Melanoma causes the majority of skin cancer-related deaths. The development of melanoma is a multi-step process in which melanocytes mutate and proliferate uncontrollably, producing a tumor of skin cells. Properly understanding the relationship between melanocytes, neighboring keratinocytes, and immune cells is essential to the advancement of diagnostic and prognostic tools and therapeutic targets. However, the biomarkers of early melanoma evolution and their origin within the tumor microenvironment are poorly defined and largely undiscovered.
To help unravel the mystery, researchers set out to identify genes that can serve as cell type-specific biomarkers for melanoma development. The study was published in the Journal of Investigative Dermatology (2022; https://doi.org/10.1016/j.jid.2021.06.041).
Using a discovery-based approach, the team used high-plex spatial RNA profiling to capture distinct gene expression patterns across cell types during melanoma development. In particular, they analyzed the expression of >1,000 RNAs in 134 regions of interest (each 200 [mu]m in diameter) enriched for melanocytes, neighboring keratinocytes, or immune cells. The issues examined came from patient-derived formalin-fixed, paraffin-embedded tissue sections from 12 melanocytic tumors, ranging from benign to malignant, using a digital spatial profiler. This approach allows studying the expression of hundreds or thousands of genes without disrupting the native architecture of the tumor.
The most remarkable observation was that damage-associated molecular pattern (DAMP) S100A8, which is a known melanoma marker thought to be expressed by immune cells, was enriched in the keratinocytes within the tumor microenvironment during melanoma growth. Immunohistochemistry (IHC) in a cohort of 252 melanomas revealed prominent keratinocyte-derived S100A8 expression in melanoma, but not in benign tumors, and established the same pattern for S100A8's binding partner S100A9. Binary logistic regression model demonstrated increased S100A8 IHC score significantly associated with invasive melanoma tumor type (OR=2.49, 95% CI: 1.93-3.21), and it remained significant after adjusting for sex, anatomic site, and age.
In an interview with Oncology Times, corresponding author of the study, Maija Kiuru, MD, PhD, explained the significance and clinical importance of their study. She is a practicing dermatologist and Associate Professor of Clinical Dermatology and Pathology at UC Davis Health.
Oncology Times: How are melanomas typically detected and what are some of the challenges when it comes to the early diagnosis of melanoma?
Kiuru: "Melanoma is typically detected by a clinical exam and the diagnosis is confirmed via a histopathological exam under the microscope. Most commonly, melanomas are irregular brown growths that show asymmetry, irregular borders, variable colors, large size or diameter, and change or evolution over time (the ABCDEs of melanoma). Often the key is that melanoma looks different than the patient's other moles ("the ugly duckling" sign).
"Early diagnosis is critical in improving outcomes of melanoma. However, one of the challenges is that melanocytic nevi, which are benign neoplasms composed of melanocytes, may act as mimics of melanoma both clinically and histopathologically. This may result in many biopsies being performed to exclude melanoma and undertreatment or overtreatment of melanoma."
Oncology Times: Are there any particular types of mutations that most commonly lead to melanomas?
Kiuru: "The most common driver mutation leading to melanoma is in the BRAF gene, present in approximately 50 percent of melanomas. Other commonly mutated genes include NRAS and NF1.
Oncology Times: What are the clinical implications of this study?
Kiuru: "In our study, using high-plex spatial gene expression profiling in situ without disrupting the native tumor architecture, we identified multiple markers of melanoma, expressed by various cell types within the tumor and its microenvironment, including melanocytes, immune cells, and keratinocytes.
"One such gene was S100A8, expressed by the keratinocyte microenvironment of melanoma but not of nevi, the benign melanocytic neoplasms. This underscores the importance of the interplay between the tumor and its microenvironment, as well as the role of the keratinocytes in tumor development and progression, and highlights the utility of spatial transcript profiling in tumor biomarker discovery. Novel and improved biomarkers can be utilized to diagnose melanoma more accurately, leading to improved patient outcomes."
Oncology Times: What types of studies still need to be conducted to further address the interplay between keratinocytes and melanocytes during melanomagenesis?
Kiuru: "In addition to functional assays in vitro and in vivo in mice, the next step is to obtain more granular gene expression data at a single-cell level. Fortunately, there are many technological advances in the field that allow high-plex single-cell analysis, including in archival tumor material, which is critical in the study of nevi and early melanoma, which are typically small tumors and less well-studied in prior single-cell RNA sequencing experiments."
Dibash Kumar Das is a contributing writer.