To highlight the impact of cytopathology on patient management and treatment Edited by Martha B. Pitman, MD

Clinician’s Corner Role and Complexity of Next-Generation Sequencing in Melanoma Michael A. Davies, MD, PhD, Department of Melanoma Medical Oncology, Department of Systems Biology, The University of Texas MD Anderson Cancer Center, Houston, Texas. Michael A. Davies, MD, PhD, is an associate professor of the Departments of Melanoma Medical Oncology and Systems Biology at The University of Texas MD Anderson Cancer Center. He is a physician-scientist whose research focus is the regulation and clinical significance of oncogenic signaling networks in melanoma. Dr. Davies has been the principal investigator of both individual and team science grants from the National Cancer Institute, the American Society of Clinical Oncology, the Melanoma Research Alliance, and the Melanoma Research Foundation, and he has led multiple clinical trials. He is a member of the Melanoma Research Foundation Breakthrough Consortium and the International Melanoma Working Group, and is the codirector of MD Anderson Cancer Center’s MelCore, one of the largest clinically annotated melanoma tissue banks in the world.

Melanoma is the most aggressive of the common forms of skin cancer. Whole exome sequencing analyses have shown that cutaneous melanoma has one of the highest mutation rates of all cancers. Indeed, the first whole genomic sequencing analysis of a melanoma identified more than 33,000 somatic changes.1 The overwhelming majority of the mutations detected in melanoma are likely nonspecific changes induced by the DNA-damaging effects of ultraviolet radiation. Thus, the challenge in melanoma is not to identify whether mutations are present but instead to determine which mutations are significant. Despite this challenge, molecular testing is now the standard of care in this disease because of the validation of several oncogenic drivers and the development of effective, personalized targeted therapy approaches for them.2 To date, the most significant molecular alterations in melanomas are missense mutations in BRAF. The BRAF gene encodes a serine-threonine kinase that is a key component of the RAS-RAF-MAPK signaling pathway (where MAPK indicates mitogen-activated protein kinase). Point mutations in BRAF are detected in 50% of cutaneous melanomas, and this makes them the most common oncogenic mutations detected in this disease. More than 90% of these mutations result in substitutions at the V600 residue of the BRAF protein, most commonly to either glutamic acid (V600E; 70%) or lysine (V600K; 20%). These substitutions increase the catalytic activity of the BRAF protein more than 200-fold and result in constitutive activation of the RAS-RAF-MAPK signaling pathway. Other missense mutations in BRAF have variable effects on the catalytic activity of BRAF, but it appears that most or all of these mutations also activate the same MAPK pathway. Testing for the BRAFV600 mutation is now an essential component of the management of stage IV melanoma because of the

Cancer Cytopathology

June 2015

approval of several agents specifically for patients with this event.3 Vemurafenib (approved in 2011) and dabrafenib (2013) are small molecule inhibitors of the BRAF protein. A key to their clinical success is the fact that they not only are selective for BRAF over other kinases but also have a higher affinity for the V600 mutant BRAF proteins than the wild-type BRAF protein. In phase 3 clinical trials, both vemurafenib and dabrafenib achieved Response Evaluation Criteria in Solid Tumors clinical responses in 50% and at least some degree of tumor shrinkage in 90% of metastatic melanoma patients with BRAFV600 mutations. More recently, combined treatment with dabrafenib and the mitogen-activated protein kinase kinase inhibitor trametinib was shown to achieve significant improvements in clinical response rates (responses: 75%; disease control: 100%), response duration, and overall survival in comparison with single-agent BRAF inhibition, leading to regulatory approval of this regimen in 2014. Because single-agent chemotherapies achieve clinical responses in only 5% to 10% of patients with metastatic melanoma, these results rapidly established targeted therapies as a new standard of care for patients with a BRAFV600 mutation. They also provided a powerful proof of concept for personalized targeted therapy approaches nowapproved for this disease. Although most clinical testing of the now-approved agents was performed in patients with BRAFV600E mutations, patients with mutations resulting in other substitutions at the V600 site also respond to the BRAF inhibitors. However, preclinical studies unexpectedly showed that the V600 mutant–selective BRAF inhibitors increase the growth of cancer cell lines that do not have a BRAFV600 mutation. Thus, the approved BRAF inhibitors represent the ultimate in personalized therapy because they provide clinical benefit to virtually all patients with the target BRAFV600 mutation, but they may actually harm patients 329

Clinician’s Corner

without it. Because of this critical difference, molecular testing is now essential for the management of patients with advanced melanoma. Notably, the existing data support the idea that the BRAFV600 mutation status is highly concordant (95%) between primary tumors and metastases. As BRAFV600 mutations are detected in 80% of benign nevi, this is likely due to the fact that oncogenic activation of BRAF is a very early event in melanomagenesis. However, there are cases in which a BRAFV600 mutation is detected in the metastasis of a patient reported to have BRAFV600 wild type disease on the basis of testing of the primary tumor. Such discrepancies can occur because melanoma patients can have multiple primary tumors, including some primaries that may regress and be undetectable despite eventually giving rise to metastases (known as unknown primary melanomas). Another potential cause of the discrepancies is technical. Some commercial testing platforms, such as the Cobas test, have very high sensitivity (>95%) for detecting the BRAFV600E mutation but miss a significant percentage of the mutations (up to 30%) that cause other clinically significant substitutions at V600. More recently, immunohistochemical assays have been developed that can detect the presence of the BRAF V600E protein in tumors. These assays can have significant clinical utility, not only because they are amenable to lesions too small for sequencing but also because they may be performed very rapidly. Because the mutant-selective BRAF inhibitors can provide clinical benefit within days (sometimes even within 24 hours), this rapid turnaround can be critical in the management of patients with widespread symptomatic disease. To date, the immunohistochemical assays detect only the V600E mutant protein, and thus sequencing-based analyses should be pursued even in the setting of a negative immunohistochemical assay. To facilitate ancillary testing of tumor deposits of metastatic melanoma, all fine-needle aspirates should, if possible, procure sufficient tissue for cellblock preparation. Mutations in many genes other than BRAF are prevalent in melanoma and may also be important for clinical management. Hotspot mutations in NRAS that encode conserved residues (G12, G13, and Q61) frequently affected in the RAS gene family are detected in 20% of cutaneous melanomas. Hotspot NRAS mutations and BRAFV600 mutations are virtually mutually exclusive in melanoma patients who have not been treated with a BRAF inhibitor. Thus, the detection of an NRAS mutation greatly increases the confidence in the results of a negative molecular test for a BRAFV600 mutation. In addition to this diagnostic benefit, multiple clinical trials are open specifically for metastatic melanoma patients with NRAS mutations. Somatic mutations in the KIT gene have also been detected in melanoma. KIT mutations are rare (

Role and complexity of next-generation sequencing in melanoma.

Role and complexity of next-generation sequencing in melanoma. - PDF Download Free
97KB Sizes 1 Downloads 3 Views