Session: 631. Chronic Myeloid Leukemia: Biology and Pathophysiology, excluding Therapy: Poster III
Hematology Disease Topics & Pathways:
Diseases, CML, Myeloid Malignancies
The family of signal-transducing adaptor proteins (STAPs), which includes STAP-1 and STAP-2, has been implicated in various intracellular signaling pathways. In 2003, we cloned STAP-2 as a c-fms interacting protein and reported that STAP-2 binds to BCR-ABL and enhances activity, leading to the activation of downstream molecules such as ERK, STAT5, BCL-xL, and BCL2. STAP-1 was cloned as a c-kit interacting protein from a hematopoietic stem cell library, but it is unknown whether STAP-1 plays a role in CML. Given the structural homology between STAP-1 and STAP-2 and the hematopoietic expression of STAP-1, we hypothesized that STAP-1 might contribute to the leukemogenesis of CML.
A STAP-1-deficient (KO) CML mouse model was developed. To generate this model, lineage (Lin)− Sca-1+ c-Kithigh (LSK) fraction isolated from bone marrow (BM) cells was infected with a retrovirus carrying BCR-ABL1 and GFP and subsequently transplanted into congeneric recipients. STAP-1 KO CML mice showed significantly longer survival than WT CML mice and displayed less severe splenomegaly and lung hemorrhages compared with WT mice. In recipient BM, absolute numbers of STAP-1 KO LSCs (GFP+ LSK cells) were significantly lower than WT LSCs. In the colony-forming assay, STAP-1 KO LSCs generated fewer colonies compared to WT LSCs. Using flow cytometric analysis, we found that STAP-1 KO LSCs had a higher apoptotic rate than WT LSCs. These findings suggest that the suppression of apoptosis induced by STAP-1 mediates longer survival of LSCs. To further understand the effects of STAP-1, we performed a gene expression analysis using RNA-sequence (RNA-seq) and compared WT and STAP-1 KO CML LSCs. When canonical pathways were analyzed with Ingenuity Pathway Analysis, various pathways associated with inflammatory cytokines were observed to be regulated in STAP-1 KO CML LSCs. Changes in mRNA expression, including that of SOS1, SOS2, FOXO3, FASLG, NFKB2, and BCL-xL, indicated that the PTEN signaling pathway, known to play a tumor suppressive role in CML, was significantly activated by STAP-1 KO (p=1.096E-3, activation Z-score=2.611). The pathway related to JAK/STAT signaling was also affected (p=2.04E-5, activation Z-score=-3.286). Downstream genes in the JAK/STAT signaling pathway, such as STAT5B and BCL-xL, were downregulated more than 2-fold in STAP-1 KO LSCs, suggesting that the deletion of STAP-1 inhibits the expression of STAT5-targeted anti-apoptotic protein and induced apoptosis of CML LSCs. To confirm the results of the RNA-seq experiment, an intracellular flow cytometric assay with CML Lin− cells was conducted. The frequency of cells positive for phosphorylated STAT5 was reduced for STAP-1 KO compared with that for WT. Quantitative PCR with CML LSCs confirmed the downregulation of BCL2 and BCL-xL, which are STAT5-targeted anti-apoptotic genes, in STAP-1 KO CML LSCs.
In conclusion, we show that STAP-1 plays a crucial role in the maintenance of CML LSCs using a murine model of CML. STAP-1 deficiency results in the reduction of phosphorylated STAT5, downregulation of anti-apoptotic genes BCL-2 and BCL-xL, and induced apoptosis of CML LSCs. These findings suggest that STAP-1 and related signaling pathways could be potential therapeutic targets for CML LSCs.
Disclosures: Ichii: Celgene K.K.: Speakers Bureau; Kowa Pharmaceutical Co.,LTD.: Speakers Bureau; Novartis Pharma K.K.: Speakers Bureau. Shibayama: Fujimoto Pharmaceutical: Honoraria, Research Funding; Takeda Pharmaceutical Co.,LTD.: Honoraria, Research Funding; Celgene K.K.: Honoraria, Research Funding; Jansen Pharmaceutical K.K: Honoraria; Ono Pharmaceutical Co.,LTD: Honoraria, Research Funding; Novartis Pharma K.K.: Honoraria, Research Funding; Mundipharma K.K.: Honoraria, Research Funding; Bristol-Meyer Squibb K.K: Honoraria, Research Funding. Oritani: Novartis Pharma: Speakers Bureau. Kanakura: Alexion Pharmaceuticals, Inc.: Consultancy, Honoraria, Research Funding.
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