant gene.
"Our key finding was that the rate of births among couples where the husband had two copies of the DEFB126 mutation was 30 percent lower than in other couples," wrote Scott Venners, assistant professor of epidemiology at the Simon Fraser University Faculty of Health Sciences.
"This most likely indicates that the DEFB126 mutation reduced the rate of conception in these couples and so it took them longer to achieve pregnancy," he told Reuters.
When helping couples conceive, doctors may consider using more direct interventions such as in vitro fertilisation or intrauterine insemination if they find that the male partner has this gene mutation, Tollner said.
(Reporting by Tan Ee Lyn; Editing by Daniel Magnowski)