Genetic and phenotypic diversity of NHE6 mutations in Christianson syndrome.

Mutations in the X-linked endosomal Na+/H+ Exchanger 6 (NHE6, also known as SLC9A6) cause a newly-recognized, neurogenetic syndrome with variable expressivity. In a systematic, large-scale resequencing screen of X-chromosome exons in pedigrees consistent with X-linked intellectual disability (XL-ID), protein-truncating mutations occurred in NHE6 at a rate of approximately 1% (2 in nearly 200 pedigrees). This high rate of mutation placed NHE6 among the top six most recurrently mutated genes in XL-ID.1 The first reports of mutations in NHE6 were associated with an Angelman-like syndrome (AS).2 The association of NHE6 with Angelman syndrome represents an exciting new opportunity to deepen our understanding of this condition through the exploration of an alternative gene pathway that may provide novel mechanistic perspectives on the disease. However, NHE6 mutations may not be uniformly associated with Angelman syndrome3, and therefore, this relationship needs to be studied in larger groups of patients with NHE6 mutations. A large South African pedigree previously reported by Christianson et al.4 was one of the pedigrees determined to have an NHE6 mutation by Gilfillan et al.2 The “Christianson syndrome” (CS) reported in the original publication was not noted to include Angelman syndrome; instead, the affected males in the pedigree were reported to have X-linked severe ID associated with craniofacial dysmorphology, mutism (i.e. non-verbal status) despite normal hearing, generalized tonic-clonic epilepsy, postnatal microcephaly, ataxia, ophthalmoplegia and cerebellar and brainstem atrophy. Christianson et al. also posited a limited life expectancy, although he studied only the single pedigree. In addition to the features described above, Christianson reported an association with autistic symptoms as has been reported subsequently.3 In parallel to the description of autistic symptoms associated with mutations in NHE6, Morrow et al.5 published mutations in the highly related endosomal protein NHE9 in autism with epilepsy. Further, in a recent transcriptome study in cortex from postmortem autism brain, we demonstrated that genes encoding NHE6 and NHE9 were dysregulated in brains from patients with idiopathic autism -- NHE6 gene expression was decreased and NHE9 gene expression is increased in a fashion that was highly correlated with gene expression decreases in synapse-related genes.6 These findings demonstrate that, in addition to the role of NHE6 in monogenic CS, NHE6 and NHE9 may play critical roles in the pathophysiology of complex autism, likely participating in a convergent cellular mechanism involving synapse and circuit development. In addition, NHE proteins have broad importance in neurology, particularly, given the spontaneous mutation in Nhe1 in the slow-wave epilepsy mouse7, and the various anti-epileptic medicines that alter regulation of proton concentrations.8, 9 The structure of NHE proteins generally involves a twelve-membrane spanning motif harboring the Na+ and H+ exchange activity that is highly conserved across family members, and the proteins also contain a large, less conserved carboxyl domain that is thought to involve protein localization and regulation.10 Other studies indicate that NHE1-5 are localized to the cell membrane, while NHE6-9 are thought to be organellar. NHE6 is localized to early, recycling and late endosomes in hippocampal neurons.11, 12 Recent studies indicate that NHE6 plays a role in neuronal arborization and synapse development through modulation of neurotrophin signaling.12 To date, all studies of mutations in NHE6 in the literature have reported the associated clinical phenotype from three or less pedigrees.2–4, 13–18 The vast majority of prior publications have reported inherited mutations. Here we report results from the prospective recruitment of twelve independent pedigrees affected by CS with confirmed mutations in NHE6. In contrast to prior literature, we find a high frequency of de novo mutations (seven to date). In the current study, we quantify the clinical features of this cohort in order to address the following questions: (1) What are the core features of CS?; (2) Quantitatively, what is the range of clinical symptoms and outcomes?; and (3) Are there important differences between the inherited mutations and the de novo mutations, either at the level of the genetic mutation or at the level of neuromedical features?