Although the majority of colorectal cancers exhibit chromosome instability (CIN), only a few genes that might cause this phenotype have been identified and no general mechanism underlying their function has emerged. To systematically identify somatic mutations in potential CIN genes in colorectal cancers, we determined the sequence of 102 human homologues of 96 yeast CIN genes known to function in various aspects of chromosome transmission fidelity. We identified 11 somatic mutations distributed among five genes in a panel that included 132 colorectal cancers. Remarkably, all but one of these 11 mutations were in the homologs of yeast genes that regulate sister chromatid cohesion. We then demonstrated that down-regulation of such homologs resulted in chromosomal instability and chromatid cohesion defects in human cells. Finally, we showed that down-regulation or genetic disruption of the two major candidate CIN genes identified in previous studies (MRE11A and CDC4) also resulted in abnormal sister chromatid cohesion in human cells. These results suggest that defective sister chromatid cohesion as a result of somatic mutations may represent a major cause of chromosome instability in human cancers.CDC4 ͉ MRE11A ͉ somatic mutation

Mutations that cause chromosome instability (CIN) in cancer cells produce ''sublethal'' deficiencies in an essential process (chromosome segregation) and, therefore, may represent a major untapped resource that could be exploited for therapeutic benefit in the treatment of cancer. If second-site unlinked genes can be identified, that when knocked down, cause a synthetic lethal (SL) phenotype in combination with a somatic mutation in a CIN gene, novel candidate therapeutic targets will be identified. To test this idea, we took a cross species SL candidate gene approach by recapitulating a SL interaction observed between rad54 and rad27 mutations in yeast, via knockdown of the highly sequence-and functionally-related proteins RAD54B and FEN1 in a cancer cell line. We show that knockdown of RAD54B, a gene known to be somatically mutated in cancer, causes CIN in mammalian cells. Using high-content microscopy techniques, we demonstrate that RAD54B-deficient human colorectal cancer cells are sensitive to SL killing by reduced FEN1 expression, while isogenic RAD54B proficient cells are not. This conserved SL interaction suggests that extrapolating SL interactions observed in model organisms for homologous genes mutated in human cancers will aid in the identification of novel therapeutic targets for specific killing of cancerous cells exhibiting CIN.cancer therapeutics ͉ chromosome instability ͉ synthetic lethality

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Figure 1 Relative expression of SEMA4C mRNA over GAPDH in stimulated tonsillar B cells (N=5).