Molecular Genetics and Pathophysiology of Adult-Onset Type II Citrullinemia

成人型II型瓜氨酸血症（CTLN 2）的特征是肝脏特异性的氨基琥珀酸合成酶（ASS）缺乏，ASS是尿素循环的限速酶。尽管CTLN 2预后差，但肝移植治疗效果显著。小林等发现CTLN 2是由7q21.3上的SLC 25 A13基因突变引起的，该基因编码钙结合线粒体溶质载体蛋白，称为citrin（Nat Genet 22：159-163，1999）。到目前为止，我们已经在129例CTLN 2和108例NICCD患者中发现了18个（包括5个已知）SLC 25 A13突变，并针对最常见的9个突变，建立了多种DNA诊断方法，使用GeneScan/SNaPshot。DNA诊断显示，在日本人群中，携带者频率为1/69（纯合子频率：1/19，000）。Citrin缺乏症被认为仅限于日本，但我们在其他国家也发现了一些病例。在我们的初步筛选研究中，我们在东亚检测到类似的载波频率（1/50-1/100）。大多数NICCD患者表现出症状，在1岁时改善。10多年甚至几十年后，一些患者发展为严重的CTLN 2，并伴有神经精神症状。由于我们发现Citrin和其亚型arthrin定位于线粒体内膜，并作为钙刺激的天冬氨酸谷氨酸载体发挥作用，因此可以通过天冬氨酸从线粒体输出到细胞质和苹果酸天冬氨酸NADH穿梭的缺陷来理解NICCD和CTLN 2的各种症状。然而，目前尚难以阐明CTLN 2中ASS蛋白的肝缺陷机制。我们产生了两种桔蛋白缺陷小鼠，现在正在检查它们是否适合作为NICCD和/或CTLN 2的动物模型。

Adult-onset type II citrullinemia (CTLN2) is characterized by a liver-specific deficiency of argininosuccinate synthetase (ASS), with is a rate-limiting enzyme of the urea cycle. Although the prognosis of CTLN2 is bad, liver transplantation is remarkably effective. Kobayashi at al. have discovered that CTLN2 is caused by mutations of the SLC25A13 gene on 7q21.3, with encodes a calcium-binding mitochondrial solute carrier protein, designated citrin (Nat Genet 22 : 159-163, 1999).In the present study we found that the citrin deficiency causes not only CTLN2 but also neonatal hepatitis with intrahepatic cholestasis, named NICCD. So far, we have identified 18 including 5 known) SLC25A13 mutations in 129 CTLN2 and 108 NICCD patients, and for most frequent 9 mutations, established multiple DNA diagnosis methods using GeneScan/SNaPshot. DNA diagnosis revealed that the carrier frequency is 1/69 (homozygote frequency : 1/19,000) in the Japanese population. Citrin deficiency was thought to be restricted to Japan, but we found some cases in other country. In our preliminary screening study, we detected a similar carrier frequency (1/50-1/100) in East Asia. Most NICCD patients show symptoms, with ameliorate by 1 year of age. More Than 10 years or even several decades later, some patients develop severe CTLN2 with neuropsychiatric symptoms. Since we found that citrin and the isoform, aralar, localize in the mitochondrial inner membrane and function as aspartate glutamate carriers stimulated by calcium, the various symptoms of NICCD and CTLN2 may be understood through the defects of aspartate export from mitochondria to cytosol and of malate aspartate NADH shuttle. It is, however, still difficult to clarify the mechanism of hepatic defect of ASS protein in CTLN2. We generated two kinds of citrin-deficient mice and are now examining them to see whether they are suitable as animal models for NICCD and/or CTLN2.