Socs2 Is Dispensable for BCR/ABL1-Induced Chronic Myeloid Leukemia-Like Disease in Mice and for Normal Hematopoietic Stem Cell Function,
Authors: Hansen Nils , Agerstam Helena , Wahlestedt Martin , Ehinger Mats , Johnels Petra , Ishiko Jun , Melo Junia V. , Alexander Warren S , Bryder David , Järås Marcus , Fioretos Thoas 
Published: October 2019

Abstract
Abstract 3743

Background:
The P210 BCR/ABL1 fusion protein initiates signaling through several down stream pathways such as STAT5, PI3K, AKT, RAS, and WNT. However, only few down stream mediators have so far been thoroughly studied in vivo in the context of BCR/ABL1-mediated induction of CML-like disorder in mice deficient for the specific genes. Suppressor of cytokine signaling 2 (SOCS2) is known as a feedback inhibitor of cytokine signaling, a negative regulator of the STAT5 pathway, and is markedly upregulated in primary bone marrow cells from patients with chronic myeloid leukemia (CML). However, it has not been clear whether SOCS2 is involved in BCR/ABL1 induced cell transformation or whether it is important for normal hematopoietic stem cell (HSC) function.

Methods:
To evaluate the stem cell function of Socs2, HSCs from wild type and Socs2 deficient mice were competitively transplanted into lethally irradiated recipients. Blood chimerism and lineage distribution were analyzed by flow cytometry at 4 and 18 weeks after transplantation. To investigate the potential role of Socs2 in CML, c-kit enriched Socs2(−/−) and wild type cells were transduced with a BCR/ABL1 expressing retroviral vector and transplanted into lethally irradiated recipients. The in vivo development of malignant disease was analyzed by peripheral blood cell counts and, upon euthanization, by flow cytometry and histopathology. Socs family gene expression was assessed by Q-RT/PCR and Stat5 phosphorylation by Western blotting on BCR-ABL1 expressing cells.

Results:
Although Socs2 was previously found to be upregulated in long-term repopulating HSCs by gene expression arrays, Socs2 deficient HSCs were indistinguishable from wild type HSCs when challenged in competitive bone marrow transplantation. Furthermore, when expressing BCR/ABL1, both Socs2 deficient and wild type cells induced a CML-like disease with an average survival of three weeks after transplantation. The leukemic mice suffered from elevated white blood cell counts and splenomegaly. To investigate if other Socs family members were upregulated to compensate for the Socs2 deficiency, we compared the expression of the Socs gene family members in Socs2(−/−) with wild type, leukemic cells. The expression levels of all other Socs genes were similar between Socs2(−/−) and Socs2 wt cells, suggesting that compensatory mechanisms of other Socs genes do not account for the lack of Socs2 function. In addition, the finding that the phosphorylation of Stat5 was unaffected in Socs2(−/−) cells indicates that BCR/ABL1-induced Stat5-phosphorylation is insensitive to Socs2 levels.

Conclusions:
Collectively, our results clarify that Socs2 is dispensable for normal HSC function and for BCR/ABL1-induced CML-like disease.

Disclosures:
No relevant conflicts of interest to declare.