Mechanisms for fidelity of chromosome distribution

     

All organisms have mechanisms to ensure that cells produced from mitotic and meiotic divisions contain the proper number of chromosomes. The cell monitors that chromosomes are copied exactly once and then distributed correctly to daughter cells. This is critical since cells containing an incomplete chromosome complement can be unviable or have abnormal growth phenotypes. Many of these mechanisms are conserved from budding yeast to man.  Due to the ease of genetic manipulations in budding yeast, we mainly use S. cerevisiae as our model organism.  My research program uses genomics, genetics and biochemistry to study mechanisms that contribute to the fidelity of chromosome distribution, in particular 1) cohesin deposition, 2) centromere/ kinetochore formation, and 2) the generation of meiotic recombination and crossing over events.

Inheritance of the correct number of chromosomes following cell division is dependent on chromosome cohesion. As DNA replicates, the cell establishes a molecular memory of sister chromatids using a protein complex called cohesin. Cohesin binds to both sister chromatids and ensures that they remain together until they are required to separate into daughter cells at the metaphase-to-anaphase transition [1]. We have used DNA microarrays to produce a genome-wide map of cohesin during mitosis and meiosis in the S. cerevisiae genome [2]. Although there does not appear to be a consensus sequence for cohesin binding, analysis of the map reveals unexpected correlations between cohesin association and genome features such as sequence, chromosome structure and transcriptional status. We are currently studying the dynamic nature of the cohesin complex and its relationship to chromatin and transcription.

 

Centromeres in all organisms contain the histone H3 variant Cse4, also known as CENP-A.  This histone variant is critical for the formation of the kinetochore, a multi-protein structure essential for microtubule attachment and therefore chromosome segregation.  We have previously shown that mutation of a kinetochore protein is essential for cohesin loading at pericentric regions [3].  We are currently studying the relationship between Cse4 and cohesin at pericentric regions.  We are generating a nucleosome-level-resolution map of Cse4 in the budding yeast genome in order to understand where it is normally found so that we can isolate factors that constrain it to these regions.

 

The fidelity of chromosome distribution following the first meiotic division is dependent on recombination between homologous chromosomes. Crossing over is essential for the proper alignment and disjunction of homologs. In S. cerevisiae these crossover events begin as a double-strand break (DSB) made by Spo11. I have used DNA microarrays to map Spo11-mediated DSBs in yeast [4, 5].  The location of DSBs is determined to some extent by a chromatin pattern that is dependent on transcription factors [5].  We are currently studying the relationship between recombination, transcription, and cohesin in meiosis and mitosis.

 

We have used functional genomics to identify a novel gene, MND1, which is essential for meiotic recombination [6]. This gene has orthologs in man and mouse. We are characterizing the precise function of the Mnd1/Hop2 complex in yeast with respect to recombination.  Our recent work indicates that Mnd1/Hop2 is required for interactions between homologous chromosomes[7] and in this way facilitates recombination [8].

Because of the unbiased nature of the approach, we expect genomics to offer unexpected insights into the processes that ensure a perfect chromosome complement following mitotic and meiotic cell divisions.  These studies should also help reveal the interplay between these mechanisms, and potentially reveal other cellular mechanisms that enhance the fidelity of chromosome distribution. Our results will help us evaluate current models for the function of centromeres, recombination, and chromosome cohesion in chromosome segregation, and will have broader implications for roles of these factors in chromosome structure.

 

1.	Gerton, J. Chromosome cohesion: a cycle of holding together and falling apart. PLoS Biol. 3, e94, 2005.
2.	Glynn, E.F., Megee, P.C., Yu, H.G., Mistrot, C., Unal, E., Koshland, D.E., DeRisi, J.L., and Gerton, J.L. Genome-Wide Mapping of the Cohesin Complex in the Yeast Saccharomyces cerevisiae. PLoS Biol. 2, E259, 2004.
3.	Weber, S.A., Gerton, J.L., Polancic, J.E., DeRisi, J.L., Koshland, D., and Megee, P.C. The kinetochore is an enhancer of pericentric cohesin binding. PLoS Biol. 2, E260, 2004.
4.	Gerton, J.L., DeRisi, J., Shroff, R., Lichten, M., Brown, P.O., and Petes, T.D. Inaugural article: global mapping of meiotic recombination hotspots and coldspots in the yeast Saccharomyces cerevisiae. Proc Natl Acad Sci U S A. 97, 11383-11390., 2000.
5.	Mieczkowski, P.A., Dominska, M., Buck, M.J., Gerton, J.L., Lieb, J.D., and Petes, T.D. Global analysis of the relationship between the binding of the Bas1p transcription factor and meiosis-specific double-strand DNA breaks in Saccharomyces cerevisiae. Mol Cell Biol. 26, 1014-1027, 2006.
6.	Gerton, J.L., and DeRisi, J.L. Mnd1p: An evolutionarily conserved protein required for meiotic recombination. Proc Natl Acad Sci U S A. 99, 6895-6900., 2002.
7.	Gerton, J.L., and Hawley, R.S. Homologous chromosome interactions in meiosis: diversity amidst conservation. Nat Rev Genet. 6, 477-487, 2005.
8.	Henry, J.M., Camahort, R., Rice, D.A., Florens, L., Swanson, S.K., Washburn, M.P., and Gerton, J. Mnd1/Hop2 Facilitates Dmc1-Dependent Interhomolog Crossover Formation in Meiosis of Budding Yeast. Mol Cell Biol. 26, 2006.

 

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