Telomere position effect in human cells

by Baur, Joseph Anthony.

Telomeres are tracts of repetitive DNA that cap the ends of linear chromosomes. Each time the chromosome is duplicated, a small amount of telomeric DNA is lost from the end due to factors inherent in the mechanism of DNA replication. The result is a net shortening of telomeres with each cell division, unless new repeats are synthesized through the action of the enzyme telomerase. Most human somatic cells lack telomerase activity and so continued cell division leads to telomere shortening. After a limited number of divisions (the “Hayflick limit”), it is believed that a few critically shortened telomeres trigger a state of growth arrest termed replicative senescence. vi Genes near telomeres in yeast and other lower organisms have been shown to be reversibly repressed, resulting in a variegated (mosaic) phenotype. This silencing has been termed telomere position effect, or TPE. Because human telomeres shorten during cell division, a similar effect in human cells could potentially be regulated by the age of the cell. In the present work, telomere position effect was demonstrated in human cells by comparing the expression of a luciferase reporter integrated either next to a telomere or at an internal site. Despite the expected high variability within each group, a ten-fold decrease in average luciferase activity was shown for the telomeric clones. Silencing was relieved by treatment with a histone deacetylase inhibitor or BrdU, indicting that reduced expression was not due to alterations in the gene itself. Elongation of telomeres by telomerase resulted in a two to ten-fold increase in silencing specifically in telomeric clones. When a fluorescent reporter was used, TPE in human cells produced a variegated phenotype, and spontaneous reactivation of the transgene could be detected in non-expressing subclones. A screen of candidate proteins identified hRap1 as a potential mediator of this effect. No effect of telomere length was detected on the expression of several endogenous subtelomeric genes. However, few candidates are currently available since knowledge concerning the detailed structure of most chromosome ends is limited at present. A more detailed analysis of subtelomeric gene expression will be an important future step since relief of silencing in these regions has the potential to play an important role in human aging. vii