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Alteration of DNA Methylation Could Be the Way to Cell Aging
Posted on: June 25, 2003

Methylation of genes, DNA repeated sequences are just a way of gene regulation. Of cause as organism is an integrated system, so the possibility that DNA methylation is involved in aging could not be ignored. By comparing germ cell to any somatic cell due to methylation difference we could decide, if this possibility will be true, or at least find some mechanisms, which can help us to go further into experiment. Let's start from a concern that increased paternal age may be associated with altered fertility and an increased incidence of birth defects in man. Earlier, the abnormalities in the fertility and in the embryos sired by older males were observed. Aging in mammals is associated with alterations in the content and patterns of DNA methylation in somatic cells; however, little is known in regard to germ cells.

Advancing age in men and women has been associated with an increased incidence of parenting abnormal offspring. Because the father donates only chromatin to his offspring, the causal factor(s) for this effect must be of a genetic or epigenetic origin. A candidate epigenetic mechanism that may be involved in male germ cell aging is DNA methylation (e.g. DNA chemical modification). The presence of 5-methylcytosine (m5C) (chemically modified structural element of DNA chain) in the sequence of somatic cell DNA is commonly correlated with decreased gene expression and is also implicated in the stability of chromosomes. Mounting evidence links the presence of m5C to the recruitment of proteins to DNA that maintain stably repressed chromatin. In this manner, patterns of DNA methylation partition the genome into transcriptionally active and inactive regions.

Age-dependent alterations of DNA methylation have been observed in mammalian somatic cells and occur in age-related disease. It is not known to what extent any of these changes may be present in germ cells. New generations of male germ cells are continually produced throughout adulthood. Age-dependent changes in DNA methylation may differ between the cells of old tissues and the cells of tissues that are renewed within an old individual.

The Brown Norway (BN) rat is an inbred strain frequently used as a model of male reproductive aging because it has a long lifespan and a low incidence of age-related disease, yet loses male reproductive function when other systems seem to be unaffected. The abnormalities in the development of embryos sired by aged BN rats were found. Similar results have been obtained when embryos are sired by males treated with the DNA demethylating agent.

For the studying of methylation, liver and purified populations of pachytene spermatocytes and round spermatids as well as caput and cauda spermatozoa were obtained from the testes and epididymidis of young and old male BN rats. A systematic search for global and gene-specific alterations of DNA methylation in germ cells and liver of male rats was done.

The study demonstrated an age-dependent epigenetic defect in male germ cells. Germ cells are some of the most highly differentiated and specialized cell types in the body. Although germ cells are in a constant state of renewal, this study clearly shows that spermatozoa are susceptible to age-dependent alterations of DNA methylation. Further studies are required to determine whether the observed defect in mature sperm is present within the spermatogonial stem cell population or whether a modification of the testicular environment is responsible for the observed age-dependent alterations. Nevertheless, the discovery of altered DNA methylation in male germ cells during aging provides a new avenue of research in the fields of aging, fertility, and male-mediated progeny health. In addition to sperm, all earlier germ cell types, including premeiotic and postmeiotic cells, contained significantly lower levels of global DNA methylation when compared to liver. These findings further underlined a fundamental epigenetic difference between germ cells and somatic tissue.

A number of methods employed revealed a region of the ribosomal DNA locus that was preferentially hypermethylated with age in both spermatozoa and liver. Further was demonstrated that a large proportion of rat ribosomal DNA is normally methylated and that regional and site-specific differences existed in the patterns of methylation between spermatozoa and liver. Might be that these patterns of ribosomal DNA methylation in spermatozoa were vulnerable to the same age-dependent alterations that was observed in normal aging liver. Failure to maintain normal DNA methylation patterns in male germ cells could be one of the mechanisms underlying age-related abnormalities in fertility and progeny outcome.

Alterations in the biology of the ribosome and the nucleolus have been proposed in aging processes. Dysfunctional regulation of ribosome biogenesis would presumably have an impact on protein synthesis and thus have broad implications in cellular aging. Alterations in the patterns of rDNA (ribosomal DNA) methylation may signify impaired function of rDNA. Rodent studies demonstrated that methylation in the rDNA promoter region is correlated with unexpressed rRNA (ribosomal RNA). A previous study revealed that in various somatic tissues of mice, age-dependent methylation occurs specifically in the region of rDNA and this resulted in a loss of rDNA function. Studies by using human tissues also show a loss of rDNA function with age. In older studies, a decrease was shown in detectable rDNA content in various aged tissues and interpreted as a loss of rDNA repeats. The studies showed that the methylation free proportion in the ribosomal DNA locus drops significantly with age to only 5.0% in spermatozoa and 2.3% in liver of total rDNA sequences.

A mechanism of age-dependent rDNA hypermethylation may have involved spreading of methylation that has been described for other genes in age-related disease, such as cancer. The non-transcribed spacers found upstream of rDNA repeats were heavily methylated, and may facilitate this process. How de novo methylation is targeted? It is one of the most fundamental unanswered questions in the study of the biology of DNA methylation. Much evidence supports the notion that repetitive sequences attract de novo methylation. This is observed in imprinted domains, transgenes, and retroviral sequences. It is possible that during aging the repetitive nature of rDNA cistrons makes them a target for de novo methylation over non-repetitive single copy genes.

The bulk of genomic DNA methylation is reprogrammed during preimplantation development. However, the methylation patterns of imprinted genes and some repetitive DNA sequences endure embryonic reprogramming. Whether or not aberrant hypermethylation of rDNA sequences from aged sperm persist into the adult offspring remains to be tested; aberrant methylation of rDNA could be partly responsible for the abnormalities observed in embryos sired by old animals.

Source: Christopher C. Oakes, Dominic J. Smiraglia, Christoph Plass, Jacquetta M. Trasler, and Bernard Robaire; Aging results in hypermethylation of ribosomal DNA in sperm and liver of male rats; PNAS, February 18, 2003, Vol. 100, No. 4: 1775-1780
Picture: court flagella spermatozoid (spermatozoid flagella abnormalities);
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