The Myc oncogene regulates the expression of multiple components of the protein synthetic machinery, including ribosomal proteins, initiation factors of translation, Pol III, and rDNA1,2. An outstanding question is whether and how increasing the cellular protein synthesis capacity can affect the multi-step process leading to cancer. We utilized ribosomal protein heterozygote mice as a genetic tool to restore increased protein synthesis in Eμ–Myc/+ transgenic mice to normal levels and show that in this context Myc's oncogenic potential is suppressed. Our findings demonstrate that the ability of Myc to increase protein synthesis directly augments cell size and is sufficient to accelerate cell cycle progression independently of known cell cycle targets transcriptionally regulated by Myc. In addition, when protein synthesis is restored to normal levels, Myc overexpressing precancerous cells are more efficiently eliminated by programmed cell death. Our findings reveal a novel paradigm that links increases in general protein synthesis rates downstream of an oncogenic signal to a specific molecular impairment in the modality of translation initiation employed to regulate the expression of selective mRNAs. We show that an aberrant increase in cap-dependent translation downstream Myc hyperactivation specifically impairs the translational switch to internal ribosomal entry site (IRES)-dependent translation required for accurate mitotic progression. Failure of this translational switch results in reduced mitotic-specific expression of the endogenous IRES-dependent form of Cdk11 (p58-PITSLRE)3-5, which leads to cytokinesis defects and is associated with increased centrosome numbers and genome instability in Eμ–Myc/+ mice. When accurate translational control is re-established in Eμ–Myc/+ mice, genome instability is suppressed. Our findings reveal how perturbations in translational control provide a highly specific outcome on gene expression, genome stability, and cancer initiation that have important implications for understanding the molecular mechanism of cancer formation at the post-genomic level.

Mutations in DKC1 cause dyskeratosis congenita (DC), a disease characterized by premature aging and increased tumor susceptibility. The DKC1 protein binds to the box H + ACA small nucleolar RNAs and the RNA component of telomerase. Here we show that hypomorphic Dkc1 mutant (Dkc1m) mice recapitulate in the first and second generations (G1 and G2) the clinical features of DC. Dkc1m cells from G1 and G2 mice were impaired in ribosomal RNA pseudouridylation before the onset of disease. Reductions of telomere length in Dkc1m mice became evident only in later generations. These results suggest that deregulated ribosome function is important in the initiation of DC, whereas telomere shortening may modify and/or exacerbate DC.

The pathogenesis of multiple myeloma (MM), an incurable tumour causing the deregulated proliferation of terminally differentiated B cells, is unknown. Chromosomal translocations (14q1) affecting band 14q32 and unidentified partner chromosomes are common in this tumour, suggesting that they may cause the activation of novel oncogenes. By cloning the chromosomal breakpoints in an MM cell line, we show that the 14q+ translocation represents a t(6;14)x(p25;q32) and that this aberration is recurrent in MM, as it was found in two of eleven MM cell lines. The translocation juxtaposes the immunoglobulin heavy-chain (IgH) locus to MUM1 (multiple myeloma oncogene 1)/IRF4 gene, a member of the interferon regulatory factor (IRF) family known to be active in the control of B-cell proliferation and differentiation. As a result, the MUM1/IRF4 gene is overexpressed--an event that may contribute to tumorigenesis, a MUM1/IRF4 has oncogenic activity in vitro. These findings identify a novel genetic alteration associated with MM, with implications for the pathogenesis and diagnostics of this tumour.

Purpose The objective of this study was to determine the incidence of the MYB-MFIB fusion in salivary adenoid cystic carcinoma (ACC), to establish the clinicopathological significance of the fusion and to analyze the expression of MYB in ACCs in the context of the MYB-NFIB fusion. Experimental Design We performed an extensive analysis involving 123 cancers of the salivary gland, including primary and metastatic ACCs, and non-ACC salivary carcinomas. MYB-NFIB fusions were identified by reverse transcription-PCR (RT-PCR) and sequencing of the RT-PCR products, and confirmed by fluorescence in situ hybridization. MYB RNA expression was determined by quantitative RT-PCR and protein expression was analyzed by immunohistochemistry. Results The MYB-NFIB fusion was detected in 28% primary and 35% metastatic ACCs, but not in any of the non-ACC salivary carcinomas analyzed. Different exons in both MYB and NFIB genes were involved in the fusions, resulting in expression of multiple chimeric variants. Notably, MYB was overexpressed in the vast majority of the ACCs, although MYB expression was significantly higher in tumors carrying the MYB-NFIB fusion. The presence of the MYB-NFIB fusion was significantly associated (p = 0.03) with patients older than 50 years of age. No correlation with other clinicopathological markers, factors and survival was found. Conclusions We conclude that the MYB-NFIB fusion characterizes a subset of ACCs and contributes to MYB overexpression. Additional mechanisms may be involved in MYB overexpression in ACCs lacking the MYB-NFIB fusion. These findings suggest that MYB may be a specific novel target for tumor intervention in patients with ACC.

TP53 , a well-known tumour suppressor gene, is frequently inactivated by mutation or deletion in a majority of human tumours1,2. A tremendous effort has been made to restore p53 activity in cancer therapies3–7. However, no effective p53-based therapy has been successfully translated into clinical cancer treatment due to the complexity of p53 signalling. Here, we demonstrate that genomic deletion of TP53 frequently encompasses neighbouring essential genes, rendering cancer cells with hemizygous TP53 deletion vulnerable to further suppression of such genes. POLR2A is identified as such a gene that is almost always co-deleted with TP53 in human cancers. It encodes the largest and catalytic subunit of RNA polymerase II complex, which is specifically inhibited by α-Amanitin8,9. Our analysis of The Cancer Genome Atlas (TCGA) and Cancer Cell Line Encyclopaedia (CCLE) databases reveals that POLR2A expression levels are tightly correlated with its gene copy numbers in human colorectal cancer (CRC). Suppression of POLR2A with α-Amanitin or small interfering RNAs, selectively inhibits proliferation, survival and tumorigenic potential of CRC cells with hemizygous TP53 loss in a p53-independent manner. Previous clinical applications of α-Amanitin have been limited due to its liver toxicity10. However, we found that α-Amanitin-based antibody drug conjugates (ADCs) are highly effective therapeutic agents with reduced toxicity11. Here, we show that low doses of α-Amanitin-conjugated anti-EpCAM (Epithelial Cell Adhesion Molecule) antibody lead to complete tumour regression in murine models of human CRC with hemizygous deletion of POLR2A. We anticipate that inhibiting POLR2A will be a novel therapeutic approach for human cancers harbouring such common genomic alterations.

Cardiac progenitor/stem cells in adult hearts represent an attractive therapeutic target for heart regeneration, though (inter)-relationships among reported cells remain obscure. Using single-cell qRT–PCR and clonal analyses, here we define four subpopulations of cardiac progenitor/stem cells in adult mouse myocardium all sharing stem cell antigen-1 (Sca1), based on side population (SP) phenotype, PECAM-1 (CD31) and platelet-derived growth factor receptor-α (PDGFRα) expression. SP status predicts clonogenicity and cardiogenic gene expression (Gata4/6, Hand2 and Tbx5/20), properties segregating more specifically to PDGFRα+ cells. Clonal progeny of single Sca1+ SP cells show cardiomyocyte, endothelial and smooth muscle lineage potential after cardiac grafting, augmenting cardiac function although durable engraftment is rare. PDGFRα− cells are characterized by Kdr/Flk1, Cdh5, CD31 and lack of clonogenicity. PDGFRα+/CD31− cells derive from cells formerly expressing Mesp1, Nkx2-5, Isl1, Gata5 and Wt1, distinct from PDGFRα−/CD31+ cells (Gata5 low; Flk1 and Tie2 high). Thus, PDGFRα demarcates the clonogenic cardiogenic Sca1+ stem/progenitor cell.

The cohesin complex is responsible for the accurate separation of sister chromatids into two daughter cells. Several models for the cohesin complex have been proposed, but the one-ring embrace model currently predominates the field. However, the static configuration of the embrace model is not flexible enough for cohesins to perform their functions during DNA replication, transcription, and DNA repair. We used coimmunoprecipitation, a protein fragment complement assay, and a yeast two-hybrid assay to analyze the protein–protein interactions among cohesin subunits. The results show that three of the four human cohesin core subunits (Smc1, Smc3, and Rad21) interact with themselves in an Scc3 (SA1/SA2)-dependent manner. These data support a two-ring handcuff model for the cohesin complex, which is flexible enough to establish and maintain sister chromatid cohesion as well as ensure the fidelity of chromosome segregation in higher eukaryotes.

Abnormalities of chromosome 1q21 are common in B cell malignancies, but their target genes are largely unknown. By cloning the breakpoints of a (1;14) (q21;q32) chromosomal translocation in a myeloma cell line, we have identified two novel genes, IRTA1 and IRTA2, encoding cell surface receptors homologous to the Fc and inhibitory receptor families. Both genes are selectively expressed in mature B cells: IRTA1 in marginal zone B cells and IRTA2 in centrocytes, marginal zone B cells, and immunoblasts. As a result of the t(1;14), IRTA1 is fused to the immunoglobulin Calpha domain to produce a chimeric IRTA1/Calpha fusion protein. In tumor cell lines with 1q21 abnormalities, IRTA2 expression is deregulated. Thus, IRTA1 and IRTA2 are novel immunoreceptors implicated in B cell development and lymphomagenesis.