Men Have Biological Clocks Too, Sperm Study Says
for National Geographic News
|June 6, 2006|
Note to men: You've got a biological clock too, and it's ticking.
It's not just women who face decreased reproductive success with age. The genetic quality of sperm deteriorates as men get older, according to a new study.
Starting in their 20s, men face steadily increasing chances of infertility, fathering an unsuccessful pregnancy, and passing on to their children a genetic mutation that causes dwarfism, according to the study.
The finding comes as more and more men are delaying fatherhood. Since 1980 U.S. birth rates have increased up to 40 percent for men aged 35 to 49 and decreased up to 20 percent for men under the age of 30, according to the research.
Studies have also shown that it takes longer for older men to conceive.
"We [now] know the probability for certain types of DNA damage goes up with age, and we can give you a mathematical probability," said Andrew Wyrobek, a researcher at the Lawrence Livermore National Laboratory in Livermore, California.
The higher the percentage of a man's sperm that has DNA damage, the less likely he will be able to successfully father a healthy child, Wyrobek added.
(See our quick overview of human genetics.)
Wyrobek is a co-lead author of the new study, which appears in today's issue of the journal Proceedings of the National Academy of Sciences.
Researchers have long known that female fertility decreases with age. The longer women delay reproduction, the greater their risk of miscarriage and giving birth to children with diseases such as Down syndrome.
Female fertility abruptly ends with the onset of menopause.
"Our research suggests that men too have a biological time clock, only it is different," Brenda Eskenzai, a study co-author at the University of California, Berkeley's School of Public Health, said in a statement.
"Men seem to have a gradual rather than an abrupt change in fertility and in the potential ability to produce viable healthy offspring."
In the study, Wyrobek, Eskenzai, and colleagues examined the genetic quality of sperm from 97 healthy, nonsmoking men between the ages of 22 and 80. The men were current and retired employees of the Livermore laboratory.
The study sample included at least 15 men from each ten-year period from 20 to 60 years of age and 25 men 60 to 80 years old.
In earlier research on the same sperm samples, the team found that sperm count, mobility, and the ability of sperm to move in a straight line declines with age. The new research shows that mobility has a high correlation with DNA fragmentation.
The new study also found that men face increased risk of fathering children with achondroplasia, a genetic mutation that causes a form of dwarfism.
The condition stunts bone growth; affected individuals have short arms and legs and grow to only about 4 feet (1.2 meters) tall.
As women grow older, they are more likely to give birth to a baby with Down syndrome. Not so for men, apparently.
However, the study found no general correlation among male age and chromosome changes that cause Down syndrome.
Even so, 4 of the 97 men were at increased risk for transmitting multiple genetic and chromosomal defects, according to the results. Age may have nothing to do with these subjects' condition, though; one was in his 20s and three were over 60.
Interestingly, Wyrobek added, the sperm in the Lawrence Livermore sample showed no increase with age in Apert syndrome, a disfiguring birth defect.
However, the likelihood of fathering a child with Apert increased with age in men from inner-city Baltimore, Maryland, tested by researchers at the Johns Hopkins Medical Center for a different study.
Wyrobek cautioned against reading too much into the Baltimore study group's results, since they were all from men who lived in the same area.
"There're other things going on besides age. It could be socioeconomic, or diet, or ethnicity."
Further studies, he added, will examine the Apert syndrome factor in greater detail.
Craig Niederberger is a urologist at the University of Illinois in Chicago. He said the research findings are interesting and warrant further study. But he cautioned that the technique used to test the DNA of sperm is new and controversial.
"Older men should not yet be concerned about fathering children. The evidence is still inconclusive," he said.
According to Niederberger, who is president of the Society for Male Reproduction and Urology, researchers need to examine the integrity of sperm DNA with other methods before sounding alarms about male infertility.
Nevertheless, he said, the finding that a genetic mutation that leads to dwarfism increases with age is cause for "some concern. We ought to pursue it."
Wyrobek said the research raises more questions than answers. But it suggests certain kinds of DNA damage and genetic defects go up with age.
Bottom line: "There are consequences of delaying fatherhood," he said.
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