Efficacy of CDK9 inhibition in therapy of post-myeloproliferative neoplasm (MPN) secondary (s) AML cells

Signaling downstream of the pathogenetic mutations in JAK2, MPL, or calreticulin, along with co-occurring mutations in chromatin modifiers or transcription factors, results in dysregulated transcriptome and proteome that is responsible for the transformation of myeloproliferative neoplasms (MPN) to sAML, as well and for conferring therapy refractoriness [ 1 , 2 ]. Although routinely used and effective in symptomatic and advanced MPN, JAK inhibitor (JAKi), e.g., ruxolitinib, and/or treatments with standard AML chemotherapy are ineffective against post-myeloproliferative neoplasm (MPN) sAML [ 3 ]. Binding and activity of transcription factors (TFs), including lineage-specific master regulators and signaling TFs such as STAT3/5, RELA, and MYC, at enhancers and promoters of their targets involves recruitment of transcriptional co-factors and epigenetic regulators, including HAT (histone acetyltransferase), bromodomain extra-terminal (BET) protein (BETP) BRD4 and pTEFb (positive transcript elongation factor b) [ 4 ]. Collectively, activities of these TFs and co-factors induce promoter-proximal pause-release of the poised RNA pol II (RNAP2) to stimulate productive mRNA transcript elongation, leading to the dysregulated transcriptome that confers the aggressive phenotype and therapy refractoriness in post-MPN sAML cells [ 2 , 5 ]. The non-cell-cycle regulatory CDK9 is the catalytic subunit of the positive transcript elongation factor b (pTEFb) [ 5 , 6 ]. In association with its regulatory subunit cyclin T1, CDK9 mediates phosphorylation of serine 2 in the tandem heptad repeats of the C-terminal domain of RNA pol II (RNAP2), which induces pause-release of RNAP2. CDK9 also phosphorylates and inactivates the negative transcription regulators NELF (negative elongation factor) and DSIF (DRB sensitivity-inducing factor), thereby further promoting RNAP2-mediated transcription [ 5 , 6 ]. CDK9 activity is required to maintain constant production of mRNAs of short-lived proteins, e.g., MCL1 and c-Myc, promoting growth and survival of AML cells [ 6 , 7 ]. Dysregulated c-Myc in AML cells, either due to amplification or protein stabilization, interacts with and recruits pTEFb to its own enhancers and promoter and those of its target genes to mediate RNAP2 pause-release [ 6 , 7 ]. Of the two protein isoforms of CDK9, the smaller 42 kDa isoform is more abundant in AML cells [ 6 ]. Selective CDK9 inhibitors that exhibit high affinity interaction with the ATP binding site of CDK9 inhibit phosphorylation of serine 2 of RNAP2, and CDK9-mediated phosphorylation of the negative regulators of RNAP2-mediated transcription, i.e., NELF and DSIF, thus inhibiting RNAP2-mediated transcription of oncogenes, e.g., c-Myc and MCL-1 [ 5 , 6 , 7 ].