A novel regulator of DNA double strand break repair fate with roles in immunity and oncogenesis

DNA双链断裂（DSB）是一种高毒性的DNA损伤形式，可以杀死细胞，并导致我们细胞中的DNA突变类型，从而引发癌症。DSB的这种细胞杀伤特性解释了为什么它们在放疗和化疗治疗中故意产生以消除癌细胞。然而，当细胞的DNA复制机制遇到问题时，正常分裂细胞中也会自发产生DSB。这些DSB通常被无错误的DNA修复途径识别和处理，防止它们引起引发癌症发展的突变。也许自相矛盾的是，在某些专门的组织中，诱变DSB修复实际上是有利的，提供了一种分子机制，通过这种机制，遗传物质可以在我们细胞基因组的不同区域之间转移，以创造遗传多样性。这些突变事件是我们适应性免疫系统可塑性的基础，重组白色血细胞中的抗体基因片段，使它们能够产生许多不同类型的抗体，以对抗我们接触的许多不同病原体。为了抵消非计划性DSB的潜在危险影响，并在它们被编程时做出适当的反应，细胞进化出两种专用的DNA修复途径：同源修复（HR）使用复制和粘贴机制来准确地修复DSB。因此，它是分裂细胞中最重要的途径，因为这是大多数DSB发生的时候，新细胞继承新突变的威胁意味着精确的DNA修复是强制性的;另一方面，非同源末端连接（NHEJ）是一种更简单的途径，可以将几乎任何两个DNA末端粘合在一起，而不关心它们的序列或它们是否正确。NHEJ通常修复非分裂细胞中的DSB，其中它们作为孤立事件发生，然而当许多DNA末端可用时，它可能是诱变性的。NHEJ在我们的免疫系统中也是至关重要的，其中来自我们染色体上不同位置产生的两个DSB的DNA末端的融合是所需的结果。由于不同细胞环境之间所需DNA修复结果的内在差异，这些修复途径之间实现正确的平衡对于确保DSB得到适当解决至关重要。最近，我们发现DNA修复途径平衡的不平衡将正常免疫功能与人类乳腺癌发展联系起来。事实上，通常促进抗体基因重排的蛋白质的一个专门子集也负责伴随人类乳腺癌肿瘤抑制因子“BRCA 1”的缺失的突变，从而驱动恶性转化。在新的工作中，我们已经确定了一种新的蛋白质，其在BRCA 1缺陷细胞中的丢失可以挽救通常困扰它们的DNA修复缺陷。然而，这会使它们对通常非常有效的抗癌药物产生抗药性。重要的是，我们发现这种蛋白质可能对正常的免疫功能至关重要，促进抗体基因中的DNA修复。我们的建议扩展了这些令人兴奋的发现，使用分子，生物化学和细胞生物学方法来质疑这种蛋白质在DSB修复中的意外功能的分子基础。我们还将在小鼠中克隆该基因，以研究其在免疫系统中的实际作用，并为体外研究提供重要试剂。这项工作的好处将是双重的：首先，它将提供对正常免疫功能的新见解，有助于了解人类免疫缺陷疾病;其次，它将揭示常见人类癌症的分子基础，也将深入了解面临个性化医疗方法的潜在耐药性机制。这些发现可能为改进诊断，治疗和更好地管理癌症的方法铺平道路。

DNA double-strand breaks (DSBs) are a highly toxic form of DNA damage that can kill cells and cause the types of mutations to the DNA in our cells that can trigger cancer. This cell-killing property of DSBs explains why they are intentionally generated in radiotherapy and chemotherapy treatments to eliminate cancer cells. However, DSBs also arise spontaneously in normal dividing cells, when the cell's DNA copying machinery encounters problems. These DSBs are typically recognised and processed by error-free DNA repair pathways, preventing them from causing the mutations that trigger cancer development. Perhaps paradoxically, in certain specialised tissues mutagenic DSB repair is actually favoured, providing a molecular mechanism by which genetic material can be transferred between different regions of the genomes in our cells to create genetic diversity. These mutagenic events are fundamental for the plasticity of our adaptive immune systems, reshuffling antibody gene segments in white blood cells so that they can generate the many different types of antibody that are needed to fight the many different pathogens that we are exposed to.To counteract the potentially hazardous effect of unscheduled DSBs and respond appropriately when they are programmed, cells have evolved two dedicated DNA repair pathways: Homologous Recombination (HR) uses a copy and paste mechanism to repair DSBs accurately. It is therefore the most important pathway in dividing cells, as this is when most DSBs occur and the threat of new cells inheriting new mutations means accurate DNA repair is mandatory; On the other hand, Non-homologous end-joining (NHEJ) is a more simple pathway that can glue nearly any two DNA ends together, not caring about their sequence or whether they are the right ones. NHEJ normally repairs DSBs in non-dividing cells where they occur as isolated events, yet when numerous DNA ends are available it can be mutagenic. NHEJ is also crucial in our immune systems, where the fusing of DNA ends originating from two DSBs generated at different locations on our chromosomes is the required outcome. Because of this intrinsic discrepancy in desired DNA repair outcome between different cellular contexts, achieving the right equilibrium between these repair pathways is vital to ensure DSBs are appropriately resolved.Recently, we have found imbalances in DNA repair pathway equilibrium to link normal immune-function to human breast cancer development. Indeed a specialised subset of proteins that normally facilitate antibody gene rearrangements, are also responsible for the mutations that accompany loss of the human breast cancer tumour suppressor 'BRCA1', driving malignant transformation. In new work, we have identified a new protein whose loss in BRCA1-deficient cells can rescue the DNA repair defect that normally afflicts them. However, this then renders them resistant to important anti-cancer drugs that are normally extremely effective at killing them. Importantly, we have found that this protein is likely to be crucial for normal immune function, promoting DNA repair in antibody genes. Our proposal extends on these exciting findings, using molecular, biochemical, and cell biology approaches to question the molecular basis of this protein's unanticipated function in DSB repair. We will also inactivate this gene in the mouse, to study its actual role in the immune system, and generate important reagents for in vitro studies. The benefit of this work will be two-fold: Firstly, it will provide new insight into normal immune function, aiding the understanding of human immunodeficiency disorders; Secondly, it will reveal the molecular basis of common human cancers, also yielding insight into potential mechanisms of drug resistance that face personalised medicine approaches. These findings may pave way to improved approaches to diagnose, treat and better manage cancer.