Single-cell transcriptomics of Marjolin ulcers

Marjolin's ulcer is a a rare and aggressive cutaneous malignancy that can arise on previously injured skin, established scars, and chronic non-healing wounds. It is most often found in burn scars, but it can also occur in other types of wounds, including venous stasis ulcers, pressure sores, and vaccination sites. The most common histological type of Marjolin’s ulcers is squamous cell carcinoma, however basal cell carcinomas, malignant melanomas, and sarcomas have also been reported. All parts of the body could potentially be affected yet the lower extremities are the anatomic sites most commonly involved. While SCCs commonly have a metastasis rate of 0.5 to 3.0 percent, those arising from burn scars metastasize at a rate in excess of 30 percent. The 5-year survival after a diagnosis of Marjolin’s ulcer was found to be 50 percent. The transformation to a malignancy can occur either chronically, over a period of more than 35 years, or occasionally within a year of the original injury. The exact mechanism of this transformation is not fully understood, but it is thought to be related to chronic keratinocyte dysfunction during the healing process of severe burn wounds. Surgical excision is the main treatment for Marjolin's ulcer and provides the best chance of survival. In a previous study, we investigated the bulk transcriptional changes that lead to Marjolin's ulcer by comparing global gene expression changes between squamous cells present in a squamous cell carcinoma versus those present within Marjolin's ulcer (MU) (Sinha et. al. JBCR 2017). This imparted novel insights into mechanisms underlying divergent clinical features of these cutaneous cancers. The goal of our current study is to characterize a new case of Marjolin's ulcer in a patient under our care by analyzing the cell types, their frequencies, and their individual transcriptional responses within a burn scar versus within Marjolin's ulcer. To achieve this, we conducted single-cell RNA sequencing on two excised tissues: 1. a sample from the center of the tumor (tumor core), and 2. another sample from the margin of tumor-free scar tissue. During surgery, we collected two full-thickness punch biopsies from the resected tissue: one from the center of the tumor (tumor core) and one from the margin of tumor-free scar tissue. To examine molecular changes within specific cell compartments, we isolated single-cells from both biopsies and processed according to 10X Genomics Chromium Single Cell 3’ Reagent Guidelines v3 Chemistry as per the manufacturer’s protocol. In brief, single cells were sorted based on forward versus side scatter gating into 0.1% BSA–PBS and partitioned into Gel Bead-In-EMulsions (GEMs) using 10x GemCodeTM Technology. Next-Generation Sequencing was performed using the Illumina NovaSeq S2 Flow cells. All raw FASTQs were aligned to the human reference genome generated using cellranger mkref pipeline. The resulting gene barcode matrix was processed using Seurat R toolkit.