2012-01-27 20:01:52 UTC

New Generation Molecular Stool Testing: A Rational Step Toward Eradication of Colorectal Cancer

Jan. 27, 2012


David A. Ahlquist, MD

Professor of Medicine, Mayo Clinic, Rochester, MN


If one started over to design the perfect tool for colorectal cancer (CRC) screening, what would it look like? Most may agree that it would be noninvasive, accurately detect both early stage CRC and advanced precancer (adenomatous and serrated lesions) throughout the colorectum, not require bowel preparation and not restrict diet or medications, accommodate work and daily routines, and be affordable and widely distributable. The arrows from our current screening quiver all fall short of the mark. Currently available conventional tools variably suffer shortcomings of invasiveness, expense, inconvenience, cumbersome and unwieldy preparation, or inaccuracy. And, we have learned that each are biased toward detection of distal disease, including fecal blood testing,1 sigmoidoscopy and even colonoscopy.2 Until CRC control has been achieved, there remains an imperative to re-imagine our approach to screening.

Molecular stool testing (MST) appears to fulfill each of the above criteria for an ideal screening tool, with the exception of documented efficacy in a large-scale, mass-screening trial. Based on the compelling biology of tumor exfoliation and buoyed by key technical advances,3 MST is now approaching its potential. Unlike earlier versions, a new generation MST incorporates a preservative buffer with stool collection to prevent DNA degradation, broadly informative marker panels to cover molecular heterogeneity, exquisite analytical sensitivity to detect small lesions, and automation to enhance precision and reduce cost.

 In a recent multicenter case-control study involving 678 patients,4 we found that a prototype MST detected 191 of 220, or 87 percent, of non-metastatic CRC and 60 of 94, or 64 percent, of adenomas greater than 1 cm at 90 percent specificity. Uniquely, MST detected both proximal and distal neoplasms with equal sensitivity. And, importantly, adenoma detection increased in proportion to polyp size, a well-known predictor of risk for CRC progression (view figure 1); 79 percent of adenomas greater than 2 cm and 91 percent of those greater than 3 cm were detected. In a pilot study, we observed that sessile serrated polyps (the other major, predominantly proximal and often overlooked type of precancer) greater than 1 cm (n=14) were detected by MST in 71 percent of cases compared to fecal blood tests in only 7 percent (p<0.01).

Several molecular approaches centered on assay of circulating blood markers have emerged as noninvasive alternatives for CRC screening. One, a plasma test for the methylated septin 9 gene, is available commercially. From peer-reviewed reports, CRC detection rates by plasma septin 9 range from 52 to 73 percent in referred patients, plasma levels increase with CRC stage5 and few data are available on precancer (adenoma) detection. In a blinded comparison of 52 patients with CRC or large adenomas,6 our group found substantially higher detection rates with MST than with septin 9. Sensitivity for large adenomas was 82 percent with MST versus 14 percent with plasma septin 9 (p=0.001); sensitivity for stage I to III CRC was 91 percent by MST versus 50 percent by septin 9 (p=0.013) (table).

Based on these findings, we constructed a conceptual model contrasting mechanisms of marker release from colorectal neoplasms into plasma and stool (view figure 2). Molecular markers enter plasma via vascular invasion that occurs with CRC and, as demonstrated,5, 6 vary according to stage. Plasma DNA markers are higher in late- than early stage CRC and may not rise above background levels with precancers, which by definition do not invade the epithelial basement membrane. In contrast, markers abundantly shower into the colon lumen from large precancers and across all stages of CRC.4, 6 In this conceptualization, the biology rationally supports detection of the critical screening targets (precancers and early stage CRC) by MST over plasma-based molecular tests.

Effective detection of advanced precancers is a requisite for CRC prevention, and the above new findings have major implications for CRC screening by MST. We have estimated that cumulative detection of a cohort of advanced precancers using MST every two to three years increases to roughly 87 percent by the second screening round and 95 percent by the third.7 Such estimates of program sensitivity compare favorably to those by colonoscopy done at the recommended 10-year interval. And, because of this capacity to detect precancers, cost-effectiveness modeling may well demonstrate that the screening window using MST could be opened to every two to five years and retain benefit — a feature that would enhance compliance.

Screening tools must be validated in the screening setting. An optimized MST that targets two methylated gene markers, mutant KRAS and a quantitative immunochemical measure of hemoglobin (fecal immunochemical test [FIT]) is currently being evaluated for approval by the FDA. In a landmark multicenter study involving more than 10,000 average-risk patients and expected to be completed in 2012, the accuracy of MST is compared to conventional FIT for hemoglobin for detection of colorectal neoplasia. If high neoplasm detection rates by MST are corroborated, then MST broadly applied to the general population and coupled with high-quality colonoscopy promises to advance our quest to eradicate CRC.

Dr. Ahlquist shares in royalties from Exact Sciences as an inventor of licensed technology. He also receives research support from Exact Sciences and serves on its advisory board.

1. Morikawa T, Kato J, Yamaji Y, Wada R, Mitsushima T, Shiratori Y. A Comparison of the immunochemical fecal occult blood test and total colonoscopy in the asymptomatic population. Gastroenterology 2005;129:422-8.
2. Baxter NN, Goldwasser MA, Paszat LF, Saskin R, Urbach DR, Rabeneck L. Association of colonoscopy and death from colorectal cancer. Ann Intern Med 2009;150:1-8.
3. Ahlquist DA. Molecular detection of colorectal neoplasia. Gastroenterology 2010;138:2127-39.
4. Ahlquist DA, Zou H, Domanico M, Mahoney DW, Yab TC, Taylor WR, Butz ML, Thibodeau SN, Rabeneck L, Paszat LF, Kinzler KW, Vogelstein B, Bjerregaard NC, Laurberg S, Sørensen HT, Berger BM, Lidgard GP. Next-Generation Stool DNA Test Accurately Detects Colorectal Cancer and Large Adenomas. Gastroenterology 2011 (in press).
5. Grutzmann R, Molnar B, Pilarsky C, Habermann JK, Schlag PM, Saeger HD, Miehlke S, Stolz T, Model F, Roblick UJ, Bruch HP, Koch R, Liebenberg V, Devos T, Song X, Day RH, Sledziewski AZ, Lofton-Day C. Sensitive detection of colorectal cancer in peripheral blood by septin 9 DNA methylation assay. PLoS One 2008;3:e3759.
6. Ahlquist DA, Taylor WE, Mahoney DW, Zou H, Domanico M, Thibodeau SN, Boardman LA, Berger BM, Lidgard GP. The Stool DNA Test is More Accurate than the Plasma Septin 9 Test in Detecting Colorectal Neoplasia. Clin Gastroenterol Hepatol 2011 (in press).
7. Berger BM, Ahlquist DA. Stool DNA Screening for Colorectal Neoplasia: Biological and Technical Basis for High Detection Rates. Pathology 2012 (in press).

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