The absence of (TCAGGG)n repeats in some telomeres, combined with variable responses to NR2F2 depletion, suggest that this nuclear receptor plays an indirect role in the alternative lengthening of telomeres

Abstract

The alternative lengthening of telomeres (ALT) facilitates telomere lengthening by a DNA strand invasion and copying mechanism. The nuclear receptors (NRs), NR2F2 and NR2C2, can bind to (TCAGGG)n variant repeats within telomeres and it has been proposed that this facilitates telomere interactions in ALT+ cells. Here we show that the frequency of cells with detectable NR2F2 and NR2C2 nuclear foci varies considerably between ALT+ cell lines and does not correlate with the level of protein expression. In addition, four of five ALT+ cell lines lack (TCAGGG)n repeats in some telomeres, indicating that direct NR binding does not play a role in ALT at these telomeres. NR2F2-depletion altered the abundance of C-circles and APBs but the direction of the response was inconsistent between three ALT+ cell lines. Moreover, transcriptome analysis following NR2F2-depletion in the ALT+ cell lines revealed different very responses. For example, NR2F2-depletion down-regulated many genes in U2OS cells, consistent with the cell cycle arrest and changes to ALT markers, but these features were not shared by the other two ALT+ cell lines. Among 86 ALT-associated genes, only MND1 showed consistent down-regulation across three NR2F2-depleted ALT+ cell lines. Altogether our data suggest that NR2F2 does not play a direct role in ALT and we speculate about an alternative role for this NR in a DNA damage response at telomeres.

Telomere Maintenance Pathway Activity Analysis Enables Tissue- and Gene-Level Inferences

“…In the RAD51-dependent pathway, telomere-bound POT1 is replaced by RPA, followed by RAD51 recruitment and strand invasion. Low expression of BLM helicase and of the nuclear receptor NR2F2 potentially limits hyperactivity of ALT in this tissue ( Conomos et al, 2014 ; Sobinoff et al, 2017 ; Zhang et al, 2021 ), although it has recently be shown that in some cases NR2F2 may not be directly involved in ALT ( Alhendi and Royle, 2020 ). Interestingly, a recent study of Episkopou et al (2019) have identified that the expression of the testis-specific Y-encoded-like protein 5 ( TSPYL5 ), a previously unrecognized APB body component, is crucial for survival of ALT + cells, as it protects replaced POT1 from proteasomal degradation.…”

Telomere maintenance pathway activity analysis enables tissue- and gene-level inferences

“…In the RAD51-dependent pathway, telomere-bound POT1 is replaced by RPA, followed by RAD51 recruitment and strand invasion. Low expression of BLM helicase and of the nuclear receptor NR2F2 potentially limits hyperactivity of ALT in this tissue (Conomos et al, 2014; Sobinoff et al, 2017; Zhang et al, 2021), although it has recently be shown that in some cases NR2F2 may not be directly involved in ALT (Alhendi and Royle, 2020). Interestingly, a recent study of Episkopou et al .…”

“…In the RAD51dependent pathway, telomere-bound POT1 is replaced by RPA, followed by RAD51 recruitment and strand invasion. Low expression of BLM helicase and of the nuclear receptor NR2F2 potentially limits hyperactivity of ALT in this tissue Zhang et al, 2021), although it has recently be shown that in some cases NR2F2 may not be directly involved in ALT (Alhendi and Royle, 2020). Interestingly, a recent study of Episkopou et al have identified that the expression of the testis-specific Y-encoded-like protein 5 (TSPYL5), a previously unrecognized APB body component, is crucial for survival of ALT + cells, as it protects replaced POT1 from proteasomal degradation (Episkopou et al, 2019).…”

Identification of Key Proteins from the Alternative Lengthening of Telomeres-Associated Promyelocytic Leukemia Nuclear Bodies Pathway

“…This ranking allowed us to elucidate the role of each gene/protein in APBs. For instance, NSMCE2 was highly amplified and overexpressed across the PCA studies ( Figure 3 b); this gene encodes a protein of the small ubiquitin-related modifier (SUMO) and it is part of the SMC5/6 complex, which is crucial for the SUMOylation of proteins [ 44 ] that is a hallmark of the APBs environment. Additionally, the knockdown of NSMCE2 in ALT + cell lines had led to a reduction of telomere length [ 45 ].…”