@article {OuyangENEURO.0046-19.2019, author = {Qing Ouyang and Lena Joesch-Cohen and Sasmita Mishra and Hasib A. Riaz and Michael Schmidt and Eric M. Morrow}, title = {Functional Assessment In Vivo of the Mouse Homologue of the Human Ala-9-Ser NHE6 Variant}, elocation-id = {ENEURO.0046-19.2019}, year = {2019}, doi = {10.1523/ENEURO.0046-19.2019}, publisher = {Society for Neuroscience}, abstract = {Christianson syndrome (CS) is an X-linked neurogenetic disorder resulting from loss-of-function (LoF) mutations in SLC9A6, which encodes the endosomal Na+/H+ exchanger 6 (NHE6). NHE6 regulates proton efflux from endosomes and, thus, participates in regulating cargo processing and trafficking. LoF mutations in NHE6 cause aberrant acidification of endosomes. While CS arises in males generally due to clear LoF mutations, other potentially hypomorphic variants have emerged, yet most of these variants have not been evaluated for functional effects, particularly in vivo. Here we characterize an SLC9A6 variant that has been previously reported in patients, yet now also appears in exome datasets of largely control individuals{\textemdash}c.25G\>T, p.A9S. By heterologous expression in cell lines, we show that human NHE6A9S is expressed and localizes in a manner comparable to control NHE6. By genome editing, we generated the equivalent NHE6 mutation in mouse{\textemdash}p.A11S{\textemdash}and determined that male NHE6A11S mice have normal brain size at 6 months of age and do not show cerebellar degeneration or defective neuronal arborization. Neurons from male NHE6A11S mice also did not demonstrate an abnormality in intra-endosomal pH in comparison to controls. These findings are in contrast to findings in NHE6-null mice previously reported and indicate that the NHE6A11S variant functions at a level equivalent to control NHE6 for many of the assays performed. These data stand in support of the population genetic data, which are also evaluated here, indicating that the A9S variant is unlikely to confer disease susceptibility with high penetrance.Significance Statement Loss-of-function mutations in SLC9A6, encoding Na+/H+ exchanger 6 (NHE6), cause Christianson syndrome. Also, missense variants of unknown consequences have emerged. Functional evaluation of these mutations contributes to an understanding of the medical relevance and cellular effects of these variants. We show that human NHE6A9S protein is synthesized and localizes normally. Mice genome edited to the equivalent variant{\textemdash}NHE6A11S{\textemdash}did not show a reduction in brain size or defects in neuronal arborization. Mutant NHE6A11S neurons also did not demonstrate a significant decrease in luminal pH of endosomes. These findings are in contrast to findings in NHE6-null mice previously reported. Combined with the population genetic data, our data indicate that the NHE6A9S variant is unlikely to confer disease susceptibility with high penetrance.}, URL = {https://www.eneuro.org/content/early/2019/11/01/ENEURO.0046-19.2019}, eprint = {https://www.eneuro.org/content/early/2019/11/01/ENEURO.0046-19.2019.full.pdf}, journal = {eNeuro} }