STAT5 Signaling Promotes Resistance to Ivosidenib in IDH1-Mutated AML
Authors: Liu Alex , Cathelin Severine , Chan Steven M 
Published: November 2021

Abstract
Mutations in isocitrate dehydrogenase 1 (IDH1) promote leukemic transformation through the production of an oncometabolite, 2-hydroxyglutarate (2-HG). Ivosidenib is an inhibitor of mutant IDH1 approved for the treatment of IDH1-mutated AML. Treatment with ivosidenib can induce terminal differentiation of leukemic blasts via suppression of 2-HG. However, ivosidenib as a single agent has limited efficacy, highlighting the need to better understand the mechanisms of drug resistance.
The study of drug resistance mechanisms has been hindered by the lack of IDH1/2-mutated AML cell lines models. To address this issue, we derived an Idh1-mutated AML cell line from bone marrow cells of a murine AML model generated by crossing mice expressing mutant Idh1R132H with mice expressing mutant Npm1 (Npm1c). This cell line (henceforth termed "OCI-mIDH1/N") undergoes partial myeloid differentiation in response to ivosidenib treatment in vitro.
The establishment of OCI-mIDH1/N enabled us to perform a genome-wide CRISPR knockout screen to identify genes that upon inactivation, increased the differentiation response to ivosidenib. Through this screen, we identified C-type lectin member 5a (Clec5a) as one of the top hits. Clec5a encodes a cell surface receptor that signals through the intracellular spleen tyrosine kinase (SYK). To confirm this hit, we generated Clec5a knockout clones of OCI-mIDH1/N cells. Consistent with results of the screen, ivosidenib treatment induced higher levels of Gr-1, a myeloid differentiation marker, on Clec5a-/- cells than on parental Clec5a+/+ cells. Next, we investigated the role in SYK. Clec5a-/- cells had lower levels of pSYK compared with Clec5a+/+ cells, and overexpression of Syk in Clec5a-/- cells reversed their sensitization to ivosidenib. Furthermore, direct inhibition of SYK with fostamatinib was sufficient to sensitize Clec5a+/+ cells to ivosidenib. Our findings show that CLEC5A-SYK signaling promotes resistance to ivosidenib-induced differentiation.
Mechanistically, we found that CLEC5A-SYK signaling drives ivosidenib resistance through STAT5 dependent expression of self-renewal genes in the HOX family. Clec5a-/- and Syk-/- OCI-mIDH1/N cells exhibited lower levels of STAT5 activation compared with wildtype cells. Furthermore, SYK inhibitor treatment downregulated pSTAT5 and decreased STAT5 occupancy at HOX gene clusters. Functionally, overexpression of constitutively active STAT5 in Clec5a-/- and Syk-/- cells reversed their heightened sensitivity to ivosidenib and increased the expression of key HOX self-renewal genes. To determine the clinical relevance of these findings, we analyzed two independent RNA-seq datasets from patients treated with IDH inhibitors (Quek et al., 2018; Wang et al., 2021). Gene set enrichment analysis revealed that poor responders expressed significantly higher levels of STAT5 target genes compared with responders in both datasets.
Mutations in the receptor tyrosine kinase (RTK) pathway have previously been shown to be associated with resistance to IDH inhibitors. Given that these pathways signal through STAT5, we hypothesized that the mechanism by the RTK mutations confer ivosidenib resistance is through STAT5 activation. To test this hypothesis, we ectopically expressed KRASG12D or PTPN11E76G in OCI-mIDH1/N cells. Expression of the oncogenes increased pSTAT5 and conferred resistance to ivosidenib-induced differentiation. Importantly, knockdown of Stat5 completely restored their sensitivity to ivosidenib, in support for our hypothesis.
Next, we tested if the STAT5 inhibitor pimozide synergizes with ivosidenib to treat patient-derived xenograft (PDX) models of human IDH1-mutated AML. In two PDX models where ivosidenib alone induced moderate differentiation, the addition of pimozide to ivosidenib significantly increased the expression of myeloid differentiation markers on AML cells. In two PDX models where ivosidenib alone induced no differentiation, pimozide alone was sufficient to induce profound differentiation of AML cells. Collectively, these results suggest that ivosidenib resistant cells shift their dependence from 2-HG to STAT5 to maintain their undifferentiated state.
In summary, our findings demonstrate that STAT5 is a critical mediator of resistance to ivosidenib, and combination therapy with pimozide and ivosidenib is a promising therapeutic approach for IDH1-mutated AML.

Disclosures
Chan: BMS: Research Funding; AbbVie: Research Funding.