Hsia, H.-Y. | Search bioRxiv

bioRxiv2025

Orphan nuclear receptors recruit TRIM28 to promote telomeric H3K9me3 for the alternative lengthening of telomeres pathway

Alternative lengthening of telomeres (ALT) is a telomere maintenance mechanism deployed in embryonic stem cells and cancer cells. High levels of the heterochromatin mark H3 lysine 9 trimethylation (H3K9me3) at telomeres are critical for ALT, but how that is achieved remains unclear. Telomeric association of orphan nuclear receptors (NRs)--such as COUP-TF1, COUP-TF2, TR2, and TR4--has been shown previously to promote ALT activation. Here, we show that orphan NRs regulate telomeric H3K9me3 through TRIM28, a corepressor of ZNF transcription factors, to drive ALT. We report that H3K9me3 is induced by telomeric association of orphan NRs in cultured human fibroblast and ALT cancer cell lines. Moreover, TRIM28 is required for the orphan NR-induced H3K9me3 and ALT phenotypes. Importantly, physical interaction of TRIM28 with orphan NRs facilitates a telomeric localization of TRIM28. A TRIM28 variant defective in orphan NR interaction fails to localize to telomeres and is unable to promote H3K9me3 and ALT phenotypes. These findings indicate that telomeric orphan NRs recruit TRIM28, driving telomeric H3K9me3 and ALT activation, emphasizing the role of changes in chromatin structure in ALT activation.

molecular biology↗

bioRxiv2022

Orphan nuclear receptors promote alternative lengthening of telomeres (ALT) through ALT-associated PML bodies

Alternative lengthening of telomeres (ALT) is a telomerase-independent telomere maintenance mechanism utilized by about 15% of cancers. Orphan nuclear receptors (NRs), such as COUP-TF1, COUP-TF2, EAR2, TR2, and TR4, associate with telomeres of ALT cells by binding to variant telomeric repeats. However, how these orphan NRs function in the ALT pathway remains to be characterized. Here, we have established an ALT-inducing cell model by tethering orphan NRs to telomeres in non-ALT BJ fibroblast cells. We demonstrate that recruitment of orphan NRs to telomeres is sufficient to initiate formation of ALT-associated promyelocytic leukemia nuclear bodies (APBs) and telomeric DNA synthesis at APBs. We found that the ability of orphan NRs to initiate APB formation and recombination is dependent on the orphan NR AF2 domain, the zinc-finger protein ZNF827, and PML protein. Depletion of orphan NRs in ALT cell lines reduced APB formation and telomeric DNA synthesis, confirming the role of orphan NRs in ALT cells. Furthermore, we found that ATRX/DAXX depletion, together with the telomeric localization of orphan NRs, induces APB formation, telomere clustering, and telomeric DNA synthesis more dramatically in non-ALT cells. Accordingly, we propose that these events in ALT, orphan NR recruitment to telomeres and ATRX/DAXX loss, operate in concert to activate the ALT pathway.

molecular biology↗