MedicalResearch.com Interview with:
Boris C. Bastian, MD, PhD
Professor of Dermatology and Pathology
Gerson and Barbara Bass Bakar Distinguished Professor in Cancer Research
University of California, San Francisco
Medical Research: What is the background for this study? What are the main findings?
Dr. Bastian: The cost of DNA sequencing has dropped substantially since the initial sequencing of the human genome in 2001. As a result, the most common cancer subtypes have now been sequenced, revealing the pathogenic mutations that drive them. A typical cancer is driven by 5-10 mutations, but we still do not understand the order in which these mutations occur for most cancers.
Determining the order in which mutations occur is challenging for cancers that are detected at a late stage. Melanomas, however, lend themselves to this type of analysis because they are pigmented and found on the surface of the skin, allowing them to be identified early. Sometimes, melanomas are even found adjacent to their remnant precursor neoplasms, such as benign nevi (also known as common moles). We performed detailed genetic analyses of 37 cases of melanomas that were adjacent to their intact precursor neoplasms. We microdissected and sequenced the surrounding uninvolved normal tissue, the precursor neoplasm, and the descendent neoplasm. By comparing the genetic abnormalities in each of the microdissected areas, we were able to decipher the order of genetic alterations for each case.
Our work reveals the stereotypic pattern of mutations as they occur in melanoma. Mutations in the MAPK pathway, usually affecting BRAF or NRAS, occur earliest, followed by TERT promoter mutations, then CDKN2Aalterations, and finally TP53 and PTEN alterations. Benign nevi typically harbor a single pathogenic alteration, whereas fully evolved melanomas harbor three or more pathogenic alterations. We also identified an intermediate stage of neoplasia with some but not all of the pathogenic mutations required for fully evolved melanoma. There has been a longstanding debate whether morphologically intermediate lesions, such as dysplastic nevi, truly constitute biological intermediates or whether they simply represent a gray zone of histopathological assessment. Our data indicates that these neoplasms are genuine biological entities. Finally, we observe evidence of UV-radiation-induced DNA damage at all stages of pathogenesis, implicating UV radiation in both the initiation and progression of melanoma.