The Sir2 histone deacetylase functions as a chromatin silencer to regulate recombination, genomic stability, and aging in budding yeast. Seven mammalian Sir2 homologs have been identified (SIRT1-SIRT7), and it has been speculated that some may have similar functions to Sir2. Here, we demonstrate that SIRT6 is a nuclear, chromatin-associated protein that promotes resistance to DNA damage and suppresses genomic instability in mouse cells, in association with a role in base excision repair (BER). SIRT6-deficient mice are small and at 2-3 weeks of age develop abnormalities that include profound lymphopenia, loss of subcutaneous fat, lordokyphosis, and severe metabolic defects, eventually dying at about 4 weeks. We conclude that one function of SIRT6 is to promote normal DNA repair, and that SIRT6 loss leads to abnormalities in mice that overlap with aging-associated degenerative processes.

Telomere dysfunction activates p53-mediated cellular growth arrest, senescence and apoptosis to drive progressive atrophy and functional decline in high-turnover tissues. The broader adverse impact of telomere dysfunction across many tissues including more quiescent systems prompted transcriptomic network analyses to identify common mechanisms operative in haematopoietic stem cells, heart and liver. These unbiased studies revealed profound repression of peroxisome proliferator-activated receptor gamma, coactivator 1 alpha and beta (PGC-1α and PGC-1β, also known as Ppargc1a and Ppargc1b, respectively) and the downstream network in mice null for either telomerase reverse transcriptase (Tert) or telomerase RNA component (Terc) genes. Consistent with PGCs as master regulators of mitochondrial physiology and metabolism, telomere dysfunction is associated with impaired mitochondrial biogenesis and function, decreased gluconeogenesis, cardiomyopathy, and increased reactive oxygen species. In the setting of telomere dysfunction, enforced Tert or PGC-1α expression or germline deletion of p53 (also known as Trp53) substantially restores PGC network expression, mitochondrial respiration, cardiac function and gluconeogenesis. We demonstrate that telomere dysfunction activates p53 which in turn binds and represses PGC-1α and PGC-1β promoters, thereby forging a direct link between telomere and mitochondrial biology. We propose that this telomere–p53–PGC axis contributes to organ and metabolic failure and to diminishing organismal fitness in the setting of telomere dysfunction.

Cultured monkey (TC7) and mouse (3T6) cells synthesize an Escherichia coli enzyme, xanthine-guanine phosphoribosyltransferase (XGPRT; 5-phospho-a-D-ribose-1-diphosphate:xanthine phosphoribosyltransferase, EC 2.4.2.22), after transfection with DNA vectors carrying the corresponding bacterial gene, Ecogpt. In contrast to mammalian cells, which do not efficiently use xanthine for purine nucleotide synthesis, cells that produce E. coli XGPRT can, synthesize GMP from xanthine via XMP. After transfection with vector-Ecogpt DNAs, surviving cells producing XGPRT can be selectively grown with xanthine as the sole precursor for guanine nucleotide formation in a medium containing inhibitors (aminopterin and mycophenolic acid) that block de novo purine nucleotide synthesis. Cells transformed for Ecogpt arise with a frequency of 10-4 to 10-5; they appear to be genetically stable in as much as there is no discernible decrease in XGPRT formation or loss in their ability to grow in selective medium after propagation in nonselective medium. (1, 7), the recovery of transformants without a selection is impractical. Therefore, our first goal was to obtain a gene whose expression in the transduced cells would allow them to be grown selectively. That purpose has been achieved by the isolation of a gene from Escherichia coli (Ecogpt) that encodes xanthineguanine phosphoribosyltransferase (XGPRT; 5-phospho-a-Dribose-l-diphosphate:xanthine phosphoribosyltransferase, EC 2.4.2.22), a purine salvage-pathway enzyme.E. coli XGPRT and the analogous mammalian enzyme, hypoxanthine phosphoribosyltransferase (HPRT; IMP pyrophosphate phosphoribosyltransferase, EC 2.4.2.8), catalyze the conversion of hypoxanthine and guanine to IMP and GMP, respectively; the bacterial enzyme also efficiently converts xanthine to XMP (8), a reaction catalyzed only very poorly by the mammalian enzyme (9). We have previously reported that infection of cultured mammalian cells with recombinant DNAs containing the Ecogpt segment induces the synthesis of bacterial XGPRT (5). Moreover, HPRT-negative cell lines transfected with appropriate vectors containing the Ecogpt gene synthesize XGPRT and grow selectively in hypoxanthine/ aminopterin/thymidine medium (5). This finding suggests that E. coli XGPRT can provide the purine salvage function ofmammalian, HPRT After 3 days at 370C in Eagle's medium containing 5% fetal calf serum, the transfected cell monolayers were treated with Abbreviations: SV40, simian virus 40; kb, kilobase(s); XGPRT, xanthineguanine phosphoribosyltransferase; GPRT, guanine phosphoribosylb transferase; HPRT, hypoxanthine phosphoribosyltransferase; T and t, SV40 large and small tumor antigens, respectively; APRT, adenine phosphoribosyltransferase; DHFR, dihydrofolate reductase; TK, thymidine kinase.

Transfection of cultured monkey kidney cells with recombinant DNA constructed with a cloned Escherichia coli gene that codes for xanthine-guanine phosphoribosyltransferase and several different SV40 DNA-based vectors, results in the synthesis of readily measurable quantities of the bacterial enzyme. Moreover, the physiological defect in purine nucleotide synthesis characteristic of human Lesch-Nyhan cells can be overcome by the introduction of the bacterial gene into these cells.