A validated quantitative assay to detect occult micrometastases by reverse transcriptase-polymerase chain reaction of guanylyl cyclase C in patients with colorectal cancer

Purpose: Guanylyl cyclase C (GCC), a receptor for bacterial diarrheagenic enterotoxins, may be a prognostic and predictive marker to detect occult micrometastases in patients undergoing staging for colorectal cancer. However, quantification of GCC expression in tissues by the quantitative reverse transcription-PCR (qRT-PCR) has not undergone analytic and clinicopathologic validation. Experimental Design: A technique to quantify GCC mRNA in tissues employing RT-PCR was developed and validated employing external calibration standards of RNA complementary to GCC. Results: GCC qRT-PCR exhibited reaction efficiencies >92%, coefficients of variations <5%, linearity >6 orders of magnitude, and a limit of quantification of >25 copies of GCC cRNA. This assay confirmed that GCC mRNA was overexpressed by colorectal tumors from 41 patients, which correlated with increased GCC protein quantified by immunohistochemistry. Analyses obtained with 164 lymph nodes from patients free of cancer and 15 nodes harboring metastases established a threshold for metastatic disease of similar to 200 GCC mRNA copies/mu g total RNA, with a sensitivity of 93% and specificity of 97%. GCC mRNA above that threshold was detected in 76 of 367 (similar to 21%) nodes free of disease by histopathology from 6 of 23 (26%) patients, suggesting the presence of occult micrometastases. Conclusions: Quantifying GCC mRNA in tissues by RT-PCR employing external calibration standards is analytically robust and reproducible, with high clinicopathologic sensitivity and specificity. This validated assay is being applied to similar to 10,000 lymph nodes in a prospective trial to define the sensitivity of GCC qRT-PCR for staging patients with colorectal cancer.