An Italian expert has discovered a new gene mutation linked to mental retardation, according to a new study. The research, headed by Patrizia D’Adamo of the San Raffaele Foundation del Monte Tabor Foundation in Milan, looked at forms of mental retardation specifically associated with the X chromosome, known as X-linked retardation.

Mental retardation in general is the commonest form of developmental disability among children and young people, and in 25-50% of cases, is caused by a defect in a gene or chromosome. X-linked retardation is considered a particularly complex disability to study because there are more than 200 known defects likely to cause retardation, stemming from problems with over 80 different genes. The Italian study, set to be published in the American Journal of Human Genetics, analyzed the DNA of various males affected by mental retardation. They then studied the DNA of the men’s relatives and discovered that, in two cases, the defect arose from a missing gene that normally contains information for a protein specific to the nerve cells, RAB39B. The absence of this protein reduced contact between the nerve cells, or synapses.

RAB39B is thought to play an important role in forming and maintaining the correct number of synapses, which transmit information between neurons. As a result, neuron communications in people affected by this genetic mutation experience are diminished, thereby reducing their cognitive and linguistic capacities and creating difficulties with social interactions.

The researchers also discovered that the seriousness of retardation, which was aggravated by autism and epilepsy, appeared to increase through the generations, although they are as yet unable to explain why. The protein identified by D’Adamo’s group is part of a family of over 60 proteins involved in transporting information inside the cell but it is the only one discovered so far that is specific to nerve cells. According to D’Adamo, the next step will be more detailed research into the molecular mechanisms that alter the number of synapses and inter-neuron communications, in order to try identify potential treatments.