A relative excess of nonneoplastic cells in frozen carcinoma samples is often a cause of false-negative results in molecular assays. Given the greater cohesiveness of epithelial tumor cells compared with nonneoplastic epithelium and mesenchymal stroma, the authors hypothesized that tumor procurement by touch imprinting would provide a simple, cost-effective method of obtaining enriched neoplastic cells compared with frozen whole-tumor samples.
Eleven adenocarcinomas with known KRAS gene mutations were tested. Two sets of 8 touch imprint (TP) slides and 1 frozen whole-tumor sample (FS), both with a corresponding hematoxylin and eosin-stained slide, were obtained from each tumor. DNA from unstained TP and FS samples was tested for KRAS exon 2 mutations by Sanger sequencing. The percentage of carcinoma cells was determined by light microscopy of hematoxylin and eosin-stained slides. The fold increase in the mutant-enriched DNA in TP versus FS samples was determined by calculating the height ratio between the mutant and wild-type peaks on the sequencing electropherogram.
Using light microscopy, TP demonstrated a 1.1-fold to 3.5-fold (mean, 1.8-fold) enrichment in neoplastic cells compared with the FS. The mutant-to-wild-type peak height ratio was 1.4-fold to 7.1-fold (mean, 3.1-fold) higher in TP compared with the corresponding FS samples. The average amount of extracted DNA ranged from 145 ng to 7.9 μg per TP slide.
The procurement of carcinoma samples by TP is rapid, simple, and inexpensive; consistently provides a tumor-enriched sample; is an excellent source of high-quality tumor DNA; and could compensate for the relatively low sensitivity of direct sequencing. Cancer (Cancer Cytopathol) 2013;121:354-360. © 2013 American Cancer Society.