A beginning of the end: new insights into the functional organization of telomeres

Abstract

Ever since the first demonstration of their repetitive sequence and unique replication pathway, telomeres have beguiled researchers with how they function in protecting chromosome ends. Of course much has been learned over the years, and we now appreciate that telomeres are comprised of the multimeric protein/DNA shelterin complex and that the formation of t-loops provides protection from DNA damage machinery. Deriving their name from D-loops, t-loops are generated by the insertion of the 3' overhang into telomeric repeats facilitated by the binding of TRF2. Recent studies have uncovered novel forms of chromosome end-structure that may implicate telomere organization in cellular processes beyond its essential role in telomere protection and homeostasis. In particular, we have recently described that t-loops form in a TRF2-dependent manner at interstitial telomere repeat sequences, which we termed interstitial telomere loops (ITLs). These structures are also dependent on association of lamin A/C, a canonical component of the nucleoskeleton that is mutated in myriad human diseases, including human segmental progeroid syndromes. Since ITLs are associated with telomere stability and require functional lamin A/C, our study suggests a mechanistic link between cellular aging (replicative senescence induced by telomere shortening) and organismal aging (modeled by Hutchinson Gilford Progeria Syndrome). Here we speculate on other potential ramifications of ITL formation, from gene expression to genome stability to chromosome structure.

Keywords: aging; chromosome looping; chromosome structure; genome stability; nuclear lamina; telomere.

Figure 1.

Model of ITL formation. Telomeric DNA (red) associates with ITSs found within non-telomeric DNA (black) to form ITLs. This association is facilitated by an interaction between TRF2 and lamin A/C and may result in heterochromatin spreading and gene inactivation in neighboring regions. In lamin A/C deficient cells (after lamin A/C knockdown or in progerin expressing cells), TRF2 no longer associates with ITSs resulting in a loss of ITL. This may result in altered chromatin state, misregulation of gene expression, loss of chromosome condensation, and telomere instability.

Figure 2.

Small ITS are recombination hot spots. (A) The average recombination rate at small ITSs (red dot) is higher than expected compared to sites placed randomly throughout the genome (black line). Recombination rates were taken from Kong et al. (B) There is no correlation between the number of ITSs in a 10kb genomic window and the frequency of recombination.

Figure 3.

ITL may facilitate chromosome condensation. (A) A schematic of the location of genomic FISH probes used for ITL analysis on mouse chromosome 12. (B) The frequency of inverted telomeric/genomic FISH signals, as depicted in D, for genomic probes shown in A. The inversion frequency decreases with increasing distance from the chromosome end. Analysis was performed on mitotic chromosomes from differentiating mESC. Error bars represent 95% confidence intervals. (C) Metaphase chromosomes show a higher frequency of inverted telomeric/MMU12-D FISH signals than prometaphase chromosomes. Analysis was performed on mitotic chromosomes from differentiating mESCs. Error bars represent 95% confidence intervals. *P < 0.001, Student's t-test. (D) A schematic of an inverted chromosome structure. (E) Metaphase, but not prometaphase chromosomes isolated from differentiating mESCs are longer than those from T cells. Values represent mean ± s.e.m. *P < 0.001, Student's t-test. (F) The inverted region of the chromosome was determined by measuring the distance from a telomere FISH signal to the end of the chromosome, as shown in D. This value was reported as a percent of the total chromosome length. T cells show a larger region of inversion than differentiating mESCs for metaphase, but not prometaphase chromosomes. Values represent mean ± s.e.m. *P < 0.001, Student's t-test.