Genetic basis of limited cell proliferation

Pathomolecular effects of homocysteine on the aging process: A new theory of aging

Homocysteine has been associated with the most common age-related diseases but never associated with the acceleration of the aging process. This theoretical paper will try to demonstrate the pro-aging effects of homocysteine at the molecular, cellular, and organ level. High homocysteine levels in homocystinuria are associated with premature disease of the cardiovascular, skeletal, neurological, and other systems. These observations are similar to those noted in the aging process and should be considered as a progeroid syndrome. There is enough scientific evidence to support that homocysteine accelerates the aging process at the cellular and at the organism level. Most importantly, decreasing homocysteine levels by dietary or pharmacological interventions could prolong maximum life span in humans and/or delay the onset of the most common age-related diseases.

Aging and longevity. A paradigm of complementation between homeostatic mechanisms and genetic control?

Aging is a universal and inevitable phenomenon that affects nearly all animal species. It can be considered the product of an interaction between genetic, environmental, and lifestyle factors, which in turn influence longevity that varies between and within species. It has been proposed not only that the aging process is under genetic control, but that it can also be considered a result of the failure of homeostasis due to the accumulation of damage. This review article discusses these issues, focusing on the function of genes that associate with aging and longevity, as well as on the molecular mechanisms that control cell survival and maintenance during aging.

Aging, increasing genomic entropy, and neurodegenerative disease

The genome, as biologic information, can be conceptualized in terms of entropy. The second law of thermodynamics dictates that entropy must increase over time. Consequently, aging can be viewed as increasing genomic entropy. Genetic instability is the biophysical correlate of increasing genomic entropy. Rates of increasing genomic entropy can be determined from age-specific mortality rate dynamics (e.g., Parkinson's disease and amyotrophic lateral sclerosis). These observations are consistent with a model of neurodegenerative disease as a manifestation of increasing genomic entropy with aging.

The origins of human ageing

The origins of human ageing are to be found in the origins and evolution of senescence as a general feature in the life histories of higher animals. Ageing is an intriguing problem in evolutionary biology because a trait that limits the duration of life, including the fertile period, has a negative impact on Darwinian fitness. Current theory suggests that senescence occurs because the force of natural selection declines with age and because longevity is only acquired at some metabolic cost. In effect, organisms may trade late survival for enhanced reproductive investments in earlier life. The comparative study of ageing supports the general evolutionary theory and reveals that human senescence, while broadly similar to senescence in other mammalian species, has distinct features, such as menopause, that may derive from the interplay of biological and social evolution.

What is the relationship between osteoarthritis and ageing?

The relationship between osteoarthritis and ageing raises important questions about what exactly defines 'normal' ageing and whether the pathogenesis of osteoarthritis shares common pathways with other age-associated dysfunctions, or whether osteoarthritis is a time-dependent disorder distinct from normal ageing with a separate causative mechanism at work. Theories of ageing now emphasize the stochastic nature of the ageing process, that is the role played by accumulation of essentially random cell and tissue damage, such as somatic mutations, oxidative damage and the formation of aberrant proteins. The role of genetic factors in determining longevity and predisposition to age-associated diseases is probably in programming the efficiency of somatic maintenance functions and in influencing the development of a durable soma. Gene-environment interactions, for example through lifestyle, can also be important. Many of the risk factors and mechanisms that are thought to contribute to osteoarthritis can be accommodated within this framework.

Osteoporosis, sedentary lifestyle, and increasing hip fractures: pathogenic relationship or differential survival bias

Osteoporosis, although a disorder of antiquity, has become more prevalent in developed countries and is a major risk factor for skeletal fracture. Accordingly, the increasing incidence of hip fracture among the elderly within developed nations has been attributed to an increased prevalence of osteoporosis. An increasingly sedentary lifestyle has been suggested as a significant contributing factor for the increased prevalence of osteoporosis. However, differential survival, reflecting changing competing mortality risks, will alter the gene pool of a surviving population cohort. Thus, the gene pool (and hence, disease susceptibilities) of 70-year-old individuals in 1990, for example, should not implicitly be assumed to be the same as 70-year-old individuals in 1950. Consequently, differences in the prevalence of osteoporosis or incidence of hip fracture between current and past elderly cohorts do not necessarily imply differences in environmental risk factors such as levels of physical activity. Instead, variation in competing mortality risks over time may produce differential survival with selection bias and "naturally" lead to increases in the incidence and prevalence of some aging-related disorders such as osteoporosis.

The search for a gene involved in the determination of limited duplicative capacity in human cells

On the assumption that EBV integration into the genome of human B lymphocyte might lead to the inactivation of a hypothetical gene determining the limited duplicative capacity and consequently participate to the cell immortalization, a search for the human-virus junction was done. This led to the identification of a site of integration in the central part of the heavy chain of the immunoglobulin region. The coincidence of the involvement of the site in lymphomatogenesis with the first complete characterization of an integration site led to the speculation that the heavy chain gene itself might be an important controller of cell duplication in the B lymphocyte.

Aging, genomic entropy and carcinogenesis: implications derived from longitudinal age-specific colon cancer mortality rate dynamics

Many types of cancer are intrinsically linked to the process of aging. Aging, from the perspective of the second law of thermodynamics, can be viewed as associated with the inevitable and natural increase in informational entropy of the genome. The molecular biologic basis of increasing genetic informational entropy is the inherent and variable instability of different regions of genome. Colon cancer cells have been shown to have characteristic acquired genetic abnormalities, most commonly, deletions in presumed tumor suppressor genes. Age-specific colon cancer mortality rates in the US from 1958 to 1988 were subjected to longitudinal Gompertzian analysis, a method that may identify and distinguish among genetic, environmental and competitive influences upon mortality. The Strehler-Mildvan modification of the Gompertz relationship between aging and mortality can be used to determine a relative measure of the rate of increase in informational entropy (a reflection of genetic instability) for those genetic factors that are involved in the pathogenesis of colon cancer.

Biomarkers of aging: correlation of DNA I-compound levels with median lifespan of calorically restricted and ad libitum fed rats and mice

I-compounds are species-, tissue-, genotype-, gender-, and diet-dependent bulky DNA modifications whose levels increase with animal age. While a few of these DNA modifications represent oxidation products, the majority of I-compounds appear to be derived from as yet unidentified endogenous DNA-reactive intermediates other than reactive oxygen species. Circadian rhythms of certain I-compounds in rodent liver imply that levels of these DNA modifications are precisely regulated. Caloric restriction (CR), the currently most effective method available to retard aging and carcinogenesis, has been previously shown to elicit significant elevations of I-compound levels in tissue DNA from Brown-Norway (BN) and F-344 rats as compared to age-matched ad libitum fed (AL) animals. The present investigation has extended this work by examining liver and kidney DNA I-compound levels in three genotypes of rats (F-344, BN, and F-344 x BN) and two genotypes of mice (C57BL/6N and B6D2F1) under identical experimental conditions in order to determine whether correlations exist between I-compound levels, measured in middle-aged animals, and median lifespan. Levels of a number of liver and kidney I-compounds were found to display genotype- and diet-dependent, statistically significant positive linear correlations with median lifespan in both species. In particular, the longer-lived hybrid F-344 x BN rats and B6D2F1 mice tended to exhibit higher I-compound levels than the parent strains. CR enhanced I-compound levels substantially in both rats and mice. Thus, I-compounds, measured at middle age, reflected the functional capability ('health') of the organism at old age, suggesting their predictive value as biomarkers of aging. The positive linear correlations between levels of certain I-compounds (designated as type I) and lifespan suggest that these modifications may be functionally important and thus not represent endogenous DNA lesions (type II), whose levels would be expected to correlate inversely with lifespan.

Prospects for the genetics of human longevity

Longevity varies between and within species. The existence of species-specific limit to human life-span and its partial heritability indicate the existence of genetic factors that influence the ageing process. Insight into the nature of these genetic factors is provided by evolutionary studies, notably the disposable soma theory, which suggests a central role of energy metabolism in determining life-span. Energy is important in two ways. First, the disposable soma theory indicates that the optimum energy investment in cell maintenance and repair processes will be tuned through natural selection to provide adequate, but not excessive, protection against random molecular damages (e.g. to DNA, proteins). All that is required is that the organism remains in a sound condition through its natural expectation of life in the wild environment, where accidents are the predominant cause of mortality. Secondly, energy is implicated because of the intrinsic vulnerability of mitochondria to damage that may interfere with the normal supply of energy to the cell via the oxidative phosphorylation pathways. Oxidative phosphorylation produces ATP, and as a by-product also produces highly reactive oxygen radicals that can damage many cell structures, including the mitochondria themselves. Several lines of evidence link, on the one hand, oxidative damage to cell ageing, and on the other hand, energy-dependent antioxidant defences to the preservation of cellular homeostasis, and hence, longevity. Models of cellular ageing in vitro allow direct investigation of mechanisms, such as oxidative damage, that contribute to limiting human life-span. The genetic substratum of inter-individual differences in longevity may be unraveled by a two-pronged reverse genetics approach: sibling pair analysis applied to nonagenarian and centenarian siblings, combined with association studies of centenarians, may lead to the identification of genetic influences upon human longevity. These studies have become practicable thanks to recent progress in human genome mapping, especially to the development of microsatellite markers and the integration of genetic and physical maps.