Exploring the Stability of Genomic Imprinting and X-Chromosome Inactivation in the Aged Brain (RNA-Seq)

DNA methylation is a common mechanism regulating monoallelic expression of different genes, including imprinted genes. How age-induced alterations of methylation levels vary across alleles and impact on monoallelic expression is unknown. In the brain, this phenomenon may contribute mechanistically to neuronal dysfunction and disease. In order to investigate the allele specific transcriptional and epigenetic signatures of aging, we used key brain areas, hippocampus and cerebellum, of juvenile and old hybrid mice obtained from BL6×CAST/EiJ (CAST) reciprocal crosses. We set out to explore whether epigenetic drift affects global DNA methylation machinery by investigating 5- methylcytosine (5-mC) and 5-hydroxymethylcytosine (5-hmC) levels. Mass spectrometry analysis showed a significant increase of 5hmC levels in hippocampus of old mice, but this was not accompanied by measurable differences in methylating and demethylating enzymes. The expression of several repetitive elements (LINEs, SINEs, IAP) was also analysed, strictly dependent on 5mC and 5hmC levels, and reporting a reduction of the intracisternal A-particle (IAP) retrotransposons in HCP of old mice. We uncovered the allelic-specific DNA methylation profile of aging using genomic imprinting as a read-out by IMPLICON and allelic expression by RNA sequencing. Here, imprinting is not substantially changed during aging, not affecting their normal monoallelic expression. This work provides the first epigenetic and transcriptional landscape of aging with allelic resolution and confirms that genomic imprinting is a stable epigenetic phenomenon across the whole lifespan.