New molecular road map for CRC

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Karen Titus

April 2017—Molecular testing for colorectal cancer is not for the faint of heart.

While that’s not news to Stan Hamilton, MD—he’s head, Division of Pathology and Laboratory Medicine, and the Frederick F. Becker distinguished chair in cancer research, University of Texas MD Anderson Cancer Center—he was reminded of this fact recently when a friend looked at the multipage molecular pathology report on his own tumor. “He called and basically said, ‘What are you guys doing?’” recalls Dr. Hamilton, noting that his friend, an engineer, is well versed in reading technical reports. “And he was completely befuddled by what he saw.”

Dr. Stan Hamilton (right) at the University of Texas MD Anderson Cancer Center with George Chang, MD, MS, a professor in the Department of Surgical Oncology and chief of the section of colon and rectal surgery. The guideline on molecular biomarkers for the evaluation of colorectal cancer “has brought the quality control aspect [of testing] front and center,” says Dr. Hamilton.

Dr. Hamilton adds, “He asked, ‘Why can’t you make this easier on us patients?’”

And that’s just the report. Left unsaid, but equally valid, is another question: Why can’t molecular testing be easier for clinicians and pathologists, too?

With colorectal cancer, ease is a distant, likely nonexistent, goal. Molecular testing options are complicated to navigate. Even when a molecular portrait of the tumor has emerged, “We don’t have a lot of active agents to choose from,” says Carmen Allegra, MD, chief of oncology and hematology, University of Florida, Gainesville.

Yet the literature continues to grow, and with it come new approaches to using older tests, as well as added data about promising biomarkers. Trying to make sense of it all is a fresh guideline on molecular biomarkers for evaluating CRC, representing the best and the brightest from the CAP, American Society for Clinical Pathology, Association for Molecular Pathology, and American Society of Clinical Oncology (Sepulveda AR, et al. Arch Pathol Lab Med. Epub ahead of print Feb. 6, 2017. doi: 10.5858/arpa.2016-0554-CP).

Unlike previous guidelines from ASCO and the National Comprehensive Cancer Network, says lead author and AMP co-chair Antonia Sepulveda, MD, PhD, that input from four key societies meant unusually heavy emphasis on laboratory testing as well as the more typical guideline coverage of required testing for targeted and conventional therapies. “This has never really been done with such a global scope,” says Dr. Sepulveda, professor and vice chair for translational research, and director, Division of Gastrointestinal Pathology, Department of Pathology and Cell Biology, Columbia University, New York City.

“If you look at the recommendations,” adds Dr. Hamilton, who was the CAP co-chair, “the vast majority of them deal with how to get the testing done. This is not an inconsequential issue. But this is not simply a lab testing guideline. It includes the clinical utility of the tests, and with expert opinion from the medical oncologists who are ordering the tests.”

Another strength, says Dr. Sepulveda, is that the guideline is based on a systematic literature review (with extensive tables to prove it) and levels of evidence, using National Academy of Medicine (formerly Institute of Medicine) standards for developing clinical practice guidelines. This should chase most bias from the recommendations, says Dr. Hamilton. Moreover, even with its 21 statements—“It’s a large and comprehensive guideline,” Dr. Sepulveda says—updates are likely. By following the academy’s standards, the new guideline should remain relevant for some time, she says.

Reflecting the complicated nature of CRC testing, “There was an enormous amount of literature—thousands of papers that we combed through,” says Dr. Allegra, the ASCO co-chair. The major tests for CRC are not new, but their application is evolving—a fact reflected in the guidelines. And in the meantime, researchers continue to find new puzzles in need of solving.

As with the city of Pittsburgh, there are three main rivers to follow in CRC testing: DNA mismatch repair, or MMR, status; BRAF mutation; and RAS mutation.

Testing for MMR, either by immunohistochemistry for the four MMR proteins (MLH1, MSH2, MSH6, and PMS2) or by microsatellite instability DNA-based testing, has evolved from its use in a small group of patients with an inherited disease to a wide variety of clinical decisions, including whether to give postoperative adjuvant therapy and how to treat patients with advanced disease. Oncologists might order that same test for different reasons, says Dr. Hamilton. “That, in fact, is part of the reason we’ve now recommended universal testing in the guideline.”

The first use to emerge was as a marker for Lynch syndrome. MMR is also used as a prognostic marker.

Most recently, researchers have recognized the value of the MSI-high (i.e. high level of microsatellite instability) in patients with advanced disease (about five to six percent of CRC patients) in predicting response to immunotherapy with immune checkpoint inhibiting drugs, specifically pembrolizumab (Le DT, et al. N Engl J Med. 2015;372[26]:2509–2520).

In patients with colorectal cancer, about 20 percent have defects or mutations in one of the DNA repair genes. In about a quarter of those patients, the mutation is based in their germline, the underlying mechanism of Lynch syndrome. As Dr. Hamilton notes, knowing MMR status is crucial not only for managing these patients but also to encourage earlier screening of at-risk family members. For the other 75 percent of patients with mismatch repair deficiency, the mutation is sporadic.