The Biology of Genetic Variants in Human Kidney Disease

描述（由申请人提供）：我们在肾脏病样肾衰竭家族中发现了隐性XPNPEP3突变（O’toole, et al. 2010）。受影响的个体肾脏大小正常，伴有罕见的小囊肿，肾脏组织学以间质纤维化和肾小管萎缩为主，与肾病（NPHP）相似，NPHP是儿科人群中终末期肾病最常见的遗传原因（Hildebrandt等，2009）。我们已经清楚地表明，XPNPEP3定位于线粒体，而不是像其他NPHP基因产物那样定位于初级纤毛/中心体/基底复合体（O'Toole, et al. 2010）。XPNPEP3的酵母同源物YER078c定位于线粒体，而不是细胞质（Naamati等，2009），是一种氨基肽酶，通常从其蛋白靶的氨基端去除单个残基（Vogtle, et al. 2009）。这种翻译后处理通过阻断进入以n端规则为特征的线粒体蛋白降解途径来稳定这些靶标并延长其半衰期（Voglte等，2009；bachmaair等，1986）。我们认为，随着XPNPEP3功能的丧失，线粒体蛋白降解的增加可以激活TOR，因为我们发现酿酒酵母YER078c缺失菌株的TOR活性增加，这通过线粒体靶向人类XPNPEP3的表达来纠正。Xpnpep3-/-小鼠在2月龄时发生mTOR激活，随后在4月龄时发生小管损伤并出现明显的细胞质空泡化。这些结果表明，线粒体蛋白XPNPEP3通过mTOR激活的一种新的线粒体机制导致肾脏病样肾病。我们认为mTOR激活可能将XPNPEP3与其他NPHP基因联系起来，其中mTOR激活在其发病机制中已知是重要的，但其激活的机制尚不清楚。我们的总体假设是，XPNPEP3功能的丧失增加了线粒体蛋白降解为肽片段，这些肽片段被转运到细胞质中，在那里它们上调了TOR的活性。正是通过线粒体途径激活mTOR，导致在受影响个体中观察到间质纤维化、肾小管萎缩和囊肿形成的肾脏表型。我们将使用酵母和小鼠Xpnpep3缺失的遗传模型来验证这一假设，以检查其在肾脏疾病进展中的作用，并研究导致mTOR激活和缺陷蛋白降解的机制。

DESCRIPTION (provided by applicant): We have identified recessive XPNPEP3 mutations in families with nephronophthisis-like renal failure (O'Toole, et. al. 2010). Affected individuals have normal kidney size with rare, small cysts and renal histology dominated by interstitial fibrosis and tubular atrophy similar to that seen in nephronophthisis (NPHP), the most common genetic cause of end-stage renal disease in the pediatric population (Hildebrandt, et. al. 2009). We have clearly shown that XPNPEP3 localizes to the mitochondria, rather than the primary cilia/centrosome/basal body complex (O'Toole, et al. 2010) as typical for other NPHP gene products. The yeast ortholog of XPNPEP3, YER078c, localizes to the mitochondria, but not to the cytosol (Naamati, et. al. 2009) and is an aminopeptidase that generally removes a single residue from the amino-terminus of its protein targets (Vogtle, et al. 2009). This post-translational processing stabilizes these targets and increases their half-life by blocking entry into a mitochondrial protein degradation pathway characterized by the N-end rule (Voglte, et. al. 2009, Bachmair, et. al. 1986). We believe that increased mitochondrial protein degradation, as may occur with loss of XPNPEP3 function, can activate TOR, as we found increased TOR activity in YER078c deletion strains of S. cerevisiae which is corrected by the expression of mitochondrially targeted human XPNPEP3. Examining Xpnpep3-/- mice, mTOR activation occurred by 2 months of age followed by the development of tubular damage with pronounced cytoplasmic vacuolization by 4 months of age. These results suggest that the mitochondrial protein XPNPEP3 causes a nephronophthsis-like kidney disease through a novel mitochondrial mechanism of mTOR activation. We believe that mTOR activation may link XPNPEP3 to other NPHP genes, where mTOR activation is known to be important in their pathogenesis, but the mechanisms underlying its activation are poorly understood. Our overall hypothesis is that loss of XPNPEP3 function increases the degradation of mitochondrial proteins to peptide fragments, which are transported into the cytosol where they upregulate TOR activity. It is the activation of mTOR through this mitochondrial pathway that leads to the renal phenotype of interstitial fibrosis, tubular atrophy and cyst formation observed in affected individuals. We will test this hypothesis using yeast and murine genetic models of Xpnpep3 deletion to examine its role in the progression of kidney disease and study the mechanisms leading to mTOR activation and defective protein degradation. PUBLIC HEALTH RELEVANCE: We believe that the mitochondrial protein XPNPEP3 causes a progressive kidney disease through a new pathway of mTOR activation involving the mitochondria. mTOR is a molecule that regulates several cell functions and its activation has been shown to be important for kidney diseases including diabetes, polycystic kidney disease, as well as, cancer. We will use yeast and mouse models of Xpnpep3 deletion to examine its role in the progression of kidney disease and study the mechanisms leading to mTOR activation.