Micronuclei arising due to loss of KIF18A form stable micronuclear envelopes and do not promote tumorigenesis

Abstract

Micronuclei, whole or fragmented chromosomes which are spatially separated from the main nucleus, are strongly associated with genomic instability and have been identified as drivers of tumorigenesis. Paradoxically, Kif18a mutant mice produce micronuclei due to unaligned chromosomes in vivo but do not develop spontaneous tumors, raising questions about whether all micronuclei contribute similarly to genomic instability and cancer. We report here that micronuclei in Kif18a mutant mice form stable nuclear envelopes. Challenging Kif18a mutant mice via deletion of the Trp53 gene led to formation of thymic lymphoma with elevated levels of micronuclei. However, loss of Kif18a had modest or no effect on survival of Trp53 homozygotes and heterozygotes, respectively. To further explore micronuclear envelope stability in KIF18A KO cells, we compared micronuclei induced via different insults in cultured cells. Micronuclei in KIF18A KO cells form stable nuclear envelopes characterized by increased recruitment of core and non-core nuclear envelope components and successful expansion of decondensing chromatin compared to those induced by microtubule drug washout or exposure to radiation. We also observed that lagging chromosomes, which lead to micronucleus formation, were positioned closer to the main chromatin masses, and further from the central spindle, in KIF18A KO cells. Our studies provide in vivo support to models suggesting that micronuclear fate depends on the sub-cellular location of late lagging chromosomes and suggest that not all micronuclei actively promote tumorigenesis.

Identification of the KIF18A alpha-4 helix as a therapeutic target for chromosomally unstable tumor cells

“…Similarly, diploid somatic cells can continue proliferating in the absence of KIF18A activity, despite the presence of chromosome alignment defects (22,26,29). In the case of the S284A mutant, we predict that enough KIF18A activity remains to support mitotic progression but not chromosome alignment.…”

A role for Nup153 in nuclear assembly reveals differential requirements for targeting of nuclear envelope constituents

“…Specifically, the role that Nup153 plays in ensuring non-core NE residents are recruited to nascent nuclei may be reduced at micronuclei and, at least in certain cases, there may not be an alternative route at interphase that allows this deficiency to recover, as we have seen happens in parent nuclei. Notably, in cases where micronuclei form via a route that does not preclude non-core protein recruitment and NE expansion, the resulting NE are more stable and these micronuclei are not associated with tumorigenesis (Sepaniac et al, 2021). Such a situation is seen in KIF18A knockout mice, in which micronuclei arise following chromosome misalignment.…”

Causes and Consequences of Nuclear Envelope Rupture and DNA Damage in Micronuclei

“…Some have found that Lamin A/C intensity and frequency of presence is comparable to that of PN in MN induced by Nocodazole and Colchicine (15,25). Contrarily, about one third of MN induced by Paclitaxel lack Lamin A/C (26), and nearly all ruptured MN in KIF18A KO cell lines were deficient for the protein (27). These differences in results may be associated with the type of treatments used to induce MN, as both studies that observe typical Lamin A/C levels at MN use microtubule depolymerizing agents.…”

“…It is commonly agreed that the midzone plays a role in aberrant mNE assembly, as treatments that limit exposure of LCs to the midzone also decrease the prevalence of mNE reassembly defects (15,27). A recent study found that generating MN with KIF18A KO, as opposed to the more commonly used nocodazole treatment, increased the frequency of lamin recruitment to MN (27).…”

“…It is commonly agreed that the midzone plays a role in aberrant mNE assembly, as treatments that limit exposure of LCs to the midzone also decrease the prevalence of mNE reassembly defects (15,27). A recent study found that generating MN with KIF18A KO, as opposed to the more commonly used nocodazole treatment, increased the frequency of lamin recruitment to MN (27). KIF18A KO generates MN by causing improper metaphase alignment of chromosomes, elongated MTs, decreased tension between kinetochores, and increased MT oscillation intensity (36,37).…”