Chromosome Segregation Is Biased by Kinetochore Size

Abstract

SummaryChromosome missegregation during mitosis or meiosis is a hallmark of cancer and the main cause of prenatal death in humans. The gain or loss of specific chromosomes is thought to be random, with cell viability being essentially determined by selection. Several established pathways including centrosome amplification, sister-chromatid cohesion defects, or a compromised spindle assembly checkpoint can lead to chromosome missegregation. However, how specific intrinsic features of the kinetochore—the critical chromosomal interface with spindle microtubules—impact chromosome segregation remains poorly understood. Here we used the unique cytological attributes of female Indian muntjac, the mammal with the lowest known chromosome number (2n = 6), to characterize and track individual chromosomes with distinct kinetochore size throughout mitosis. We show that centromere and kinetochore functional layers scale proportionally with centromere size. Measurement of intra-kinetochore distances, serial-section electron microscopy, and RNAi against key kinetochore proteins confirmed a standard structural and functional organization of the Indian muntjac kinetochores and revealed that microtubule binding capacity scales with kinetochore size. Surprisingly, we found that chromosome segregation in this species is not random. Chromosomes with larger kinetochores bi-oriented more efficiently and showed a 2-fold bias to congress to the equator in a motor-independent manner. Despite robust correction mechanisms during unperturbed mitosis, chromosomes with larger kinetochores were also strongly biased to establish erroneous merotelic attachments and missegregate during anaphase. This bias was impervious to the experimental attenuation of polar ejection forces on chromosome arms by RNAi against the chromokinesin Kif4a. Thus, kinetochore size is an important determinant of chromosome segregation fidelity.

Micronucleus formation in human cancer cells is biased by chromosome size

“…The findings by Drpic et al and Worrall and coworkers are fully in tune with our NCH149 data, which show a significantly increased rate of large chromosomes sequestered into micronuclei in human cancer cells. Mechanistically, both cohesion fatigue and chromosome lagging related to kinetochore size might contribute.…”

“…The authors demonstrated that merotelic attachments in Indian muntjac cells lead to anaphase lagging chromosomes and sequestration of about half of the lagging chromosomes into micronuclei. This effect was further increased when erroneous attachments were enhanced by monastrol treatment . Similarly, it has most recently been shown that the largest chromosomes 1 and 2 are particularly prone to missegregation in human retinal pigment epithelium cells (RPE1) following mitotic spindle disruption by release from a nocodazole‐mediated mitotic arrest .…”

“…In the study on Indian muntjac cells by Drpic and coworkers, chromosomes with large kinetochores were more prone to erroneous merotelic kinetochore–microtubule attachments, with rates of erroneous attachments of 7.0% versus 1.6% for chromosomes with large and small kinetochores, respectively. This is in the same order of magnitude as micronucleus entrapment rates in NCH149 cells in our study.…”

“…Although recurrent patterns of chromosomal aberrations are observed in different tumor types, it is thought that the chances of missegregation are equal for all chromosomes, with the prevalence of specific karyotypes being largely determined by viability and selection. In contrast to this common belief, a recent report has described a higher rate of merotelic microtubule‐kinetochore attachments, lagging chromosomes, and micronucleus formation for chromosomes with larger kinetochores in Indian muntjac cells, which were chosen for their small chromosome set of 2 n = 6 …”

kSHREC ‘Delta’ reflects the shape of kinetochore rather than intrakinetochore tension

“…While typical mammalian kinetochores appear as small spots in conventional fluorescence LM (Figure 1A,A’), IM kinetochores form thin lines with the length greater than 1 µm (Figure 1B,B’). Molecular composition of IM kinetochores is similar to that of conventional kinetochores [19] and major kinetochore components can be visualized in both via expression of fluorescently-labeled proteins or antibody staining with similar efficiency (Figure 1A,B). At the EM level, IM kinetochores display the typical trilaminar plate with the widths of the layers similar to that observed in other mammalian cells [14, 20, 21].…”