Certain genetic abnormalities in a man's chromosomes appear to be linked to intellectual disabilities in his offspring, especially if he fathered them late in life, a new study suggests.
These abnormalities are caused by what are called "copy number variations" in genes. These include missing, repeated, inverted or misplaced DNA sequences, the researchers explained.
Paternal age also important
"While it is commonly known that the risk of birth defects such as Down Syndrome increases with maternal age, this study shows that an increased paternal age is also an important factor," said study author Jayne Hehir-Kwa, from the department of human genetics at the University of Nijmegen in the Netherlands.
She said that scientists "are still trying to understand how many of these genetic mutations actually occur. This study has provided an important starting point, providing insight into how these mutations are formed."
The type of genetic mutations studied were typically found in children with an IQ less than 70, Hehir-Kwa said. These kids often have other congenital abnormalities such as heart defects, she added.
Dads are more often the source of these gene aberrations than moms. In about 70% of cases, deletions and duplications of DNA responsible for intellectual disability were inherited from the father, Hehir-Kwa said.
Paternal age key
"In many cases these fathers are, on average, significantly older, by two years, than fathers of children without intellectual disability," she said.
However, because these outcomes remain very rare and the incidence of these mutations in men isn't known, it is not yet possible to assign a specific level of risk for having a child with an intellectual disability, the researcher noted.
"To be able to directly translate these findings into patient management is still too soon at this stage," Hehr-Kwa said.
The report was published in the online edition of the Journal of Medical Genetics.
For the study, Hehir-Kwa's team looked for copy number variations in over 3,400 people with intellectual disabilities between 2006 and 2010.
They found that 227 of those studied had new copy number variations, which were not inherited.
An in-depth analysis of the parents of 118 of the participants revealed that the variations of 90 of the 118 came for the father. And 75% of these variations were missing DNA sequences.
Among the group with non-repetitive DNA sequences - which accounted for most variations - older fathers were significantly associated with a child with intellectual disability, the researchers found.
No tests at this time
Based on these findings, the researchers conclude that copy number variations occur more in fathers than mothers and that age also plays an important role.
Dr Robert Marion, chief of genetics and development medicine and director of the Centre for Congenital Disorders at the Children's Hospital at Montefiore Medical Centre in New York City, said that "the study provides some evidence for what we had suspected."
"This study verifies that older fathers are more at risk to have children with intellectual disability, and the basis for this is changes in the DNA that lead to copy number variants," Marion said.
Another expert, Dr Stephanie Sacharow, an assistant professor of clinical and translational genetics at the University of Miami Miller School of Medicine, added that men cannot be tested for these variations at this time. That's because the variations are only found in sperm, which is constantly changing.
"Over time, men are more likely to have new mutations. This is not something that can be predicted based on a man's age, because most older men will have normal children," she stressed. "But the risk to an older man of having a genetic issue which could lead to intellectual disability is going to be higher as he ages." - (HealthDay News, October 2011)
For more information on genetic birth defects, visit the U.S. National Library of Medicine.
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