The inv(16) Cooperates with ARF Haploinsufficiency to Induce Acute Myeloid Leukemia

Abstract

The inv(16) is one of the most frequent chromosomal translocations associated with acute myeloid leukemia (AML) and creates a chimeric fusion protein consisting of most of the runt-related X1 co-factor, core binding factor ␤ fused to the smooth muscle myosin heavy chain MYH11. Expression of the ARF tumor suppressor is regulated by runt-related X1, suggesting that the inv (16) The gene encoding the runt-related-X1 (RUNX1, 3 also known as acute myeloid leukemia 1 (AML1)) transcription factor is one of the most frequently mutated genes in human B-cell acute lymphoblastic leukemia (ALL) and AML. RUNX1 is inactivated by bi-allelic mutations in cases of AML with very immature blasts (1, 2) and is also disrupted by the t(12;21), which is the most frequent translocation associated with childhood B-cell ALL (3). Furthermore, RUNX1 is also targeted by many chromosomal translocations in AML, including t(3;21), t(16;21), and t(8;21), and the latter is one of the most frequent translocations (12-15%) in AML (4). Finally, the functions of RUNX1 are also indirectly compromised by the inv(16), which disrupts the gene encoding the RUNX1 co-factor, core binding factor ␤ (CBF␤). CBF␤ is a small protein that allosterically regulates the ability of RUNX1 to bind to DNA and that blocks the ubiquitin-mediated degradation of RUNX1 (6 -8). The inv(16) occurs in roughly 8% of AML patients and encodes a fusion protein that contains most of CBF␤ fused to a smooth muscle myosin heavy chain (MYH11) (4, 5). (For simplicity, we will refer to the inv(16) fusion protein as the "IFP.") Recent insights suggest that RUNX1 regulates the transcription of genes that can predispose hematopoietic stem cells or progenitor cells to oncogenic transformation. One such target is the p14 ARF (alternative reading frame) tumor suppressor, which regulates the p53-dependent oncogene checkpoint by antagonizing MDM2 functions, thereby stabilizing and activating p53 (9 -13). In murine cells, p19Arf (the murine homolog of human p14 ARF , referred to here collectively as ARF) levels increase in response to various oncoproteins, including v-Abl, c-Myc, Ras, E2F-1, and E1A (11, 14 -17). Thus, loss of ARF impairs p53-mediated growth arrest and/or apoptosis in response to oncogenes.The development of an inv(16) animal model in which to test the mechanism of action of the IFP has been extremely difficult. Previous murine models of the inv(16) have involved the expression of the IFP coupled with either ENU mutagenesis, retroviral insertional mutagenesis (18,19), the removal of two tumor suppressors (20), or the coexpression of oncogenes (20,21). However, only the use of chimeric mice carrying a "knock-in" inv(16) allele plus random mutagenesis yielded an AML that mimicked the human disease (18,19).Given that RUNX1 and RUNX1-ETO (Eight-Twenty-One) regulate the activity of the ARF promoter (22), we reasoned that the IFP may also influence Arf expression and that this pathway would be operational in myeloid leukemogenesis. We demonstrated, in a bone marrow transplant (BMT) mo...

A Pathway for the Control of Anoikis Sensitivity by E-Cadherin and Epithelial-to-Mesenchymal Transition

“…This provides a novel rationale for the loss of function of this gene in tumors. In this connection, a loss of heterozygosity of the CDKN2a locus promotes tumor progression significantly in both mouse and human contexts, revealing a haploinsufficiency effect and validating the importance of the ϳ2-fold regulation of p14ARF expression observed here (24,40,52,62). The pro-anoikis effect of p14ARF also may explain the advantage to tumor cells of overexpressing TBX2/3 and/or NRAGE proteins-repressors of p14ARF that counteract anoikis and promote anchorage-independent growth (10, 42, 96)-or other repressors of p14ARF (49).…”

Mechanisms that link the oncogenic epithelial–mesenchymal transition to suppression of anoikis

“…This observation, and the correlation between the expression of ankyrin-G and p14ARF in breast cancer cell lines, suggests that p14ARF is an important determinant of anoikis sensitivity (Kumar et al, 2011). This may also constitute an underlying reason for the widespread loss of p14ARF in human cancers and the observation that even heterozygosity for wild-type p14ARF correlates strongly with tumor incidence (Hewitt et al, 2002;Moreno-Miralles et al, 2005). Although p14ARF was originally shown to induce apoptosis through p53, p53-independent mechanisms that involve its interaction with the Myc proto-oncogene, the generalized EMT-mediating co-repressor protein, C-terminal binding protein (CTBP) or the mitochondrial p32 protein have now been shown (Itahana and Zhang, 2008;Paliwal et al, 2006;Qi et al, 2004).…”

Methylation-Independent Silencing of the Tumor Suppressor INK4b (p15) by CBFβ-SMMHC in Acute Myelogenous Leukemia with inv(16)

“…Until recently, the chimeric protein CBFh-SMMHC had been known primarily for its ability to block differentiation presumably by inhibiting transcription of myeloid-specific genes through dominant repression of CBF transcription factors (31,32). Very recently, it was shown that CBFh-SMMHC can suppress expression of ARF by impairing RUNX1-mediated activation of the promoter in HeLa cells (33). These data along with ours suggest that CBFh-SMMHC induces leukemia in part by affecting the cell cycle and apoptosis through inhibition of the p53 and the Rb pathways.…”