SUMMARY The sirtuin gene family (SIRT) is hypothesized to regulate the aging process and play a role in cellular repair. This work demonstrates that SIRT3−/− mouse embryonic fibroblasts (MEFs) exhibit abnormal mitochondrial physiology as well as increases in stress-induced superoxide levels and genomic instability. Expression of a single oncogene (Myc or Ras) in SIRT3−/− MEFs results in in vitro transformation and altered intracellular metabolism. Superoxide dismutase prevents transformation by a single oncogene in SIRT3−/− MEFs and reverses the tumor permissive phenotype as well as stress-induced genomic instability. In addition, SIRT3−/− mice develop ER/PR-positive mammary tumors. Finally, human breast and other human cancer specimens exhibit reduced SIRT3 levels. These results identify SIRT3 as a genomically expressed, mitochondrial localized tumor suppressor.

Tumour suppressor genes (TSGs) inhibiting normal cellular growth are frequently silenced epigenetically in cancer. DNA methylation is commonly associated with TSG silencing, yet mutations in the DNA methylation initiation and recognition machinery in carcinogenesis are unknown. An intriguing possible mechanism for gene regulation involves widespread non-coding RNAs such as microRNA, Piwi-interacting RNA and antisense RNAs. Widespread sense-antisense transcripts have been systematically identified in mammalian cells, and global transcriptome analysis shows that up to 70% of transcripts have antisense partners and that perturbation of antisense RNA can alter the expression of the sense gene. For example, it has been shown that an antisense transcript not naturally occurring but induced by genetic mutation leads to gene silencing and DNA methylation, causing thalassaemia in a patient. Here we show that many TSGs have nearby antisense RNAs, and we focus on the role of one RNA in silencing p15, a cyclin-dependent kinase inhibitor implicated in leukaemia. We found an inverse relation between p15 antisense (p15AS) and p15 sense expression in leukaemia. A p15AS expression construct induced p15 silencing in cis and in trans through heterochromatin formation but not DNA methylation; the silencing persisted after p15AS was turned off, although methylation and heterochromatin inhibitors reversed this process. The p15AS-induced silencing was Dicer-independent. Expression of exogenous p15AS in mouse embryonic stem cells caused p15 silencing and increased growth, through heterochromatin formation, as well as DNA methylation after differentiation of the embryonic stem cells. Thus, natural antisense RNA may be a trigger for heterochromatin formation and DNA methylation in TSG silencing in tumorigenesis.

The term epigenetics was coined in the context of developmental studies, but the meaning of the term has evolved over time. Epigenetic modulators of gene expression are now known to include DNA methylation, chromatin modifications and noncoding RNAs. The observation that epigenetic changes can be transmitted transgenerationally makes the science of epigenetics very relevant to the field of environmental and molecular toxicology. Heavy metals constitute an important class of environmental contaminants that have been known to influence gene expression directly by binding various metal response elements in the target gene promoters. Recent research suggests that metals can also influence gene expression through epigenetic mechanisms; this adds a new twist to the complexity of metal-mediated gene expression. Here, we review recent studies that investigate the epigenetic, gene expression, and biological effects of various inorganic and organic forms of heavy metals, such as cadmium, arsenic, nickel, chromium, methylmercury, lead, copper and organotin compounds.

The ribosome-associated molecular chaperone complexes RAC (Ssz1p/Zuo1p) and Ssb1p/Ssb2p expose a link between protein folding and translation. Disruption of the conserved nascent peptide-associated complex results in cell growth and translation fidelity defects. To better understand the consequences of deletion of either RAC or Ssb1p/2p, experiments relating to cell growth and programmed ribosomal frameshifting (PRF) were assayed. Genetic analyses revealed that deletion of Ssb1p/Ssb2p or of Ssz1p/Zuo1p resulted in specific inhibition of ؊1 PRF and defects in Killer virus maintenance, while no effects were observed on ؉1 PRF. These factors may provide a new set of targets to exploit against viruses that use ؊1 PRF. Quantitative measurements of growth profiles of isogenic wild-type and mutant cells showed that translational inhibitors exacerbate underlying growth defects in these mutants. Previous studies have identified ؊1 PRF signals in yeast chromosomal genes and have demonstrated an inverse relationship between ؊1 PRF efficiency and mRNA stability. Analysis of published DNA microarray experiments reveals conditions under which Ssb1, Ssb2, Ssz1, and Zuo1 transcript levels are regulated independently of those of genes encoding ribosomal proteins. Thus, the findings presented here suggest that these trans-acting factors could be used by cells to posttranscriptionally regulate gene expression through ؊1 PRF.Programmed ribosomal frameshifting (PRF) is a posttranscriptional regulatory mechanism in which elongating ribosomes are directed to shift the reading frame in response to specific cis-acting signals in mRNAs. Many viruses, including human immunodeficiency virus type 1, use PRF to optimize the stoichiometric ratios between their structural and enzymatic proteins (5, 11). Changes in PRF efficiency alter those ratios, inhibiting virion morphogenesis (9). In particular, retroviruses appear to be very susceptible to such changes (4, 21, 33, 37). Thus, elucidation of the molecular mechanisms underlying PRF can aid in the rational design of antiviral therapeutics (reviewed in reference 8). Though first discovered in viruses, it is also becoming apparent that programmed Ϫ1 ribosomal frameshifting (Ϫ1 PRF) is also used to regulate expression of cellular genes (reviewed in reference 34). Given the widespread use of PRF, it is important to understand the molecular mechanisms underlying PRF and to identify cellular factors that might be used to regulate these processes.Ribosomes can be directed to shift by one base in either the 5Ј or 3Ј direction depending on the cis-acting signal. Programmed Ϫ1 ribosomal frameshifting is the result of a net shift of the translational reading frame by 1 base in the 5Ј direction. The shift is typically directed by a tripartite cis-acting mRNA element composed of a (from 5Ј to 3Ј) heptameric slippery site, a spacer, and a thermodynamically stable structure, typically an mRNA pseudoknot (reviewed in reference 5). It is generally accepted that Ϫ1 PRF requires a change in the forward kinetics...