Abstract 2541: MicroRNA-497~195 cluster suppresses acute lymphoblastic leukemia growth by targeting CCND3/CDK4 and inhibiting cell cycle progression
Authors: Boldrin Elena , Gaffo Enrico , Boer Judith , Demir Salih , Zinngrebe Julia , Claus Rainer , Plass Christoph , den Boer Monique L. , Debatin Klaus-Michael , te Kronnie Geertruy , Bortoluzzi Stefania , Meyer Lüder Hinrich 
Published: September 2020

Abstract
Acute lymphoblastic leukemia (ALL) is the most common malignancy of childhood. Relapse is associated with poor prognosis, requiring further investigation on high-risk leukemias. MicroRNAs (miRNAs) are involved in the regulation of physiological processes. Deregulated miRNAs have been described to play a role in the initiation and progression of leukemia.
Here, we investigated miRNA expression profiles comparing leukemia samples of patients with good and poor outcome, also characterized by slow or rapid engraftment, respectively, upon transplantation into NOD/SCID mice.
We found downregulation of the miR-497~195 cluster in B-cell precursor (BCP)-ALL patient-derived xenograft samples with rapid engraftment and early relapse. Accordingly, in an independent cohort, patients with a high expression of miR-497 or miR-195 showed higher event-free survival. We identified promoter methylation as the mechanism responsible for miR-497~195 downregulation, as it was associated with low miR-497~195 levels in xenografts and treatment with the demethylating agent Decitabine increased miR-497~195 expression in BCP-ALL cell lines.
To study the role of miR-497~195 in BCP-ALL, we stably overexpressed the cluster in three xenograft samples. MiR-497~195 overexpression impaired engraftment of leukemic cells in NOD/SCID mice, as compared to control cells, prolonging recipient survival. Gene expression profiling and ex vivo analysis of miR-497~195 overexpressing xenograft cells showed that inhibition of cell proliferation is a mechanism responsible for the miRNA cluster tumor suppressive role.
Mechanistically, CDK4 and CCND3 were downregulated at mRNA and protein levels upon miR-497~195 overexpression, leading to decreased RB1 phosphorylation, reduced transcription of E2F target genes, and inhibition of the entry in S phase. CDK4 mediated G1/S transition is also inhibited by CDKN2A and CDKN2B, which are deleted in 20% of BCP-ALL. Deletion of CDKN2A/B and concomitant low miR-497~195 expression were particularly associated with a high proportion of early relapse cases in our cohort, indicating that impaired regulation of G1/S transition is critical for ALL aggressiveness. Importantly, targeting this pathway with the CDK4/CDK6 inhibitor Palbociclib impaired ex vivo growth of BCP-ALL xenograft samples, indicating a possible therapeutic strategy.
Altogether, we showed that the tumor-suppressive cluster miR-497~195 is downregulated by epigenetic repression in BCP-ALL and that low expression is associated with early relapse and rapid engraftment. Low miR-497~195 expression might cooperate with deletion of CDKN2A/B promoting tumor progression, through loss of CCND3/CDK4-dependent control of cell cycle regulation.
Citation Format: Elena Boldrin, Enrico Gaffo, Judith Boer, Salih Demir, Julia Zinngrebe, Rainer Claus, Christoph Plass, Monique L. den Boer, Klaus-Michael Debatin, Geertruy te Kronnie, Stefania Bortoluzzi, Lüder Hinrich Meyer. MicroRNA-497~195 cluster suppresses acute lymphoblastic leukemia growth by targeting CCND3/CDK4 and inhibiting cell cycle progression [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2541.