Novel mutations in human SLC12A transporters: Challenges of unique clinical cases
Vanderbilt university medical center, Nashville
Invited by I. Medina and J-L. Gaiarsa

Abstract : Electroneutral cation-chloride cotransporters participate to a variety of cellular processes, including maintenance and regulation of cell volume, Cl- homeostasis, and epithelial transport. With the relative ease and low cost of whole exome sequencing, single clinical cases with unique mutations in SLC12A transporters are being identified. We recently identified the first patient with a loss-of-function mutation in NKCC1. The now 16-year old NKCC1 patient demonstrated symptoms at 6 months of age and gradually lost gastrointestinal and bladder function. She is also hypo-insufficient for all major endocrine glands, i.e. the thyroid, parathyroid, adrenal, and pancreatic glands. While her symptoms are consistent with dysautonomia, her central nervous system seems to be unaffected. She carries a deletion of 11 bases in exon 22 of SLC12A2, leading to the truncation of the carboxyl-terminus of NKCC1. We demonstrated that the mutant transporter is functionally inactive but able to dimerize with wild-type cotransporter. While this interaction failed to produce dominant-negative effects in Xenopus laevis oocytes, we demonstrated that the mutant transporter is expressed in epithelial cells and trafficked to the apical pole, including the apical membrane. Likely due to dimerization, some wild-type NKCC1 is also observed on the apical membrane. Using CRISPR/cas9, we also reproduced the 11 bp deletion in mouse. The mouse does not exhibit an overt deleterious phenotype. However, analysis of salivary gland and intestine revealed mislocalization of the cotransporter and accumulation of mucus in the intestine. Additional NKCC1 mutations have been found in the UK in several 5 year-old patients with sensorineural deafness, cognitive deficits, and developmental abnormalities. We hypothesize that the transporter is critical during development and reduction of expression leads to CNS and other abnormalities. The haploinsufficiency leads to variable developmental CNS phenotypes.