Whole exome sequencing identifies three recessive FIG4 mutations in an apparently dominant pedigree with Charcot-Marie-Tooth disease

Abstract

Charcot-Marie-Tooth disease (CMT) is genetically heterogeneous and classification based on motor nerve conduction velocity and inheritance is used to direct genetic testing. With the less common genetic forms of CMT, identifying the causative genetic mutation by Sanger sequencing of individual genes can be time-consuming and costly. Next-generation sequencing technologies show promise for clinical testing in diseases where a similar phenotype is caused by different genes. We report the unusual occurrence of CMT4J, caused by mutations in FIG4, in a apparently dominant pedigree. The affected proband and her mother exhibit different disease severities associated with different combinations of compound heterozygous FIG4 mutations, identified by whole exome sequencing. The proband was also shown to carry a de novo nonsense mutation in the dystrophin gene, which may contribute to her more severe phenotype. This study is a cautionary reminder that in families with two generations affected, explanations other than dominant inheritance are possible, such as recessive inheritance due to three mutations segregating in the family. It also emphasises the advantages of next-generation sequencing approaches that screen multiple CMT genes at once for patients in whom the common genes have been excluded.

Keywords: Autosomal recessive CMT; CMT4J; Charcot–Marie–Tooth; FIG4; Whole exome sequencing.

Figure 1

Pedigree, genotypes and results of quantitative Taqman polymerase chain reaction (A) Pedigrematograms of family members showing those with the c.122T>C (★) and c.290-2A>T (⬇) mutations. I.1 appears homozygous for the c.122T>C mutation. (C) qRTPCR with intronic primers flanking exon 2, as per Figure 4 in Nicholson et al [9]. An additional cycle is required for detection of FIG4 in patient I-1 compared with the control, indicative of reduced abundance of the exon 2 sequence in the patient DNA. The difference is a little less than one cycle, as also seen in Nicholson et al, Figure 4.