NEWS AND VIEWS From spindle checkpoint to cancer

Christoph Lengauer & Zhenghe Wang

Truncating and missense mutations in BUB1B, encoding the mitotic spindle checkpoint protein BUBR1, have been identified in individuals with mosaic variegated aneuploidy. This finding supports the idea that there is a genetic basis for aneuploidy in cancers.

Most human cancer cells are aneuploid, mean- showed a defect in the mitotic spindle check- ing they have an aberrant number of chromo- point. This finding was not unexpected, as oth- somes. Whether mutations are ers had previously shown that deficiency of the 123 45 responsible for this aneuploidy is debated1. In spindle checkpoint components resulted in yeast, mutations in several , including unequal segregation of into 2 genes involved in segregation and 67891011 12 daughter cells and thereby led to aneuploidy . regulation of the mitotic spindle checkpoint, The mitotic checkpoint serves as a surveillance can result in aneuploidy. In humans, two mechanism that ensures the faithful transmis- mitotic spindle checkpoint genes, BUB1 and 13 14 15 16 17 18 sion of chromosomes from a mother cell to its BUB1B (encoding BUBR1) have been reported two daughter cells during mitosis. It warrants to be mutated infrequently in colorectal cancers 19 20 21 22 X Y that all chromosomes congress to the with chromosomal instability and in other ane- metaphase plate before separating into the two http://www.nature.com/naturegenetics uploid tumor types2,3. But convincing func- Figure 1 Karyotype of a cell from an individual newly forming daughter cells, and that the tional studies that could establish a causal with MVA. Instead of two copies (normal for a chromosome attach properly to connection between BUB1B mutations and diploid cell), there are three copies each of the spindle microtubules. Many important chromosomal instability have been missing. chromosomes 9, 12, 21 and X. members of this checkpoint have been identi- On page 1159, Sandra Hanks and colleagues4 fied, including the genes Bub1, Bub3, Mad1l1, report hereditary mutations in BUB1B in indi- Mad2l1, Mxd3 and Mps1 (ref. 10). Most of viduals with a rare recessive disease called formed lymphoblasts all showed that at least these regulatory components seem to function mosaic variegated aneuploidy (MVA) and pro- 10% of cells were trisomic with respect to as inhibitors of a multisubunit E3 ubiquitin vide such an important piece of evidence. many different chromosomes. Trisomies ligase, known as the promoting com- involving all chromosomes except chrom- plex or cyclosome (APC/C), which presum- MVA syndrome somes 5, 10, 13, 14 and 17 were observed. The ably controls the exit from metaphase. In 1986, Vigfusson et al.5 observed substantial existence of this mosaic trisomy in four differ- Mutational analysis of one of the mitotic developmental delay and apparent mental ent tissues and in repeated cultures over a 3-y checkpoint genes, BUB1B, in eight families © 2004 Nature Publishing Group retardation in a male pigtail macaque mon- period suggested that it was due to a genetic with MVA by Hanks et al.4 showed that five key. They found that 75.7% of his lymphocyte abnormality resulting in mitotic instability. families carry mutations in BUB1B in both nuclei were trisomic, but with the supernu- So far, only 15 cases of MVA have been alleles. Four families have either nonsense or merary chromosome varying from cell to cell. reported worldwide7. It is not clear whether frameshift mutations in one allele that result Examination of cultured skin fibroblasts all cases represent the same condition, as in the premature truncation of the encoded showed a similar unusual karyotype. some clinical and cytogenetic differences exist BUBR1 protein, deleting its kinase domain A few years later, several humans, including among them. For example, mitoses of some and thus inactivating its function. The two pairs of siblings, were found to have a of the individuals have a higher frequency of reported mutations of the second allele were similar cytogenetic phenomenon. Cells of separated centromeres and splayed chro- missense mutations. Five (of six) missense these individuals frequently form micronuclei matids in the presence of colcemid, a cytoge- mutations were located in the kinase domain and have abnormal numbers of several chro- netic phenomenon referred to as premature of BUBR1. They probably also result in a loss mosomes in multiple tissues, called mosaic centromere division8. The two most common of function, although functional studies are aneuploidies (Fig. 1). For example, in 1991, clinical abnormalities of individuals with required to prove the case. the case of a 17-year-old girl with severe MVA are microcephaly and intrauterine The paper by Hanks et al.4 is a perfect microcephaly and mental retardation was growth retardation. Also, the risk of malig- illustration of the idea that uncovering the reported6. Karyotype analyses of her phyto- nancy for the affected individuals is high. underlying genetic basis of a rare disease hemagglutinin-stimulated lymphocytes, cul- Almost one-third of the young affected indi- with increased cancer risk can contribute to tured skin fibroblasts, direct and cultured viduals reported developed leukemia, rhab- the understanding of general aspects of bone marrow and Epstein-Barr virus–trans- domyosarcoma or Wilms tumor7. tumorigenesis.

Christoph Lengauer and Zhenghe Wang are at Aneuploidy and mitotic spindle Chromosomal instability and cancer the Sidney Kimmel Comprehensive Cancer checkpoint Like the supernumerary chromosomes that Center, Johns Hopkins University School of Matsuura et al.9 recently reported that skin vary from cell to cell in individuals with Medicine, Baltimore, Maryland 21231, USA. fibroblasts cultured from six individuals with MVA, most human cancers are aneuploid, e-mail: [email protected] both premature centromere division and MVA and chromosomal instability presumably

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drives tumor progression by accelerating was a somatic deletion that was predicted to 1. Jallepalli, P.V. & Lengauer, C. Nat. Rev. Cancer 1, 109–117 (2001). the rate of gain or loss of individual chro- remove part of the protein’s kinase domain— 2. Cahill, D.P. et al. Nature 392, 300–303 (1998). mosomes. The molecular mechanisms of analogous to the truncating mutations now 3. Gemma, A. et al. Genes Chromosomes Cancer 29, chromosomal instability in human cancers found in individuals with MVA. 213–218 (2000). largely remain to be elucidated. Mutant mice that express low levels of 4. Hanks, S. et al. Nat. Genet. 36, 1159–1161 (2004). Several years ago, we hypothesized that BubR1 protein develop progressive aneu- 5. Vigfusson, N.V. et al. Cytogenet. Cell Genet. 42, mutations in human spindle checkpoint genes ploidy but form tumors only after being chal- 154–158 (1986). could cause chromosomal instability in can- lenged with a carcinogen12.These findings, as 6. Warburton, D., Anyane-Yeboa, K., Taterka, P., Yu, cers11. We cloned the two human homologs of well as the low frequency of BUB1 and BUB1B C.Y. & Olsen, D. Ann. Genet. 34, 287–292 (1991). 7. Jacquemont, S., Boceno, M., Rival, J.M., yeast Bub1, called BUB1 and BUB1B, and mutations in cancers and the lack of gene Mechinaud, F. & David, A. Am. J. Med. Genet. 109, found that the two genes were mutated, albeit mutations found in any other members of the 17–21 (2002). rarely, in chromosomal instability tumors of mitotic checkpoint machinery thus far, led to 8. Plaja, A. et al. Am. J. Med. Genet. 98, 216–223 2 (2001). individuals with colorectal cancer . This study questions regarding the importance and 9. Matsuura, S. et al. Am. J. Hum. Genet. 67, provided the first evidence that gene mutations applicability of our original observations. The 483–486 (2000). can result in a defective spindle checkpoint in belief that spindle checkpoint defects cause 10. Musacchio, A. & Hardwick, K.G. Nat. Rev. Mol. humans and thus might be responsible for aneuploidy in human tumors, however, is Cell. Biol. 3, 731–741 (2002). 11. Lengauer, C., Kinzler, K.W. & Vogelstein, B. Nature aneuploidy in human cancers. One BUB1B revived by pinning down BUB1B mutations 396, 643–649 (1998). mutation in an aneuploid colorectal cancer as the underlying genetic alteration in MVA. 12. Dai, W. et al. Cancer Res. 64, 440–445 (2004).

Complementing complexity http://www.nature.com/naturegenetics Trudy F C Mackay

One challenge in modern biology is to understand the detailed genetic basis of variation for quantitative traits, including complex behaviors. A new study shows that historical recombination in outbred strains combined with functional complementation tests can identify pleiotropic genes with small effects on naturally occurring variation for anxiety-related behaviors in mice.

Most variation between individuals in behav- of this issue, Binnaz Yalcin and colleagues5 fore, the phenotype of a single individual is ior, physiology, morphology, disease suscepti- describe a combined genetic and functional not a reliable indicator of QTL genotype. bility and reproductive fitness can be strategy for identifying genes corresponding Third, even after whittling down the QTL © 2004 Nature Publishing Group attributed to the segregation of multiple to QTLs with small effects. interval to a region containing several posi- quantitative trait loci (QTLs) with individu- tional candidate genes, one must determine ally small effects, whose expression is condi- The elusive QTLs which of them corresponds to the QTL. tional on the environment1. Understanding There are three main problems facing high- Guessing (the ‘candidate gene’ approach) has the genetic and environmental factors that resolution QTL mapping. First, an increas- worked in cases where the genetic basis under- cause this variation is of fundamental impor- ingly large number of recombination events in lying the trait phenotype is well understood. tance for medicine, agriculture, evolution and each QTL interval are required to map the But the availability of complete genome the emerging discipline of functional QTLs to decreasingly smaller intervals. sequences for many organisms means it is pos- genomics. Genome scans for QTLs have Second, individual QTLs have small effects sible to peruse the gene list in a candidate QTL become a cottage industry in the past 15 that are sensitive to the environment; there- region, inevitably finding many predicted years, facilitated by the discovery of abun- dant, highly polymorphic molecular markers in many species and the development of Figure 1 The power of haplotype mapping sophisticated statistical methods of analy- compared with single-marker analysis. The 2,3 sis . Identifying the genes underlying QTL haplotypes of four inbred strains are depicted peaks by positional cloning has proven elusive for two SNPs, which flank a causal QTL that is in all but a handful of cases where the QTLs associated with variation in high (H) and low had larger than usual effects4. On page 1197 (L) EMO between the strains but which has not been genotyped. There is no association between mean anxiety levels between the two SNPs considered individually. Reconstructing Trudy F. C. Mackay is in the Department of the haplotypes partitions the strains into two Genetics, Campus Box 7614, North Carolina groups with divergent phenotypes, indicating State University, Raleigh, North Carolina there is a QTL between the typed SNPs that 27612, USA. affects EMO. e-mail: [email protected]

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