Age -associated reactivation of the X chromosome.

Abstract

Deterioration of genetic information with age may involve both damage to genomic DNA itself (genetic mutation) or damage to the regulated use and accessibility of the genome (epigenetic mutation). In eutherian mammals, all except one of the X chromosomes are inactivated in a cell. Since the coordinate repression of a large number of X-linked genes is mediated by epigenetic gene silencing, X inactivation is an attractive model for the study of the stability of epigenetic gene regulation. An experimental strategy was designed to determine: (1) if the transcriptional inactivation status of the X chromosome changes with age, (2) if age-related X reactivation is tissue specific, and (3) if X chromosome reactivation occurs in <italic>cis</italic> along the whole X chromosome. Mice carrying Searle's X:autosome translocation T(X; 16)16H developed with nonrandom inactivation of the nontranslocated X chromosome and were collected in aged cohorts across the adult lifespan (2 to 24 months). Using an optimized fluorescent-labeled primer extension (FluPE) assay, sequence differences between homologous genes on the active and inactive X chromosomes were used to quantify the relative levels of allelic mRNA present in steady state pools from whole tissues. Our results show a significant age-related reactivation of transcription from the <italic>Atp7a</italic> gene producing a maximum of 5% of total <italic> Atp7a</italic> transcripts from the presumptive inactive X chromosome. Reactivation is not restricted to a single tissue. Gene expression from the inactive X chromosome was observed in both kidney and spleen RNA. The limited allelic differences between the X chromosomes of T(X; 16)16H and other inbred strains prevented the examination of reactivation as a <italic>cis</italic>-coordinated control phenomenon.