Response to Human Nedd4L rs4149601 G Allele Generates Evolutionary New Isoform I With C2 Domain
We highly appreciate the comments of Ishigami et al1 and are sorry about our mistaken description regarding the rs4149601 (G/A) A allele of NEDD4L generating isoform I2 (which should be generated by the G allele).
In contrast to the previous publication,3 we found that the A allele of variant rs4149601 was associated with hypertension,2 consistent with the study in black subjects.4 The full-length C2 domain is very important for targeting epithelial sodium channel for degradation by ubiquitination by NEDD4L. Functional study has shown that the subjects carrying variant rs4149601 would have a decrease in the amount of C2 domain NEDD4L produced by isoform I.5 In this case, ubiquitination of the epithelial sodium channel could be reduced, and the activity of the epithelial sodium channel-Na+ transport and blood pressure are expected to be increased.
The results of Ishigami et al1 cited Reference6 do not support the above conception, because isoform I alone does not significantly change the relative epithelial sodium channel current, indicating that the story of splice mutation is much more complex in vivo than what we observed in vitro. The G allele has been shown to display leaky splice site selection with a mixture of splice products, whereas the A allele only produces 1 truncated protein.6 The rs4149601 polymorphism is a variant at the splice junction of a putative exon 1 in NEDD4L. Splice site mutation has been considered to result in different splice errors, including the following: (1) producing a premature stop codon, loss of an exon, or inclusion of an intron; (2) reducing specificity, resulting in variation in the splice location, causing insertion or deletion of amino acids, or a loss of the reading frame; and (3) displacing a splice site, leading to inclusion or exclusion of RNA, and, in turn, exons longer or shorter. So far, most reported splicing mutations in mutation databases have been described only at the genomic sequence level, and their effects at the protein level cannot be accurately predicted, because it is still largely unknown which of these 3 splicing pathway are followed as a consequence of the mutation in vivo. Clearly, much more studies are needed. The response of Ishigami et al1 to our work is very helpful for stimulating further research efforts.
Sources of Funding
This study was supported by the National High-Tech Research and Development Program of China (2006AA02A406 to R.H.).
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