2002 Global Researcher Conference Proceeding

April 26 - 28, 2002

Conference: 2002 Global Researcher Conference
Title: Gene Mutation Analysis of NDI patients in Japan
Authors: Sasaki, Sei; Asai, Tomoki; Uchida, Shinichi; Kuwahara, Michio
Institutions: Tokyo Medical & Dental University, Tokyo Medical and Dental University, Graduate School, Tokyo Medical and Dental University School of Medicine

It is now clear that gene mutation analysis is critically important for NDI patients, because this information allows determination of cellular defects that cause the inability of collecting duct to AVP and allows development of therapeutic strategies and new drugs. It may also be important to know the characteristics of NDI gene mutations in an ethnic group other than Caucasian.

We have started gene mutation analysis of NDI patients in Japan in collaboration with the Japanese NDI patients association. We have analyzed 26 families and found mutations in the vasopressin receptor 2 gene (V2R) in 15 families (58%) and in the AQP2 gene in 5 families (19%). No mutations of V2R or AQP2 were found in the remaining 6 families. It would be likely that AQP2 mutations are more popular in Mongolian than in Caucasian. Significant number of the patients who do not have any mutation in V2R nor AQP2 suggests a possibility that other genes are responsible for NDI. Of 5 AQP2 gene-mutated NDI families, 4 showed an autosomal dominant inheritance, i.e., the mutations are present only in one allele. Clinical picture of these dominant type NDI may be different from those of recessive type; dominant patients seem to have milder urine concentrating defect. One dominant type patient (28 month old, male) could concentrate urine up to 500 mOsm after water deprivation and DDAVP treatment.

All gene mutations that cause dominant type NDI in our cases were present in the C-terminus of AQP2, and they were 1, 7, or 10 bp deletions. The mutations are predicted to encode a ~60 amino-acid longer protein, and all mutant proteins shared the same C-terminal tail. Yeast expression system showed that water channel function of these mutants was normal, but in the oocyte expression system, the mutants could not confer water permeability due to little surface expression. Co-expression of wild and mutant AQP2 showed the presence of dominant-negative effects. Further analysis for the mechanisms of cellular defects of these mutants in MDCK cells is underway and such studies will lead to discovery of the treatment of the patients.

Sasaki, et al., have analyzed 26 Japanese NDI families. Fifteen of these families had mutations in the vasopressin-2 receptor (V2R) gene. Six NDI families had no mutation of either the V2R gene or the AQP2 gene. Sasaki suggests that AQP2 mutations appear more often in Mongolian than Caucasian populations. He also suggests that mutations in other genes could also cause NDI.

The inheritance pattern for four out of the five NDI families with AQP2 was dominant, rather than recessive. This means that patients in these families only had to inherit the mutated gene in only a single dose (as opposed to two) for it to produce NDI symptoms. These dominant patients appear to have a less severe urine concentrating defect than the other NDI patients. All these AQP2 mutations of the patients with dominant NDI were present in the tail end (the C-terminus) of the AQP2 protein. These proteins were functional but could not travel from the cell interior to the cell membrane so they could perform their function.