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 and determine the timing of micronucleus rupture. We demonstrate that these results are not due to chromosome-specific differences in spindle position or initial protein recruitment during post-mitotic nuclear envelope assembly. Micronucleus size strongly correlates with lamin B1 levels and nuclear pore density in intact micronuclei, but, unexpectedly, lamin B1 levels do not completely predict nuclear lamina organization or 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

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

“…We found that the micronuclei that result from ensheathed chromosomes had disrupted envelopes 8 h after 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.…”

Polar Chromosomes—Challenges of a Risky Path

“…First, a polar chromosome can remain unaligned through metaphase and anaphase and mis-segregate while still stuck at the polar region (Figure 7, route 1). Interestingly, the frequencies of unaligned chromosomes in anaphase across different cell types are not known, although they are expected to be low [124,178]. This is because cells have intricate systems to avoid the occurrence of unaligned chromosomes, but if such chromosomes persist, they must pass the control of SAC [11] (Figure 7, route 1).…”

“…This phenomenon could drastically affect the propensity to propagate unaligned chromosomes over generations. However, a recent study indicated that micronuclear stability is determined more by the identity and length of chromosomes trapped within the micronucleus than by their mis-segregation position [178]. Regardless of the mechanisms, if an unaligned polar chromosome satisfies the SAC and results in aneuploidy, it is expected that daughter cells would struggle in proliferation independently of aneuploidy, as prolonged prometaphase activates p53-p21 dependent apoptotic response that blocks further daughter cell proliferation [197,198].…”

“…However, it is currently unclear if chromosome mis-segregations and whole-chromosome aneuploidies are biased and to what extent toward certain chromosomes, with much evidence supporting [188,[209][210][211][212] and some discouraging [213] such a hypothesis in vertebrate model systems after different spindle perturbations. The reported biases are related to the size of the chromosome or kinetochore [178,209,210], the duration of prometaphase and cohesion fatigue [209], or the level of centromere-associated proteins [211,212].…”