The C-terminal domain of the Bloom syndrome DNA helicase is essential for genomic stability

BACKGROUND: Bloom syndrome is a rare cancer-prone disorder in which the cells of affected persons have a high frequency of somatic mutation and genomic instability. Bloom syndrome cells have a distinctive high frequency of sister chromatid exchange and quadriradial formation. BLM, the protein altered in BS, is a member of the RecQ DNA helicase family, whose members share an average of 40% identity in the helicase domain and have divergent N-terminal and C-terminal flanking regions of variable lengths. The BLM DNA helicase has been shown to localize to the ND10 (nuclear domain 10) or PML (promyelocytic leukemia) nuclear bodies, where it associates with TOPIIIα, and to the nucleolus. RESULTS: This report demonstrates that the N-terminal domain of BLM is responsible for localization of the protein to the nuclear bodies, while the C-terminal domain directs the protein to the nucleolus. Deletions of the N-terminal domain of BLM have little effect on sister chromatid exchange frequency and chromosome stability as compared to helicase and C-terminal mutations which can increase SCE frequency and chromosome abnormalities. CONCLUSION: The helicase activity and the C-terminal domain of BLM are critical for maintaining genomic stability as measured by the sister chromatid exchange assay. The localization of BLM into the nucleolus by the C-terminal domain appears to be more important to genomic stability than localization in the nuclear bodies.

背景：布卢姆综合征（Bloom syndrome）是一种罕见的癌症易感遗传病，患者体细胞具有高频体细胞突变（somatic mutation）和基因组不稳定性（genomic instability）特征。该综合征患者的细胞存在标志性的高频率姐妹染色单体交换（sister chromatid exchange, SCE）与四射体形成（quadriradial formation）现象。布卢姆综合征中功能发生改变的蛋白BLM属于RecQ DNA解旋酶家族（RecQ DNA helicase family），该家族成员的解旋酶结构域（helicase domain）平均同源性达40%，且各自拥有长度不一、序列差异显著的N端侧翼区域与C端侧翼区域。现有研究表明，BLM DNA解旋酶可定位于核结构域10（nuclear domain 10, ND10）或称PML（早幼粒细胞白血病，promyelocytic leukemia）核体，在此处与TOPIIIα结合，同时也可定位于核仁。结果：本研究证实，BLM的N端结构域负责将蛋白靶向至核体，而C端结构域则引导蛋白定位至核仁。相较于会升高SCE频率与染色体异常的解旋酶结构域及C端突变，BLM N端结构域的缺失对姐妹染色单体交换频率与染色体稳定性的影响微乎其微。结论：以姐妹染色单体交换实验作为检测手段，BLM的解旋酶活性与C端结构域是维持基因组稳定性的核心要素。相较于BLM定位于核体的过程，其通过C端结构域靶向核仁的机制对基因组稳定性的贡献更为关键。