Functionally distinct roles for different miR-155 expression levels through contrasting effects on gene expression, in acute myeloid leukaemia.

Enforced expression of microRNA-155 (miR-155) in myeloid cells has been shown to have both oncogenic or tumour-suppressor functions in acute myeloid leukaemia (AML). We sought to resolve these contrasting effects of miR-155 overexpression using murine models of AML and human paediatric AML data sets. We show that the highest miR-155 expression levels inhibited proliferation in murine AML models. Over time, enforced miR-155 expression in AML in vitro and in vivo, however, favours selection of intermediate miR-155 expression levels that results in increased tumour burden in mice, without accelerating the onset of disease. Strikingly, we show that intermediate and high miR-155 expression also regulate very different subsets of miR-155 targets and have contrasting downstream effects on the transcriptional environments of AML cells, including genes involved in haematopoiesis and leukaemia. Furthermore, we show that elevated miR-155 expression detected in paediatric AML correlates with intermediate and not high miR-155 expression identified in our experimental models. These findings collectively describe a novel dose-dependent role for miR-155 in the regulation of AML, which may have important therapeutic implications.

MOZ (MYST3, KAT6A) inhibits senescence via the INK4A-ARF pathway.

Cellular senescence is an important mechanism that restricts tumour growth. The Ink4a-Arf locus (also known as Cdkn2a), which encodes p16(INK4A) and p19(ARF), has a central role in inducing and maintaining senescence. Given the importance of cellular senescence in restraining tumour growth, great emphasis is being placed on the identification of novel factors that can modulate senescence. The MYST-family histone acetyltransferase MOZ (MYST3, KAT6A), first identified in recurrent translocations in acute myeloid leukaemia, has been implicated in both the promotion and inhibition of senescence. In this study, we investigate the role of MOZ in cellular senescence and show that MOZ is a potent inhibitor of senescence via the INK4A-ARF pathway. Primary mouse embryonic fibroblasts (MEFs) isolated from Moz-deficient embryos exhibit premature senescence, which was rescued on the Ink4a-Arf(-/-) background. Importantly, senescence resulting from the absence of MOZ was not accompanied by DNA damage, suggesting that MOZ acts independently of the DNA damage response. Consistent with the importance of senescence in cancer, expression profiling revealed that genes overexpressed in aggressive and highly proliferative cancers are expressed at low levels in Moz-deficient MEFs. We show that MOZ is required to maintain normal levels of histone 3 lysine 9 (H3K9) and H3K27 acetylation at the transcriptional start sites of at least four genes, Cdc6, Ezh2, E2f2 and Melk, and normal mRNA levels of these genes. CDC6, EZH2 and E2F2 are known inhibitors of the INK4A-ARF pathway. Using chromatin immunoprecipitation, we show that MOZ occupies the Cdc6, Ezh2 and Melk loci, thereby providing a direct link between MOZ, H3K9 and H3K27 acetylation, and normal transcriptional levels at these loci. This work establishes that MOZ is an upstream inhibitor of the INK4A-ARF pathway, and suggests that inhibiting MOZ may be one way to induce senescence in proliferative tumour cells.

Neither loss of Bik alone, nor combined loss of Bik and Noxa, accelerate murine lymphoma development or render lymphoma cells resistant to DNA damaging drugs.

The pro-apoptotic BH3-only protein, BIK, is widely expressed and although many critical functions in developmental or stress-induced death have been ascribed to this protein, mice lacking Bik display no overt abnormalities. It has been postulated that Bik can serve as a tumour suppressor, on the basis that its deficiency and loss of apoptotic function have been reported in many human cancers, including lymphoid malignancies. Evasion of apoptosis is a major factor contributing to c-Myc-induced tumour development, but despite this, we found that Bik deficiency did not accelerate Eµ-Myc-induced lymphomagenesis. Co-operation between BIK and NOXA, another BH3-only protein, has been previously described, and was attributed to their complementary binding specificities to distinct subsets of pro-survival BCL-2 family proteins. Nevertheless, combined deficiency of Bik and Noxa did not alter the onset of Eµ-Myc transgene induced lymphoma development. Moreover, although p53-mediated induction of Bik has been reported, neither Eµ-Myc/Bik(-/-) nor Eµ-Myc/Bik(-/-)Noxa(-/-) lymphomas were more resistant than control Eµ-Myc lymphomas to killing by DNA damaging drugs, either in vitro or in vivo. These results suggest that Bik, even in combination with Noxa, is not a potent suppressor of c-Myc-driven tumourigenesis or critical for chemotherapeutic drug-induced killing of Myc-driven tumours.