Mutations in noncoding regions of GJB1 are a major cause of X-linked CMT

Abstract

Objective: To determine the prevalence and clinical and genetic characteristics of patients with X-linked Charcot-Marie-Tooth disease (CMT) due to mutations in noncoding regions of the gap junction β-1 gene (GJB1).

Methods: Mutations were identified by bidirectional Sanger sequence analysis of the 595 bases of the upstream promoter region, and 25 bases of the 3' untranslated region (UTR) sequence in patients in whom mutations in the coding region had been excluded. Clinical and neurophysiologic data were retrospectively collected.

Results: Five mutations were detected in 25 individuals from 10 kindreds representing 11.4% of all cases of CMTX1 diagnosed in our neurogenetics laboratory between 1996 and 2016. Four pathogenic mutations, c.-17G>A, c.-17+1G>T, c.-103C>T, and c.-146-90_146-89insT were detected in the 5'UTR. A novel mutation, c.*15C>T, was detected in the 3' UTR of GJB1 in 2 unrelated families with CMTX1 and is the first pathogenic mutation in the 3'UTR of any myelin-associated CMT gene. Mutations segregated with the phenotype, were at sites predicted to be pathogenic, and were not present in the normal population.

Conclusions: Mutations in noncoding DNA are a major cause of CMTX1 and highlight the importance of mutations in noncoding DNA in human disease. Next-generation sequencing platforms for use in inherited neuropathy should therefore include coverage of these regions.

Figure 1. Pedigrees for the families reported in this study

Black symbols = affected; empty symbols = unaffected; dot symbols = affected by history; diagonal line = deceased; arrow = index case.

Figure 2. GJB1 gene structure with mutations in noncoding regions highlighted

(A) Structural organization of GJB1. (B) Base numbering at each junction between regions according to the Human Genome Variation Society. (C) GJB1 has 2 tissue-specific promoters (P1 and P2) that are alternatively spliced. In liver and pancreas, GJB1 transcription is driven via promoter 1 (P1) upstream of the noncoding exon, exon 1a, whereas in neural tissue it is driven via the nerve-specific promoter 2 (P2) upstream of noncoding exon 1b.^, The P1- and P2-expressed mRNAs have different 5′ untranslated regions (UTRs) but an identical open reading frame (ORF) region and 3′ UTR. (D) The EGR2 (E1, E2, and E3) and SOX10 (S1 and S2) binding sites of the P2 promoter region that function synergistically to regulate Cx32 expression in the nervous system. ^a Variants included in this study. ^b Novel variants.