Scientists have discovered a genetic anomaly responsible for severe learning difficulties.

Using a new technique for detailed examination of human chromosomes, researchers at the Wellcome Trust Sanger Institute in Cambridge have identified a missing piece of genetic information which they believe may be a common cause of learning difficulties. The results were published online yesterday in Nature Genetics.

“It seems that people with some forms of severe learning difficulties have a small, but important section of chromosome 17 missing, which appears to have been deleted during meiosis, the process by which the egg and sperm are produced,” explains Dr Charles Shaw-Smith, a University of Cambridge, Department of Medical Genetics, researcher based at the Institute.

“The deletion occurs in a region of the genome known for its complexity of organisation, and also for its association with neurodegenerative disorders, including rare heritable forms of dementia. However, this is the first time that the region has been associated with learning difficulties in children.”

During meiosis, chromosomes are copied before lining up to exchange material. However, if the copying mechanism goes wrong, the chromosomes line up out of synch and the genetic material gets mis-paired. Dr Shaw-Smith likens this to lining up the buttons and holes wrongly on a shirt so that they don’t pair up properly. It is during this process that genetic material can get deleted.

Dr Shaw-Smith’s team, led by Dr Nigel Carter at the Wellcome Trust Sanger Institute, used a technique known as “array-based comparative genomic hybridization” to identify the missing piece of genetic information. This technique allows human chromosomes to be studied at much higher resolution than was previously possible.

Identification of patients with this condition was greatly helped by the DECIPHER online database (http://decipher.sanger.ac.uk), developed through a second Cambridge University/Sanger Institute collaboration, whereby information about patients with rare chromosome disorders is logged and made available to the international clinical genetics community. This led to the identification of the deletion of part of chromosome 17, also identified by groups in the USA and the Netherlands.

“We believe that the loss of genetic material on chromosome 17 may prove to be a relatively common cause of learning difficulties,” says Dr Shaw-Smith.

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