The Biology of Genetic Variants in Human Kidney Disease

描述（由申请人提供）：我们已经鉴定了肾单位结核样肾衰竭家族中的隐性XPNPEP 3突变（O 'Toole，et. 2010）。受影响的个体具有正常的肾脏大小，具有罕见的小囊肿，并且肾组织学以间质纤维化和肾小管萎缩为主，类似于在肾单位肾结核（NPHP）中所见，这是儿科人群中终末期肾病的最常见遗传原因（Hildebrandt，et. 2009）。我们已经清楚地表明，XPNPEP 3定位于线粒体，而不是其他NPHP基因产物典型的初级纤毛/中心体/基体复合体（O 'Toole等，2010）。XPNPEP 3的酵母直向同源物YER 078 c定位于线粒体，但不定位于细胞溶质（Naamati，et. 2009），并且是通常从其蛋白质靶标的氨基末端去除单个残基的氨肽酶（Vogtle等人，2009）。这种翻译后加工稳定了这些靶标，并通过阻断进入以N端规则为特征的线粒体蛋白降解途径来增加其半衰期（Voglte，et. 2009，Bachmair，et. 1986）。我们认为，增加的线粒体蛋白降解，如可能发生的XPNPEP 3功能的丧失，可以激活TOR，因为我们发现增加的TOR活性的YER 078 c缺失菌株的S。酿酒酵母，其通过表达靶向人XPNPEP 3来校正。检查Xpnpep 3-/-小鼠，mTOR活化在2月龄时发生，随后在4月龄时发生肾小管损伤，伴有明显的细胞质空泡化。这些结果表明，线粒体蛋白XPNPEP 3通过mTOR激活的新线粒体机制引起肾单位硬化样肾病。我们认为mTOR激活可能将XPNPEP 3与其他NPHP基因联系起来，已知mTOR激活在其发病机制中很重要，但对其激活的机制知之甚少。 我们的总体假设是，XPNPEP 3功能的丧失增加了线粒体蛋白降解为肽片段，肽片段被转运到细胞溶质中，在那里它们上调TOR活性。正是通过这种线粒体途径激活mTOR，导致在受影响个体中观察到的间质纤维化、肾小管萎缩和囊肿形成的肾表型。我们将使用Xpnpep 3缺失的酵母和小鼠遗传模型来检验这一假设，以检查其在肾脏疾病进展中的作用，并研究导致mTOR激活和蛋白质降解缺陷的机制。

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.