Identification Of Cd44 As A Ras-Mek-Regulated Invasion Receptor That Is Overexpressed In Neoplastic Mast Cells And Triggers Disease Expansion In Advanced Systemic Mastocytosis

CD44, also known as Hermes antigen, is a multifunctional invasion receptor that mediates homing and expansion of normal and neoplastic stem- and progenitor cells in various organs including the bone marrow (BM). Mast cells (MC) and their progenitors also express CD44. However, little is known about the impact and regulation of CD44 in neoplastic cells in systemic mastocytosis (SM). We examined the expression, regulation, and functional role of CD44 in neoplastic cells in SM. As assessed by multi-color flow cytometry, CD34 + /CD38 - stem cells (SC), CD34 + /CD38 + progenitor cells (PC), and KIT + /CD34 - MC invariably expressed CD44 in all SM variants, including patients with indolent SM (ISM, 11/11), SM with associated hematologic neoplasm (SM-AHN, 7/7), aggressive SM (ASM, 3/3), and MC leukemia (MCL, 8/8). Expression of CD44 on SC, PC, and MC increased significantly with the aggressiveness of SM. Moreover, soluble CD44 levels measured in the sera of patients with SM by ELISA were found to correlate with the WHO type of SM. In particular, significantly higher levels of soluble CD44 were measured in advanced SM compared to ISM, cutaneous mastocytosis (CM), or healthy controls (p KIT WT , ROSA KIT D816V , MCPV-1.1, MCPV-1.2, MCPV-1.3, and MCPV-1.4 expressed cytoplasmic and cell surface CD44. To study the mechanisms underlying CD44 expression on MC, we applied various targeted drugs. Incubation with the demethylating agents decitabine (0.1-5 µM) or 5-azacytidine (0.1-5 µM) for 96 hours resulted in a dose-dependent upregulation of CD44 surface expression compared to baseline levels (100%) in all MC lines examined (decitabine, 5 µM: 210±50% in HMC-1.1, 294±76% in HMC-1.2, 236±56% in ROSA KIT WT , 210±54% in ROSA KIT D816V , 242±47% in MCPV-1.1, 179±23% in MCPV-1.2, 207±17% in MCPV-1.3, and 152±5% in MCPV-1.4 cells, p KIT WT , 292±136% in ROSA KIT D816V , 365±55% in MCPV-1.1, 345±106% in MCPV-1.2, 325±87% in MCPV-1.3, and 278±38% in MCPV-1.4 cells, p KIT WT , 31±3% in ROSA KIT D816V , 62±6% in MCPV-1.1, 82±13% in MCPV-1.2, 84±5% in MCPV-1.3, and 87±1% in MCPV-1.4 cells, p KIT WT , and 80±3% in ROSA KIT D816V , 62±5% in MCPV-1.1, 72±4% in MCPV-1.2, 73±4% in MCPV-1.3, and 63±9% in MCPV-1.4 cells, p KIT WT cells with KRAS WT or oncogenic KRAS G12V . In all transduced cell lines, KRAS overexpression resulted in upregulation of CD44 surface expression compared to empty vector (100%) (HMC-1.2: 143±17% with KRAS WT , 249±53% with KRAS G12V , p KIT WT : 170±13% with KRAS WT , 403±64% with KRAS G12V , p In conclusion, CD44 is expressed in neoplastic MC as well as in neoplastic stem- and progenitor cells in patients with advanced SM. Our data also suggest that CD44 is an important mediator of evolution and of malignant spread of neoplastic MC into various organs in SM. Future studies will show whether CD44 can serve as a therapeutic target in patients with advanced SM. Disclosures Hoermann: Gilead: Research Funding; Novartis: Honoraria; Amgen: Honoraria; Ariad: Honoraria. Sperr: Novartis: Honoraria; Amgen: Honoraria, Research Funding. Arock: Agensys, Inc: Research Funding. Valent: Ariad: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Amgen: Honoraria; Deciphera Pharmaceuticals: Research Funding; Celgene: Honoraria, Research Funding.