Nucleation capacity and presence of centrioles define a distinct category of centrosome abnormalities that induces multipolar mitoses in cancer cells

Abstract

Analysis of centrosome number and structure has become one means of assessing the potential for aberrant chromosome segregation and aneuploidy in tumor cells. Centrosome amplification directly causes multipolar catastrophic mitoses in mouse embryonic fibroblasts (MEFs) deficient for the tumor suppressor genes Brca1 or Trp53. We observed supernumerary centrosomes in cell lines established from aneuploid, but not from diploid, colorectal carcinomas; however, multipolar mitoses were never observed. This discrepancy prompted us to thoroughly characterize the centrosome abnormalities in these and other cancer cell lines with respect to both structure and function. The most striking result was that supernumerary centrosomes in aneuploid colorectal cancer cell lines were unable to nucleate microtubules, despite the presence of gamma-tubulin, pericentrin, PLK1, and AURKA. Analysis by scanning electron microscopy revealed that these supernumerary structures are devoid of centrioles, a result significantly different from observations in aneuploid pancreatic cancer cell lines and in Trp53 or Brca1 deficient MEFs. Thus, multipolar mitoses are dependent upon the ability of extra gamma-tubulin containing structures to nucleate microtubules, and this correlated with the presence of centrioles. The assessment of centrosome function with respect to chromosome segregation must therefore take into consideration the presence of centrioles and the capacity to nucleate microtubules. The patterns and mechanisms of chromosomal aberrations in hematologic malignancies and solid tumors are fundamentally different. The former is characterized by specific chromosome translocations, whose consequence is the activation of oncogenes. Most carcinomas, however, reveal variations in the nuclear DNA content. The observed genomic imbalances and gross variations in chromosome number can result from unequal chromosome segregation during mitotic cell division. It is therefore fundamental to elucidate mechanisms involved in distribution of the genome to daughter cells. Prior to cell division, the centrosome organizes microtubules and the mitotic spindle. Deciphering the consequences of alterations in centrosome number, structure, and function is an important step towards understanding how a diploid genome is maintained. Although extra centrosomes have now been observed in carcinomas and were correlated with aneuploidy, a careful functional investigation of these structures and their role in generating chromosome imbalances may lead to the identification of distinct mechanistic pathways of genomic instability. Understanding these pathways will also be important in determining whether they are potential molecular targets of therapeutic intervention.