Combined JAK/STAT5A and BCR-ABL Inhibition Impairs Blast Crisis Chronic Myeloid Leukemia Stem Cell Self-Renewal
Authors: Recart Angela C Court , Goff Daniel J , Sadarangani Anil , Mason Cayla N , Shih Alice Y. , Wall Russell , Leu Heather S. , Ma Wenxue , Chun Hye-Jung , Marra Marco A. , Barrett Christian L , Frazer Kelly A , Jamieson Catriona 
Published: October 2019

Abstract
Abstract 910
In blast crisis transformation of CML (BC CML), the leukemia stem cells (LSC), via the acquisition of both enhanced survival and self-renewal capacity, become increasingly resistant to BCR-ABL targeted tyrosine kinase inhibition and thus often contribute to relapse after treatment, pointing to the need for alternative therapeutic strategies and a better understanding of the molecular mechanisms underlying disease progression.
Janus kinase 2 (JAK2) plays an important role in BCR–ABL + cell survival and has profound effects on self-renewal and lineage commitment of normal and leukemic hematopoietic stem cells, through the activation of the transcription factor signal transducer and activator of transcription 5 (STAT5).
To determine if JAK/STAT signaling pathway activation is related to CML progression, LSC from human Chronic Phase (CP CML) and BC CML samples were sorted using FACS Aria (Lin-CD34+CD38+) and analyzed using splice-isoform specific q-RT-PCR. Our results showed that, compared to CP CML, BC LSC harbor enhanced mRNA expression of BCR-ABL, JAK2 and STAT5A isoforms, confirming that progression of CP to BC, in CML LSC, is marked by activation of JAK/STAT pathway.
Therefore, we investigated the response of BC CML LSC to a clinical grade JAK2 inhibitor, SAR302503 (Sanofi, Cambridge, MA) alone or in combination with a potent BCR-ABL inhibitor, dasatinib, in vivo. After two weeks of treatment, RAG2−/−gc−/− mice intrahepatic transplanted with BC LSC, showed a significant (p<0.05) reduction of engraftment levels, after combination therapy with SAR302503 and dasatinib, compared to vehicle treated mice, in four different patient samples. In all the hematopoietic tissues analyzed, SAR302503 alone (60 mg/kg/b.i.d.) did not have an effect reducing the leukemic burden. Dasatinib alone (50mg/kg/day) reduced the LSC population in the liver, spleen, and peripheral blood, but the bone marrow retained a significant percentage of BC LSC. However, combination treatment was able to reduce the LSC in the BM significantly (p=0.0006) compared to dasatinib alone.
To test whether the combination therapy can impair self-renewal capacity of the BC CML LSC in vivo, we immunomagnetic bead selected CD34+ cells from BM and spleens of treated mice, and serially transplanted an equal number into secondary recipients. We observed a significant (p<0.0001) reduction of engraftment of LSC on the mice transplanted with combination treated cells compared with vehicle treated cells. Interestingly, secondary mice transplanted with cells treated with dasatinib showed 37.5% engraftment in the spleen and 46.4% in BM, while the level of engraftment for mice transplanted with combination treatment was only 1% and 3% for spleen and BM, respectively. Moreover, mice serially transplanted with combination treated cells, had a significant (p=0.0002) increased survival time. BC CML LSC are enriched for the granulocyte macrophage progenitor (GMP) population, which has been shown to harbor LSC serial transplantation potential. Our results showed that secondary recipient transplanted with combination treated cells, presented a significantly lower proportion of the GMP population, compared with vehicle (p<0.0001) and dasatinib (p=0.02) treated cells. Together, these results suggest that the combination therapy, using a Jak2 inhibitor with a BCR-ABL inhibitor, can abolish LSC self-renewal capacity and thereby potentially prevent relapse.
Validation studies, using nanoproteomic analysis, confirmed that LSC sorted cells from mice treated with SAR302503 had lower expression levels of p-JAK2 (Tyr 1007-08) and p-STAT5A (Tyr 694) compared with vehicle treated mice (51% and 64% of reduction, respectively), while no changes are observed for total JAK2 protein or B2M between both conditions.
Full transcriptome sequencing and q-RT-PCR analysis, on sorted CML LSC from mice treated with SAR302503 in combination with dasatinib, confirmed that STAT5A specific isoforms decresed after treatment, suggesting JAK/STAT pathway could be used as biomarker of response and could explain the impairment of self-renewal in the combination therapy.

Disclosures:
No relevant conflicts of interest to declare.