Dynamic nuclear lamina-chromatin interactions during G1 progression

A large fraction of heterochromatin in the metazoan genome is associated with the nuclear lamina (NL) in interphase nuclei. This heterochromatin is often referred to as Lamina-Associated Domains (LADs) and are often mapped from cell populations asynchronously progressing through the cell cycle. We and others have recently reported that LADs are largely stable during G1, S, or G2 phases of the cell cycle, and appear similar to LADs mapped from bulk cell populations. LADs in senescent cells, however, are reported to be quite different from proliferating cells, and it remains unclear how senescent cell LADs are established. As cells finish mitosis and re-enter G1, reassembly of the nuclear envelope and NL appears to precede mitotic chromosome decondensation. Therefore, the initial NL interactions with the decondensing chromatin may be quite different from those reported in asynchronous or FACS isolated G1, S, or G2 populations. By developing a modified version of the Tyramide-Signal Amplification sequencing (TSA-seq), which we call chromatin pull down-based Tyramide Signal Amplification-sequencing (cTSA-seq), we uncover a dynamic NL-chromatin interaction as cells progress through G1. The appearance of stable LADs coincides with sufficient chromatin decondensation and active gene expression in G1. Interestingly, early G1 NL-chromatin interactions, which are found toward the telomeric ends of human chromosomes, are similar to those found in oncogene-induced senescent cells. We find that the assembly of LADs during the formation of the G1 nucleus is gradual and that the arrest of NL-chromatin interactions in early G1 may contribute to genome disorganization of senescence cells.