The enzyme Arginase ARG plays a role in the

The enzyme Arginase 1 (ARG1) plays a role in the hepatic urea cycle by hydrolyzing L-arginine to L-ornithine and urea [13]. In the context of macrophages and MDSCs, ARG1 expression redirects L-arginine metabolism to abolish cytotoxic nitric oxide production [14], suppress T-cell function [15], [16], as well as increase levels of ornithine, polyamines and proline involved in wound healing [14]. In solid cancers, the presence of expanded ARG1-expressing MDSC and tumour-associated macrophage populations have been associated with decreases in patient survival [17], [18], [19]. Recent studies in chronic myeloid leukemia (CML) have demonstrated the presence of ARG1-expressing MDSCs in this neoplasm [20], [21]. However, little is known about ARG1 in MDS and CMML. Interrogating the immune environment and its connection to genetic lesions has the potential to reveal leukemogenic events that could be exploited for the development of novel therapeutics [3]. We previously reported that an Inpp5d-deficient murine model of low-grade MDS/CMML demonstrated pathological macrophage skewing characterized by overexpression of immunosuppressive and pro-tumorigenic effectors such as Arg1 [14]. We have also identified overexpression of this target in hyper-inflammatory macrophages from the MDS/CMML Tet2-knockout mouse model [22] and the advantage Tet2-deficient progenitor Glucose Uptake Fluorometric Assay Kit may have in such an evironment [23]. Here, we identify overexpression of ARG1 in human MDS and CMML BM samples with lower-grade features. We further link ARG1 overexpression with mutations in the epigenetic regulators TET2 and DNMT3A, both of which are early driver mutations that likely lead to alterations in the immune microenvironment, setting the stage for myeloid cancer initiation.
Materials and methods
Results
Discussion Work done in our laboratory has shown that Tet2-knockout macrophages variably overexpress Arg1 in a murine model of MDS/CMML [22]. Moreover, high macrophage Arg1 expression in the absence of Tet2 correlates with impaired resolution of inflammation by macrophages and a corresponding increase in plasma levels of proinflammatory cytokines and chemokines [22]. These findings are thought to reflect an inflammatory phenotype similar to what has been seen in the early stages of human MDS/CMML [3]. We postulated that overexpression of Arg1 is related to an inflammatory phenotype, with Arg1 induced as part of a homeostatic response [22]. Moving into the human context, we wanted to determine whether a connection existed between development of MDS/CMML and ARG1 overexpression. Additionally, a potential association between the presence of human TET2 mutations and high ARG1 protein expression was also explored. In the current study, we demonstrated high levels of arginase activity and ARG1 staining in studies performed on MDS/CMML BM samples. Additionally, we showed that this phenomenon was associated with the presence of earlier disease (lower-grade/lower-risk/depth of cytopenias) and TET2/DNMT3A mutations. Links between overexpression of ARG1 and solitary TET2 or DNMT3A mutations would be interesting to examine with greater sample sizes. While our findings that high ARG1 levels were connected to either DNMT3A (a methyltransferase) or TET2 (a demethylase) lesions initially appeared counterintuitive, recent evidence indicates that these two proteins have some overlapping functionalities. In one study, murine Tet2 and Dnmt3a were shown to act cooperatively at specific sites within the genome [33]. Supporting this hypothesis, upon mining published supplemental data from a recent study of Dnmt3a-deficient mice [34], we noted upregulation of Arg1, Ccr5, Tgfbi, Socs3, Bst2, Ccl6, Ifit2, Ccl2, Cxcl1 and Ccl7 in Dnmt3a-knockout mouse peritoneal macrophages, all of which we also found upregulated in Tet2-knockout PMΦs [22]. If this is the case in humans, it follows that DNMT3A- and TET2-mutant cases may share some similar aberrant gene expression patterns [33] involving ARG1 dysregulation.