MECHANISMS RESPONSIBLE FOR GENOMIC INSTABILITY IN PREMATURE AGING LAMINOPATHIES

描述（由申请人提供）：a型纤层蛋白是一种中间丝蛋白，为系住染色质和调节核结构和功能的蛋白质复合物提供支架。在LMNA基因突变与各种退行性疾病（广泛称为板层病、癌症和衰老）相关后，对板层蛋白的兴趣增加了。a型纤层蛋白改变与疾病之间的因果关系以及所涉及的分子途径尚不清楚。我们的数据揭示了a型纤层蛋白在端粒维持和53BP1稳定中的新功能，53BP1是DNA损伤反应途径的中介。a型纤层蛋白的缺失导致端粒缩短、端粒染色质缺陷、DNA修复受损、基因组不稳定性增加以及功能失调端粒的非同源末端连接缺陷，这一过程需要53BP1。其中一些表型可能与53BP1缺乏一致。为了支持这一观点，53BP1重组为a型层板缺陷细胞挽救了DNA修复缺陷。阐明53BP1降解的分子机制可能为治疗提供新的靶点。我们的初步数据表明53BP1可以被蛋白酶组织蛋白酶L和蛋白酶体降解。有趣的是，早衰小鼠模型（Zmpste24-/-）显示明显上调Cathepsin L，显示低水平的53BP1，这表明Cathepsin L介导的53BP1降解的解除可能是板层病的常见事件。重要的是，我们发现用维生素D（一种已知的胱抑素D[8]的激活剂，内源性组织蛋白酶L的抑制剂）治疗可以恢复53BP1的水平，提供了一种防止53BP1降解的假设策略，改善板层病的表型。待验证的假设是，a型纤层蛋白的改变引起Cathepsin L的上调，导致53BP1蛋白不稳定，DNA修复和端粒稳定性缺陷，从而导致基因组不稳定和一些纤层蛋白相关疾病的表型。我们的假设将在三个具体目标下进行检验。在Aim 1下，我们将阐明导致层状病变中53BP1不稳定的分子机制，并评估靶向该途径并改善DNA修复的方法。在Aim 2下，我们将确定层状病中组织蛋白酶L/53BP1通路的解除管制对端粒稳定性的影响程度。在Aim 3中，我们将确定维生素D对Cathepsin L/53BP1通路的调节是否能改善纤层蛋白相关疾病的体内病理生理。本研究不仅对阐明a型纤层蛋白影响端粒功能和DNA修复的分子机制具有重要意义，而且对了解纤层蛋白相关疾病（包括退行性纤层病、早衰综合征和癌症）的病理生理基础具有重要意义。我们的研究结果可以转化为针对53BP1降解途径的新治疗策略的开发，以改善这些疾病相关的表型。

DESCRIPTION (provided by applicant): A-type lamins are intermediate filament proteins that provide a scaffold for tethering chromatin and protein complexes regulating nuclear structure and function. Interest in lamins increased after the association of mutations in the LMNA gene with a variety of degenerative disorders broadly termed laminopathies, cancer and aging. The causal relationship between alterations of A-type lamins and disease and the molecular pathways involved are poorly understood. Our data revealed novel functions for A-type lamins in the maintenance of telomeres and in the stabilization of 53BP1, a mediator in the DNA damage response pathway. Loss of A-type lamins leads to telomere shortening, telomeric chromatin defects, impaired DNA repair, increased genomic instability, and defects in the non-homologous end-joining of dysfunctional telomeres, a process that requires 53BP1. Some of these phenotypes would be consistent with 53BP1 deficiency. In support of this notion, reconstitution of 53BP1 into A-type lamins-deficient cells rescued defects in DNA repair. Elucidating molecular mechanism responsible for degradation of 53BP1 might provide novel targets for therapy. Our preliminary data indicates that 53BP1 is degraded by the protease Cathepsin L and by the proteasome. Interestingly, a mouse model of progeria (Zmpste24-/-) shown to markedly upregulate Cathepsin L, exhibits low levels of 53BP1, suggesting that deregulation of the Cathepsin L-mediated degradation of 53BP1 might be a common event in laminopathies. Importantly, we found that treatment with vitamin D, a known activator of Cystatin D [8], the endogenous inhibitor of Cathepsin L, rescues 53BP1 levels, providing a putative strategy to prevent 53BP1 degradation, ameliorating phenotypes of laminopathies. The hypothesis to be tested is that alterations in A-type lamins causing upregulation of Cathepsin L lead to destabilization of 53BP1 protein and defects in DNA repair and telomere stability, contributing to genomic instability and the phenotypes of some lamin-related diseases. Our hypothesis will be tested under three specific aims. Under Aim 1, we will elucidate the molecular mechanisms leading to destabilization of 53BP1 in laminopathies and evaluate ways to target this pathway and improve DNA repair. Under Aim 2, we will determine the extent to which deregulation of the Cathepsin L/53BP1 pathway in laminopathies affects telomere stability. Under Aim 3, we will determine whether regulation of the Cathepsin L/53BP1 pathway by vitamin D ameliorates the pathophysiology of lamin-related diseases in vivo. The studies performed here will be of significance not only for elucidating the molecular mechanisms by which A-type lamins impact on telomere function and DNA repair, but also for understanding the underlying basis of the pathophysiology of lamin-related diseases, including degenerative laminopathies, premature aging syndromes and cancer. The results of our study could translate into the development of novel therapeutic strategies that target 53BP1 degradation pathway to improve the phenotypes associated to these diseases. PUBLIC HEALTH RELEVANCE: Understanding the cellular functions of A-type lamins, structural components of the nucleus, has become a highly topical subject due to their implication in a number of degenerative disorders broadly named laminopathies, as well as in cancer and aging. The molecular pathways by which defects in A-type lamins contribute to disease are poorly understood. Genomic instability caused by defective repair of DNA damage has been proposed to contribute to the pathogenesis of laminopathies. This proposal aims to unravel molecular mechanisms that are altered upon loss of A-type lamins leading to increased genomic instability. In addition, we will evaluate ways to target these molecular mechanisms with the goal of identifying treatments that ameliorate the phenotypes of laminopathies. Both, studies with primary cells (in vitro) and with mouse models of laminopathies (in vivo) will be conducted. The knowledge obtained from this research will be of great possible significance for the development of therapeutic approaches to treat lamin-related diseases.