Depolymerized lamins link nuclear envelope breakdown to mitotic transcriptional quiescence

Abstract

The nuclear envelope, a defining feature of eukaryotic cells, restricts DNA-dependent processes including gene transcription to the nucleus. The nuclear lamina is an integral component of the animal nuclear envelope, composed of polymers of nuclear lamin proteins1,2. Upon mitosis, the nuclear lamina disassembles, the nuclear envelope breaks down, and transcription becomes quiescent3,4. We report here a direct molecular link between nuclear lamina disassembly and mitotic transcriptional quiescence. We found that, at the G2 cell-cycle phase immediately preceding mitosis, nuclear lamin A/C (LMNA) became phosphorylated at Ser22 and depolymerized from the nuclear lamina. Depolymerized LMNA accumulated in the nuclear interior and physically associated with active cis-regulatory elements genome-wide. Depolymerized LMNA-associated sites were overrepresented near genes repressed by LMNA, suggesting that depolymerized LMNA participates in transcriptional repression at G2. Consistently, depolymerized LMNA-target genes underwent a steep expression decline from S to G2/M. Furthermore, LMNA deletion caused inappropriate RNA Polymerase II (Pol II) accumulation downstream of Pol II pause sites at promoters and enhancers genome-wide, leading to inappropriate and excessive transcriptional elongation. A subset of depolymerized LMNA-target genes were upregulated in fibroblasts of patients with Hutchinson-Gilford progeria, a premature aging disorder caused by LMNA mutations5, raising the possibility that defects in depolymerized LMNA-mediated mitotic transcriptional quiescence contribute to progeria pathogenesis. These observations support a model in which depolymerized LMNA targets active regulatory elements to promote RNA Pol II pausing preceding mitosis, coupling nuclear envelope breakdown to mitotic transcriptional quiescence.