senescence

 DNA somatic mutations, poor DNA repair and aging

As cells get older, the amount of DNA damage accumulates overtaking the rate of repair and resulting in a reduction of protein synthesis.

As proteins in the cell are used for numerous vital functions, the cell becomes slowly impaired and eventually dies. When enough cells in an organ reach such a state, the organ itself will become compromised and the symptoms of disease begin to manifest.

Experimental studies in animals, where genes associated with DNA repair were silenced, resulted in accelerated aging, early manifestation of age related diseases and increased susceptibility to cancer.

In studies where the expression of certain DNA repair genes was increased resulted in extended lifespan and resistance to carcinogenic agents in cultured cells.

 Longevity genes and DNA repair

Most lifespan influencing genes affect the rate of DNA damageCertain genes are known to influence variation in lifespan within a population of organisms. Studies in model organisms such as yeast, worms, flies and mice have identified single genes, which when modified, can double lifespan (eg. a mutation in the age-1 gene of the nematode Caenorhabditis elegans).

These genes are known to be associated specifically with cell functions other than DNA repair, but when the pathways that they influence are followed to their final destination, it was observed that they mediate one of three functions: increasing the rate of DNA repair, increasing the rate of antioxidant production, or decreasing the rate of oxidant production.

Therefore, the common pattern across most lifespan influencing genes is in their downstream effect of altering the rate of DNA damage.

 Caloric restriction increases DNA repair

Caloric restriction (CR) has been shown to increase lifespan and decrease age related disease in all organisms where it has been studied, from single celled life such as yeast, to multicellular organisms such as worms, flies, mice and primates.

The mechanism by which CR works is associated with a number of genes related to nutrient sensing which signal the cell to alter metabolic activity when there is a shortage of nutrients, particularly carbohydrates.

When the cell senses a decrease in carbohydrate availability, activation of the lifespan influencing genes DAF-2, AGE-1 and SIR-2 is triggered.

The reason why a shortage of nutrients will induce in a cell a state of increased DNA repair and an increase in lifespan is suggested to be associated with an evolutionarily conserved mechanism of cellular hibernation.

Essentially this permits a cell to maintain a dormant state until conditions that are more favorable are met. During this period, the cell must decrease its normal rate of metabolism and one of the ways it can accomplish this is by reducing genomic instability.

Thus, the cellular rate of aging is mutable and can be influenced by environmental factors such as nutrient availability, which mediate their effect by altering the rate of DNA repair.

 oxydative stress
 energy and caloric consomption
 mitochondrial DNA mutations

 proteasome

• See Carrard G, et al: Impairment of proteasome structure and function in aging. Int J Biochem Cell Biol 34:1461, 2002.

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