Aging results in hypermethylation of ribosomal DNA in sperm and liver of male rats

Abstract

There is a concern that increased paternal age may be associated with altered fertility and an increased incidence of birth defects in man. In previous studies of aged male rats, we have found abnormalities in the fertility and in the embryos sired by older males. 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. A systematic search for global and gene-specific alterations of DNA methylation in germ cells and liver of male rats was done. Restriction landmark genomic scanning, a method used to determine specific methylation patterns of CpG island sequences, has revealed a region of the ribosomal DNA locus that is preferentially hypermethylated with age in both spermatozoa and liver. In contrast, all single copy CpG island sequences in spermatozoa and in liver remain unaltered with age. We further demonstrate that a large proportion of rat ribosomal DNA is normally methylated and that regional and site-specific differences exist in the patterns of methylation between spermatozoa and liver. We conclude that patterns of ribosomal DNA methylation in spermatozoa are vulnerable to the same age-dependent alterations that we observe 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.A dvancing age in men and women has been associated with an increased incidence of parenting abnormal offspring (1). 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.The presence of 5-methylcytosine (m 5 C) in the CpG dinucleotide sequence of somatic cell DNA is commonly correlated with decreased gene expression and is also implicated in the stability of chromosomes (2). Mounting evidence links the presence of m 5 C to the recruitment of proteins to DNA that maintain stably repressed chromatin (3). In this manner, patterns of DNA methylation partition the genome into transcriptionally active and inactive regions. In mammals, the majority of cytosine bases are methylated at CpG sites. Genomic DNA is relatively CpGpoor due to an accumulation of m 5 C 3 thymine transitions. ''CpG islands'' are regions that are predominantly unmethylated and thus retain their CpG content. CpG islands are commonly found associated with genes.Age-dependent alterations of DNA methylation have been observed in mammalian somatic cells (4, 5) and occur in age-related disease (6). 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 str...

Paternal age and reproduction

“…Genomic imprinting regulates whether the paternally or maternally inherited allele is expressed by silencing the reciprocal allele using methylation-induced blockage of target sequences or other mechanisms controlled by methylation (La Salle and Trasler, 2006). Influence of paternal age on methylation patterns was shown in rats (Oakes et al, 2003), and postulated in the human for diseases such as Huntington disease, Alzheimer's disease, autism or schizophrenia (Farrer et al, 1991(Farrer et al, , 1992Reichenberg et al, 2006;Perrin et al, 2007).…”

Age paternel et risques pour la descendance

“…On ne dispose, actuellement, d'aucune explication concernant la pathogenie des anomalies chromosomiques des gametes liees a I'~ge paternel, a moins d'envisager qu'elles pourraient etre dues & des mutations ou ~ des facteurs epigenetiques divers [34]. En outre, si I'on a la preuve que des anomalies ponctuelles comme les SAD sont bien dues des mutations, on en est pareillement reduit aux hypotheses pour expliquer des variations continues telles que les variations fonctionnelles cerebrales.…”

“…A cSte des mutations, le vieillissement du m~le entrainerait aussi une hypermethylation de I'ADN ribosomal des spermatozoides (et du foie) dans les sequences CpG, ce qui pourrait etre prejudiciable au conceptus [34].…”

“…Si la methylation de C934G jouait un rSle dans ce doublement de frequence, on aurait I& une explication a I'origine paternelle du syndrome d'Apert puisque la methylation de la cytosine des sites CpG est plus importante dans les cellules germinales m&les que dans leurs homologues femelles [15]. Ce qui renvoie ace que nous signalions plus haut concernant I'influence du vieillissement du male sur I'augmentation de la methylation de I'ADN ribosomal [34].…”

“…L'hypermethylation de I'ADN ribosomal des cellules germinales m~les en fonction du vieillissement pourrait egalement jouer un rSle dans cette degradation [34]. Des travaux ont par ailleurs montre que I'ADN mitochondrial etait plus fragile que celui du noyau vis-&-vis des mutations, en particulier vis-a-vis de celles dues aux radicaux libres, qui s'accumulent avec I'age [49].…”