Deletional mutations are the basic cause of aging: historical perspectives

DNA double-strand breaks: a potential causative factor for mammalian aging?

Aging is a pleiotropic and stochastic process influenced by both genetics and environment. As a result the fundamental underlying causes of aging are controversial and likely diverse. Genome maintenance and in particular the repair of DNA damage is critical to ensure longevity needed for reproduction and as a consequence imperfections or defects in maintaining the genome may contribute to aging. There are many forms of DNA damage with double-strand breaks (DSBs) being the most toxic. Here we discuss DNA DSBs as a potential causative factor for aging including factors that generate DNA DSBs, pathways that repair DNA DSBs, consequences of faulty or failed DSB repair and how these consequences may lead to age-dependent decline in fitness. At the end we compare mouse models of premature aging that are defective for repairing either DSBs or UV light-induced lesions. Based on these comparisons we believe the basic mechanisms responsible for their aging phenotypes are fundamentally different demonstrating the complex and pleiotropic nature of this process.

The role of DNA damage and repair in aging: new approaches to an old problem

DNA damage and mutations have been implicated as key causal events in the biological process of aging. In this context, it has been hypothesized that the complex of genome maintenance systems acts as a longevity assurance system by signaling and repairing damage or removing cells that are beyond repair. In the past, various approaches have been taken to clarify the importance of preserving genome integrity for healthy aging. Here I will briefly review these approaches in the context of the progress made in improving our understanding of the interrelationship between DNA damage, genome maintenance and mutations.

Declining cellular fitness with age promotes cancer initiation by selecting for adaptive oncogenic mutations

Age is the single most important prognostic factor in the development of many cancers. The major reason for this age-dependence is thought to be the progressive accumulation of oncogenic mutations and epigenetic changes. Similarly, mutagens are thought to be carcinogenic primarily by engendering oncogenic mutations. Yet while the accumulation of heritable somatic changes is expected to augment the incidence of oncogenic mutations, a major effect of increased mutation load is reduced fitness. We propose that the fitness of progenitor cell compartments substantially impacts on the selective advantage conferred by particular mutations. We hypothesize that reduced cellular fitness within aged stem cell pools can select for adaptive oncogenic events and thereby promote the initiation of cancer. Thus, certain oncogenic mutations may be adaptive within aged but not young stem cell pools. We further argue that accumulating genetic alterations with age or mutagen exposure might promote cancer not only by causing oncogenic hits within cells but also by leading to eventual reduction in stem cell fitness, which then selects for oncogenic events. Therefore, initial stages of cancer development may not be limited by the incidence of initiating oncogenic changes, but instead by contexts of reduced cellular fitness that select for these changes.

Impact of genome instability on transcription regulation of aging and senescence

Genomic instability has been implicated as a major stochastic mechanism of aging. Using a transgenic mouse model with chromosomally integrated lacZ mutational target genes, mutations were found to accumulate with age at an organ- and tissue-specific rate. Also, the spectrum of age-accumulated mutations was found to differ greatly from organ-to-organ; while initially similar, mutation spectra of different tissues diverged significantly over the lifetime. To explain how genomic instability, which is inherently stochastic, can be a causal factor in aging, it is proposed that randomly induced mutations may adversely affect normal patterns of gene regulation, resulting in a mosaic of cells at various stages on a trajectory of functional decline, eventually resulting in cell death or neoplastic transformation. To directly address this question, we demonstrate that it is now possible to analyze single cells, isolated from old and young tissues, for specific alterations in gene expression.

Large genome rearrangements as a primary cause of aging

In his introductory chapter of the Mutation Research special issue on 'Genetic Instability and Aging', the late Bernard Strehler provided some historical perspectives on the long-standing hypothesis that aging is primarily caused by changes in the genome of somatic cells (Strehler, 1995, Mutat. Res. 338 (1995) 3). Based on his own findings of a loss of ribosomal RNA gene copies in postmitotic tissues of dogs as well as humans during aging, his main conclusion was that deletional mutations are more likely than point mutations to be a main causal factor in aging. To directly assess the levels of different types of spontaneous mutations in organs and tissues during aging, we have used a mouse model harboring a chromosomally integrated cluster of lacZ-containing plasmids that can be recovered and analyzed in Escherichia coli. Our results indicate the accumulation of mutations in some but not all organs of the mouse with significant differences in mutational spectra. In addition to point mutations, genome rearrangements involving up to 66 Mb of genomic DNA appeared to be a major component of the mutational spectra. Physical characterization of the breakpoints of such rearrangements indicated their possible origin by erroneous, non-homologous DNA double-strand break repair. Based on their increased occurrence during aging in some tissues and their often very large size, we have designed a model for an aging tissue in terms of a cellular mosaic with a gradual increase in genome rearrangements that leads to functional senescence, neoplastic transformation or death of individual cells by disrupting nuclear architecture and patterns of gene regulation.

mtLOH (mitochondrial loss of heteroplasmy), aging, and 'surrogate self'

In tribute to Dr Strehler, an attempt is made to use a style of reasoning found in some of his later papers as an outline of this article. First, general arguments in favor of the involvement of somatic mutations in mtDNA in the aging process are presented. Second, evidence is provided in support of a general tendency of mitochondrial genomes to reach homoplasmic state at the cellular level, for which we propose the term mitochondrial loss of heteroplasmy (mtLOH). This process is likely to facilitate the involvement of mtDNA mutations in the aging process by streamlining the phenotypic expression of the mutant genotype. Third, preliminary evidence of the very high incidence of clonal deletions in pigmented neurons of substantia nigra is reported. This observation highlights the possibility that accumulation of mtDNA mutations specific in certain cell types of a complex tissue may account for the involvement of mtDNA mutations in the aging process despite the relatively low average incidence of these mutations in the tissue as a whole. High incidence of mtDNA deletions in pigmented neurons evokes Strehler's idea that efforts to delay aging may not be the most cost-efficient way of preserving 'self awareness and a joyful sense of life', as he put it. A potential alternative suggested by Strehler, i.e. creation of a 'surrogate self' by computer simulation may deserve more attention than it currently enjoys.

Spectra of spontaneous frameshift mutations at the hisD3052 allele of Salmonella typhimurium in four DNA repair backgrounds

To characterize the hisD3052 -1 frameshift allele of Salmonella typhimurium, we analyzed approximately 6000 spontaneous revertants (rev) for a 2-base deletion hotspot within the sequence (CG)4, and we sequenced approximately 500 nonhotspot rev. The reversion target is a minimum of 76 bases (nucleotides 843-918) that code for amino acids within a nonconserved region of the histidinol dehydrogenase protein. Only 0.4-3.9% were true rev. Of the following classes, 182 unique second-site mutations were identified: hotspot, complex frameshifts requiring DeltauvrB + pKM101 (TA98-specific) or not (concerted), 1-base insertions, duplications, and nonhotspot deletions. The percentages of hotspot mutations were 13.8% in TA1978 (wild type), 24.5% in UTH8413 (pKM101), 31.6% in TA1538 (DeltauvrB), and 41.0% in TA98 (DeltauvrB, pKM101). The DeltauvrB allele decreased by three times the mutant frequency (MF, rev/10(8) survivors) of duplications and increased by about two times the MF of deletions. Separately, the DeltauvrB allele or pKM101 plasmid increased by two to three times the MF of hotspot mutations; combined, they increased this MF by five times. The percentage of 1-base insertions was not influenced by either DeltauvrB or pKM101. Hotspot deletions and TA98-specific complex frameshifts are inducible by some mutagens; concerted complex frameshifts and 1-base insertions are not; and there is little evidence for mutagen-induced duplications and nonhotspot deletions. Except for the base substitutions in TA98-specific complex frameshifts, all spontaneous mutations of the hisD3052 allele are likely templated. The mechanisms may involve (1) the potential of direct and inverted repeats to undergo slippage and misalignment and to form quasi-palindromes and (2) the interaction of these sequences with DNA replication and repair proteins.

Halcyon days with Bill Arnold

The circumstances that led to the discovery that plants luminesce after they are illuminated are described, as are other discoveries that would not have been possible were it not for the fortuitous association I had with my dear and most admirable friend, W.A. Arnold, to whom this special issue is dedicated.