Molecular and functional analysis of CIC-5 in idiopathic low-molecular-weight proteinuria

CLCN5基因突变已在三种高钙尿性肾结石疾病中被证实，即Dent病、X连锁隐性肾结石和X连锁隐性低磷血症。最近，一些低分子蛋白尿(LMWP)的日本儿童表现出与英国家庭登特氏病患者相似的症状，也有报道称CLCN5基因发生了突变。本研究调查了五个不相关的LMWP日本家系，其中两个没有LMWP以外的任何体征，其中三个有LMWP以外的几个体征，即高钙尿、氨基酸尿症、低磷血症和软骨病。在两个仅有LMWP的家系中的三名患者中发现了一个无义(E118X)和一个错义(W22G)突变。1例低磷性软骨病患者发现CLCN5基因第5~8外显子缺失，1例LMWP伴轻度高钙尿症患者发现无义突变(R347X)。在1例LMWP、氨基酸尿症和低钾血症患者中，未发现CLCN5基因外显子和外显子-内含子边界突变。除了在这些无义和缺失突变中预测的氯离子通道功能丧失外，错义突变W22G在非洲爪哇卵母细胞表达系统中也证实了功能丧失。这些结果阐明了CLCN5基因的四个新突变，并提示CLCN5功能丧失突变并不一定导致疾病早期的高钙尿和肾钙沉着，LMWP是ClC-5氯通道紊乱的早期和基本表现。

Mutations in the CLCN5 gene have been demonstrated in three disorders of hypercalciuric nephrolithiasis, i.e., Dent's disease, X-linked recessive nephrolithiasis, and X-linked recessive hypophosphatemic rickets. Recently, a number of Japanese children with low-molecular-weight proteinuria (LMWP) showing symptoms similar to those shown by patients with Dent's disease in British families have also been reported to have mutations in the CLCN5 gene. The present study examines five unrelated Japanese families with LMWP, two of which lacked any signs other than LMWP, and three of which had several signs other than LMWP, i.e., hypercalciuria, aminoaciduria, hypophosphatemia, and rickets. One nonsense (E118X) and one missense (W22G) mutations were found in three patients in the two families having only LMWP. One genomic deletion including exon 5 to 8 in the CLCN5 gene was found in a patient with hypophosphatemic rickets, and a nonsense mutation (R347X) was found in one patient with LMWP and slight hypercalciuria. No mutation of the exons and exon-intron boundaries in the CLCN5 gene was found in one patient with LMWP, aminoaciduria, and hypokalemia. In addition to the predicted loss of chloride channel function in these nonsense and deletion mutations, the loss of function in the missense mutation W22G was confirmed in the Xenopus oocyte expression system. These results clarified four novel mutations in the CLCN5 genes and additionally suggested that the loss-of-function mutation of the CLCN5 does not necessarily lead to hypercalciuria and nephrocalcinosis in the early stage of the disease, and that LMWP is an early and essential manifestation of disorders of the CLC-5 chloride channel.