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Intellectual disability: The cause is often in the genes - less often than previously thought, children inherit the mutation from their parents
In many cases, parents of a mentally handicapped child do not have a significantly increased risk that their next child will also be born with a mental handicap.
Genetic factors have long been underestimated as a cause of intellectual disability. “Rather, it was believed that developmental delays and intellectual disabilities are caused by mothers' misconduct during pregnancy, by infectious diseases or by birth defects. In fact, genes play an essential role in the development of mental retardation, ”says Prof. Dr. André Reis from the Human Genetic Institute in Erlangen. For those affected and their relatives, the question of whether the predisposition for an intellectual disability runs in the family is of great importance. "Many parents of mentally handicapped children do without further children because they are concerned about passing on a genetic defect," says Professor Reis. “Here we can give the all-clear in many cases.” Scientists from the Mental Retardation Network have genetically examined more than 2,200 patients with intellectual disabilities and their families. They were able to identify different mutations that impair brain function and thus trigger the disability, but were not passed on from parents to children. “In a much larger proportion of patients than previously thought, these mutations appear new, i.e. spontaneously.” Such new mutations arise after the egg cell has been fertilized and are not anchored in the genome of the mother or father. "In these cases, we can now tell the parents that they are not at a significantly higher risk of intellectual disabilities in the family than any other parent," explains Dr. Sabine Endele, project manager of the Mental Retardation Network, which deals with the elucidation of genetic causes of mental retardation and is funded by the Federal Ministry of Education and Research (BMBF) as part of the National Genome Research Network NGFN-Plus in the program of medical genome research.
Mutations change the flow of electricity in the brain
Very different mutations can cause mental retardation. "For example, there are defects that affect entire chromosome sections or those in which only a single gene is changed," explains Professor Reis. Most mutations have in common that they change the molecular signal transmission between nerve cells. The researchers of the Mental Retardation Network, for example, have found mutations in the genes in patients that code for certain ion channels in the brain. "If, for example, the GRIN2B gene, which carries the information for an ion channel regulated by neurotransmitters, the NMDA receptor, has mutated, the conductivity of the ion channel changes," says Dr. Endele. Electrical impulses can then no longer be transmitted correctly in the brain and, as a result, brain function is impaired.
Hope for therapies
"Even though we have been able to identify around ten new genetic defects as the cause of intellectual disability in the past few years, we still do not know the cause of their disability in almost half of the patients," says Professor Reis. However, new methods of genome research open up the possibility for researchers to investigate the underlying genetic changes more systematically, reliably and faster than before. Professor Reis: “Every mutation that we identify as the cause of mental retardation is a glimmer of hope for the families concerned. Because the more we know about the molecular causes, the sooner possible starting points for the therapy of genetically determined forms of mental retardation can be developed. ”Substances against the molecular causes of Fragile X syndrome, one of the most common forms of the genetic mental retardation. After successful testing in animal experiments, the substances are currently being investigated in therapy studies with patients. "This gives us reason to hope that drug therapies could also be possible for other forms of intellectual disability," said Professor Reis. Without genetic research over many years to elucidate the genetic cause, this would not be possible.
Definition of intellectual disabilitiesIt is estimated that two to three percent of the population suffer from a mild and up to 0.5 percent from a severe form of intellectual disability. According to the American Association on Intellectual and Developmental Disabilities, an intellectual disability is defined as a substantial limitation of cognitive and social skills that manifests itself in childhood. Associated with it are a persistently restricted ability to understand new or complex information, a reduced ability to learn and limited independence. In severe cases of intellectual disability, the patients are completely in need of care and can hardly communicate with their fellow human beings. By determining the intelligence quotient (IQ), the degree of limitation can be measured. Experts speak of an IQ of 70 to 85 of a learning disability, an IQ of under 70 of a slight intellectual disability and an IQ of under 50 of a moderate to severe intellectual disability.
Prof. Dr. André Reis
Human Genetics Institute
university hospital Erlangen
Schwabach facility 10
Tel .: 09131 85-22020
Fax: 09131 85-23232
Email: [email protected]
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