Telomere shortening in human fibroblasts is not dependent on the size of the telomeric-3′-overhang

Telomeres shorten in human somatic cells with each round of DNA replication, and this shortening is thought to ultimately trigger replicative senescence. Telomere shortening is caused partly by the inability of semiconservative DNA replication to copy a linear strand of DNA to its very end. Post-replicative processing of telomeric ends, producing single-stranded G-rich 3' overhangs, has also been suggested to contribute to telomere shortening. This suggestion implies that a positive correlation should exist between the length of 3' overhangs and the rate of telomere shortening. We confirmed shortening of overhangs as human lung (MRC5) and foreskin (BJ) fibroblasts approach senescence by measuring overhang length using in-gel hybridization. However, a large study of fibroblast strains from 21 donors maintained under conditions which lead to two orders of magnitude of variation in telomere shortening rate failed to show any correlation between telomere overhang length and shortening rate, suggesting that overhang length is neither a cause nor a correlate of telomere shortening.