A novel mutation in LRSAM1 causes axonal Charcot-Marie-Tooth disease with dominant inheritance

Engeholm et al. BMC Neurology 2014, 14:118 http://www.biomedcentral.com/1471-2377/14/118 CA SE R EPOR T Open Access A novel mutation in LRSAM1 causes axonal Charcot-Marie-Tooth disease with dominant inheritance Maik Engeholm1,2* , Julia Sekler1,2 , David C Schöndorf1,2,3 , Vineet Arora1,2,3 , Jens Schittenhelm4 , Saskia Biskup1,5 , Caroline Schell1 and Thomas Gasser1,2 Abstract Background: Charcot-Marie-Tooth disease (CMT) refers to a heterogeneous group of genetic motor and sensory neuropathies. According to the primary site of damage, a distinction is made between demyelinating and axonal forms (CMT1 and 2, respectively, when inherited as an autosomal dominant trait). Leucine-rich repeat and sterile alpha motif-containing protein 1 (LRSAM1) is a ubiquitin-protein ligase with a role in sorting internalised cell-surface receptor proteins. So far, mutations in the LRSAM1 gene have been shown to cause axonal CMT in three different families and can confer either dominant or recessive transmission of the disease. Case presentation: We have identified a novel mutation in LRSAM1 in a small family with dominant axonal CMT. Electrophysiological studies show evidence of a sensory axonal neuropathy and are interesting in so far as giant motor unit action potentials (MUAPs) are present on needle electromyography (EMG), while motor nerve conduction studies including compound motor action potential (CMAP) amplitudes are completely normal. The underlying mutation c.2046+1G>T results in the loss of a splice donor site and the inclusion of 63 additional base pairs of intronic DNA into the aberrantly spliced transcript. This disrupts the catalytically active RING (Really Interesting New Gene) domain of LRSAM1. Conclusions: Our findings suggest that, beyond the typical length-dependent degeneration of motor axons, damage of cell bodies in the anterior horn might play a role in LRSAM1-associated neuropathies. Moreover, in conjunction with other data in the literature, our results support a model, by which disruption of the C-terminal RING domain confers dominant negative properties to LRSAM1. Keywords: Axonal CMT, LRSAM1, Anterior horn cell disease, Splice site mutation, RING domain, Exome sequencing Background CMT comprises a clinically and genetically heterogeneous group of inherited motor and sensory neuropathies [1]. With an overall prevalence of approximately 1 in 2,500 individuals, CMT is the most common genetic disorder of the PNS [2]. In a majority of cases, CMT is transmitted as an autosomal dominant trait and is further classified as CMT1 or CMT2 according to the primary site of damage *Correspondence: 1 Department of Neurology and Hertie Institute for Clinical Brain Research, Hoppe-Seyler-Str. 3, 72076 Tübingen, Germany 2 German Center for Neurodegenerative Diseases (DZNE), Otfried-Müller-Str. 27, 72076 Tübingen, Germany Full list of author information is available at the end of the article (demyelinating and axonal, respectively) [1,3]. In other families, CMT is transmitted as an X-linked or autosomal recessive trait. These latter cases are commonly classified as CMT4 when they show a demyelinating phenotype, while axonal forms are referred to as autosomal recessive CMT2 (AR-CMT2) [4]. Since the identification of a duplication of the peripheral myelin protein 22 locus as the cause of CMT1A more than 20 years ago [5,6], mutations in more than 60 genes implicated in a variety of different cellular functions have been associated with various forms of CMT [3]. Beyond their relevance for clinical classification, these genes identify molecules and molecular pathways that play a primary role for the structural and functional integrity of PNS neurons and their myelin © 2014 Engeholm et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Engeholm et al. BMC Neurology 2014, 14:118 http://www.biomedcentral.com/1471-2377/14/118 sheath, respectively, and are potential targets for future therapeutic interventions. Three different mutations in the LRSAM1 gene have been shown to cause axonal CMT in humans. Guernsey et al. [7] reported a large, multiply consanguineous family from Eastern Canada, in which axonal CMT was inherited as an autosomal recessive trait. The clinical presentation included moderate weakness and wasting, predominantly affecting distal lower limb muscles, with an onset in early adulthood. Needle EMG revealed signs of denervation and reinnervation, and sensory nerve action potentials (SNAPs) were reduced or absent. Homozygosity mapping yielded a splice site mutation c.1913-1G>A in the LRSAM1 gene giving rise to a premature stop codon 20 bp inside the penultimate exon. Subsequently, two further mutations in LRSAM1 were identified in a Dutch and a Sardinian family with dominant axonal CMT [8,9]. In both studies, the clinical and electrophysiological findings were very similar to that reported by Guernsey et al. [7]. Both mutations, p.Leu708ArgfsX28 and p.Ala683ProfsX3, disrupt the RING domain of LRSAM1. LRSAM1 is a E3 ubiquitin-protein ligase highly conserved throughout vertebrate evolution [10]. Alternative splicing gives rise to three different isoforms in humans, the largest of which consists of 723 amino acids and harbours an N-terminal leucine-rich repeat domain, an ezrinradixin-moezin domain, a coiled-coil region, a sterile alpha motif domain and a C-terminal C3HC4-type RING finger domain (Figure 1e). In human and mouse, LRSAM1 is highly expressed in motor neurons of the spinal cord and cell bodies of sensory neurons of dorsal root ganglia [8,11]. Moreover, some expression is observed in the central nervous system [10,11]. In a cell culture system, LRSAM1 has been shown to interact with and mediate monoubiquitination of the Tumour susceptibility gene 101 protein (TSG101) [10]. TSG101 is a component of the ESCRT (Endosomal Sorting Complexes Required for Transport)-1 complex, which is involved in the sorting of endocytic ubiquitinated cargoes into lumenal vesicles of late endosomes [12]. Upon LRSAM1-mediated ubiquitination, TSG101 relocalises from these multivesicular bodies to a detergent-soluble compartment and loses its ability to direct internalised receptor proteins to the lysosome [10]. In a mouse model, loss of LRSAM1 results in decreased motor NCVs and motor axon counts following acrylamide challenge, while neuromuscular function is otherwise unimpaired [11]. Here, we report a novel mutation in LRSAM1 in a small family with autosomal dominant axonal CMT. We present an in-depth electrophysiological examination of three a (...truncated)