Aging chromosome telomeres: parallel studies with terminal repeat and telomere associated DNA probes.

Human chromosome telomeres consist of tandemly repeated (TTAGGG)n sequences with variant and more complex telomere-associated DNA sequences proximal to the terminal repeats. Terminal restriction fragment (TRF) sizes have been evaluated by Southern blot analysis using a terminal repeat probe, (TTAGGG)3 that will simultaneously detect all telomeres and with a telomere-associated DNA probe, TelBamll, that identifies a specific sub-group of chromosome ends. For DNA extracted from in vitro aging fibroblasts, a progressive reduction in the size of the TRFs could be demonstrated using both probes. For both fibroblasts and adult lymphocyte DNA, there were differences in the size of the fragments detected with the two probes. Studies were carried out to determine whether this difference might, in part, be attributable to variability in terminal repeat lengths as well as heterogeneity in the location of terminal restriction enzyme recognition sites. Using the (TTAGGG)3 probe to identify all telomeres, the terminal repeat lengths from lymphocytes of two adults appeared to be highly variable with sizes upto 20 kb. For the sub-group of telomeres identified by TelBamll the terminal repeat lengths were estimated to be 2-4 kb and appeared to show relatively little size diversity. If it assumed that the molecular weights of the DNA fragments identified in these studies do accurately reflect individual telomere structures, then it can be concluded that some specific telomere repeat arrays are substantially shorter than others. Variation in terminal repeat length may be related to the extent that telomeres participate in chromosome rearrangement.