Requirement for CDK6 in MLL-rearranged acute myeloid leukemia

Abstract

Key Points• CDK6 is a critical effector of MLL fusions in myeloid leukemogenesis.• Genetic and pharmacologic inhibition of CDK6 overcome the differentiation block associated with MLLrearranged AML.Chromosomal rearrangements involving the H3K4 methyltransferase mixed-lineage leukemia (MLL) trigger aberrant gene expression in hematopoietic progenitors and give rise to an aggressive subtype of acute myeloid leukemia (AML). Insights into MLL fusionmediated leukemogenesis have not yet translated into better therapies because MLL is difficult to target directly, and the identity of the genes downstream of MLL whose altered transcription mediates leukemic transformation are poorly annotated. We used a functional genetic approach to uncover that AML cells driven by MLL-AF9 are exceptionally reliant on the cell-cycle regulator CDK6, but not its functional homolog CDK4, and that the preferential growth inhibition induced by CDK6 depletion is mediated through enhanced myeloid differentiation. CDK6 essentiality is also evident in AML cells harboring alternate MLL fusions and a mouse model of MLL-AF9-driven leukemia and can be ascribed to transcriptional activation of CDK6 by mutant MLL. Importantly, the context-dependent effects of lowering CDK6 expression are closely phenocopied by a small-molecule CDK6 inhibitor currently in clinical development. These data identify CDK6 as critical effector of MLL fusions in leukemogenesis that might be targeted to overcome the differentiation block associated with MLL-rearranged AML, and underscore that cell-cycle regulators may have distinct, noncanonical, and nonredundant functions in different contexts. (Blood. 2014;124(1):13-23) Introduction A substantial proportion of acute myeloid leukemia (AML) cases harbor balanced translocations of chromosome 11q23, and AML with t(9;11)(p22;q23) is recognized as a distinct entity by the World Health Organization Classification of Tumors of Hematopoietic and Lymphoid Tissues.1,2 On the molecular level, t(11q23) results in fusion of the MLL gene, which encodes an H3K4 methyltransferase, to a broad spectrum of partner genes, such as MLLT3 (also called AF9), MLLT4 (AF6), MLLT1 (ENL), and MLLT10 (AF10) on chromosomes 9p22, 6q27, 19p13.3, and 10p12, respectively. 3,4 A key functional feature of mixed-lineage leukemia (MLL) rearrangements is their ability to confer leukemia-initiating activity to hematopoietic stem and progenitor cells (HSPC). 5,6 MLL fusions are characterized by loss of the C-terminal H3K4 methyltransferase domain, and their leukemogenic activity is dependent on both features of the remaining N-terminal portion, such as a binding motif for the menin tumor suppressor that mediates the contact between MLL and chromatin as well as aberrant transactivation of target genes through heterologous domains contributed by the various partner proteins.7 For example, MLL fusions involving AF9, ENL, and AF10, which account for the majority of MLLrearranged AML, recruit multiprotein complexes essential for transcriptional activation/elongation...

CDK6 as a key regulator of hematopoietic and leukemic stem cell activation

“…5,7 CDK6 has also been shown to be critical in acute myeloid leukemia (AML) and acute lymphoblastic leukemia driven by mixed lineage leukemia fusion proteins. 9,10 There is considerable interest in targeting CDK4/6 in cancer therapy, and the Food and Drug Administration nominated CDK4/6 inhibitors as the "breakthrough therapeutic advance" in 2013.…”

Diagnosis and management of AML in adults: 2017 ELN recommendations from an international expert panel

“…289,290 Other examples are targeting of BRD4, a member of the BET family of bromodomain epigenetic readers, 291 or of KMT2A (MLL)-rearranged leukemias. 292,293 In a randomized trial conducted in patients with relapsed and refractory AML, the topoisomerase II inhibitor vosaroxin in combination with IDAC demonstrated a small survival benefit in patients older than 60 years (7.1 vs 5.0 months); a benefit was not shown in younger patients, potentially due to the higher transplant rate (45.8% ,60 years vs 20.2% $60 years). 222 Finally, targeted immunotherapy is an important novel approach.…”

Palbociclib treatment of FLT3-ITD+ AML cells uncovers a kinase-dependent transcriptional regulation of FLT3 and PIM1 by CDK6

“…[17][18][19][20][21][22][23][24][25][26][27][28][29] CDK6 is particularly important in AML and acute lymphoblastic leukemia (ALL) driven by mixed-lineage leukemia fusion proteins. [29][30][31] The protumorigenic functions of CDK6, but not of CDK4, go well beyond driving the cell cycle: in lymphoid and myeloid leukemia, CDK6 is part of transcriptional complexes that promote leukemogenesis and tumor formation. 32,33 CDK6 also directly regulates the transcription of factors that link neoplastic progression to the reprogramming of stem cells.…”