Artículo 6 “Films of lutetium bisphthalocyanine nanowires as

3.2.1 Constitutional genetics of ageing

Genetic markers of ageing can be considered in two categories. First constitutional genes which might determine the rate/initiation of one or more of the processes of ageing, and secondly acquired somatic damage to DNA which could result in a loss of cellular functions.

For reason's outlined above measurement of ageing is complex but that one's own genetic makeup influences longevity is in little doubt. Each species has a characteristic mean and maximum life-span, and in man there is a positive correlation between the life-span of parents and their offspring (Goldstein, 1971). Twin studies also support a role for nuclear genes in determining life-span (Martin, 1994). Identification of genes that could be involved is complicated by the diverse changes see in ageing, though some specific associations with extreme old age have been reported: polymorphisms in the genes encoding apolipoprotein E, a major protein in lipid metabolism, and that for angiotensin converting enzyme important in salt and water homeostasis

and control of cell growth, have been found in high frequency in French centogenerians (van Bockxmeer, 1994). Other alleles in the same genes are implicated in the pathogenesis of cardiovascular and neurodegeneratice disease, but the mechanisms contributing to longevity remain indeterminate.

3.2.2 The somatic mutation theorv

First put forward by Szilard in 1959 this theory proposes that the genome is subject to a constant barrage of "ageing hits" throughout life, progressively damaging and inactivating genes in a random fashion. At a critical threshold of damage the probability of an individual dying would approach 1. Along the pathway to this the phenotypic features of ageing would accumulate in a random manner in any given individual. Consistent with this theory is the increase in chromosomal breaks and aneuploidy with advancing age (Goldstein, 1971), but though causes of death and disease accumulate with time they bear no determinate relation to the number of mutations in the body (Comfort, 1969). In addition this theory would predict that the life-span of a species is proportional to the DNA content of it s genome which has not been borne out. Hence random somatic mutation cannot be a sole cause of ageing and death, and such a theory takes no account of known non-genetic disease processes.

3.2.3 Telomeres and telomerase in ageing

One aspect of ageing which is much studied is that of replicative senescence i.e. the loss of proliferative ability in normal (un-transformed) somatic cells (Goldstein, 1990). Human fibroblasts for instance have a finite replicative capacity in vitro, which shows little correlation with the chronological

age of the donor. Much evidence now exists suggesting that the cause of this cellular senescence is the gradual loss of telomeres (Harley, 1991). Telomeres play a critical role in chromosome structure and function, preventing aberrant recombination and attaching the chromosome ends to the nuclear envelope

(Greider, 1990). They are composed of DNA repeats at the 3' end of the chromosome, and are maintained by a particular DNA polymerase called telomerase. Chromosomes lacking telomeres are very unstable and could contribute to dramatic changes in cell function. It has been shown that telomeres shorten during in vitro ageing of many cultured somatic cells, and

that this is accompanied by increasing chromosomal aberrations (Harley, 1991). Immortal cell lines have levels of telomerase activity which seems to balance telomere loss, and more recently it has been shown that initial telomere length predicts replicative capacity of human fibroblasts in culture (Allsopp et al.

1992). The telomere theory of cellular ageing then hypothesizes a progressive loss of telomeres due to incomplete DNA replication and absence of telomerase in somatic cells providing a mitotic clock that ultimately signals cell cycle exit. Only a weak correlation exist between donor age and telomere length but this might represent very different starting points for individuals on which a constant rate of telomere loss is superimposed (Martin at ai. 1993).

Monozygotic twins show greater concordance for telomere length than dizygotic twins, regardless of environment, so genetic factors presumably influence telomere length (Martin, 1994). Further work on the mechanisms by which telomere loss might contribute to the in vivo situation is likely to emerge

over the next few years, though the limitations of the somatic mutation theory will inevitably apply here also.

3.2.4 The error catastrophe theorv of ageing

This postulates that increasing errors in transcription and translation lead to a protein synthetic machinery with progressively lower fidelity, and to the eventual accumulation of a lethal proportion of aberrant proteins in the cell (Goldstein, 1971). Thus cells from older donors would be expected to show higher error frequencies. Senescent cells in culture would also show high error rates, whereas immortal/transformed cells might be expected to have low rates if protein synthetic errors are a major determinant of cell life-span. Studies of

mis-translation on human cells however have not supported this theory as cells from older donors, or senescent cells in culture did not show significantly elevated rates of mis-translation compared to fetal/young cells (Harley et al.

1980).

3.2.6 Apoptosis and ageing

Apoptosis has long been recognized as having a wide incidence throughout the animal kingdom particularly during development and reproduction. The definition encompasses an orderly and characteristic sequence of structural changes seen during physiological cell death, defining built-in cellular mechanisms which can be triggered by extra-cellular factors (Wyllie at a/. 1980). The first stage is nuclear and cytoplasmic condensation

and breaking up of the cell into a number of membrane bound, ultra-structurally well preserved fragments. These apoptotic bodies are then shed from epithelial lined surfaces or taken up by other cells where they are rapidly degraded by lysosomal enzymes. Apoptosis as an active inherently programmed phenomenon has a clear role for instance in embryo-genesis: neurones dying during synapto-genesis, lymphocytes dying during receptor repertoire selection, degeneration of the Müllerian duct in male mammals. The notion that in adult organisms such a "suicide program" might exist is more recent (e.g. elimination of lymphocytes possessing self-reactive receptors, elimination of potential cancer cells) - reviewed by Raff (1992). Evidence also exists for apoptotic mechanisms in neurological diseases (Bredesen, 1996). If this process is so widespread might it not also contribute to tissue decline in ageing? There is evidence that the body's cell mass significantly declines with age, particularly in several areas of the brain (Morris and McManus, 1991), but as yet reports of a role for apoptosis in ageing remain appealing but speculative (Goya, 1986).