KEYNOTE ADDRESS�Ku80-deletion causes early ageing and suppresses cancer

Abstract

Ageing is widespread cellular decline resulting in a loss of fitness that is both pleiotropic and stochastic and influenced by both genetics and environment. As a result the fundamental underling causes of ageing are diverse and controversial. One potential ageing target is nuclear DNA, as it is a permanent blueprint that controls cellular processes. Thus, DNA replication and genome maintenance are highly regulated events that ensure faithful reproduction and maintenance of the blueprint and these pathways assure sufficient longevity for reproduction and survival of the species. As a consequence, imperfections or defects in maintaining the genome may contribute to ageing. Therefore, genome maintenance pathways are longevity-assurance mechanisms that sustain an organism long enough to reproduce and propagate. Chief among these mechanisms are those that respond to damaged DNA. There are two basics responses to genomic damage: DNA repair and cell cycle checkpoints. Both are considered to be tumour suppressors and are categorized as either caretakers or gatekeepers, respectively. Interestingly, observations of human and mouse pre-mature ageing models suggest these anti-tumour pathways impact the ageing process. Caretakers suppress cancer by repairing DNA damage caused by defects in replication or by a variety of agents including endogenously produced reactive by-products of oxygen metabolism and exogenous agents naturally encountered in our environment. As a consequence DNA is subject to a variety of insults that cause a diverse range of lesions and phenotypic outcomes. There are many forms of DNA damage including base lesions and double-strand breaks (DSBs) with the latter being more toxic. Cancer-causing chromosomal rearrangements may result if DSBs are not repaired properly. Additionally, an accumulation of these rearrangements may contribute to ageing since they increase in some cell types as humans and mice age. Furthermore, early ageing models suggest that defects in repairing DSBs lead to early ageing in humans and mice. Non-homologous end joining (NHEJ) is an important pathway for repairing DNA DSBs and is considered a caretaker. The Ku heterodimer (composed of Ku70 and Ku80) binds to DNA ends to initiate NHEJ, and defects in either Ku70 or Ku80 lead to increased levels of DNA DSBs and chromosomal rearrangements, leading many to believe Ku is a caretaker. Ku-mutant mice display increased GCRs, but without increased cancer. Instead, these mice show early ageing and shortened life span. Thus, Ku's role as a caretaker is uncertain as the low cancer levels may be due to Ku80-deletion or, instead, the low cancer levels may simply be a consequence of the shortened life span that prohibits sufficient time for tumours to develop. Gatekeepers respond to DNA damage by halting the cell cycle long enough for the DNA to be repaired. If the damage is irreparable, gatekeepers induce either apoptosis or senescence. These responses are deleterious to the cell but protect the organism from cancer as one potential outcome of genetic mutations is uncontrolled proliferation. p53 is critical for checkpoints and is the best-known gatekeeper because it is mutated in over half of all cancers. In addition, p53 activity influences many aspects of the Ku-mutant phenotype suggesting that Ku-deletion leads to persistent p53-mediated responses and presenting the possibility that low cancer levels and early ageing are caused by elevated gatekeeper responses. Our hypothesis is that Ku-mutant mice exhibit low cancer levels and, perhaps, ageing due to persistent p53-mediated responses to inefficiently repaired DNA. To test this hypothesis, Ku80-mutant mice were crossed to cancer-prone mice with either non-functional or functional gatekeeper responses. Ku80-mutant mice were crossed to p53-mutant mice to determine if Ku80-deletion exacerbates oncogenesis when gatekeeper responses are diminished. Ku80-mutant mice deleted for p53 exhibit early onset and high levels of two forms of cancer: pro-B cell lymphoma and medulloblastoma, thus supporting the hypothesis. Ku80-mutant mice were also crossed to APCMIN mice to determine if Ku80-deletion ameliorates oncogenesis gatekeeper responses are intact. APCMIN mice exhibit high levels of intestinal adenomas and adenocarcinomas but have normal p53-mediated responses to DNA damage. APCMIN mice, deleted for Ku80, exhibit about 67% fewer tumours than APCMIN mice with Ku80. Thus, deletion of Ku80 suppresses tumour formation, again supporting the hypothesis. Ku80-mutant cells and tissues were tested for p53-mediated DNA damage responses, levels of DNA damage, and mutations. Ku80-mutant fibroblasts exhibit elevated levels of p53-mediated DNA damage responses that increase p21-mediated cellular senescence. In addition, there are elevated levels DNA damage as seen by increased 53BP1 foci and elevated levels of chromosomal rearrangements. Thus, these data support the hypothesis that Ku80-deletion reduces tumors by elevating DNA damage gatekeeper responses to inefficiently repaired DNA. These data also support the possibility that the Ku80-mutant ageing phenotype is also due to elevated gatekeeper responses.