Broken Chromosomes, Broken Promises. (makes no sense but sounds good!)
Channels, Tethers and Threads: “Hail Mary” mechanisms that rescue cells with broken chromosomes
The Sullivan Lab is interested in the structural events that drive cell division and how these events are coordinated. We investigate the mechanisms that drive and coordinate cytokinesis and chromosome segregation upon exit from metaphase. Of particular interest to our lab is the adaptive cellular responses to broken, lagging, and rearranged chromosomes. While many of the adaptive mechanisms that function during interphase and metaphase have been well studied, it had been assumed that few if any functioned during anaphase and telophase. Work from our lab and others over the past decade has revealed that eukaryotic cells maintain a sophisticated and diverse set of mechanisms that function during the anaphase-telophase transition to ensure that chromosome fragments lacking a kinetochore (acentrics) are successfully transmitted and incorporated into daughter nuclei. These include cellular adaptations such as DNA threads and tethers, cell and spindle elongation, and channels in the nuclear envelope. We are particularly excited by our most recent finding that even in undamaged anaphase cells, threads of DNA connect separated anaphase chromosomes. These threads function in capturing wayward chromosome fragments.
A Worst Case Scenario: Exiting metaphase with a broken chromosome fragment. These fragments form dangerous micronuclei that destabilize the entire genome. We examine a set of adaptive “Hail Mary” mechanisms designed to capture and incorporate the chromosome fragments into daughter nuclei.
Major Questions
1. How does a chromosome fragment lacking a kinetochore congress, align during metaphase, undergo sister separation, and segregate to the poles? In spite of the fact that the kinetochore plays a key role in all these events, we find acentric chromosomes also successfully execute these events. This reveals unsuspected kinetochore independent forces acting on the chromosome arms.
2. How is chromosome fragment incorporated into the nucleus AFTER nuclear envelope assembly? Acentric segregation is delayed such that they are “locked out” of the new formed telophase daughter nuclei. Incredibly, we find acentrics are sucked into the nucleus through a hole in the nuclear envelope.
3. What is the fate of the chromosome fragment once incorporated into the nucleus? We can produce Drosophila in which 80% of the cells enter metaphase with an unrepaired break at the base of their X-chromosome, yet adult survival rates are normal. This indicates that the chromosome fragment is successfully incorporated into the genome. How, when and where this occurs is unknown.
4. What is the origin of DNA threads and tethers? The origin strands of DNA, both naked and coated with proteins, is a mystery as they appear to form after the completion of DNA replication. They often emanate from the telomeres and thus may consist of repeated telomeric sequences.
5. What are the mechanisms that ectopically localize cell cycle regulators (Bub, Polo, AuroraB, Incenp) to the tether? Upon double strand break induction, these four proteins ectopically localize to the DNA tethers. The mechanisms that guide this dramatic change in localization are unknown.
6. What is the function of these that coat the tethers? Our studies reveal that AuroraB is required for maintaining a gap in the nuclear envelope allowing the chromosome fragment to enter. The function of the other three proteins remains unknown.