The advent of microarray-based comparative genomic hybridization (array CGH) promises to revolutionize clinical cytogenetics because of its ability to rapidly screen the genome at an unprecedented resolution. Yet, the ability of array CGH to detect and evaluate low-level mosaicism is not known. Our laboratory has analyzed over 3,600 clinical cases with the SignatureChip which we developed for the detection of microdeletions, microduplications,
aneuploidy, unbalanced translocations, and subtelomeric and pericentromeric copy number alterations. Here, we report 18 cases of mosaicism detected by array CGH in a routine diagnostic setting, 14 of which were not known to us at the time of the analysis. These 14 cases represent approximately 8% of all abnormal cases identified in our laboratory. For each case, fluorescence in situ hybridization (FISH) analysis was performed on PHA-stimulated cultures after mosaic
chromosome abnormalities were suspected by array CGH. In all cases, FISH confirmed the mosaic
chromosome abnormalities which included a variety of marker chromosomes, autosomal
trisomies, terminal and interstitial deletions, and derivative chromosomes. Interestingly, confirmatory FISH analyses on direct blood smears indicated that the percentage of abnormal cells in unstimulated cultures was in some cases different than that found in PHA-stimulated cells. We also report the detection of a previously unsuspected case of an
isochromosome 12p (associated with Pallister-Killian syndrome) by array CGH using genomic
DNA extracted from peripheral blood. These results support a growing body of data that suggests that stimulated peripheral blood cultures likely distort the percentage of abnormal cells and may, for some
chromosome abnormalities, make their detection unlikely by conventional analysis. Thus, array CGH, which is based on genomic
DNA extracted directly from uncultured peripheral blood, may be more likely to detect low-level mosaicism for unbalanced
chromosome abnormalities than traditional cytogenetic techniques.