Establishment of centromere identity is dependent on nuclear spatial organization
Wu. W., McHugh, T., Kelly, D.A., Pidoux, A.L., and Allshire, R.C.
Centromeric heterochromatin is needed to trigger the assembly of CENP-A chromatin on adjacent centromeric DNA in fission yeast. Here we show that heterochromatin tends to associate with fission yeast spindle pole bodies (SPBs) where centromeres cluster in interphase. The requirement for heterochromatin in establishing CENP-A chromatin and assembling functional kinetochores on naïve centromeric DNA is bypassed by placing or tethering centromeric DNA near SPB-centromere clusters. Thus, nuclear positioning relative to a compartment surrounding SPBs influences centromere identity.
Summary of Paper by Lori Koch
The centromere is a special region of a chromosome that attaches to dynamic fibres called microtubules that move the chromosome into a new cell during cell division. Exactly how centromeres come to be marked remains one of the most intriguing mysteries of molecular biology. The histone variant CENP-A replaces histone H3 at centromeres however it is unclear exactly how CENP-A loading is directed. In their recent publication in Current Biology, scientists in the Allshire group led by Dr. Weifang Wu present evidence to support the notion that position within the nucleus is an epigenetic mechanism that influences centromere identity. The fission yeast Schizosaccharomyces pombe has three chromosomes with core centromere DNA flanked by repeating DNA sequences called outer repeats coated in H3K9 methylation-dependent heterochromatin. To identify the minimal requirements for functional centromere formation, the researchers created a system to test for de novo CENP-A incorporation into naïve centromere DNA within minichromosomes freshly transformed into yeast cells. Using fluorescence in situ hybridization (FISH), they determined the position of their minichromosomes within cells. In many types of cells including fission yeast, the centromeres often cluster next to the centrosome and the scientists wondered whether artificial minichromosomes also did this. They found that minichromosomes carrying centromere DNA flanked by outer repeat heterochromatin almost always localized next to the fission yeast centrosomes, spindle-pole bodies (SPBs), within the nucleus. However, when the outer repeats lacked H3K9 methylation/heterochromatin due to loss of the Clr4 methyltransferase enzyme, the minichromosome did not co-localize with SPBs, suggesting that the presence of outer repeat heterochromatin directs the association of naïve centromere DNA with SPBs. Using qChIP, they found that CENP-A was loaded onto minichromosomes carrying both centromeric DNA and outer repeats but not otherwise. Interestingly, they found that artificially tethering minichromosomes to the SPB led to CENP-A loading even in the absence of outer repeats or methylated outer repeats. They also performed experiments inserting centromere DNA at different loci on endogenous chromosomes and found that the amount of CENP-A loading depends on distance from the functional centromere. Overall, their data supports a model in which proximity to a centromere-rich region, such as the SPB, within the cell promotes CENP-A loading.