Orphan nuclear receptors recruit TRIM28 to promote telomeric H3K9me3 for the ALT 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 this 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 facilitate ALT activation. Here, we show that orphan NRs regulate telomeric H3K9me3 through TRIM28, a corepressor of ZNF transcription factors, to promote 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 H3K9 methylation and ALT phenotypes. Importantly, physical interaction of TRIM28 with orphan NRs induces 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 for telomeric H3K9me3 and ALT activation, emphasizing the role of chromatin structure in ALT activation.