Complex Neurological Phenotype in Female Carriers of NHE6 Mutations

Roles of Endomembrane Alkali Cation/Proton Exchangers in Synaptic Function and Neurodevelopmental Disorders

Endomembrane alkali cation (Na⁺, K⁺)/proton (H⁺) exchangers (eNHEs) are increasingly associated with neurological disorders. These eNHEs play integral roles in regulating the luminal pH, processing, and trafficking of cargo along the secretory (Golgi and post-Golgi vesicles) and endocytic (early, recycling, and late endosomes) pathways, essential regulatory processes vital for neuronal development and plasticity. Given the complex morphology and compartmentalization of multipolar neurons, the contribution of eNHEs in maintaining optimal pH homeostasis and cargo trafficking is especially significant during periods of structural and functional development and remodeling. While the importance of eNHEs has been demonstrated in a variety of non-neuronal cell types, their involvement in neuronal function is less well understood. In this review, we will discuss their emerging roles in excitatory synaptic function, particularly as it pertains to cellular learning and remodeling. We will also explore their connections to neurodevelopmental conditions, including intellectual disability, autism, and attention deficit hyperactivity disorders.

Donor Splice Site Variant in SLC9A6 Causes Christianson Syndrome in a Lithuanian Family: A Case Report

Background and Objectives: The pathogenic variants of SLC9A6 are a known cause of a rare, X-linked neurological disorder called Christianson syndrome (CS). The main characteristics of CS are developmental delay, intellectual disability, and neurological findings. This study investigated the genetic basis and explored the molecular changes that led to CS in two male siblings presenting with intellectual disability, epilepsy, behavioural problems, gastrointestinal dysfunction, poor height, and weight gain. Materials and Methods: Next-generation sequencing of a tetrad was applied to identify the DNA changes and Sanger sequencing of proband’s cDNA was used to evaluate the impact of a splice site variant on mRNA structure. Bioinformatical tools were used to investigate SLC9A6 protein structure changes. Results: Sequencing and bioinformatical analysis revealed a novel donor splice site variant (NC_000023.11(NM_001042537.1):c.899 + 1G > A) that leads to a frameshift and a premature stop codon. Protein structure modelling showed that the truncated protein is unlikely to form any functionally relevant SLC9A6 dimers. Conclusions: Molecular and bioinformatical analysis revealed the impact of a novel donor splice site variant in the SLC9A6 gene that leads to truncated and functionally disrupted protein causing the phenotype of CS in the affected individuals.

Novel SLC9A6 Variation in Female Carriers With Intellectual Disability and Atypical Parkinsonism

Background and Objectives

Variations in SLC9A6 cause the X-linked neurologic disorder Christianson syndrome in males. Meanwhile, female carriers with SLC9A6 variations may remain asymptomatic or develop intellectual disability, behavioral problems, and psychiatric illnesses. Only a few female carriers have been reported to have associated atypical parkinsonism in late life.

Methods

We present a Japanese family with a novel SLC9A6 variation identified by quad whole-exome sequencing analysis and a reverse phenotyping strategy. The molecular and cellular impacts of the W89R variation in vitro were examined.

Results

The missense variation (c.265T>C, p.Trp89Arg) in SLC9A6 cosegregated with atypical parkinsonism and intellectual disability in female carriers of this family. The female carriers in this family presented with bradykinesia, rigidity, and tremor, predominately on the right side. We found that the W89R variation changed membrane traffic of NHE6-harboring vesicles, indicating potential involvement in the disease pathogenesis.

Discussion

This study might have revealed an example of a monogenic origin of atypical parkinsonism in females with SLC9A6 variations and draw attention to this understudied female-specific phenotype in clinical practice.

Case Report: Christianson Syndrome Caused by SLC9A6 Mutation: From Case to Genotype-Phenotype Analysis

Christianson syndrome (CS) is an X-linked neurodevelopmental syndrome characterized by microcephaly, epilepsy, ataxia, and severe generalized developmental delay. Pathogenic mutations in the SLC9A6 gene, which encodes the Na⁺/H⁺ exchanger protein member 6 (NHE6), are associated with CS and autism spectrum disorder in males. In this study, whole exome sequencing (WES) and Sanger sequencing revealed a novel de novo frameshift variant c.1548_1549insT of SLC9A6 in a 14-month-old boy with early-onset seizures. According to The American College of Medical Genetics and Genomics (ACMG)/the Association for Molecular Pathology (AMP) guidelines, the variant was classified as pathogenic. The proband presented with several core symptoms of typical epilepsy, including microcephaly, motor delay, distal muscle weakness, micrognathia, occasional unprovoked laughter, swallowing and speech difficulties. Electroencephalography (EEG) showed spikes-slow waves in frontal pole, frontal, anterior temporal and frontal midline point areas. Gesell development schedules (GDS) indicated generalized developmental delay. We also summarized all the reported variants and analyzed the correlation of genotype and phenotype of CS. Our study extends the mutation spectrum of the SLC9A6 gene, and it might imply that the phenotypes of CS are not correlated with SLC9A6 genotypes.

Christianson syndrome: A novel splicing variant of SLC9A6 causes exon skipping in a Chinese boy and a literature review

BACKGROUND

Variants in the endosomal solute carrier family 9 member A6 (SLC9A6)/(Na⁺ ,K⁺ )/H⁺ exchanger 6 (NHE6) gene have been linked to epilepsy, speech loss, truncal ataxia, hyperkinesia, and postnatal microcephaly.

METHODS

In the present study, we evaluated genetic alterations in a 3-year-old Chinese boy displayed features of epilepsy, psychomotor retardation, microcephaly, low body weight, difficulty in feeding, excessive movement, attention loss, ataxia, and cerebellar atrophy and his healthy family using WES method. The identified variant was further confirmed by Sanger sequencing method. Finally, minigene assays were used to verify whether the novel SLC9A6 intronic variant influenced the normal splicing of mRNA.

RESULTS

We identified a novel hemizygous splicing variant [NM_001042537.1: c.1463-1G>A] in SLC9A6 by trio-based exome sequencing. The minigene expression in vitro confirmed the splicing variant altered a consensus splice acceptor site of SLC9A6 intron 11, resulting in skipping over exon 12.

CONCLUSIONS

Our finding extends the catalog of pathogenic intronic variants affecting SLC9A6 pre-mRNA splicing and provides a basis for the genetic diagnosis of CS.

Assorted dysfunctions of endosomal alkali cation/proton exchanger SLC9A6 variants linked to Christianson syndrome

Genetic screening has identified numerous variants of the endosomal solute carrier family 9 member A6 (SLC9A6)/(Na+,K+)/H+ exchanger 6 (NHE6) gene that cause Christianson syndrome, a debilitating X-linked developmental disorder associated with a range of neurological, somatic, and behavioral symptoms. Many of these variants cause complete loss of NHE6 expression, but how subtler missense substitutions or nonsense mutations that partially truncate its C-terminal cytoplasmic regulatory domain impair NHE6 activity and endosomal function are poorly understood. Here, we describe the molecular and cellular consequences of six unique mutations located in the N-terminal cytoplasmic segment (A9S), the membrane ion translocation domain (L188P and G383D), and the C-terminal regulatory domain (E547*, R568Q, and W570*) of human NHE6 that purportedly cause disease. Using a heterologous NHE6-deficient cell expression system, we show that the biochemical, catalytic, and cellular properties of the A9S and R568Q variants were largely indistinguishable from those of the WT transporter, which obscured their disease significance. By contrast, the L188P, G383D, E547*, and W570* mutants exhibited variable deficiencies in biosynthetic post-translational maturation, membrane sorting, pH homeostasis in recycling endosomes, and cargo trafficking, and they also triggered apoptosis. These findings broaden our understanding of the molecular dysfunctions of distinct NHE6 variants associated with Christianson syndrome.