Two independent groups have identified DNA probes that soonshould lead to the ability to diagnose carriers of the mostcommon inherited form of mental retardation.

Fragile X syndrome, which affects one in 1,500 males and onein 2,500 females, causes X chromosomes to break easily. Thebreakage causes mental retardation in most patients.

Current cytological assays for Fragile X carriers are tediousand unreliable. Thirteen labs currently test for Fragile Xsyndrome, using restriction fragment-length polymorphisms.According to Dr. Ray Fenwick of the Baylor College of Medicine,such assays are often not sensitive enough to distinguishbetween affected and healthy individuals.

Two reports in today's Science pinpoint DNA probes thatidentify an unstable region on the X chromosome of Fragile Xcarriers characterized by amplifications and insertions of theDNA sequences.

Both a French research team led by I. Oberle at the INSERM inCedex, France, and an Australian/American team led by S. Yu atAdelaide Children's Hospital in Australia found that Fragile Xcarriers contain unstable DNA sequences of various lengthswithin the identified region and that the size of the region canchange between generations.

The French group showed that larger insertions andmethylation of key sites in the region correlate with greaterseverity of mental retardation in affected males.

-- Carol Talkington Verser, Ph.D. Special to BioWorld

(c) 1997 American Health Consultants. All rights reserved.

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