Genetic Analysis Sheds Light on Mental Retardation.(Statistical Data Included)(Brief Article)

CAMBRIDGE, MASS.--When Dr. Jeff Milunsky determined the reason for a child's mental retardation, he brought an end to the mother's 8-year search for an answer.

A new genetic technique revealed that the boy was missing one end of chromosome 1. This defect is known as a subtelomeric rearrangement, Dr. Milunsky said at a meeting on behavioral pediatrics that was sponsored by Boston Medical Center.

Subtelomeric rearrangements are the second leading genetic cause of mental retardation, noted Dr. Milunsky, who serves as director of clinical genetics at the Center for Human Genetics, Boston. (Down syndrome is the number one cause of mental retardation.)

Many brain-related genes lie near the ends of chromosomes, next to protective end sequences that are called telomeres, and when these genes are deleted or rearranged, mental retardation can result, Dr. Milunsky said.

A normal-resolution karyotype scans genetic material for changes in chromosomal structure, such as those visible in Down syndrome. High-resolution studies make it possible to detect more subtle chromosome rearrangements, he explained.

But such diagnostic techniques are not always sufficient. Subtelomeric rearrangements and several other well-known syndromes are not visible with normal- or high-resolution chromosomal analysis.

That's where fluorescent in situ hybridization (FISH) analysis comes in. The technique is used when the smallest genetic rearrangements are suspected, Dr. Milunsky said.