About Guanylyl Cyclase C (GCC)
The Clinical Dilemma
Despite best efforts to identify metastases in lymph nodes, about 25%-30% of pN0 patients end up with recurrent disease. This high rate of recurrence suggests that occult metastases are present but not identified by current methods. In an attempt to reduce the chance for recurrence, many clinicians treat stage II patients with adjuvant chemotherapy in spite of the absence of clear clinical evidence of disease spread. Recent data from 74,028 stage II colon cancer patients indicate that 17.7% of stage IIA and 36.6% of stage IIB patients received adjuvant chemotherapy (ACT) (from 2003-2005) with a >10% survival advantage over surgery alone. Currently, debate exists among major national cancer organizations on whether or not to treat stage II patients. The American Society of Clinical Oncology (ASCO) does not recommend routine ACT for patients with stage II colon cancer but suggests that for “high risk” cases, ACT may be considered. The National Comprehensive Cancer Network (NCCN) recommends consideration of ACT for “low risk” T3N0M0 tumors and treatment with ACT for “high risk” T3N0M0 tumors and all T4N0M0 tumors.
A more sensitive and thorough method is needed to complement traditional histopathology to identify those stage II patients who may harbor clinically significant occult metastases and who could be more appropriately staged as a stage III patient. Common practice is for a single 5µm section of LN to be examined histologically. This practice has been criticized, “The microscopic examination of only 1 section of a lymph node (standard practice) is clearly inadequate for the detection of MM and possibly for macrometastases as well”.
Molecular methods, such as quantitative RT-PCR, which are up to 100,000X more sensitive than traditional microscopy and can sample larger portions of the lymph nodes simultaneously, have been shown to be clinically important in identifying cancer missed by histopathology alone. One argument against many of the molecular markers today such as CEA and CK20 is their lack of sensitivity and specificity for colorectal cancer. A new marker, guanylyl cyclase C (GCC), may hold the answer as an extremely specific and conserved marker for the gastrointestinal tract and metastatic CRC.
The Biology of Guanylyl Cyclase C
Role as modulator of water transport in the intestine
Guanylyl cyclase C is a transmembrane receptor protein found exclusively in the lining of the intestine from the duodenum to the rectum. It is involved in multiple functions including water transport, crypt morphology and suppression of tumorigenesis. Like the other five members of the mammalian guanylyl cyclase family, GCC converts GTP into cGMP, a second messenger for intracellular communication and regulation. GCC is the receptor for heat stable bacterial enterotoxin, the cause of traveler’s diarrhea. The binding of enterotoxin to GCC stimulates the intracellular conversion of GTP to cGMP which ultimately activates the Cystic Fibrosis Transmembrane Conductance Regulator protein (CFTR) promoting chloride efflux, driving water into the lumen of the intestine, and causing the diarrhea so characteristic of the bacterial infection.
Role as modulator of crypt morphology & tumorigenesis
In addition to being the receptor for enterotoxin, GCC also binds the paracrine hormones guanylin and uroguanylin. These hormones appear to be involved in the regulation of the crypt morphology of the intestine and GCC as well as tumorigenesis. “In the intestine, guanylyl cyclase C (GCC) and its paracrine ligands organize and regulate the homeostatic integrity of the crypt-villus axis, forming a hormonal tumor suppressor signaling sequence, whose dysfunction defines the initiation of neoplastic transformation and create a permissive niche for tumor progression.”
Specificity in Normal Mucosa and Metastatic Disease
Originally identified as the receptor for enterotoxin (cause of traveler’s diarrhea), GCC’s expression in the GI tract makes it an ideal marker for metastatic disease. GCC is highly specific for cells lining the gastrointestinal tract from the duodenum to the rectum. In addition, its expression is conserved in CRC and metastatic disease but is not found in other kinds of cancer. Birbe et al examined tissue samples for the presence of GCC from 142 normal patients (including normal colon, rectum, esophagus and stomach), 108 patients with inflammation, metaplasia or dysplasia, and 139 primary colorectal tumors and colonic adenocarcinomas with metastases to various sites. GCC was detected in all primary adenocarcinomas of the colon, was detected in normal colon and rectum from the duodenum to the rectum, but was not detected in normal esophagus or stomach. Expression was conserved in inflammatory bowel disease, premalignant lesions, and in primary and metastatic adenomcarcinomas. The highly specific expression of GCC (confined to the intestine) and its conservation of expression in cancer, make it an ideal candidate as a marker of occult CRC metastasis.
Clinical Studies of GCC as a Marker of Occult Metastases
In 1999, Cagir et al examined the link between recurrence and presence of GCC in lymph nodes. Eleven stage B patients free of disease for over 6 years formed a control group while 10 stage B patients who developed recurrence within 3 years formed a case group. Total RNA was extracted from the patients’ formalin-fixed paraffin-embedded lymph nodes and interrogated using RT-PCR for the presence of GCC mRNA. All eleven control patients were GCC-negative while all patients who recurred were GCC-positive. This important study paved the way for a large multi-center prospective trial sponsored by the National Cancer Institute to investigate the use of GCC in identifying occult metastases and correlating with disease recurrence. In that trial, over 400 hundred stage I, II and III patients were enrolled over a period of five years. In the first phase of the trial, lymph nodes were harvested and bisected, with one half examined by RT-PCR for the presence of GGC mRNA while the other half was examined by traditional histopathology. Detection of GCC in the lymph nodes was correlated with recurrence over a five year period. This study has been recently completed and the Company’s collaborators at Thomas Jefferson University are presenting the results at major medical meetings troughout the year.
The second phase of the prospective trial is still under way and will examine the patients’ blood for the presence of GCC mRNA over time as a way to monitor recurrence and response to therapy. Results are expected to be published in the next two years.