Late-onset hereditary axonal neuropathies

Approach to Neuropathic Pain

Neuropathic pain is a common chief complaint encountered by neurologists and primary care providers. It is caused by disorders involving the somatosensory nervous system. The clinical evaluation of neuropathic pain is challenging and requires a multifaceted systematic approach with an emphasis on a thorough history and physical examination to identify characteristic signs and symptoms. Ancillary laboratory investigations, targeted imaging, and electrodiagnostic studies further help identify underlying etiologies to guide specific treatments. Management of neuropathic pain encompasses treating the underlying pathology as well as symptomatic control with nonpharmacological, pharmacological, and interventional therapies. Here, we present an approach to help evaluate patients with neuropathic pain.

A Novel Mutation of the Membrane Metallo-Endopeptidase Gene Related to Late-Onset Hereditary Polyneuropathy: Case Report and Review of the Literature

The advent of next generation sequencing has revolutionized diagnostic approaches to hereditary polyneuropathies. Recently, mutations on the membrane metallo-endopeptidase (MME) gene, encoding neprilysin, have been related to the development of late-onset Charcot-Marie-Tooth disease type 2 (CMT2). Here, we report the first Greek patient presenting with a slowly progressive late-onset axonal polyneuropathy and a novel, likely pathogenic, heterozygous variant in the MME gene. In addition, we have performed a systematic review of all published case reports of patients with MME mutations. The results of the studies show that MME variants can be inherited as both fully penetrant autosomal-recessive and incompletely penetrant autosomal-dominant traits. A number of heterozygous variants characterized as incompletely penetrant impose an increased risk of developing a CMT2-like phenotype late in life, identical to the case study described here. Greater mutation numbers in different populations and mutation-specific functional studies will be essential to identify the pathogenicity and inheritance of more MME variants.

Novel MFN2 Missense Mutation Induces Hereditary Axonal Motor and Sensory Neuropathy in a Saudi Arabian Family

Hereditary axonal motor and sensory neuropathy or Charcot-Marie-Tooth type 2 (CMT2) is a common inherited peripheral neuropathy. Major symptomatologic signs vary from minimal to significant weakness and loss of sensation, feet usually affected more than hands. It may also cause visual acuity impairment, hearing loss, and skeletal deformity. CMT2 classification is based on the clinical, electrophysiological, and genetic inheritance pattern. Dominant CMT2 is classified from CMT2A to CMT2N and recessive CMT2 into CMT2B1 and CMT2B2. CMT2A is the most frequent subtype of CMT2 and caused by mutations in the mitofusin 2 (MFN2) gene. We hereby report a Saudi Arabian CMT2A patient with a variant c.58C>T of the MFN2 gene mutation.

Characterising the phenotype and mode of inheritance of patients with inherited peripheral neuropathies carrying MME mutations

Background

Mutations in the metalloendopeptidase (MME) gene were initially identified as a cause of autosomal recessive Charcot-Marie-Tooth disease type 2 (CMT2). Subsequently, variants in MME were linked to other late-onset autosomal dominant polyneuropathies. Thus, our goal was to define the phenotype and mode of inheritance of patients carrying changes in MME.

Methods

We screened 197 index cases with a hereditary neuropathy of the CMT type or distal hereditary motor neuropathy (dHMN) and 10 probands with familial amyotrophic lateral sclerosis (fALS) using a custom panel of 119 genes. In addition to the index case subjects, we also studied other clinically and/or genetically affected and unaffected family members.

Results

We found 17 variants in MME in a total of 20 index cases, with biallelic MME mutations detected in 13 cases from nine families (three in homozygosis and six in compound heterozygosis) and heterozygous variants found in 11 families. All patients with biallelic variants had a similar phenotype, consistent with late-onset axonal neuropathy. Conversely, the phenotype of patients carrying heterozygous mutations was highly variable [CMT type 1 (CMT1), CMT2, dHMN and fALS] and mutations did not segregate with the disease.

Conclusion

MME mutations that segregate in an autosomal recessive pattern are associated with a late-onset CMT2 phenotype, yet we could not demonstrate that MME variants in heterozygosis cause neuropathy. Our data highlight the importance of establishing an accurate genetic diagnosis in patients carrying MME mutations, especially with a view to genetic counselling.

Evaluation of the dynactin 1 gene in Leonbergers and Labrador Retrievers with laryngeal paralysis

OBJECTIVE To sequence exons and splice consensus sites of the dynactin subunit 1 (DCTN1) gene in Leonbergers and Labrador Retrievers with clinical laryngeal paralysis. ANIMALS 5 unrelated Leonbergers with laryngeal paralysis, 2 clinically normal Leonbergers, 7 unrelated Labrador Retrievers with laryngeal paralysis, and 2 clinically normal Labrador Retrievers. PROCEDURES Primers were designed for the entire coding regions of the DCTN1 gene, a noncoding exon at the 5´ end of the gene, and a 900-bp single-nucleotide polymorphism (SNP)-rich region located 17 kb upstream of the DCTN1 gene by use of the CanFam3 assembly of the canine genome sequence. Sequences were generated and compared between clinically normal and affected dogs. The SNPs flanking the DCTN1 gene as well as a previously identified nonsynonymous SNP in exon 32 were genotyped in affected and clinically normal Leonbergers and Labrador Retrievers. RESULTS None of the affected dogs were homozygous for any mutation affecting coding regions or splicing consensus sequences. Of the 16 dogs tested for the missense SNP in exon 32, all were homozygous for the reference allele, except for 2 affected and 1 clinically normal Labrador Retriever and 1 clinically normal Leonberger. The DCTN1 gene sequences (5 dogs) and haplotypes of polymorphic markers surrounding the DCTN1 gene (all dogs) were not consistent with the hypothesis that laryngeal paralysis was associated with inheritance of the same DCTN1 disease-causing allele within all Labrador Retrievers or Leonbergers evaluated. CONCLUSIONS AND CLINICAL RELEVANCE Mutations in the DCTN1 gene did not appear to cause laryngeal paralysis in Leonbergers or Labrador Retrievers.

Rare Variants in MME, Encoding Metalloprotease Neprilysin, Are Linked to Late-Onset Autosomal-Dominant Axonal Polyneuropathies

Axonal polyneuropathies are a frequent cause of progressive disability in the elderly. Common etiologies comprise diabetes mellitus, paraproteinaemia, and inflammatory disorders, but often the underlying causes remain elusive. Late-onset axonal Charcot-Marie-Tooth neuropathy (CMT2) is an autosomal-dominantly inherited condition that manifests in the second half of life and is genetically largely unexplained. We assumed age-dependent penetrance of mutations in a so far unknown gene causing late-onset CMT2. We screened 51 index case subjects with late-onset CMT2 for mutations by whole-exome (WES) and Sanger sequencing and subsequently queried WES repositories for further case subjects carrying mutations in the identified candidate gene. We studied nerve pathology and tissue levels and function of the abnormal protein in order to explore consequences of the mutations. Altogether, we observed heterozygous rare loss-of-function and missense mutations in MME encoding the metalloprotease neprilysin in 19 index case subjects diagnosed with axonal polyneuropathies or neurodegenerative conditions involving the peripheral nervous system. MME mutations segregated in an autosomal-dominant fashion with age-related incomplete penetrance and some affected individuals were isolated case subjects. We also found that MME mutations resulted in strongly decreased tissue availability of neprilysin and impaired enzymatic activity. Although neprilysin is known to degrade β-amyloid, we observed no increased amyloid deposition or increased incidence of dementia in individuals with MME mutations. Detection of MME mutations is expected to increase the diagnostic yield in late-onset polyneuropathies, and it will be tempting to explore whether substances that can elevate neprilysin activity could be a rational option for treatment.

Inherited neuropathies: clinical overview and update

Inherited neuropathy is a group of common neurologic disorders with heterogeneous clinical presentations and genetic causes. Detailed neuromuscular evaluations, including nerve conduction studies, laboratory testing, and histopathologic examination, can assist in identification of the inherited component beyond family history. Genetic testing increasingly enables definitive diagnosis of specific inherited neuropathies. Diagnosis, however, is often complex, and neurologic disability may have both genetic and acquired components in individual patients. The decision of which genetic test to order or whether to order genetic tests is often complicated, and the strategies to maximize the value of testing are evolving. Apart from rare inherited metabolic neuropathies, treatment approaches remain largely supportive. We provide a clinical update of the various types of inherited neuropathies, their differential diagnoses, and distinguishing clinical features (where available). A framework is provided for clinical evaluations, including the inheritance assessment, electrophysiologic examinations, and specific genetic tests.

Second-generation antiepileptic drugs and pregnancy: a guide for clinicians

When treating pregnant women with antiepileptic drugs (AEDs), clinicians have to balance potential fetal adverse effects against the risks of uncontrolled maternal disease. Only recently have emerging scientic data provided a rational basis for treatment decisions considering both aspects. The focus of research is currently moving from the first to the second AED generation. Lamotrigine is relatively well studied, and data on other novel AEDs, such as levetiracetam, oxcarbazepine, topiramate, zonisamide, gabapentin and pregabalin, are in progress. Safety issues appear to be favorable for lamotrigine, and preliminary results are also promising for levetiracetam and oxcarbazepine. Drugs metabolized by uridine-diphospate glucuronosyl transferase or excreted unchanged by the kidneys are particularly susceptible to increased body clearance during pregnancy. Lamotrigine is subject to both mechanisms, and therapeutic serum levels may sometimes be difficult to maintain. The authors review the recommendations and clinical research on modern AED treatment during pregnancy, highlighting current experience with second-generation drugs.

Hereditary predominantly motor neuropathies

PURPOSE OF REVIEW

We review recent advances in Charcot-Marie-Tooth disease (CMT), the most frequent inherited neuromuscular disorder.

RECENT FINDINGS

During the last year further progresses have occurred in this field and concerned identification of novel mutations in recently identified genes, allowing better definition of associated phenotypes; increased knowledge on pathophysiologic mechanisms of the different CMT types, with the contribution of cellular and animal model studies; studies on the natural history of CMT and attempts at developing appropriate outcome measures to assess disease course and intervention efficacy; trials with ascorbic acid in CMT type 1A; and studies on new possible therapeutic strategies.

SUMMARY

Such advances have implications on clinical management of CMT and are modifying the clinical approach to CMT, by improving diagnostic tools, allowing better definition of prognosis, and increasing the hope for future effective treatments. Research on CMT is important as is shedding light on important pathways that regulates the normal function of axonal transport, vesicular trafficking, and also revealing new aspects of intracellular organelles' function and interactions.

Not all neuropathy in diabetes is of diabetic etiology: differential diagnosis of diabetic neuropathy

Diabetic peripheral neuropathy is the most common peripheral neuropathy in the developed world; however, not all patients with diabetes and peripheral nerve disease have a peripheral neuropathy caused by diabetes. Several (although not all) studies have drawn attention to the presence of other potential causes of a neuropathy in individuals with diabetes; 10% to 50% of individuals with diabetes may have an additional potential cause of a peripheral neuropathy and some may have more than one cause. Neurotoxic medications, alcohol abuse, vitamin B(12) deficiency, renal disease, chronic inflammatory demyelinating neuropathy, inherited neuropathy, and vasculitis are the most common additional potential causes of a peripheral neuropathy in these series. The most common disorders in the differential diagnosis of a generalized diabetic peripheral neuropathy are discussed in this article. Prospective studies to investigate the prevalence of other disorders that might be responsible for a peripheral neuropathy in individuals with diabetes are warranted.

Geriatric neurogenetics: oxymoron or reality?

BACKGROUND

Primary genetic diseases are generally associated with pediatric and young adult populations. Little information is available about the occurrence of single-gene mendelian diseases in elderly populations.

OBJECTIVE

To describe the occurrence of single-gene neurogenetic disorders in a group of elderly patients.

DESIGN

Retrospective review of neurogenetic cases in an academic medical center.

SETTING

Academic university and Veterans Affairs medical centers.

PATIENTS

Eight elderly patients with single-gene neurogenetic diseases were studied. These patients included an 87-year-old man and an 85-year-old man with Huntington disease, an 84-year-old woman with limb-girdle muscular dystrophy type 2A, a 78-year-old man with spinocerebellar ataxia type 14, an 86-year-old man with spinocerebellar ataxia type 5, an 85-year-old man with a presenilin 1 familial Alzheimer disease mutation, an 87-year-old man with autosomal dominant hereditary neuropathy, and a 78-year-old man with spinocerebellar ataxia type 6. Three patients had no family history of neurologic disease.

MAIN OUTCOME MEASURES

Medical histories, physical examination results, and genetic testing results.

CONCLUSIONS

Single-gene mendelian neurogenetic diseases can be found in the oldest old population (> 85 years). Such cases are currently underrecognized and will become more commonly observed in the future. This phenomenon is a result of (1) the aging of the general population, (2) better recognition of the highly variable ages at onset of genetic diseases, and (3) the availability of specific DNA-based genetic testing.