Measuring and modeling the dynamics of mitotic error correction

Abstract

Accurate chromosome segregation is essential to human health. Chromosome segregation is mediated by the spindle, a bipolar structure of microtubules that aligns chromosomes and separates sister chromatids to opposite daughter cells. Issues with chromosome segregation can arise from the initial configuration of chromosomes and microtubules, perturbed correction of erroneous attachments, and reduced functionality of the spindle assembly checkpoint. The relative contributions of these factors to the overall result of chromosome segregation, and how these factors are impacted by various mitotic proteins, is not well understood.

We developed an experimental method and coarse-grained model to measure the dynamics of mitotic error correction. We measured the rate of error correction in human tissue culture cells using distributions of divided kinetochore counts, and found that error correction can be well-described as a chromosome-autonomous process, in which errors are corrected at a constant rate over time. We measured an identical error correction rate using the distribution of anaphase timing, and found that anaphase timing can be well-described as a slowest first passage time problem. We showed that this coarse-grained model can also explain the perturbed dynamics of error correction when changing the initial configuration of chromosomes and when changing microtubule dynamics. This project is described in Chapter 1.

In Chapter 2, we explored how a faulty spindle assembly checkpoint can work in concert with perturbed error correction to cause errors in chromosome segregation. We incorporated the possibility of a faulty spindle assembly checkpoint in our model, in which a cell can only enter premature anaphase if every erroneous chromosome fails to block anaphase. This model can successfully explain the observed anaphase onset time distribution, spontaneous kinetochore count difference distribution, and error correction time course in multiple human tissue culture cell lines with genetic and small molecule perturbations of mitotic proteins. In Chapter 3, we tested what kind of information about underlying error rates and allocation bias can be extracted from kinetochore count data.

This work provides a quantitative method to distinguish between perturbations that affect the initial error state, error correction rate, and spindle assembly checkpoint function and can be used to measure these parameters across different cell types and conditions.