Truncating SLC5A7 mutations underlie a spectrum of dominant hereditary motor neuropathies

Abstract

Objective: To identify the genetic cause of disease in 2 previously unreported families with forms of distal hereditary motor neuropathies (dHMNs).

Methods: The first family comprises individuals affected by dHMN type V, which lacks the cardinal clinical feature of vocal cord paralysis characteristic of dHMN-VII observed in the second family. Next-generation sequencing was performed on the proband of each family. Variants were annotated and filtered, initially focusing on genes associated with neuropathy. Candidate variants were further investigated and confirmed by dideoxy sequence analysis and cosegregation studies. Thorough patient phenotyping was completed, comprising clinical history, examination, and neurologic investigation.

Results: dHMNs are a heterogeneous group of peripheral motor neuron disorders characterized by length-dependent neuropathy and progressive distal limb muscle weakness and wasting. We previously reported a dominant-negative frameshift mutation located in the concluding exon of the SLC5A7 gene encoding the choline transporter (CHT), leading to protein truncation, as the likely cause of dominantly-inherited dHMN-VII in an extended UK family. In this study, our genetic studies identified distinct heterozygous frameshift mutations located in the last coding exon of SLC5A7, predicted to result in the truncation of the CHT C-terminus, as the likely cause of the condition in each family.

Conclusions: This study corroborates C-terminal CHT truncation as a cause of autosomal dominant dHMN, confirming upper limb predominating over lower limb involvement, and broadening the clinical spectrum arising from CHT malfunction.

Figure 1. Family pedigrees, clinical photographs, and multispecies alignment showing the effect of the 3 reported mutations

(A–B) Pedigrees of families A and B showing affected (black), unaffected (white), and possibly affected (grey) individuals. Those clinically investigated are indicated by * and those who had genetic testing are indicated by the test result, where (+/−) represents individuals heterozygous for the SLC5A7 mutation and (−/−) represents individuals homozygous for wild type. (C) Hands of individual B:III:1 showing pronounced atrophy of the intrinsic hand muscles. (D) Species amino acid sequence alignment of the CHT C-terminal region, depicting the outcomes of the K499Nfs13, H521Qfs2, and K510Nfs2 alterations (arrows indicate the position of polypeptide truncation, and additional aberrant amino acids are highlighted in yellow). The primary endocytic motif (SEENMDKTILV-1° Motif) is highlighted in blue (Ser522 to Val532), and the secondary endocytic motif (DELAL-2° Motif) is highlighted in green (Asp540 and Leu544).

Figure 2. Schematic displaying the 3 described CHT mutant proteins alongside wild type molecule (Adapted from reference 2, using Microsoft Powerpoint Software)

Schematic indicating the transmembrane architecture and C-terminal truncation of both sequence variants identified in the current study alongside the previously published dHMN-VII-associated CHT sequence variant (A: H521Qfs*2; B: K510Nfs*2; C: K499Nfs*13; D: wild type). Dark circles represent amino acids that are conserved across human CHT, mouse Cht1, and the C. elgans CHT ortholog. Potential protein kinase A phosphorylation sites are indicated by light circles containing a letter P, and potential protein kinase C phosphorylation sites are indicated by dark circles containing a letter P. Potential N-glycosylation sites are indicated by gray “tree-like” structures.