Karen Blyth

The runx genes: gain or loss of function in cancer

The RUNX genes have come to prominence recently because of their roles as essential regulators of cell fate in development and their paradoxical effects in cancer, in which they can function either as tumour-suppressor genes or dominant oncogenes according to context. How can this family of transcription factors have such an ambiguous role in cancer? How and where do these genes impinge on the pathways that regulate growth control and differentiation? And what is the evidence for a wider role for the RUNX genes in non-haematopoietic cancers?

Serine starvation induces stress and p53-dependent metabolic remodelling in cancer cells

Cancer cells acquire distinct metabolic adaptations to survive stress associated with tumour growth and to satisfy the anabolic demands of proliferation. The tumour suppressor protein p53 (also known as TP53) influences a range of cellular metabolic processes, including glycolysis, oxidative phosphorylation, glutaminolysis and anti-oxidant response. In contrast to its role in promoting apoptosis during DNA-damaging stress, p53 can promote cell survival during metabolic stress, a function that may contribute not only to tumour suppression but also to non-cancer-associated functions of p53. Here we show that human cancer cells rapidly use exogenous serine and that serine deprivation triggered activation of the serine synthesis pathway and rapidly suppressed aerobic glycolysis, resulting in an increased flux to the tricarboxylic acid cycle. Transient p53-p21 (also known as CDKN1A) activation and cell-cycle arrest promoted cell survival by efficiently channelling depleted serine stores to glutathione synthesis, thus preserving cellular anti-oxidant capacity. Cells lacking p53 failed to complete the response to serine depletion, resulting in oxidative stress, reduced viability and severely impaired proliferation. The role of p53 in supporting cancer cell proliferation under serine starvation was translated to an in vivo model, indicating that serine depletion has a potential role in the treatment of p53-deficient tumours.

Acetyl-CoA Synthetase 2 Promotes Acetate Utilization and Maintains Cancer Cell Growth under Metabolic Stress

Summary A functional genomics study revealed that the activity of acetyl-CoA synthetase 2 (ACSS2) contributes to cancer cell growth under low-oxygen and lipid-depleted conditions. Comparative metabolomics and lipidomics demonstrated that acetate is used as a nutritional source by cancer cells in an ACSS2-dependent manner, and supplied a significant fraction of the carbon within the fatty acid and phospholipid pools. ACSS2 expression is upregulated under metabolically stressed conditions and ACSS2 silencing reduced the growth of tumor xenografts. ACSS2 exhibits copy-number gain in human breast tumors, and ACSS2 expression correlates with disease progression. These results signify a critical role for acetate consumption in the production of lipid biomass within the harsh tumor microenvironment.

Gene therapy - X-SCID transgene leukaemogenicity

Arising from: Woods, N.-B., Bottero, V., Schmidt, M., von Kalle, C. & Verma, I. M. 440, 1123 (2006); see also communication from Pike-Overzet et al.; Woods et al. replyGene therapy has been remarkably effective for the immunological reconstitution of patients with severe combined immune deficiency, but the occurrence of leukaemia in a few patients has stimulated debate about the safety of the procedure and the mechanisms of leukaemogenesis. Woods et al. forced high expression of the corrective therapeutic gene IL2RG, which encodes the γ-chain of the interleukin-2 receptor, in a mouse model of the disease and found that tumours appeared in a proportion of cases. Here we show that transgenic IL2RG does not necessarily have potent intrinsic oncogenic properties, and argue that the interpretation of this observation with respect to human trials is overstated.

N-WASP coordinates the delivery and F-actin–mediated capture of MT1-MMP at invasive pseudopods

N-WASP is critical for cancer cell invasion through its promotion of the trafficking and capture of MT1-MMP in invasive pseudopods.

Runx2 : A novel oncogenic effector revealed by in vivo complementation and retroviral tagging

The Runx2 (Cbfa1, Pebp2αA, Aml3) gene was previously identified as a frequent target for transcriptional activation by proviral insertion in T-cell lymphomas of CD2-MYC transgenic mice. We have recently shown that over-expression of the full-length, most highly expressed Runx2 isoform in the thymus perturbs T-cell development, leads to development of spontaneous lymphomas at low frequency and is strongly synergistic with Myc. To gain further insight into the relationship of Runx2 to other lymphomagenic pathways, we tested the effect of combining the CD2-Runx2 transgene either with a Pim1 transgene (Eμ-Pim1) or with the p53 null genotype, as each of these displays independent synergy with Myc. In both cases we observed synergistic tumour development. However, Runx2 appeared to have a dominant effect on the tumour phenotype in each case, with most tumours conforming to the CD3+, CD8+, CD4+/− phenotype seen in CD2-Runx2 mice. Neonatal infection of CD2-Runx2 mice with Moloney murine leukaemia virus (Moloney MLV) also led to a dramatic acceleration of tumour onset. Analysis of known Moloney MLV target genes in these lymphomas showed a high frequency of rearrangement at c-Myc or N-Myc (82%), and a significant number at Pim1 or Pim2 (23%), and at Pal1/Gfi1 (18%). These results indicate that Runx2 makes a distinct contribution to T-cell lymphoma development which does not coincide with any of the oncogene complementation groups previously identified by retroviral tagging.

Limited Mitochondrial Permeabilization Causes DNA Damage and Genomic Instability in the Absence of Cell Death

Summary During apoptosis, the mitochondrial outer membrane is permeabilized, leading to the release of cytochrome c that activates downstream caspases. Mitochondrial outer membrane permeabilization (MOMP) has historically been thought to occur synchronously and completely throughout a cell, leading to rapid caspase activation and apoptosis. Using a new imaging approach, we demonstrate that MOMP is not an all-or-nothing event. Rather, we find that a minority of mitochondria can undergo MOMP in a stress-regulated manner, a phenomenon we term “minority MOMP.” Crucially, minority MOMP leads to limited caspase activation, which is insufficient to trigger cell death. Instead, this caspase activity leads to DNA damage that, in turn, promotes genomic instability, cellular transformation, and tumorigenesis. Our data demonstrate that, in contrast to its well-established tumor suppressor function, apoptosis also has oncogenic potential that is regulated by the extent of MOMP. These findings have important implications for oncogenesis following either physiological or therapeutic engagement of apoptosis.

TIGAR Is Required for Efficient Intestinal Regeneration and Tumorigenesis

Summary Regulation of metabolic pathways plays an important role in controlling cell growth, proliferation, and survival. TIGAR acts as a fructose-2,6-bisphosphatase, potentially promoting the pentose phosphate pathway to produce NADPH for antioxidant function and ribose-5-phosphate for nucleotide synthesis. The functions of TIGAR were dispensable for normal growth and development in mice but played a key role in allowing intestinal regeneration in vivo and in ex vivo cultures, where growth defects due to lack of TIGAR were rescued by ROS scavengers and nucleosides. In a mouse intestinal adenoma model, TIGAR deficiency decreased tumor burden and increased survival, while elevated expression of TIGAR in human colon tumors suggested that deregulated TIGAR supports cancer progression. Our study demonstrates the importance of TIGAR in regulating metabolism for regeneration and cancer development and identifies TIGAR as a potential therapeutic target in diseases such as ulcerative colitis and intestinal cancer.

A full-length Cbfa1 gene product perturbs T-cell development and promotes lymphomagenesis in synergy with myc.

The Cbfa1/PEBP2αA/AML3 gene plays an essential role in osteogenesis but is also expressed in the T-cell lineage where it has been implicated in lymphoma development as a target for retroviral insertional mutagenesis. As lymphoma cells with til-1 insertion express at least five distinct Cbfa1 isoforms, it is important to establish which, if any, have intrinsic oncogenic potential. We have generated transgenic mice in which the most abundant lymphoma isoform (G1/p57) is expressed under the control of the CD2 locus control region. Co-precipitation analysis of transgenic thymus revealed high levels of Cbfa1 protein in an abundant complex containing the binding cofactor Cbfb. CD2-Cbfa1-G1 mice displayed abnormal T-cell development, with a pronounced skew towards CD8 SP cells in the thymus and developed a low incidence of spontaneous lymphomas (6% at 12 months) with cells of similar phenotype. Strongly synergistic tumour development was seen when CD2-Cbfa1-G1 mice were crossed with lines carrying myc transgenes (CD2-myc or tamoxifen-regulatable CD2-mycERTM) and Cbfa1 was found to rescue expression of the CD2-myc transgene in preleukaemic mice. However, synergy did not appear to be due to a dominant block of myc-induced apoptosis by Cbfa1 as explanted primary tumours and cell lines from CD2-Cbfa1-G1/CD2-mycERTM mice showed accelerated death on induction with tamoxifen at similar rates to CD2-mycERTM controls. Moreover, thymocytes from preleukaemic CD2-Cbfa1-G1 mice showed reduced survival in vitro and increased sensitivity to the inhibitory effects of TGF-β. This study demonstrates that a full-length Cbf α-chain gene can act as an oncogene without fusion to a heterologous protein.

Pyruvate carboxylation enables growth of SDH-deficient cells by supporting aspartate biosynthesis

Succinate dehydrogenase (SDH) is a heterotetrameric nuclear-encoded complex responsible for the oxidation of succinate to fumarate in the tricarboxylic acid cycle. Loss-of-function mutations in any of the SDH genes are associated with cancer formation. However, the impact of SDH loss on cell metabolism and the mechanisms enabling growth of SDH-defective cells are largely unknown. Here, we generated Sdhb-ablated kidney mouse cells and used comparative metabolomics and stable-isotope-labelling approaches to identify nutritional requirements and metabolic adaptations to SDH loss. We found that lack of SDH activity commits cells to consume extracellular pyruvate, which sustains Warburg-like bioenergetic features. We further demonstrated that pyruvate carboxylation diverts glucose-derived carbons into aspartate biosynthesis, thus sustaining cell growth. By identifying pyruvate carboxylase as essential for the proliferation and tumorigenic capacity of SDH-deficient cells, this study revealed a metabolic vulnerability for potential future treatment of SDH-associated malignancies.