U.S. scientists have identified the specific brain defect that causes fragile X syndrome -- the most common inherited form of mental retardation.

   Emory University School of Medicine Professor Stephen Warren identified the genetic mutation leading to fragile-X syndrome in 1991, but the nature of the deficiency caused by the mutation was not known.

   In the new research, Warren and colleagues determined fragile X is caused by a mutation in the FMR1 gene on the X chromosome. A region of the mutated FMR1 gene repeats a trinucleotide sequence of DNA bases -- CGG -- between 200 and 1,000 times, rather than the normal 6 to 55 repeats.

   The abnormal trinucleotide repeats cause the absence of the FMR protein normally produced by the gene.

   "This is quite exciting, progressing from the identification of the gene in 1991 to now believing we will be able to treat a previously untreatable condition," said Warren. "Our next steps will be to continue screening and identifying the best drugs to try and correct the deficiencies that result from fragile X syndrome."

   The study, which included Mika Nakamoto, Vijayalaxmi Nalavadi, Michael Epstein and Usha Narayanan, appears in the Proceedings of the National Academy of Sciences.

--