Chromosome length and gene density contribute to micronuclear membrane stability

Abstract

Micronuclei are derived from missegregated chromosomes and frequently lose membrane integrity, leading to DNA damage, innate immune activation, and metastatic signaling. Here we demonstrate that two characteristics of the trapped chromosome, length and gene density, are key contributors to micronuclei membrane stability in human cells. Chromosome length is proportional to micronuclei size, and gene density has an additive effect with micronucleus size on membrane stability. We demonstrate that these results are not due to chromosome-specific differences in spindle position or initial nuclear pore complex recruitment during post-mitotic nuclear envelope assembly. We find that chromosome length and micronuclei size strongly correlate with lamin B1 and nuclear pore density in intact micronuclei. Unexpectedly, lamin B1 levels do not predict nuclear lamina organization and membrane stability. Instead, small gene-dense micronuclei have decreased nuclear lamina gaps compared to large micronuclei, despite very low levels of lamin B1. Our data strongly suggest that nuclear envelope composition defects previously correlated with membrane rupture only partly explain membrane stability in micronuclei. We propose that an unknown factor linked to gene density has a separate function that inhibits the appearance of nuclear lamina gaps and delays membrane rupture until late in the cell cycle.

Endomembranes promote chromosome missegregation by ensheathing misaligned chromosomes

“…We found that the micronuclei that result from ensheathed chromosomes had disrupted envelopes 8 h post-release from CENP-E inhibition. Rupture of micronuclei has been shown to lead to DNA damage, and activation of innate immune and cell invasion pathways (Ly et al, 2017;Hatch et al, 2013;Mammel et al, 2021;Bakhoum et al, 2018). The presence of ER in the micronuclear space of disrupted micronuclei indicates that ensheathing may increase the likelihood of rupture, we speculate that this may occur by endomembranes physically interfering with envelope reformation at the micronucleus.…”

“…Moreover, the levels of either mCherry-BAF or LBR-mCherry were correlated with GFP-Sec61β. To confirm that these micronuclei had disrupted nuclear envelopes, we stained for H3K27Ac, a modification to Histone H3 that is removed by exposure to the cytoplasm (Mammel et al, 2021). Intact micronuclei had similar H3K27Ac signals to the corresponding main nucleus, whereas in micronuclei that were disrupted, the signal was lost (Figure 5A).…”

The fate of extra centrosomes in newly formed tetraploid cells: should I stay, or should I go?

“…As a result, chromosomes trapped in micronuclei are prone to incomplete replication, DNA breaks, and chromosomal shattering leading to complex rearrangements (i.e., chromothripsis) (Crasta et al, 2012;Hatch et al, 2013;Zhang et al, 2015;Liu et al, 2018;Tang et al, 2022). There is evidence micronucleus instability can cause punctuated bursts of numerical and structural chromosomal alterations within a few cell divisions and promote inflammation, epithelial-tomesenchymal transition, and metastasis (Mackenzie et al, 2017;Bakhoum et al, 2018;Umbreit et al, 2020), but we are still learning about the cellular events leading to micronuclei rupture in normal and cancer cells (Sepaniac et al, 2021;Agustinus et al, 2022;Mammel et al, 2022;Papathanasiou et al, 2022). Chromothripsis is more frequent in WGD+ than WGD− cancers (Notta et al, 2016;Cortés-Ciriano et al, 2020), likely reflecting an increase in both the rates and tolerance of genomic alterations in tetraploid cells.…”