Genetic and phenotypic diversity of NHE6 mutations in Christianson syndrome

Abstract

Objective: Recently, Christianson syndrome (CS) has been determined to be caused by mutations in the X-linked Na(+) /H(+) exchanger 6 (NHE6). We aimed to determine the diagnostic criteria and mutational spectrum for CS.

Methods: Twelve independent pedigrees (14 boys, age = 4-19 years) with mutations in NHE6 were administered standardized research assessments, and mutations were characterized.

Results: The mutational spectrum was composed of 9 single nucleotide variants, 2 indels, and 1 copy number variation deletion. All mutations were protein-truncating or splicing mutations. We identified 2 recurrent mutations (c.1498 c>t, p.R500X; and c.1710 g>a, p.W570X). Otherwise, all mutations were unique. In our study, 7 of 12 mutations (58%) were de novo, in contrast to prior literature wherein mutations were largely inherited. We also report prominent neurological, medical, and behavioral symptoms. All CS participants were nonverbal and had intellectual disability, epilepsy, and ataxia. Many had prior diagnoses of autism and/or Angelman syndrome. Other neurologic symptoms included eye movement abnormalities (79%), postnatal microcephaly (92%), and magnetic resonance imaging evidence of cerebellar atrophy (33%). Regression was noted in 50%, with recurrent presentations involving loss of words and/or the ability to walk. Medical symptoms, particularly gastrointestinal symptoms, were common. Height and body mass index measures were below normal ranges in most participants. Behavioral symptoms included hyperkinetic behavior (100%), and a majority exhibited high pain threshold.

Interpretation: This is the largest cohort of independent CS pedigrees reported. We propose diagnostic criteria for CS. CS represents a novel neurogenetic disorder with general relevance to autism, intellectual disability, Angelman syndrome, epilepsy, and regression.

Figure 1. Christianson syndrome pedigrees

(A) Pedigrees for each CS family (#1–12) are shown, with male members (square) and female members (circles). Affected male probands are shaded and heterozygous female carriers are represented by dots inside circles. The cDNA and protein coordinates for each mutation (derived from Ensembl transcript ENST00000370695 and Ensembl protein sequence ENSP00000359729, respectively) are also shown beneath each pedigree. Note that the probands from families 3 and 7 share a recurrent de novo mutation with no known biological relationship; in addition, the probands from families 9 and 12 have a recurrent mutation which was maternally inherited in each respective family, with no known biological relationship between families. (B) Genomic deletion in NHE6 (SLC9A6) identified in patient 10. The deletion spanned 120.7 kb (ChrX:135098247-135218928; GRCh37/hg19).

Figure 2. Patient growth trajectories

(A) Postnatal head growth of current Christianson syndrome patients. The 50^th (blue) and 3^rd (red) head circumference percentiles are shown as a reference. Head circumference < 3^rd percentile is indicative of microcephaly. Head circumference percentiles are reported according to Rollins et al. (B) Height trajectories of current Christianson syndrome patients. Height percentiles were plotted according to CDC recommendations, whereby the 2006 WHO child growth standards were used for measurements taken ≤ 24 months of age and the 2000 CDC growth charts for measurements taken > 24 months. (C) Weight trajectories of current Christianson syndrome patients. Weight percentiles were plotted according to CDC recommendations, whereby the 2006 WHO child growth standards were used for measurements taken ≤ 24 months of age and the 2000 CDC growth charts for measurements taken > 24 months. Birth weight and percentile for probands are as follows: patient 1 (3.6 kg, 50–75%), patient 2 (3.2 kg, 25–50%), patient 3 (4.0 kg, 90–95%), patient 4 (2.1 kg, <2%), patient 5 (3.5 kg, 50–75%), patient 6 (3.1 kg, 25–50%), patient 8a (3.2 kg, 25–50%), patient 8b (3.8 kg, 75–90%), patient 11a (3.7 kg, 75–90%), patient 11b (3.5 kg, 50–75%), patient 12 (3.0 kg, 10–25%). (D) Body Mass Index (BMI) scores of current Christianson syndrome patients. BMI percentile values were plotted according to the 2000 CDC growth charts. The 85^th (blue) and 5^th (red) percentiles were chosen as reference percentiles since they designate overweight and underweight categories, respectively.

Figure 3. Neurological investigations in Christianson syndrome patients

(A) EEG referential montage of CS patient (11a) showing diffuse slow spike-and-wave complexes with lack of normal background features, characteristic electrographic findings of Lennox-Gastaut Syndrome. (B and C) MRI results of CS patient showing moderate to severe atrophy of the cerebellar hemispheres and vermis (arrow) in one patient in the current study at two distinct ages. (B) Sagittal T1-weighted MRI of patient at 4 years (1.5T). (C) Sagittal T1-weighted MRI at 10 years (1.5T).

Figure 4. Patient mutations in NHE6 protein

Mutations from all families in the present report (n = 12) are shown in the NHE6 protein. The NHE6 protein is a twelve-membrane spanning motif with the Na⁺/H⁺ exchange occurring between transmembrane segments S4 and S5. While most mutations are located in the transmembrane domain, two mutations (3 and 7) are located in the carboxyl domain. cDNA positions are based on Ensembl transcript ENST00000370695 and peptide positions are based on ENSP00000359729. Probands from families #1–7 exhibit de novo mutations, whereas probands from families #8–12 exhibit inherited mutations.