A robust protocol for measuring a specific class of genetic elements called "trinucleotide repeats" has been optimized at the National Institute of Standards and Technology (NIST) to help clinical laboratories accurately identify Fragile X syndrome, the most common cause of inherited mental retardation. The research is part of a NIST effort to develop standards for measuring the expansion of these trinucleotide repeats. The NIST protocol responds to the guidelines recently issued by the American College of Medical Genetics, which recognized that Fragile X is one of the most frequently ordered genetic tests and that there are many testing methods with different strengths and weaknesses.

Fragile X syndrome results from the repetition of a particular sequence of three chemical units on the X chromosome. About 30 repeats is normal; higher numbers, especially in the range of 60 to 200, indicate an unstable "premutation" repeat length. As the number of repeats increases in successive generations, the production of a certain protein is shut off and symptoms of the disease appear or become more severe. Thus, accurate measurement of the size of the affected region of the chromosome is important as a diagnostic indicator of the disease and likelihood in future generations.

Current methods become less reliable when the number of repeats exceeds 100. The NIST protocol establishes specific conditions for the creation of multiple copies of the genetic material and their analysis. NIST initially focused on repeat sizes of about 30 to 110; future work will assess methods for measuring larger repeat elements.

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Contact: Michael E. Newman
michael.newman@nist.gov
301-975-3025
National Institute of Standards and Technology (NIST)
13-May-2002

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