Contribution of dysfunctional telomeres to primary osteoarthritis

原发性骨关节炎(OA)具有非常强的遗传成分；然而，只有少数的OA风险等位基因已被识别。在这项建议中，我们将重点放在端粒缩短作为促成原发骨性关节炎的表观遗传学因素。端粒对邻近基因的沉默效应被称为端粒位置效应，并被端粒缩短所破坏，这是以前在OA患者中检测到的。本应用的目的是证明或驳斥端粒缩短与骨性关节炎发病机制有关的观点，并显示SIRT6激活剂的治疗效果。我们的中心假设是端粒缩短通过上调端粒近端基因的表达来促进骨性关节炎的发展，而端粒近端基因的表达被SIRT6拮抗。我们还提出，软骨和滑膜中端粒的缩短会触发关节中的端粒相关分泌表型(TasP)和端粒重复序列RNA(Terra)的表达。这一假说是在现有文献和初步数据的基础上提出的，显示SIRT6在短端骨性关节炎软骨中的表达增加，显示在50例膝关节骨性关节炎患者关节置换手术中，患侧与非患侧软骨的端粒显著缩短，表明端粒非常短的软骨中端粒附近的差异表达基因丰富，表明NMN对软骨细胞衰老有抑制作用。在初步数据的指导下，这一假说将通过追求三个特定目标来检验：1)确定SIRT6激活剂，如烟酰胺单核苷酸(NMN)在受影响软骨细胞中的作用；2)确定端粒的延伸是否可以沉默端粒近端上调的基因；以及3)确定Terra转录产物是否存在于OA患者的滑液外小体中，以及它们的水平是否对软骨细胞的炎症反应有直接影响。这项研究具有重要意义，因为它表明端粒缩短影响的端粒位置效应是导致软骨老化和骨关节炎病理的新的重要机制。这个项目的新奇之处在于，它探索了一种有助于骨性关节炎病因的新的表观遗传学机制。

Primary osteoarthritis (OA) has a very strong genetic component; however, only a small number of OA risk alleles have been identified. In this proposal we focus on telomere shortening as an epigenetic factor contributing to primary OA. The silencing effect of telomeres on the genes located nearby is known as telomere position effect and is disrupted by telomere shortening, which was detected previously in patients with OA. The objective of this application is to prove or refute the concept that telomere shortening has a mechanistic relationship to the pathogenesis of OA and to show a therapeutic effect of SIRT6 activators. Our central hypothesis is that telomere shortening contributes to the development of OA through upregulated expression of telomere-proximal genes, which is antagonized by SIRT6. We also propose that telomere shortening in cartilage and synovium triggers telomere-associated secretory phenotype (TASP) in the joint and expression of telomeric repeat-containing RNA (TERRA). This hypothesis has been formulated on the basis of the current literature and preliminary data revealing increased SIRT6 expression in OA cartilage with short telomeres, showing significant shortening of telomeres in affected versus unaffected cartilage obtained from 50 patients with knee OA during joint replacement surgery, indicating enrichment in differentially expressed genes located close to telomeres in cartilage with very short telomeres and showing a suppressive effect of NMN on chondrocyte senescence. Guided by preliminary data, this hypothesis will be tested by pursuing three specific aims: 1) Determine the effect of SIRT6 activators such as nicotinamide mononucleotide (NMN) on progression of OA in affected chondrocytes; 2) Determine if telomere extension can silence upregulated telomere-proximal genes; and 3) Determine if TERRA transcripts are present in synovial fluid exosomes of OA patients and if their level has a direct effect on the inflammatory response of chondrocytes. The proposed research is significant because it indicates telomere position effect affected by telomere shortening as the new important mechanism contributing to cartilage aging and OA pathology. The novelty of this project is that it explores a new epigenetic mechanism contributing to etiology of OA.