BROCHROMITO

Mitosis with broken chromosomes

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 Obiettivo del progetto (Objective)

'The mechanisms that safeguard cells against aneuploidy are of great interest as aneuploidy contributes to tumorigenesis. To gain insight into these mechanisms, we studied the behavior of cells entering mitosis with damaged chromosomes. We used the endonuclease I-CreI to generate acentric chromosomes in Drosophila larvae. While I-CreI expression produces acentric chromosomes in the majority of neuronal stem cells, remarkably, it has no effect on adult survival. Our live studies reveal that acentric chromatids segregate efficiently to opposite poles. The acentric chromatid poleward movement is mediated through DNA tethers decorated with BubR1, Polo, INCENP and Aurora-B. Reduced BubR1 or Polo function results in abnormal segregation of acentric chromatids, a decrease in acentric chromosome tethering and a great reduction in adult survival. We propose that BubR1 and Polo facilitate the accurate segregation of acentric chromatids by maintaining the integrity of the tethers that connect acentric chromosomes to their centric partners. Since this is a new phenomenon, our understanding of the mechanisms of the tether-based acentric segregation and prevention of aneuploidy is limited. However, this discovery opens a new field of research and raises many exciting questions such as: Part 1. What is the structure and composition of the tether? Aim 1: Is the DSB repair machinery involved in tether formation? Aim 2: Can we identify new components required for tether formation and function? Part 2. What is the role and regulation of BubR1 on the tether? My work has revealed a novel localization and function for the conserved kinase BubR1 on the chromatin tether. Aim 3: How is BubR1 recruited to the tether Aim 4: What are the molecular pathways by which BubR1 functions on the tether?'

Introduzione (Teaser)

Mitosis, cell division for growth and repair involves a complex sequence of events resulting in duplication of the genetic material. Groundbreaking research is elucidating cellular mechanisms of function and dysfunction.

Descrizione progetto (Article)

During mitosis, long, thin strands of tangled DNA called chromatin thicken and separate into pairs of duplicated chromosomes (consisting of two sister chromatids). The chromosomes attach to a spindle of microtubules along the equator of the cell via a centromere and the chromatids are pulled to opposite poles of the cell. The cell is then cleaved in two to form two genetically identical new cells.

In a fruit fly model system (Drosophila larva) in which acentric chromosomes (without centromeres) were induced, chromosomes still tethered to the spindle and segregated to the poles, and adult survival was not impaired. However, decreased function of the proteins BubR1 and Polo impaired tethering, segregation and survival. Scientists investigated the cellular mechanisms with EU funding of the project 'Mitosis with broken chromosomes' (BROCHROMITO).

Investigators cloned truncated versions of BubR1 and identified four domains in Drosophila that they then studied in transgenic flies. Important insight was obtained regarding their mediation of formation of a DNA tether. Scientists investigated two types of repair intermediates within the cells and showed that homologous recombination employing Mus309 (RecQ Helicase homolog) is involved in the model system used here.

Groundbreaking results were obtained regarding a novel signalling pathway between chromatin and the cell cleavage machinery. Cell elongation is an adaptive response for clearing long chromatid arms from the cleavage plane. The length of the chromatid arms of acentric chromosomes is transiently increased during mitosis, putting them at risk of essentially being chopped during cleavage of the cytosol in the final step of mitosis. Scientists demonstrated a concomitant elongation of the cytosol (mediated via the Rho Guanine-nucleotide exchange factor) that prevents an abnormal number of chromosomes in the cell - aneuploidy.

BROCHROMITO has successfully demonstrated two novel pathways that overcome aneuploidy in daughter cells in the Drosphila model system. One helps form a tether to the spindle despite a missing centromere, and the other elongates the cytosol during cleavage to account for 'dangling' elongated arms of chromatids.

Outcomes will have important impact on genetics and disease therapies, in particular cancers where aneuploidy contributes to tumourigenesis.