Probing PML body function in ALT cells reveals spatiotemporal requirements for telomere recombination

Abstract

Promyelocytic leukemia (PML) bodies (also called ND10) are dynamic nuclear structures implicated in a wide variety of cellular processes. ALT-associated PML bodies (APBs) are specialized PML bodies found exclusively in telomerase-negative tumors in which telomeres are maintained by recombination-based alternative (ALT) mechanisms. Although it has been suggested that APBs are directly implicated in telomere metabolism of ALT cells, their precise role and structure have remained elusive. Here we show that PML bodies in ALT cells associate with chromosome ends forming small, spatially well-defined clusters, containing on average 2-5 telomeres. Using an innovative approach that gently enlarges PML bodies in living cells while retaining their overall organization, we show that this physical enlargement of APBs spatially resolves the single telomeres in the cluster, but does not perturb the potential of the APB to recruit chromosome extremities. We show that telomere clustering in PML bodies is cell-cycle regulated and that unique telomeres within a cluster associate with recombination proteins. Enlargement of APBs induced the accumulation of telomere-telomere recombination intermediates visible on metaphase spreads and connecting heterologous chromosomes. The strand composition of these recombination intermediates indicated that this recombination is constrained to a narrow time window in the cell cycle following replication. These data provide strong evidence that PML bodies are not only a marker for ALT cells but play a direct role in telomere recombination, both by bringing together chromosome ends and by promoting telomere-telomere interactions between heterologous chromosomes.ALT-associated PML bodies (APBs) ͉ alternative lengthening of telomeres (ALT) ͉ Herpes simplex virus ICP0 ͉ telomere clusters P ML (promyelocytic leukemia) nuclear bodies, which are present in most cells, have been extensively studied and a wealth of information concerning their composition, structure, dynamics and function is available (1). The PML protein is essential for the formation of PML bodies and provides the structural scaffold to which other proteins bind. Over 60 additional proteins have been localized to PML bodies spatially or temporally [see the information on PML bodies from the Nuclear Protein Database, (2)], and as a consequence, PML bodies have been implicated in the regulation of virtually every biological function including DNA damage responses.In immortal cells that maintain telomeres by recombinationbased alternative lengthening mechanisms (ALT), a special variety of PML bodies is found that, in addition to the proteins normally associated with PML bodies, contain telomeric DNA, telomerespecific proteins, and DNA recombination and repair proteins (3). The structure and the role of these ALT-associated PML bodies (APBs) are unknown. There is, however, a clear association between the presence of APBs and the utilization of ALT for telomere maintenance. However, in those studies disruptions of APBs have often been achiev...

PML induces compaction, TRF2 depletion and DNA damage signaling at telomeres and promotes their alternative lengthening

“…We found here that APB assembly in U2OS cells was inhibited by an 'open' telomeric chromatin state, as the knockdown of several repressive chromatin modifiers, as well as chromatin decondensation initiated by HDAC inhibition or HMGN5 recruitment, resulted in a The formation of a PML subcompartment at telomeres induces telomeric chromatin compaction and clustering of telomeres, and possibly also ECTRs, as proposed previously (Cho et al, 2014;Draskovic et al, 2009). As a result of APB formation, TRF2 becomes partly depleted at associated telomeres.…”

“…5). First, formation of a PML subcompartment at telomeres induces telomeric chromatin compaction and clustering of telomeres, and possibly also ECTRs, as proposed previously (Cho et al, 2014;Draskovic et al, 2009). Second, as a result of APB formation, TRF2 becomes partly depleted at associated telomeres.…”

“…artificially enlarged APBs (Draskovic et al, 2009) and that damaged telomeres preferentially cluster with telomeres that are associated with PML in ALT-positive cells (Cho et al, 2014). Notably, long-term PML knockdown induced telomere shortening and significantly increased the number of chromosomal ends where a telomere repeat signal was absent.…”

Telomere Maintenance Mechanisms in Soft Tissue Sarcomas

“…Others have found that disruption of APBs prevents telomere maintenance by ALT and leads to loss of culture viability, arguing for an active role of APBs in telomere maintenance by ALT (Jiang et al, 2005). The spatio-temporal dynamics of telomeric DNA association with PML bodies have been recently described, and the authors of this study conclude that telomere recombination takes place in these structures (Draskovic et al, 2009). Furthermore, new PML bodies form at telomeric DNA regardless of which TMM is active (Brouwer et al, 2009) and APBs form transiently in human diploid fibroblasts following high LET radiation (Berardinelli et al, 2010), suggesting that the association of telomeric DNA with PML bodies may be a component of a DNA damage response.…”

Telomeric function of mammalian telomerases at short telomeres

“…6A). hTR localization with ALT-associated PML bodies (APBs) could also occur because APBs are formed by the coalescence of extremities with long and short telomeres (Draskovic et al, 2009). Localization of hTR to telomeres in h/h5 cells suggests that the telomerase complex in these cells can be recruited to telomeres.…”