BLM-mediated Homologous Recombination Regulation and Tumor Suppression

描述(申请人提供)：布鲁姆综合征(BS)是一种常染色体隐性遗传病，其特征是身材矮小、对阳光敏感、对感染和呼吸道疾病的易感性增加，以及对多种癌症的强烈易感性。平均死亡年龄为27岁，最常见的原因是与癌症有关。在细胞水平上，BS表现出高度的姐妹染色单体交换、同源重组和异位重组水平，表明同源重组(HR)去调节。已有研究认为，HR事件的异常导致BS基因组的整体不稳定，表现为染色体的异常和重排，包括断裂、四分叉和易位。BLM是由BS突变的基因编码的蛋白质，是RecQ解旋酶家族的成员。BLM具有解开DNA底物的能力，包括那些类似HR中间体的底物，它与I型拓扑异构酶Topo IIIα一起作用，溶解双Holliday结(DHJ)，产生非交叉重组体。BLM还与一种新的多肽BLAP75结合，并与hRad51重组酶结合。重要的是，我们的初步研究发现，BLAP75对BLM-Topo IIIα介导的反应具有戏剧性的刺激作用。本研究项目包括三个特定目标下的分子研究，旨在破译BLM-Topo IIIα-BLAP75(BTB)复合体在处理HR中间体、确定HR途径选择和调节hRad51活性方面的生物学意义。具体地说，我们将(1)描述BLM、Topo IIIα和BLAP75在HR调节所需的BTB复合体中的功能作用；(2)确定BLAP75介导的配体相互作用对BTB复合体功能的贡献；(3)检验有关BLM-hRad51相互作用的功能意义的假说。我们的研究结果将从机制上阐明BLM依赖的HR控制机制，并将为BS患者的基因组不稳定性和癌症易感性提供解释。

DESCRIPTION (provided by applicant): Bloom's syndrome (BS), an autosomal recessive disorder, is characterized by short stature, sunlight sensitivity, an increased susceptibility to infections and respiratory illness, and a strong disposition to a wide spectrum of cancers. The mean age of death is 27 years and the cause is most frequently cancer-related. At the cellular level, BS exhibits highly elevated levels of sister chromatid exchanges, interhomologue recombination, and ectopic recombination, indicative of homologous recombination (HR) deregulation. It has been proposed that abnormal HR events lead to the general destabilization of the genome in BS, manifested as chromosome aberrations and rearrangements including breaks, quadriradials, and translocations. BLM, the protein encoded by the gene mutated in BS, is a member of the RecQ helicase family. BLM has the ability to unwind DNA substrates including those that resemble HR intermediates, and it works in conjunction with Topo III alpha, a type IA topoisomerase, to dissolve the double Holliday junction (DHJ) to yield non-crossover recombinants. BLM also associates with a novel polypeptide BLAP75 and binds the hRad51 recombinase. Importantly, our preliminary studies have found a dramatic stimulatory effect of BLAP75 on the reactions mediated by BLM-Topo III alpha. This research project comprising molecular studies under three specific aims strives to decipher the biological significance of the BLM-Topo III alpha-BLAP75 (BTB) complex with regard to the processing of HR intermediates, defining HR pathway choice, and regulating the activity of hRad51. Specifically, we will (1) delineate the functional roles of BLM, Topo III alpha, and BLAP75 in the BTB complex needed for HR modulation, (2) define the contributions of BLAP75-mediated ligand interactions to BTB complex functions, and (3) test hypotheses concerning the functional significance of the BLM-hRad51 interaction. The results from our research endeavors will shed mechanistic light on BLM-dependent control mechanisms of HR and will provide an explanation for the genome instability and cancer susceptibility in BS patients.