According to a new study, one in 500 men may have an extra sex chromosome (X or Y), but few of them are aware of it. The study, published June 9 in the journal Genetics in Medicine, used data from more than 207.000 men who contributed information to the UK Biobank, a collection of genetic and health data from half a million UK-based participants.
Males normally have one X and one Y-shaped sex chromosome in each cell, but study participants included 213 males with an extra X chromosome and 143 males with an extra Y chromosome.
Only a small percentage of these men said they had been diagnosed with a chromosomal abnormality or mentioned it in their medical records.
Only 23% of XXY men had a known diagnosis, while only 0,7 percent of XYY men had a diagnosis. (According to the Genetic and Rare Diseases Information Center, the possible symptoms of carrying an extra Y chromosome can be quite minor, which may explain the disparity in diagnosis rates.)
D., a pediatric endocrinologist in the Medical Research Council (MRC) Epidemiology Unit at the University of Cambridge and one of the senior authors of the paper. “We were surprised at how often this happens,” Ken Ong told The Guardian. “It was supposed to be pretty rare.”
According to the National Human Genome Research Institute, about 100.000 to 100 out of every 200 men are XXY, while 100.000 to 18 out of every 100 people are thought to be XYY, the authors report.
In total, 0.17 percent, or about one in 580, of study participants had an extra sex chromosome.
The study authors emphasized in their report that the rate found in the study may be slightly lower than that observed in the general population. This is because UK Biobank participants are healthier than the general population and have a lower-than-average incidence of hereditary diseases.
Based on this, the authors estimate that approximately one in 500 men, or 0,2 percent, in the general population has an extra sex chromosome.
Extra sex chromosomes can increase the risk of certain health disorders, and the researchers found that this increased risk was reflected in the health data of Biobank volunteers.
According to the National Human Genome Research Institute, Klinefelter syndrome (KS) -- or having an extra X chromosome as a male -- has been linked to reproductive problems such as infertility and delayed puberty. According to the study, XXY boys were four times more likely to be childless than XY boys and were three times more likely to enter puberty late.
The authors observed that a condition known as 47,XYY syndrome, or having an extra Y chromosome as a male, was not linked to an increased likelihood of reproductive problems in study participants. According to the Genetic and Rare Diseases Information Center, the syndrome has previously been associated with learning disabilities, delays in the acquisition of speech and motor skills, and other symptoms, particularly low muscle elasticity. In the Biobank study, these symptoms were not specifically studied.
But the study revealed a possible link between additional sex chromosomes and other diseases. Both XXY and XYY men had an increased incidence of type 2 diabetes, plaque formation in artery walls (atherosclerosis), blood clots in veins (venous thrombosis) and pulmonary arteries (pulmonary embolism), and chronic obstructive pulmonary disease (COPD).
“It is unknown why both KS and 47,XYY can show significant similarities in conferring significantly greater risks for many common diseases,” the authors report. They said future research will need to examine the factors that contribute to these higher risks.
The scope of the study was limited as it only included men of European descent between the ages of 40 and 70.
D., associate professor of human genetics at the University of Exeter School of Medicine and co-senior author of the study. "Our study is important because it starts with genetics and tells us the potential health implications of having an extra sex chromosome in an older population," said Anna Murray.