High expression of ABCG2 induced by EZH2 disruption has pivotal roles in MDS pathogenesis.

Both proto-oncogenic and tumor-suppressive functions have been reported for enhancer of zeste homolog 2 (EZH2). To investigate the effects of its inactivation, a mutant EZH2 lacking its catalytic domain was prepared (EZH2-dSET). In a mouse bone marrow transplant model, EZH2-dSET expression in bone marrow cells induced a myelodysplastic syndrome (MDS)-like disease in transplanted mice. Analysis of these mice identified Abcg2 as a direct target of EZH2. Intriguingly, Abcg2 expression alone induced the same disease in the transplanted mice, where stemness genes were enriched. Interestingly, ABCG2 expression is specifically high in MDS patients. The present results indicate that ABCG2 de-repression induced by EZH2 mutations have crucial roles in MDS pathogenesis.

SETBP1 mutations drive leukemic transformation in ASXL1-mutated MDS.

Mutations in ASXL1 are frequent in patients with myelodysplastic syndrome (MDS) and are associated with adverse survival, yet the molecular pathogenesis of ASXL1 mutations (ASXL1-MT) is not fully understood. Recently, it has been found that deletion of Asxl1 or expression of C-terminal-truncating ASXL1-MTs inhibit myeloid differentiation and induce MDS-like disease in mice. Here, we find that SET-binding protein 1 (SETBP1) mutations (SETBP1-MT) are enriched among ASXL1-mutated MDS patients and associated with increased incidence of leukemic transformation, as well as shorter survival, suggesting that SETBP1-MT play a critical role in leukemic transformation of MDS. We identify that SETBP1-MT inhibit ubiquitination and subsequent degradation of SETBP1, resulting in increased expression. Expression of SETBP1-MT, in turn, inhibited protein phosphatase 2A activity, leading to Akt activation and enhanced expression of posterior Hoxa genes in ASXL1-mutant cells. Biologically, SETBP1-MT augmented ASXL1-MT-induced differentiation block, inhibited apoptosis and enhanced myeloid colony output. SETBP1-MT collaborated with ASXL1-MT in inducing acute myeloid leukemia in vivo. The combination of ASXL1-MT and SETBP1-MT activated a stem cell signature and repressed the tumor growth factor-ß signaling pathway, in contrast to the ASXL1-MT-induced MDS model. These data reveal that SETBP1-MT are critical drivers of ASXL1-mutated MDS and identify several deregulated pathways as potential therapeutic targets in high-risk MDS.

TGF-ß-induced apoptosis of B-cell lymphoma Ramos cells through reduction of MS4A1/CD20.

Transforming growth factor-ß (TGF-ß) exhibits growth inhibitory effects on various types of tumor cells, including B-cell lymphoma cells. In the present study, the role of TGF-ß in the survival of Epstein-Barr virus-negative B-cell lymphoma Ramos cells was investigated. As TGF-ß-induced apoptosis of Ramos cells in vitro and in vivo, we attempted to identify novel target gene(s) responsible for their survival. Oligonucleotide microarray analysis and chromatin immunoprecipitation revealed that Smad proteins directly regulated the transcription of membrane-spanning 4-domains, subfamily A, member 1 (MS4A1), also known as CD20, in Ramos cells upon TGF-ß stimulation. In addition, immunohistochemical analysis using clinical samples from B-cell lymphoma patients showed an inverse correlation between the expression of MS4A1/CD20 and phosphorylation of Smad3. Although knockdown of MS4A1/CD20 in Ramos cells resulted in an increase of apoptotic cells, Ramos cells stably expressing MS4A1/CD20 were resistant to TGF-ß-induced apoptosis. This suggests that MS4A1/CD20 is responsible for TGF-ß-induced apoptosis of B-cell lymphoma cells. Moreover, downregulation of MS4A1/CD20 by TGF-ß attenuated the effects of the monoclonal anti-MS4A1/CD20 antibody, rituximab, on Ramos cells. Our findings suggest that the sensitivity of B-cell lymphoma cells to rituximab may be affected by TGF-ß signaling.