Molecular cloning of the gene for cancer-prone syndrome characterized by abnormal mitotic spindle checkpoint.

癌症易感综合征（PCS综合征）是一种罕见的常染色体隐性遗传病，其特征是生长发育迟缓、小头畸形、儿童癌症、所有染色体的染色单体过早分离以及各种三体和单体的镶嵌现象。双等位基因BUB1B突变最近在MVA综合征（可能与PCS综合征相同）的8个家族中的5个中被报道。我们在此描述了7个日本PCS综合征家族中BUB1B（编码BubR1）的分子分析。在研究的所有七个家族中都发现了单等位基因BUB1B突变：在四个家族中发现了一个单碱基缺失（1833delT）；一个剪接位点突变，一个无义突变，一个错义突变，每个家族都有。来自1833delT突变和剪接位点突变患者的转录本由于无义介导的mRNA衰变而显著减少。在研究的7个家族中，第二个等位基因未发现突变，但BUB1B的RT-PCR和Western blot以及BubR1的单倍型分析显示，它们的转录本略有减少。2例患者细胞中的BubR1蛋白表达和着丝点定位均降低。它们的后期促进复合体特异性激活因子p55cdc的表达水平正常，但其着丝点关联被取消。微细胞介导的15号染色体（含BUB1B）转移到细胞中恢复了正常的BubR1水平、p55cdc的着丝点定位以及对colcolid处理的正常反应。这些发现表明，BubR1参与了p55cdc介导的有丝分裂检查点信号传导，并提示，BubR1表达（或活性）降低50%参与了PCS综合征。

Cancer-prone syndrome of premature chromatid separation with mosaic variegated aneuploidy (PCS syndrome) is a rare autosomal recessive disorder characterised by growth retardation, microcephaly, childhood cancer, premature chromatid separation of all chromosomes and mosaicism for various trisomies and monosomies. Biallelic BUB1B mutations were recently reported in five of eight families with MVA syndrome (probably identical to the PCS syndrome). We here describe molecular analysis of BUB1B (encoding BubR1) in seven Japanese families with the PCS syndrome. Monoallelic BUB1B mutations were found in all seven families studied : a single base deletion (1833delT) in four families ; and a splice site mutation, a nonsense mutation, and a missense mutation in one family each. Transcripts derived from the patients with the 1833delT mutation and the splice site mutation were significantly reduced due to nonsense-mediated mRNA decay. No mutation was found in the second alleles in the seven families studied, but RT-PCR of BUB1B and Western blot and haplotype analysis of BubR1 indicated a modest decrease of their transcripts. BubR1 in the cells from two patients showed both reduced protein expression and diminished kinetochore localization. Their expression level of p55cdc, a specific activator of anaphase-promoting complex, was normal but its kinetochore association was abolished. Microcell-mediated transfer of chromosome 15 (containing BUB1B) into the cells restored normal BubR1 levels, kinetochore localization of p55cdc, and the normal responses to colcemid treatment. These findings indicate the involvement of BubR1 in p55cdc-mediated mitotic checkpoint signaling, and suggest that >50% decrease in expression (or activity) of BubR1 is involved in the PCS syndrome.