Suppressor of Cytokine Signaling (SOCS) Molecules Are Critical to Balance Oncogenic Signaling Strength in Ph⁺ ALL.

SOCS (suppressors of cytokine signaling) are a family of intracellular proteins, including SOCS1–7 and CISH (cytokine induced SH2 domain protein). They play key roles in negative regulation of cytokine signal transduction, including JAK/STAT pathway. While BCR-ABL1 signaling involves STAT5 in Ph⁺ acute lymphoblastic leukemia (ALL) cells, STAT5 signaling also results in its own feedback inhibition via CISH, SOCS2 and SOCS3. Interestingly, CISH, SOCS2 and SOCS3 are highly expressed in Ph⁺ALL and represent the focus of this study.

Comparing microarray data for human Ph⁺ ALL (n=15) and normal human pre-B cells (n=8), we found that mRNA levels of SOCS2 and CISH were 10-fold (p<0.0001) and 3-fold (p<0.0001) upregulated in Ph⁺ ALL cells compared to normal pre-B cells respectively. As measured by qRT-PCR and Western blot, TKI-treatment rapidly decreased the expression of SOCS2, SOCC3 and CISH in Ph⁺ALL cells, indicating that high expression levels of CISH, SOCS2 and SOCS3 are induced by tyrosine kinase activity. These findings suggest that high levels of CISH/SOCS inhibitors represent a novel aspect of a leukemia-specific gene expression pattern. Importantly, we found that high expression levels of SOCS2 and CISH are predictive of poor outcome in two clinical trials for patients with ALL. In the COG trial (P9906; n=207) for children with high-risk ALL, we found that patients with minimal residual disease (MRD) findings on day 29 had 4- to 7-fold higher mRNA levels of SOCS2, SOCS3 or CISH (n=207; p=0.004), and patients with high expression of SOCS2, SOCS3 or CISH had decreased overall survival rate (p=0.009) and relapse free survival rate (p=0.011) compared to patients with low expression of SOCS2, SOCS3 or CISH. These results collectively indicate that inhibitory signaling from CISH/SOCS2/SOCS3 molecules contributes to the course of disease of human Ph⁺ALL.

To study the function of SOCS2, SOCS3 and CISH in genetic experiments, we transformed pre-B cells from mutant mice. For inducible deletion of Socs3 and Cish, pre-B cells from Socs3^fl/fl or Cish^fl/fl mice were transformed by BCR-ABL1 and gene deletion was induced by 4-OHT-mediated activation of Cre recombinase. Inducible deletion of Socs3 or Cish resulted in upregulation of p-STAT5, increased ROS (reactive oxygen species), accumulation of Arf, p53 and p21, cellular senescence and subsequent cell death of leukemia cells. Studying BCR-ABL1 transformed Cish^fl/fl ALL cells in vivo, we found that genetic deletion of Cish resulted in rapid leukemia regression and prolonged survival of recipient NOD-SCID mice (13 days vs 30 days; n=7; p<0.001). To study the role of Socs2 in Ph⁺ ALL, pre-B cells from SOCS2^−/− and wildtype mice were transformed by BCR-ABL1. Compared to wildtype ALL cells, SOCS2^−/− ALL cells had upregulated p-STAT5 and increased ROS, formed 6-fold fewer colonies in methyl cellulose, underwent G0/G1 cell cycle arrest and cellular senescence. Interestingly, similar observations were made, when a constitutively active mutant of STAT5 (Stat5^CA) was expressed in Ph⁺ ALL cells. Stat5^CA leukemia cells underwent apoptosis within 3 days, which indicates that excessively high levels of STAT5 signaling are toxic to Ph⁺ALL cells.

These results indicate that SOCS family members are critical negative regulators of oncogenic signaling strength in BCR-ABL1 ALL. These findings are of particular relevance, because they identify SOCS molecules as members of a fundamentally novel class of therapeutic targets. The concept of pharmacological perturbation of oncogenic signaling equilibrium in leukemia cells by inhibition (e.g. TKI-treatment) or exaggeration of signaling strength (e.g. blockade of SOCS molecules) may lead to the discovery of multiple additional therapeutic targets and broaden our repertoire of currently available pathways for therapeutic intervention.

No relevant conflicts of interest to declare.