Regulation of Self-Renewal through Modulation of Let-7 Stem Cell Regulatory Family of microRNAs in Chronic Myeloid Leukemia Stem Cells

Introduction

Chronic myeloid leukemia (CML) was one of the first malignancies shown to be initiated in hematopoietic stem cells by the BCR-ABL1 oncogene and sustained in blast crisis (BC) by progenitor cells that co-opt stem cell properties and behave like leukemia stem cells (BC LSCs). The BCR-ABL fusion oncogene encodes a constitutively active tyrosine kinase BCR-ABL. Although tyrosine kinase inhibitor (TKI) therapy targeting BCR-ABL suppresses CML during the chronic phase (CP), progenitors undergo expansion as a consequence of subsequent genetic and epigenetic alterations that fuel blast crisis transformation, BC LSC generation and TKI resistance. Self-renewing human BC LSCs harbor increased expression of Inflammation responsive adenosine deaminase acting on RNA (ADAR1), which can alter transcript as well as microRNA (miRNA) maturation, splicing and translation by Adenosine (A)-to-Inosine (I) editing of double stranded RNA. miRNAs are a family of small non-coding RNA molecules that regulate gene expression at a post-transcriptional level by inhibiting protein translation and/or reducing mRNA stability. Eukaryotic cells employ miRNAs in diverse biological processes including cell proliferation, differentiation, pluripotency and self-renewal. The stem cell pluripotency RNA binding protein LIN28B plays critical roles in BC transformation of CML. In this study we sought to characterize CML related-oncogenes, such as BCR-ABL, JAK2 and ADAR1, alone or in stromal co-culture in terms of their ability to regulate LSC self-renewal through modulation of let-7 /LIN28B stem cell transcriptional regulatory axis.

Methods

MiRNAs were extracted from purified CD34⁺ cells derived from CP and BC CML patient samples as well as cord blood by RNeasy microKit (QIAGEN) and let-7 expression was evaluated by qRT-PCR using miScript Primer assay (QIAGEN). CD34+ cord blood (n=3) were transduced with lentiviral human BCR-ABL, JAK2, let-7a, wild type ADAR1 and ADAR1 mutant, which lacks a functional deaminase domain. Then, 72 hours after transduction, lentivirally transduced cells were plated on irradiated SL/M2 cells. After 5 days of culture, cells were collected for RNA and microRNA extraction. Transduction efficiency and LIN28B levels were evaluated by qRT-PCR and let-7 expression was quantified by qRT-PCR using miScript primer assay. Hematopoietic Progenitor and Replating assaywere performed on lenti-let-7a-overexpressing CB cells to assess differentiation, survival and self-renewal capacity.

Results

Lentiviral overexpression of human BCR-ABL in CD34+ CB did not induce any significant change in let-7 family members and LIN28B expression in absence of stromal co-culture. However, stromal co-culture of BCR-ABL overexpressing CB led to the significant downregulation of members of the let-7 family as well as to upregulation of their target gene LIN28B, thus suggesting that extrinsic microenvironmental cues are necessary for modulating let-7 family levels in presence of BCR-ABL. Notably, qRT-PCR of CB transduced with JAK2 showed significant upregulation of ADAR1 in the absence of stroma, thus suggesting that JAK2 might be a mediator of inflammatory cytokine-driven ADAR1 activation. Lentiviral overexpression of both human JAK2 and ADAR1 significantly reduced the expression of let-7 family members and induced up-regulation of LIN28B. Interestingly, lentiviral overexpression of ADAR1 mutant did not induce any significant change in most let-7 family members. Finally, lentiviral overexpression of let-7a induced significant reduction in survival and self-renewal.

Conclusion

These finding suggest that BCR-ABL requires extrinsic signals from the niche to modulate self-renewal of BC LSCs. Conversely, lentiviral JAK2 overexpression induces activation of aberrant RNA editing and subsequent reduction of let-7 family members in the absence of the niche. Interestingly, experiments with ADAR1 mutant, suggest that ADAR1 downregulates most of the let-7 family members in a RNA–editing dependent way manner. In summary these findings suggest a novel mechanism for BC LSC generation that may have utility in prognostication and selective LSCs targeting.