Altered DNA Assay Promising as a Stool Screening Approach

ROCHESTER, Minnesota—In a blinded pilot study, conducted at the Mayo Clinic, a new DNA-based colorectal cancer fecal screening tool detected 91% of colorectal cancers and 82% of adenomas 1 cm in size or larger, with a specificity of 93%.

“If these pilot observations hold up in further clinical studies and the testing can be simplified, then the long-held goal of a noninvasive, sensitive, and specific fecal screening tool may be within reach,” said lead investigator David A. Ahlquist, MD, of the Division of Gastroenterology and Hepatology.

The test, being developed by EXACT Laboratories, Maynard, Massachusetts, uses a sequence-specific hybrid capture technique to isolate human DNA from stool. The assay targets “a spectrum of DNA alterations that occur with colorectal neoplasia,” Dr. Ahlquist said. He explained that colorectal neoplasms are genetically heterogeneous, and no single mutation has been identified that is expressed across all colorectal neoplasms. Thus, the new test has several targets: point mutations of any of 15 mutational hot spots on the K-ras, APC, and p53 genes; Bat-26, a marker of microsatellite instability; and highly amplifiable, or “long,” DNA (L-DNA).

The researchers analyzed freezer-archived stools from 22 patients with colorectal cancer, 11 with adenomas 1 cm or larger, and 28 with normal colons. The stool samples had been collected a few days before cathartic preparation for a scheduled colonoscopy.

Analyzable human DNA was recovered in all study subjects, Dr. Ahlquist said. Using the full panel of markers, the test detected 20 of 22 cancers (91%) and 9 of 11 adenomas 1 cm or larger (82%), with a specificity of 93% (Gastroenterology 119:1219-1227, 2000).

The test yielded only two false-positives, both a result of K-ras mutations, he said. When K-ras markers were excluded from the assay, sensitivity for cancer remained the same, but decreased to 73% for adenomas; however, specificity increased to 100%.

Dr. Ahlquist pointed out that the fecal DNA assay panel found “a remarkably high proportion (73% to 82%) of adenomas 1 cm or larger, whereas single stool Hemoccult testing in the same asymptomatic patients detected none.”

The L-DNA marker proved to be the most informative component of the assay panel. “It alone detected 61% of neoplasms,” he said.

Bat-26 was positive only in stools of patients with proximally located cancers, “reflecting a site-association well established from tissue studies,” Dr. Ahlquist said. He noted that neither Bat-26 nor p53 were positive for adenomas, “which would be in accord with previous tissue studies reporting that these two markers are rarely expressed before adenomatous transformation to high-grade dysplasia or frank cancer has occurred.”

Dr. Ahlquist concluded: “The results by this multitarget DNA assay approach must be corroborated in larger clinical studies, but the clear implications are for improved cancer prevention and, perhaps, less frequent screening.”