The overall theme of our research is to understand molecular mechanisms of chromosome segregation. Our goal is to understand how the genome is accurately transmitted through mitosis and meiosis. This is important because altered chromosome number (aneuploidy) causes miscarriages, infertility and birth defects and is a characteristic of cancer.
We focus on the pericentromere and adaptations to the kinetochore during meiosis. The kinetochore is a complex molecular machine that assembles on the centromere and couples chromosomes to microtubules. The pericentromere is a specialized chromosomal domain that surrounds the centromere, is highly enriched in the chromosome-linking cohesin complex and plays multiple critical roles in ensuring the accuracy of chromosome segregation. Recruitment of shugoshin by the pericentromere builds a regulatory platform that monitors segregation.
Our work is question and hypothesis-driven: we are not limited to a specific methodology but take a multi-disciplinary approach, employing a wide range of cell biology, genetics, biochemistry, biophysics and 'omics techniques, integrating new tools and approaches as necessary.
In a bit more detail....
During mitosis, duplicated sister chromatids are pulled apart to produce identical daughter cells.
Meiosis generates gametes through two consecutive segregation events: maternal and paternal chromosomes are separated in meiosis I and sister chromatids are segregated during meiosis II. Accordingly, sister kinetochores attach to microtubules from opposite poles in mitosis and meiosis II (bioriented), but to the same pole during meiosis I (monooriented). How these distinct orientations are specified and safeguarded remains poorly understood.
Our work has focused on the chromosomal region surrounding the kinetochore, the pericentromere, which plays multiple underappreciated roles in directing and monitoring chromosome segregation. We aim to elucidate the molecular underpinnings of the interplay between pericentromeres and kinetochores to understand how chromosomes are oriented in mitosis and meiosis. We use budding and fission yeast to uncover fundamental mechanisms; to reveal conserved principles we have recently initiated studies on Xenopus oocytes. Our ultimate goal is to gain an in-depth molecular knowledge of how kinetochores and pericentromeres confer directionality to chromosome movement in mitosis and meiosis.
Our current research is aimed at addressing three broad questions:
(1) How is the pericentromere functionally and geometrically organised to orient chromosomes?
Cohesin is a ring-shaped complex that organises the genome to direct its expression, repair and segregation. The budding yeast pericentromere, which is highly enriched with cohesin, has emerged as a paradigm to understand how cohesin organises chromosome domains. We discovered a specialized, kinetochore-driven cohesin loading pathway which initates the enrichment of cohesin in the pericentromere. This represents the first description of how cohesin loading is targeted to specific chromosomal sites. Our current work is aimed at understanding how the boundaries of the pericentromere are defined and determining the importance of "targeted cohesin" loading for genome function in a variety of contexts.
(2) How does the pericentromere act as a signalling platform to monitor and regulate chromosome segregation?
The pericentromeric adaptor protein, shugoshin plays an important role in promoting and sensing the proper attachment of chromosomes to microtubules during mitosis. Shugoshin recruits the chromosome-organising complex, condensin, to the pericentromere to facilitate the capture of sister kinetochores by microtubules from opposite poles. One this has occured (called sister kinetochore biorientation), shugoshin dissociates from the pericentromere to signal the bioriented state. We are currently investigating mechanisms of shugoshin regulation in both mitosis and meiosis, where shugoshin plays an essential role in spatially controlling cohesin loss.
(3) How are kinetochores adapted to direct the specialized pattern of chromosome segregation during meiosis?
Meiosis I is a unique segregation event because the maternal and paternal chromosomes, called homologs, are partitioned. This contrasts with mitosis and meiosis II where the sister chromatids are segregated and, accordingly, requires several meiosis I-specific modifications to the chromosome segregation machinery. Our work has established the importance of the kinetochore in setting up the unique features of meiosis I and we now aim to understand the molecular basis of these modifications.
(a) How are sister kinetochores oriented towards the same pole during meiosis I?
Our findings have provided evidence that sister kinetochores are fused into a single unit during meiosis I in budding yeast, in a manner dependent on a complex called monopolin. However many questions remain as to the molecular mechanism of sister kinetochore fusion by monopolin and the identify of the factors that direct the co-orientation of sister kinetochores in other organisms. We are investigating the underlying mechanism of sister kinetochore fusion by the monopolin complex in budding yeast and have initiated studies to identify factors that might confer a similar function in vertebrate oocytes. Understanding how safeguarding mechanisms recognise that homologs, rather than sister chromatids, are attached to opposite poles during meiosis I is a major unanswered question in cell biology and an important priority for our future research.
(b) How is cohesin loss regulated during meiosis?
Cohesin holds the sister chromatids together from the time of their synthesis in S phase until the time of their segregation during mitosis. During mitosis, separase cleaves cohesin along the length of chromosomes to trigger their segregation. In mitosis, all cohesin along chromosomes is cleaved in a single step. In contrast, during meiosis I, cohesin is cleaved only on chromosome arms, but protected in the pericentromere until meiosis II. Protection of pericentromeric cohesin requires shugoshin in complex with protein phosphatase 2A. However, additional factors are also known to be required for cohesin protection. We are investigating how these factors influence cohesin protection through shugoshin-PP2A and elucidating how cohesin regulation is altered to bring about the specialized segregation pattern during meiosis.
(c) How do kinetochores influence the placement of chiasmata?
Meiotic recombination generates chiasmata, or crossovers, that are essential in holding the homologs together to ensure their accurate segregation during meiosis I. It has been long-established that recombination is repressed surrounding centromeres. Furthermore, pericentromeric crossovers are known to be detrimental for chromosome segregation and increase the risk of Downs syndrome. We recently demonstrated that kinetochores, and the enrichment of pericentromeric cohesin, are important in preventing crossover recombination, providing an explanation for this long-established phenomenon. We are currently working to dissect the molecular details of this kinetochore-dependent crossover repression.