Haploinsufficiency of SGO1 results in deregulated centrosome dynamics,enhanced chromosomal instability and colon tumorigenesis

Chromosome instability (CIN) is found in 85% of colorectal cancers. Defects in mitotic processes are implicated in high CIN and may be critical events in colorectal tumorigenesis. Shugoshin-1 (SGO1) aids in the maintenance of chromosome cohesion and prevents premature chromosome separation and CIN. In addition, integrity of the centrosome may be compromised due to the deficiency of Cohesin and Sgo1 through the disengagement of centrioles. We report here the generation and characterization of SGO1-mutant mice and show that haploinsufficiency of SGO1 leads to enhanced colonic tumorigenesis. Complete disruption of SGO1 results in embryonic lethality, whereas SGO1^+/− mice are viable and fertile. Haploinsufficiency of SGO1 results in genomic instability manifested as missegregation of chromosomes and formation of extra centrosomal foci in both murine embryonic fibroblasts and adult bone marrow cells. Enhanced CIN observed in SGO1-deficient mice resulted in an increase in formation of aberrant crypt foci (ACF) and accelerated development of tumors after exposure to azoxymethane (AOM), a colon carcinogen. Together, these results suggest that haploinsufficiency of SGO1 causes enhanced CIN, colonic preneoplastic lesions and tumorigenesis in mice. SGO1 is essential for the suppression of CIN and tumor formation.Key words: SGO1, mouse genetics, chromosomal instability, centrosome, colon cancerSince its discovery several years ago, Shugoshin 1 (SGO1) has emerged as a crucial regulator of the cell cycle.^1–6 At cellular and molecular levels, SGO1 functions as a protector of centromeric cohesion of sister chromatids in higher eukaryotes.^5–7 Depletion of SGO1 by small interfering RNA (siRNA) leads to premature sister chromatid separation.^5–8 During mitosis, SGO1 localizes to centromeres in a manner that appears to be dependent on Bub1, Aurora B and survivin.^7–13 SGO1 works in concert with protein phosphatase 2A (PP2A) to protect centromeric cohesion during mitosis and meiosis.^14,15 It is implicated in microtubule dynamics and required for tension generation at the kinetochore.^2,6 In addition to the function of SGO1 in centromeres, sSGO1, a major splice variant of SGO1, has an important function in centrosome dynamics through mediating centriole cohesion.¹⁶ A recent study supports the centrosomal function of Sgo1 in further detail.¹⁷ Importantly, both cohesin and Sgo1 are shown to be involved in engagement of centrioles and thus in centrosomal integrity.¹⁷ Given the importance of centromeric cohesion and centrosome dynamics in the maintenance of chromosomal stability during cell division, it is conceivable that deregulated function of SGO1 would lead to major chromosomal instability.Chromosomal instability has long been appreciated as a driving force for tumorigenesis, since aneuploidy is prevalent in the majority of solid tumors.^18–20 However, several recent studies show that in certain physiological contexts, chromosomal instability induced by deregulated checkpoint genes is not always associated with tumor development in a straightforward manner.^21–24 For example, haploinsufficiency of CENP-E, a spindle checkpoint component, results in enhanced aneuploidy formation and a modest increase in spontaneous tumors in spleen and lung. However, CENP-E^+/− mice develop fewer tumors when these mice are challenged with carcinogens or in a genetically susceptible background, suggesting that chromosomal instability may suppress tumorigenesis in a context-dependent manner.²²Defects in the chromosome cohesion system and SGO1 may play a critical role in genomic instability, and cancers in human colon. Barber et al. attempted to identify genes involved in CIN in human colon cancer with a tumor DNA sequencing approach. They identified 11 somatic mutations distributed among five genes in a part that included 132 colorectal cancers. All but one of these 11 mutations were in the homologs of yeast genes that regulate sister chromatid cohesion, strongly suggesting a critical relationship between chromosome cohesion and CIN in colon cancer.²⁵ Consistently, SGO1 downregulation is implicated in human colon cancer. Among 46 colorectal cancer cases, hSGO1 mRNA expression was decreased in the tumor tissue in comparison with the corresponding normal tissue (p = 0.032).²⁶ However, direct evidence linking SGO1 to colonic tumor development was lacking. Furthermore, no genetic studies have been reported in mouse models with regard to functions of SGO1 in the maintenance of chromosomal stability and acceleration or suppression of tumor development.To determine the physiological function of SGO1, we have generated SGO1 haploinsufficient (+/−) mutant mice. Mouse embryonic fibroblasts (MEFs) from SGO1^+/− animals were found to contain lower levels of SGO1 than MEFs from wild-type embryos. SGO1 deficiency resulted in increased number of spindle centrosomal foci, enhanced chromosome missegregation and formation of micronuclei at an enhanced rate. Moreover, SGO1^+/− animals were prone to higher preneoplastic lesions and rapid development of colonic tumors after exposure to a colon carcinogen.