A cytogenetic study of nonpolymalformed patients with mental retardation of clinically undefined etiology: application of a high resolution banding technique.

We performed a cytogenetic study on 140 nonpolymalformed patients with mental retardation of clinically undefined origin, using a high resolution banding technique, to determine how much chromosome abnormalities contribute to the etiology of this condition. A total of 15 patients (10.7%) were found to have autosomal or sex chromosomal abnormalities. Autosomal abnormalities included partial monosomy (5 cases), reciprocal translocation (one case), 13/14 robertsonian translocation (3 cases), unbalanced translocation (one case), inverted duplication of 15q (one case) and mosaic trisomy 21 (one case). Sex chromosomal abnormalities comprised structural rearrangement of the short arm of the X chromosome (one case) and 47, XXY in a pure or mosaic form (two cases). It should be noted that four out of the 5 cases of partial monosomy had subtle interstitial deletions, which might have been unidentified by the conventional G-banding method alone. In one case of the robertsonian translocation 46,XY,t(13;14)/45,XY,t(13;14), a small deletion was thought to have occurred in the cells with a chromosome number of 45. Comparison of clinical features of the 15 chromosomally abnormal patients with those of patients with normal karyotypes did not show any clinical parameter indicative of chromosome imbalance. These results suggest that a subtle chromosomal deletion is specific to mental retardation associated with few malformations. We believe that diagnostic evaluation of mentally retarded patients, even if nonmalformed, should include chromosome analysis using a high resolution banding technique.