Genetics of inherited peripheral neuropathies and the next frontier: looking backwards to progress forwards.

Inherited peripheral neuropathies (IPNs) encompass a clinically and genetically heterogeneous group of disorders causing length-dependent degeneration of peripheral autonomic, motor and/or sensory nerves. Despite gold-standard diagnostic testing for pathogenic variants in over 100 known associated genes, many patients with IPN remain genetically unsolved. Providing patients with a diagnosis is critical for reducing their 'diagnostic odyssey', improving clinical care, and for informed genetic counselling. The last decade of massively parallel sequencing technologies has seen a rapid increase in the number of newly described IPN-associated gene variants contributing to IPN pathogenesis. However, the scarcity of additional families and functional data supporting variants in potential novel genes is prolonging patient diagnostic uncertainty and contributing to the missing heritability of IPNs. We review the last decade of IPN disease gene discovery to highlight novel genes, structural variation and short tandem repeat expansions contributing to IPN pathogenesis. From the lessons learnt, we provide our vision for IPN research as we anticipate the future, providing examples of emerging technologies, resources and tools that we propose that will expedite the genetic diagnosis of unsolved IPN families.

A deep intronic variant in MME causes autosomal recessive Charcot-Marie-Tooth neuropathy through aberrant splicing.

BACKGROUND: Loss-of-function variants in MME (membrane metalloendopeptidase) are a known cause of recessive Charcot-Marie-Tooth Neuropathy (CMT). A deep intronic variant, MME c.1188+428A>G (NM_000902.5), was identified through whole genome sequencing (WGS) of two Australian families with recessive inheritance of axonal CMT using the seqr platform. MME c.1188+428A>G was detected in a homozygous state in Family 1, and in a compound heterozygous state with a known pathogenic MME variant (c.467del; p.Pro156Leufs*14) in Family 2. AIMS: We aimed to determine the pathogenicity of the MME c.1188+428A>G variant through segregation and splicing analysis. METHODS: The splicing impact of the deep intronic MME variant c.1188+428A>G was assessed using an in vitro exon-trapping assay. RESULTS: The exon-trapping assay demonstrated that the MME c.1188+428A>G variant created a novel splice donor site resulting in the inclusion of an 83 bp pseudoexon between MME exons 12 and 13. The incorporation of the pseudoexon into MME transcript is predicted to lead to a coding frameshift and premature termination codon (PTC) in MME exon 14 (p.Ala397ProfsTer47). This PTC is likely to result in nonsense mediated decay (NMD) of MME transcript leading to a pathogenic loss-of-function. INTERPRETATION: To our knowledge, this is the first report of a pathogenic deep intronic MME variant causing CMT. This is of significance as deep intronic variants are missed using whole exome sequencing screening methods. Individuals with CMT should be reassessed for deep intronic variants, with splicing impacts being considered in relation to the potential pathogenicity of variants.

Two Novel Variants in PI4KA in a Family Presenting With Hereditary Spastic Paraparesis: A Case Report.

Objectives: To report novel biallelic PI4KA variants in a family presenting with pure hereditary spastic paraparesis. Methods: Two affected sisters presented with unsolved hereditary spastic paraparesis and underwent clinical and imaging assessments. This was followed by short-read next-generation sequencing. Results: Analysis of next-generation sequencing data uncovered compound heterozygous variants in PI4KA (NM_058004.4: c.[3883C>A];[5785A>C]; p.[(His1295Asn);(Thr1929Pro)]. Using ACMG guidelines, both variants were classified as likely pathogenic. Discussion: Here, next-generation sequencing revealed 2 novel compound heterozygous variants in the phosphatidylinositol 4-kinase alpha gene (PI4KA) in 2 sisters presenting with progressive pure hereditary spastic paraparesis. Pathogenic variants in PI4KA have previously been associated with a spectrum of disorders including autosomal recessive perisylvian polymicrogyria, with cerebellar hypoplasia, arthrogryposis, and pure spastic paraplegia. The cases presented in this study expand the phenotypic spectrum associated with PI4KA variants and contribute new likely pathogenic variants for testing in patients with otherwise unsolved hereditary spastic paraparesis.