Molecular Alterations in Adult Histiocytic Neoplasms

Introduction Histiocytic neoplasms are rare disorders of the monocyte-macrophage-dendritic cell lineage that pose significant management challenges to the clinicians. Over the last five years, there has been emerging data on the role of MAPK/ERK pathway (RAS-RAF-MEK-ERK) in their pathogenesis. In this study, we report the molecular alterations among patients with histiocytic neoplasms seen at our institution. Methods We pursued target-capture next generation sequencing (NGS) on tissue specimen of patients with various histiocytic neoplasms- Erdheim-Chester disease (ECD), Langerhans cell histiocytosis (LCH), Rosai-Dorfman disease (RDD), histiocytic sarcoma (HS), and Langerhans cell sarcoma (LCS). We used three NGS platforms, Tempus® xO (1,714 genes), Tempus® xT (596 genes), or Foundation One® CDX (324 genes). All platforms used formalin-fixed paraffin-embedded tumor tissue specimens for detection of substitutions, insertion and deletion alterations, and copy number alterations, and gene rearrangements. The Tempus assays also utilized matched blood or saliva samples for germline DNA sequencing in addition to whole-transcriptome RNA sequencing. The tumor cellularity threshold for pursuing RNA sequencing was 30% so the test was not pursued specimens for lower cellularity. Results We included 31 cases with histiocytic neoplasms in this study. The disease distribution was as follows: ECD (n=6), LCH (n=7), RDD (n=12), ECD-RDD overlap (n=3), HS (n=2), and LCS (n=1). Among the 13 patients with ECD and LCH, BRAF-V600E mutations were found in 8 (62%) cases. We found several novel DNA alterations in this study in all disease groups: ECD (BRAFV471F), LCH (MAP2K1 in-frame deletion), and RDD (frameshift mutations of DNMT3A, SMARCA4, KMT2A, and CDC73 truncation). In one LCH patient without any DNA alterations, we found MAPK1 and MAP2K1 overexpression on RNA sequencing. We also had two cases with HS, both of which were found to have KRAS exon 3 mutations. One of these cases had concomitant JAK2V617F+ myelofibrosis and the tissue specimen for HS demonstrated TET2 and SRSF2 mutations as well. Our cohort included one patient with multifocal lymph node LCS that demonstrated NRAS and TP53 mutations, who did not respond to MEK-inhibitor therapy. Alterations in epigenetic regulators (TET2, DMNT3A, KMT2A, ASXL1) were found in 5 (16%), potentially suggesting clonal hematopoiesis in a subgroup of histiocytosis. Conclusions Histiocytic neoplasms demonstrate diverse somatic oncogenic alterations. In our study, we noted several novel alterations beyond the MAPK-ERK pathway in RDD that may point toward other potentially pathogenic mechanisms for these rare disorders. In cases where the DNA sequencing does not reveal pathogenic alterations, transcriptome analysis may aid in the diagnosis and management. OffLabel Disclosure: Trametinib use for Langerhans cell sarcoma