bubr1 is an orthologue of yeast mad3 that binds and inhibits the multisubunit e3 ligase apc / c in sac signaling ( chao et al . , 2012 ; taylor et al . , yeast mad3 and mammalian bubr1 share homology at the n - terminus : tpr ( tetratricopeptide repeat ) domain is required for recruitment to kinetochore by binding to blinkin / knl1 ( kiyomitsu et al . knl1 is a core component of kmn network , essential for kinetochore - microtubule interaction for chromosome alignment ( takeuchi and fukagawa , 2012 ; varma and salmon , 2012 ) ; glebs ( gle-2-binding sequence ) motif is for bub3 binding ( larsen et al . , 2007 ) ; ken boxes and d - box found in mammalian bubr1 are the degrons required for binding to cdc20 and apc / c ( burton and solomon , 2007 ; lara - gonzalez et al . , 2011 ; tian et al . , 2012 unlike yeast mad3 , mammalian bubr1 has kinase domain similar to bub1 at the c - terminus ( fig . an interesting idea was that cenp - e , the plus end - directed microtubule motor , binds to bubr1 and activates the kinase activity for bubr1 ( mao et al . , 2003 ; 2005 ) . however , numerous following works failed to confirm the kinase activity of bubr1 . recently , it was suggested that the c - terminal kinase domain of bubr1 is a pseudo - kinase domain , resulting from the amplification from bub1 , and thus bubr1 does not possess any kinase activity ( suijkerbuijk et al . , 2012a ) . mitotic kinases such as cdk1 , plk1 , and aurora b phosphorylate bubr1 at distinct sites . the outcome of the phosphorylation at each site is not fully understood , but is expected to be crucial in mitotic checkpoint signaling . bubr1 has two distinct functions in mitosis : regulation of chromosome congression and sac ( elowe et al . , 2010 ) . phosphorylation of bubr1 at serines and threnines of t680 , t620 , d670 , d676 , t680 are involved in tension sensing , kinetochore - microtubule interaction , and mitotic exit ( elowe et al . , 2007 ; 2010 ; huang et al . , 2008 ; suijkerbuijk et al . , it appears that the spindle checkpoint regulation may be a more ancient function and the role in the stabilization of kinetochore - microtubule attachment has evolved in vertebrates ( fig . the kard domain , constituted of many plk1-mediated phosphorylation sites are required for binding to pp2a - b56 , counteracting the excessive aurora b kinase activity for chromosome alignment in metaphase ( kruse et al . treatment of microtubule poisons result in hyperphosphorylation of bubr1 , indicating that phosphorylations at multiple sites are important in exhibiting mitotic functions of bubr1 . in addition to phosphorylation , bubr1 is acetylated at lysine 250 in prometaphase ( choi et al . , 2009 ) . acetylated bubr1 functions as an inhibitor of apc / c and the deacetylated bubr1 becomes the substrate of apc / c . polyubiquitination and subsequent degradation of deacetylated bubr1 are followed by cyclin b destruction , leading to anaphase onset and mitotic exit . when acetylationmimetic mutant of bubr1 is expressed , it blocks the anaphase onset . on the contrary , the expression of acetylation - deficient form allows shortening of mitotic timing with unequal chromosome segregation . the result indicates that acetylation / deacetylation of bubr1 determines the mitotic timing and modulates apc / c activity . thus , bubr1 acetylation is a crucial part of sac signaling ( choi et al . , 2009 ) . sac activity can be divided into two phases : sac signal generation and maintenance of sac . . then , mad2 undergoes conformational change from open mad2 ( o - mad2 ) to closed mad2 ( c - mad2 ) . c - mad2 then binds to cdc20 , promoting the mitotic checkpoint complex ( mcc ) formation ( musacchio and salmon , 2007 ) , composed of mad2 , bubr1 , bub3 , cdc20 . it is now known that mad2 only loads cdc20 to bubr1 and bubr1 is the true inhibitor for apc / c - cdc20 ( han et al . mcc formation is thought to be the key to sustain sac by titrating out cdc20 from apc / c ( musacchio and salmon , 2007 ; sudakin et al . , 2001 ) , and also continuously ubiquitinating and degrading cdc20 when sac is on ( nilsson et al . , 2008 ) . progress has been made recently on the understanding of how mcc disassembly is promoted , leading to checkpoint silencing . apc15 , one of the components of apc / c mediates the autoubiquitination of cdc20 , resulting in the degradation and disassembly of mcc ( foster and morgan , 2012 ; mansfeld et al . , 2011 ; uzunova et al . , 2012 ) deacetylation of bubr1 ( choi et al . , 2009 ) leads to ubiquitination by apc / c and degradation of it , thus can also participate in the disassembly of mcc . indeed , it was shown that cells isolated from the mouse heterozygous of acetylation - deficient allele ( k243r/+ ) failed to retain mcc upon nocodazole treatment , whereas the checkpoint signal generation , measured by mad1/mad2 localization at unattached kinetochores were intact . these results suggest that the early onset destruction of acetylation - deficient bubr1 may serve as the cue to disassembly of mcc ( park et al . the role of p31 has been identified in sac silencing : it binds to c - mad2-cdc20 complex , thereby titrating out mad2 , promoting the disassembly of mcc ( miniowitz - shemtov et al . , 2012 ; however , an elegant biochemical study have shown that it is bubr1-cdc20 complex that truly inhibits apc / c activity , and extracting mad2 from the complex does not inhibit apc / c to a potent level , suggesting that the role of p31 in sac silencing may be secondary in sac silencing ( han et al . , 2013 ) . loss of bubr1 results in the failure of the stable maintenance of chromosome - spindle attachment ( lampson and kapoor , 2005 ) . loss of cenp - e , the plus end - directed motor protein that binds to bubr1 at kinetochore , leads to the increase of monotelic chromosomes near the pole ( kapoor et al . , when compared to the loss of cenp - e , bubr1 loss leads to a more profound defects in chromosome congression , indicating a direct role of bubr1 in kinetochore - microtubule ( kt - mt ) interaction than cenp - e . furthermore , impaired kinetochore - microtubule attachment is restored in bubr1-depleted cells when aurora b activity is inhibited , suggesting that bubr1 antagonizes aurora b s activity in kt - mt attachment ( lampson and kapoor , 2005 ) . erroneous microtubule attachment is corrected by the activity of aurora b. located at the inner kinetochore , aurora b phosphorylates multiprotein complexes at the outer kinetochore , the kmn network ( knl1-mis12-ndc80 ) and sca1-dam complexes , destabilizing the kinetochore - microtubule attachments ( chan et al . , 2012 ; , 2006 ; welburn et al . , 2010 ) . once amphitelic attachment is achieved , aurora b activity must be counteracted to stabilize kt - mt attachment and chromosome alignment at the metaphase plate ( congression ) . in brief , kt - mt attachment and sac are well harmonized in mitosis by the activity of aurora b : sac is on when chromosomes are not attached to microtubules in a bipolar manner ( aurora b active ) ; and turns off when chromosomes - spindle attachment is satisfied ( aurora b inactive ) . therefore , phosphatases pp1 ( kim et al . , 2010 ) and pp2a ( tanno et al . , 2010 ) have been suggested to antagonize activity of aurora b to promote chromosome congression . bubr1 recruits b56 subunit of pp2a to the kinetochore , contributing to stable kt - mt attachment ( kruse et al . , 2013b ; suijkerbuijk et al . , 2012b ) notably , tension - sensitive phosphorylations by plk1 at the kard domain ( s670 , s676 , t680 ) are required for binding pp2a - b56 to bubr1 at kinetochore ( fig . pp2a activity at the kinetochore antagonizes the excessive activity of aurora b in disassembly of microtubules at the kinetochore , thus stabilizing kt - mt attachments and achieving chromosome congression ( kruse et al . , 2013b ; suijkerbuijk et al . , 2012b ; xu et al . , 2013 bubr1 acetylation at k250 is also required for recruiting pp2a - b56 to the kinetochore . interaction between bubr1 and pp2a - b56 is acetylation - dependent , and cells isolated from the mice heterozygous for acetylation - deficient bubr1 ( k243r/+ ) exhibit severe problems in chromosome congression ( park et al . , 2013 ) . how acetylation at k250 is coordinated with plk1 s phosphorylation at kard domain is not known . nevertheless , the result suggests an interesting hypothesis that there may be an acetylation - phosphorylation code in sensing kt - mt attachments to sac signaling , and finally to sac silencing . mosaic variegated aneuploidy ( mva ) and/or premature chromatid separation ( pcs ) are rare genetic disorders that display aneuploidy and premature sister chromatid separations ( pmsc ) in metaphase chromosome spreads . mva patients are characterized by severe microcephaly , growth defects , mental retardation , and neoplasia ( jacquemont et al . , 2002 ; kawame et al . , 1999 notably , bubr1 mutation is associated with mva with cancer predisposition ( de voer et al . , however , it is unclear whether specific mutations are linked with distinct phenotypes in mva of heterologous symptoms . taken together with the bubr1 mutations found in somatic cancers ( bolanos - garcia and blundell , 2011 ) , it is noteworthy that dysfunctional bubr1 contributes to tumorigenesis . the reason for this may be that sac is essential in cell proliferation , therefore is detrimental when deleted . indeed , homozygous deletions of genes coding for mad2 ( dobles et al . , 2000 ) , bub1 ( jeganathan et al . , 2007 ) , bubr1 ( wang et al . , 2004 ) in mice exhibit early embryonic lethality , indicating that severe mitotic failure are indeed detrimental . mice knockout of bubr1 allele are lethal in early embryonic stage ; heterozygous mice are viable and display megakaryopoiesis with aneuploidy but not spontaneous cancers ( wang et al . , 2004 ) . paradoxically , gradual reduction of bubr1 level in mice of hypomorphic bubr1 allele exhibit senescence and premature aging but not spontaneous cancer ( baker et al . , 2004 ) . apparently , both haploinsufficient ( wang et al . , 2004 ) and hypomorphic bubr1 allele display aneuploidy , indicating that aneuploidy alone does not impose on tumorigenesis . notably , mice heterozygous of cenpe - null allele ( weaver et al . , 2007 ) imply that aneuploidy can be oncogenic to a certain degree , but is tumor suppressive when combined with more genetic insults . indeed , aneuploidy in mammalian cells induce proliferation defects and metabolic abnormalities , suggesting that the burden of having to produce too much nucleic acids and proteins are toxic to cellular fitness ( williams et al . , 2008 ) , consistent with the phenotypes observed from down s syndrome ( table 1 ) . however , when bubr1 haploinsufficient mice are crossed to apc mice , tumorigenesis in the large intestine increases to ten - fold , with the decrease of the polyps in the small intestines ( rao et al . , 2005 ) , suggesting that there is more than aneuploidy in bubr1 mutation , compared to cenp - e haploinsufficiency . it is worth mentioning that the depletion of bubr1 has far more profound effects in chromosome congression , compared to cenpe depletion , as mentioned above ( lampson and kapoor , 2005 ) . reduction of bubr1 protein level predisposes to early onset aging in mice ( baker et al . , 2004 ) , which is also observed in the monoallelic insertional mutation of bubr1 at nucleotide 2118 position ( wijshake et al . , 2012 this allale ( bubr1 ) was created after the genetic mutation in one of the mva patients . however , the mutation lies after the kard domain ( fig . 1 ) and affects the c - terminus pseudokinase domain , which might not have crucial functions in sac . nevertheless , bubr1 and bubr1 mice links bubr1 mutation with aging - related mva phenotype , albeit the absence of tumor formation ( summarized in table 1 ) . notably , cancer predisposition is observed in mice heterozygous of acetylation - deficient bubr1 allele ( k243r/+ ) : k243r/+ mice are succumbed to high incidence of spontaneous tumorigenesis , most abundantly lymphoma ( park et al . , 2013 ) . in fact , this is the first mouse model to show that a single mutation in sac components can lead to high - incidence spontaneous tumorigenesis . it is important to speculate the difference of k243r/+ mice from those of other bubr1 mouse models in cancer predisposition . critical difference may rely on the fact that acetylation of k250 ( k243 in mice ) has dual roles : one in stable chromosome - spindle attachment and the other in the stable maintenance of mcc until checkpoint is satisfied . homozygous mutations are detrimental that it exhibits embryonic lethality at embryonic day 6 , even before the homozygous deletion of bubr1 allele ( table 1 ) . in k243r/+ heterozygous cells , the wild - type and mutant protein co - exist in 1:1 ratio ; k243r protein localizes to the kinetochore as much as the wild - type bubr1 does , inhibiting the recruitment of pp2a - b56 to kinetochores , thus k243r allele is a dominant negative allele . in comparison , bubr1 in bubr1 cells localized more to the kinetochores with the decrease of the cytosolic portion , therefore the defect was not as severe as k243r/+ , explaining the absence of spontaneous tumorigenesis in bubr1 mice . it is worth mentioning that the acetylation of bubr1 at k250 requires brca2 , hence bubr1 acetylation is a tumor suppressive mechanism ( choi et al . , 2012 ) . what makes k243r/+ mice tumor - prone and others do nt ? it is conceivable to think that the massive chromosome missegregation resulting from the combined effects of failure in ktmt attachment and weakened sac instigated genetic instability , thus cancer , in k243r/+ mice . as observed from bubr1 haploinsufficiency , cells try to cope with the reduced level of bubr1 in mitosis . however , the aging phenotypes in hypomorphic bubr1 mouse and the bubr1 mice suggest that reduced level of bubr1 may have effects in interphase , leading to senescence and aging . recently , it was shown that bubr1 acetylation status modulates the outgrowth of dendrite in neuronal cells ( watanabe et al . , 2014 ) and also the lifespan ( north et al . , 2014 ) , indicating that bubr1 has many more functions than unerstood . nevertheless , lessons from k243r/+ mice indicate that bubr1 mutations linked with its critical functions in mitosis can predispose to tumorigenesis . it will be important to assign different human genetic mutations with different functions of bubr1 .
alteration in chromosome numbers and structures instigate and foster massive genetic instability . as boveri has seen a hundred years ago ( boveri , 1914 ; 2008 ) , 
 aneuploidy is hallmark of many cancers . 
 however , whether aneuploidy is the cause or the result of cancer is still at debate . 
 the molecular mechanism behind aneuploidy includes the chromo - some mis - segregation in mitosis by the compromise of spindle assembly checkpoint ( sac ) . 
 sac is an elaborate network of proteins , which monitor that all chromosomes are bipolarly attached with the spindles . 
 therefore , the weakening of the sac is the major reason for chromosome number instability , while complete compromise of sac results in detrimental death , exemplified in natural abortion in embryonic stage . 
 here , i will review on the recent progress on the understanding of chromosome mis - segregation and cancer , based on the comparison of different mouse models of bubr1 , the core component of sac .