Cytogenetic surveillance of mentally-retarded school children in southern Taiwan.

Background And Purpose: Mental retardation (MR), defined as having an IQ of less than 70, is present in approximately 2 to 3% of the population. Data on chromosomal abnormalities, an important cause of MR, are limited in the Taiwanese literature. This study evaluated the frequency and pattern of chromosomal abnormalities in school children with MR in southern Taiwan.

Methods: Peripheral blood samples of 419 children were collected from November 1999 to January 2003. Those with Prader-Willi syndrome (PWS), Angelman syndrome (AS) or fragile-X syndrome were excluded from the study. Metaphase chromosome preparations were obtained from peripheral blood cultures, and trypsin-giemsa (GTG) banded chromosomes were examined at the level of 500 to 600 bands. Fluorescence in situ hybridization was done for cases whose karyotypes could not be determined by conventional cytogenetic analysis.

Results: Of the 419 enrollees with MR, 10 had mild MR, while most had moderate to profound MR. Chromosomal abnormalities were found in 22.43% of the cases, with trisomy 21 being the major chromosomal abnormality, occurring in 77 cases (18.38%, 77/419 cases). Sex chromosome aneuploidies were found in 3 cases (0.72%, 3/419 cases). Structural abnormalities of autosomes were found in 13 cases (3.10%, 13/419 cases), including deletion, markers, unbalanced translocations, and inversions. One subject was found to have monosomy 20 mosaicism (0.24%, 1/419).

Conclusions: Chromosomal abnormalities occurred in a high proportion of mentally retarded school children from southern Taiwan, with trisomy 21 being the most prevalent. These findings indicate the need for increased attention to prenatal, perinatal and postnatal screening in this population.