IAP RETROTRANSPOSONS IN THE MOUSE-LIVER AS REPORTERS OF AGING

IAP are endogenous retrovirus-like elements present at a thousand copi es in the murine genome. They can modulate the level of expression of the tagged genes into which they have inserted, and conversely their a ctivity could be influenced by the level of activity of the genes and/ or DNA sequences into which they are embedded. In this report, we have analysed by Northern blots the pattern of expression of the IAP-relat ed transcripts in the organs of young and ageing mice. We show that IA P transcripts of unexpected size (namely 10 kb and 6 kb) are induced i n the liver of ageing mice from all inbred and hybrid strains tested. These transcripts are not detected in young mice, and their intensity disclose variations depending on the strain, as those observed for the two canonical 7.2 and 5.4 kb IAP transcripts. It is suggested that th ese age-dependent IAP transcripts originate from unique sites within t he mouse genome that are 'tagged' by an IAP sequence, which would be s ensitive both to strain-dependent cellular factors acting at the level of all IAPs, and to an age-dependent liver-specific cellular factor a nd/or DNA state, responsible for the position-dependent effect. These age-dependent transcripts should allow the identification of putative genes or factors of 'senescence'.