Scientists in the United States have identified a genetic defect that appears to cause mental retardation and epilepsy as well as malformations of the face and hands, according to a study published Sunday.
The still unnamed syndrome, linked to a tiny segment of missing DNA code, accounts for one out of 330 cases of retardation of unknown origin and may affect one person out of 40,000 in the general population, said the study.
Evan Eichler of the University of Washington School of Medicine led a team of 33 researchers from the United States, Italy and Britain in screening the entire genomes of 757 individuals with mental retardation.
Two unrelated persons with very similar symptoms were found to be lacking a string of 1.5 million base pairs of genetic code, located on Chromosome 15 and spanning six different genes.
There are approximately three billion base pairs -- chemical 'rungs' assembled from four different compounds -- in the human genome.
One of those genes, known as CHRNA7, is responsible for a protein critical for transmitting messages across brain cells, and has also been associated with epilepsy and schizophrenia.
Once they knew which part of the genome to explore, Eichler then screened 1,040 other individuals with mental retardation using data from the Greenwood Genetic Center in South Carolina, half of European ancestry, half of them African-American.
Seven other individuals were found with precisely the same telltale genetic defect, and shared many of the same symptoms. Of the nine cases found, all showed mild to moderate retardation, and seven had epilepsy seizures or abnormal readings for electrical activity in the brain.
All also shared certain abnormal facial characteristics, and seven of the nine had joint defects.
The newly-identified condition occurs with the same frequency as three other syndromes related to mental retardation: Williams, Angelman and Prader-Willi.
The researchers predicted that other minor syndromes are likely to emerge through high-resolution scans for 'sub-microscopic' deletions in the human genetic code.