Molecular Genetics and Pathophysiology of Adult-Onset Type II Citrullinemia

成人起病的II型瓜氨酸血症(CTLN2)的特点是肝脏特异性缺乏精氨酸琥珀酸合成酶(ASS)，是尿素循环的限速酶。虽然CTLN2期预后不佳，但肝移植疗效显著。小林在al.已发现CTLN2是由7q21.3上的SLC25A13基因突变引起的，编码一种钙结合的线粒体溶质载体蛋白Citrin(NAT Genet 22：159-163,1999)。到目前为止，我们已经在129例CTLN2和108例NICCD患者中发现了18个突变，其中包括5个已知的SLC25A13突变，对于最常见的9个突变，我们建立了使用GeneScan/SNAPSHOT的多重DNA诊断方法。DNA检测显示，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，但我们在其他国家发现了一些病例。在我们的初步筛查研究中，我们在东亚检测到类似的载频(1/50-1/100)。大多数NICCD患者都有症状，并在1岁时有所改善。10多年甚至几十年后，一些患者出现严重的CTLN2伴神经精神症状。由于我们发现Citrin及其异构体Aralar定位于线粒体内膜，并在钙刺激下作为天冬氨酸载体发挥作用，NICCD和CTLN2的各种症状可以通过天冬氨酸从线粒体输出到胞浆和苹果酸天冬氨酸NADH穿梭的缺陷来理解。然而，目前仍难以阐明CTLN2中ASS蛋白在肝脏中的缺陷机制。我们产生了两种Citrin缺陷小鼠，现在正在对它们进行检查，看看它们是否适合作为NICCD和/或CTLN2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.