Securin Is Required for Chromosomal Stability in Human Cells

Abstract

Abnormalities of chromosome number are the most common genetic aberrations in cancer. The mechanisms regulating the fidelity of mitotic chromosome transmission in mammalian cells are therefore of great interest. Here we show that human cells without an hSecurin gene lose chromosomes at a high frequency. This loss was linked to abnormal anaphases during which cells underwent repetitive unsuccessful attempts to segregate their chromosomes. The abnormal mitoses were associated with biochemical defects in the activation of separin, the sister-separating protease, rendering it unable to cleave the cohesin subunit Scc1 efficiently. These results illuminate the function of mammalian securin and show that it is essential for the maintenance of euploidy. Introduction Genetic instability is now widely recognized as an essential factor in the evolution of cancer (Loeb, 1991; Lengauer et al., 1998). In the vast majority of solid tumors, this instability appears to involve gains and losses of whole chromosomes or large parts thereof, leading to aneuploidy (Lengauer et al., 1997; Duesberg et al., 1999). Recent evidence suggests that this form of chromosomal instability (CIN) is in some cases associated with alterations in a cell cycle checkpoint that monitors the integrity of the spindle apparatus, a structure critical for proper bipolar segregation of duplicated sister chromatids at mitosis (Cahill et al., 1999). A small fraction of CIN cancers are associated with dominant mutations in the human homolog of the yeast spindle checkpoint gene BUB1 (Cahill et al., 1998; Imai et al., 1999; Gemma et al., 2000). Likewise, mutations in the mouse BUB1 gene have been shown to disrupt the mitotic spindle checkpoint (Taylor and McKeon, 1997; Lee et al., 1999). Efforts to study the mitotic spindle checkpoint pathway through genetic approaches have been hampered by the extremely early embryonic lethality of mice homozygously deleted for MAD2 and BUB3, preventing the evaluation of chromosome loss rates in proliferating somatic cells (Dobles et al., 2000; Kalitsis et al., 2000). However, recent evidence suggests that disruption of a single MAD2 allele can result in a modest increase in chromosomal instability associated with premature anaphase entry (Michel et al., 2001).