Mutations in SEPT9 cause hereditary neuralgic amyotrophy.

Hereditary neuralgic amyotrophy (HNA) is an autosomal dominant recurrent neuropathy affecting the brachial plexus. HNA is triggered by environmental factors such as infection or parturition. We report three mutations in the gene septin 9 (SEPT9) in six families with HNA linked to chromosome 17q25. HNA is the first monogenetic disease caused by mutations in a gene of the septin family. Septins are implicated in formation of the cytoskeleton, cell division and tumorigenesis.

Duplication within the SEPT9 gene associated with a founder effect in North American families with hereditary neuralgic amyotrophy.

Hereditary neuralgic amyotrophy (HNA) is an autosomal dominant disorder associated with recurrent episodes of focal neuropathy primarily affecting the brachial plexus. Point mutations in the SEPT9 gene have been previously identified as the molecular basis of HNA in some pedigrees. However in many families, including those from North America demonstrating a genetic founder haplotype, no sequence mutations have been detected. We report an intragenic 38 Kb SEPT9 duplication that is linked to HNA in 12 North American families that share the common founder haplotype. Analysis of the breakpoints showed that the duplication is identical in all pedigrees, and molecular analysis revealed that the duplication includes the 645 bp exon in which previous HNA mutations were found. The SEPT9 transcript variants that span this duplication contain two in-frame repeats of this exon, and immunoblotting demonstrates larger molecular weight SEPT9 protein isoforms. This exon also encodes for a majority of the SEPT9 N-terminal proline rich region suggesting that this region plays a role in the pathogenesis of HNA.

CC2D2A is mutated in Joubert syndrome and interacts with the ciliopathy-associated basal body protein CEP290.

Joubert syndrome and related disorders (JSRD) are primarily autosomal-recessive conditions characterized by hypotonia, ataxia, abnormal eye movements, and intellectual disability with a distinctive mid-hindbrain malformation. Variable features include retinal dystrophy, cystic kidney disease, and liver fibrosis. JSRD are included in the rapidly expanding group of disorders called ciliopathies, because all six gene products implicated in JSRD (NPHP1, AHI1, CEP290, RPGRIP1L, TMEM67, and ARL13B) function in the primary cilium/basal body organelle. By using homozygosity mapping in consanguineous families, we identify loss-of-function mutations in CC2D2A in JSRD patients with and without retinal, kidney, and liver disease. CC2D2A is expressed in all fetal and adult tissues tested. In ciliated cells, we observe localization of recombinant CC2D2A at the basal body and colocalization with CEP290, whose cognate gene is mutated in multiple hereditary ciliopathies. In addition, the proteins can physically interact in vitro, as shown by yeast two-hybrid and GST pull-down experiments. A nonsense mutation in the zebrafish CC2D2A ortholog (sentinel) results in pronephric cysts, a hallmark of ciliary dysfunction analogous to human cystic kidney disease. Knockdown of cep290 function in sentinel fish results in a synergistic pronephric cyst phenotype, revealing a genetic interaction between CC2D2A and CEP290 and implicating CC2D2A in cilium/basal body function. These observations extend the genetic spectrum of JSRD and provide a model system for studying extragenic modifiers in JSRD and other ciliopathies.

Joubert syndrome (and related disorders) (OMIM 213300).

Joubert syndrome (JS) and related disorders are characterized by the 'molar tooth sign' (cerebellar vermis hypoplasia and brainstem anomalies) on MRI, hypotonia, developmental delay, ataxia, irregular breathing pattern and abnormal eye movements. Combinations of additional features such as polydactyly, ocular coloboma, retinal dystrophy, renal disease, hepatic fibrosis, encephalocele, and other brain malformations define clinical sub-types. Recent identification of the NPHP1, AHI1, and CEP290 genes has started to reveal the molecular basis of JS, which may implicate the primary cilium in these disorders. Additional genes remain to be identified.

Inherited focal, episodic neuropathies: hereditary neuropathy with liability to pressure palsies and hereditary neuralgic amyotrophy.

Hereditary neuropathy with liability to pressure palsies (HNPP; also called tomaculous neuropathy) is an autosomal-dominant disorder that produces a painless episodic, recurrent, focal demyelinating neuropathy. HNPP generally develops during adolescence, and may cause attacks of numbness, muscular weakness, and atrophy. Peroneal palsies, carpal tunnel syndrome, and other entrapment neuropathies may be frequent manifestations of HNPP. Motor and sensory nerve conduction velocities may be reduced in clinically affected patients, as well as in asymptomatic gene carriers. The histopathological changes observed in peripheral nerves of HNPP patients include segmental demyelination and tomaculous or "sausage-like" formations. Mild overlap of clinical features with Charcot-Marie-Tooth (CMT) disease type 1 (CMT1) may lead patients with HNPP to be misdiagnosed as having CMT1. HNPP and CMT1 are both demyelinating neuropathies, however, their clinical, pathological, and electrophysiological features are quite distinct. HNPP is most frequently associated with a 1.4-Mb pair deletion on chromosome 17p12. A duplication of the identical region leads to CMT1A. Both HNPP and CMT1A result from a dosage effect of the PMP22 gene, which is contained within the deleted/duplicated region. This is reflected in reduced mRNA and protein levels in sural nerve biopsy samples from HNPP patients. Treatment for HNPP consists of preventative and symptom-easing measures. Hereditary neuralgic amyotrophy (HNA; also called familial brachial plexus neuropathy) is an autosomal-dominant disorder causing episodes of paralysis and muscle weakness initiated by severe pain. Individuals with HNA may suffer repeated episodes of intense pain, paralysis, and sensory disturbances in an affected limb. The onset of HNA is at birth or later in childhood with prognosis for recovery usually favorable; however, persons with HNA may have permanent residual neurological dysfunction following attack(s). Episodes are often triggered by infections, immunizations, the puerperium, and stress. Electrophysiological studies show normal or mildly prolonged motor nerve conduction velocities distal to the affected brachial plexus. Pathological studies have found axonal degeneration in nerves examined distal to the plexus abnormality. In some HNA pedigrees there are characteristic facial features, including hypotelorism. The prognosis for recovery of normal function of affected limbs in HNA is good, although recurrent episodes may cause residual deficits. HNA is genetically linked to chromosome 17q25, where mutations in the septin-9 (SEPT9) gene have been found.

Search for genes involved in Joubert syndrome: evidence that one or more major loci are yet to be identified and exclusion of candidate genes EN1, EN2, FGF8, and BARHL1.

Joubert syndrome (JS) is a rare autosomal recessive malformation syndrome involving agenesis or dysgenesis of the cerebellar vermis with accompanying brainstem malformations. JS is further characterized by hypotonia, developmental delay, intermittent hyperpnea, and abnormal eye movements. The biochemical and molecular basis of JS remains unknown, although several genes that are crucial in the development of the cerebellum have been proposed as attractive candidate genes. JS is clinically heterogeneous; this, together with previous linkage analyses, suggests that there may also be genetic heterogeneity. A locus for JS was previously identified on chromosome 9q34 by linkage analysis in a consanguineous family of Arabian origin. A putative second JS locus was recently suggested when a deletion on chromosome 17p11.2 was observed in a patient with Smith-Magenis syndrome and JS phenotype. We have investigated a cohort of apparently unrelated North American JS pedigrees for association with the loci on chromosomes 9q34 and 17p11.2 and excluded them in all cases where data were informative. Analysis of an additional 21 unrelated JS patients showed no evidence of homozygosity at the 9q34 and 17p11.2 loci that would suggest inheritance of founder JS mutation(s) or unreported consanguinity. Together, these data suggest that one or more major loci for JS remain to be identified. Consequently, we undertook mutation analysis of several functional candidate genes, EN1, EN2, and FGF8, in a total of 26 unrelated JS patients. Our data suggest that all of these genes may be excluded from a direct pathogenic role in JS. The BARHL1 gene, which localizes to chromosome 9q34 and has previously been proposed as a strong positional candidate gene for JS, was also investigated and excluded from involvement in JS that is linked to chromosome 9q34.

Cerebral and cerebellar motor activation abnormalities in a subject with Joubert syndrome: functional magnetic resonance imaging (MRI) study.

Joubert syndrome is an autosomal recessive disorder characterized by hypotonia, ataxia, developmental delay, and a distinctive hindbrain malformation involving the cerebellum and brain stem, visualized radiographically on magnetic resonance imaging (MRI) as the "molar tooth sign." In postmortem brains from subjects with Joubert syndrome, there is an apparent absence of decussation of both corticospinal and superior cerebellar tracts, although the functional significance has not been elucidated. We sought to explore the cerebral and cerebellar activation pattern elicited by finger tapping in an adolescent with Joubert syndrome and in a normal control subject using functional MRI. In contrast to the typical highly lateralized activation seen in our control subject, the subject with Joubert syndrome demonstrated striking bilateral activation of the sensorimotor and cerebellar cortex. Although our functional MRI data do not indicate a clear absence of decussation, the abnormal activation pattern observed suggests altered brain functional organization in relation to anatomic differences. Malformation of the hindbrain could result in recruitment of alternative pathways, similar to what has been observed following ischemic injury to the developing or mature central nervous system.

Charcot-Marie-Tooth disease: electrophysiology, molecular genetics and clinical management.

Over the past decade there has been a huge increase in the understanding of the molecular basis of Charcot-Marie-Tooth disease (CMT). Additionally there has been a better delineation of the neurophysiological deficits and clinical problems associated with CMT. This paper reviews the current molecular basis of CMT and the electrophysiological, clinical and phenotypical characteristics of the various subtypes, followed by a discussion of novel and promising therapeutic interventions that potentially could be used as part of a treatment regimen for CMT. These interventions may involve attempts to slow down or stop neurodegenerative processes through nerve growth factors, limiting oxidative stress by using antioxidants, or normalizing gene expression through genetic manipulation. Other potential therapeutic target areas include the progesterone receptor on myelin-forming Schwann cells, the immune system via modulation of nerve inflammation, and enhancing glutathione transferase activity. While ongoing molecular research continues to identify more of the mutant genes and proteins that cause the various disease subtypes, the focus of clinical research should continue to be on developing pharmaceutical and rehabilitative therapies to reverse nerve degeneration and ultimately improve the functioning of people with CMT.

Genetic evaluation of inherited motor/sensory neuropathy.

Inherited disorders of peripheral nerves represent a common group of neurologic diseases. Charcot-Marie-Tooth neuropathy type 1 (CMT1) is a genetically heterogeneous group of chronic demyelinating polyneuropathies with loci mapping to chromosome 17 (CMT1A), chromosome 1 (CMT1B), chromosome 16 (CMT1C) and chromosome 10 (CMT1D). CMT1A is most often associated with a tandem 1.5-megabase (Mb) duplication in chromosome 17p11.2-p12. In rare patients it may result from a point mutation in the peripheral myelin protein-22 (PMP22) gene. CMT1B is associated with point mutations in the myelin protein zero (Po or MPZ) gene. Mutations in the SIMPLE gene cause CMT1C, and CMT1D is the result of mutations in the early response 2 (ERG2 or Krox-20) gene. An X-linked form of CMT1 (CMT1X) maps to Xq13 and is associated with mutations in the connexin32 (Cx32) gene. Charcot-Marie-Tooth neuropathy type 2 (CMT2) is an axonal neuropathy that maps to chromosome 1p35-p36 (CMT2A), chromosome 3q13-q22 (CMT2B), chromosome 7p14 (CMT2D), chromosome 8p21 (CMT2E), chromosome 1q22-q23 (CMT2F) or chromosome 3q13 (CMT2G). Two X-linked forms of CMT2 have been reported (CMT2XA and CMT2XB), but the genes remain unidentified. An area that has recently expanded is the identification of autosomal recessive forms of CMT type 1 and 2. Of the eight recessive forms of CMT1 that have been identified to date, only two have been fully characterized at the molecular level (CMT1 AR B 1 and CMT1 AR D). Point mutations were found in the myotubularin-related protein-2 (MTM2) gene for CMT1 AR B1. CMT1 AR D is the result of point mutations in the N-myc downstream-regulated gene 1 (NDRG1). Dejerine-Sottas disease (DSD), also called hereditary motor and sensory neuropathy type III (HMSNIII), is a severe, infantile-onset demyelinating polyneuropathy syndrome that may be associated with point mutations in either the PMP22 gene, PO gene, EGR2 gene or the PRX gene (for the recessive form). It shares considerable clinical and pathological features with CMT1. Hereditary neuropathy with liability to pressure palsies (HNPP) is an autosomal dominant disorder that results in a recurrent, episodic demyelinating neuropathy. HNPP is associated with a 1.5-Mb deletion in chromosome 17p11.2-p12 that results in reduced expression of the PMP22 gene. CMT1A and HNPP are reciprocal duplication/deletion syndromes that originate from unequal crossover during germ cell meiosis. Other rare forms of demyelinating peripheral neuropathies map to chromosome 8q, 10q and 11q.

Electrodiagnostic evaluation of hereditary motor and sensory neuropathies.

Electrodiagnosis can classify hereditary motor and sensory neuropathies (HMSN) into two basic types: primarily demyelinating with secondary axonal loss and primarily axonal. For the most part, the various forms of HMSN show uniform symmetric nerve conduction slowing, in contrast to acquired neuropathies, which may be multifocal with nonuniform conduction velocity slowing and temporal dispersion. Nevertheless, there are exceptions. This article reviews the available literature and describes the electrodiagnostic approach to HMSN, detailing potential sources of error that can lead to misinterpretation of data.

The central nervous system phenotype of X-linked Charcot-Marie-Tooth disease: a transient disorder of children and young adults.

We describe 2 patients with X-linked Charcot-Marie-Tooth disease, type 1 (CMTX1) disease and central nervous system manifestations and review 19 cases from the literature. Our first case had not been previously diagnosed with Charcot-Marie-Tooth disease, and the second case, although known to have Charcot-Marie-Tooth disease, was suspected of having CMTX1 after presentation with central nervous system manifestations. The most common central nervous system manifestations were transient and included dysarthria, ataxia, hemiparesis, and tetraparesis resembling periodic paralysis. Of the 21 patients, 19 presented at 21 years of age or younger, implicating CMTX1 with transient central nervous system manifestations as a disorder that predominantly affects children and adolescents. CMTX1 should be included in the differential diagnosis of patients who present with transient central nervous system phenomena, including stroke-like episodes, tetraparesis suggestive of periodic paralysis, dysarthria, ataxia, or combinations of these deficits. Reversible, bilateral, nonenhancing white matter lesions and restricted diffusion on magnetic resonance imaging are characteristic features of the central nervous system phenotype of CMTX1.

Charcot-Marie-Tooth disease.

The family of hereditary peripheral neuropathies that makes up Charcot-Marie-Tooth disease (CMT) comprises some of the most common neuromuscular disorders. Over the past decade, understanding of the molecular basis of CMT has increased enormously. In addition, the neurophysiologic deficits and clinical problems associated with CMT are more clearly delineated, and the precise genetic cause of many types of CMT has now been determined. Advances in molecular biology and genetic manipulation techniques have allowed the development of animal models of some of these CMT types, allowing more productive scientific exploration of possible treatments. Recent treatment advances that have been effective in animal models include oral supplementation with curcumin and vitamin C (ascorbic acid), and the use of onapristone, a progesterone antagonist. Human trials with vitamin C are currently in progress. While ongoing molecular genetic research continues to identify more of the mutant genes and proteins that cause the various disease subtypes, clinical research should continue to focus on developing pharmaceutical and rehabilitative therapies to ameliorate nerve degeneration and ultimately improve function for patients with CMT. These patients optimally should be managed in a comprehensive, multidisciplinary setting involving neurologists, physiatrists, orthopedic surgeons, physical and occupational therapists, and orthotists. Treatment should be aimed at maximizing independence and quality of life.

Senataxin, the yeast Sen1p orthologue: characterization of a unique protein in which recessive mutations cause ataxia and dominant mutations cause motor neuron disease.

A severe recessive cerebellar ataxia, Ataxia-Oculomotor Apraxia 2 (AOA2) and a juvenile onset form of dominant amyotrophic lateral sclerosis (ALS4) result from mutations of the Senataxin (SETX) gene. To begin characterization this disease protein, we developed a specific antibody to the DNA/RNA helicase domain of SETX. In murine brain, SETX concentrates in several regions, including cerebellum, hippocampus and olfactory bulb with a general neuronal expression profile, colocalizing with NeuN. In cultured cells, we found that SETX was cytoplasmically diffuse, but in the nucleus, SETX was punctate, colocalizing with fibrillarin, a marker of the nucleolus. In differentiated non-cycling cells, nuclear SETX was not restricted to the nucleolus but was diffuse within the nucleoplasm, suggesting cell-cycle-dependent localization. SETX missense mutations cluster within the N-terminus and helicase domains. Flag tagging at the N-terminus caused protein mislocation to the nucleoplasm and failure to export to the cytoplasm, suggesting that the N-terminus may be essential for correct SETX localization. We report here the first characterization of SETX protein, which may provide future insights into a new mechanism leading to neuron death.

SIMPLE interacts with NEDD4 and TSG101: evidence for a role in lysosomal sorting and implications for Charcot-Marie-Tooth disease.

Mutation of the SIMPLE gene (small integral membrane protein of the lysosome/late endosome) is the molecular basis of Charcot-Marie-Tooth disease type 1C (CMT1C), a demyelinating peripheral neuropathy. Although the precise function of SIMPLE is unknown, prior reports suggest it localizes to the lysosome/late endosome. Furthermore, murine Simple interacts with Nedd4 (neural precursor cell expressed, developmentally downregulated 4), an E3 ubiquitin ligase that is important for regulating lysosomal degradation of plasma membrane proteins. To bring insights into the biochemical function of human SIMPLE, we confirmed that human SIMPLE interacts with NEDD4 and also report a novel interaction with tumor susceptibility gene 101 (TSG101), a class E vacuolar sorting protein. TSG101 is known to function downstream of NEDD4, sorting ubiquitinated substrates into multivesicular bodies (MVBs), which then deliver their cargo into the lysosomal lumen for degradation. Given the interaction with NEDD4 and TSG101, and the localization of SIMPLE along the lysosomal degradation pathway, we hypothesize that SIMPLE plays a role in the lysosomal sorting of plasma membrane proteins. We examine three CMT1C-associated SIMPLE mutations and show that they do not affect the interaction with NEDD4 or TSG101, nor do they lead to altered subcellular localization.

Prenatal diagnosis in pregnancies at risk for Joubert syndrome by ultrasound and MRI.

OBJECTIVES: To describe the prenatal imaging findings in fetuses at risk for Joubert syndrome (JS), review the literature and propose a protocol for prenatal diagnosis of JS using ultrasound and MRI. METHODS: We reviewed prenatal ultrasound and fetal MRI studies in two pregnancies at 25% recurrence risk for JS and correlated these findings with gross neuropathology in one affected fetus. RESULTS: While abnormalities such as occipital encephalocele or enlarged cisterna magna have been identified before mid-trimester, the definitive diagnosis of JS, based on core cerebellar findings, has only been possible after 17 weeks' gestation. CONCLUSIONS: With longitudinal monitoring, it is possible to diagnose JS in at-risk pregnancies before 24 weeks' gestation. On the basis of our data and review of the literature, we propose a protocol for monitoring pregnancies at risk for JS, utilizing serial ultrasounds combined with fetal MRI at 20-22 weeks' gestation to maximize the accuracy of prenatal diagnosis.

Evidence of a founder effect and refinement of the hereditary neuralgic amyotrophy (HNA) locus on 17q25 in American families.

Hereditary neuralgic amyotrophy (HNA) is an autosomal dominant disorder that is associated with episodic recurrent brachial plexus neuropathy. A mutation for HNA maps to chromosome 17q25. To refine the HNA locus further, we carried out genetic linkage studies in seven pedigrees with a high density set of DNA markers from chromosome 17q25. All pedigrees demonstrated linkage to chromosome 17q25, and an analysis of recombinant events placed the HNA locus within an interval of approximately 1 Mb flanked by markers D17S722 and D17S802. In order to test the power of linkage disequilibrium mapping, we compared genotypes of 12 markers from seven pedigrees that were from the United States and that showed linkage to chromosome 17q25. The haplotypes identified a founder effect in six of the seven pedigrees with a minimal shared haplotype that further refines the HNA locus to an interval of approximately 500 kb. These findings suggest that, for the pedigrees from the United States, there are at least two different mutations in the HNA gene.

Joubert syndrome: a haplotype segregation strategy and exclusion of the zinc finger protein of cerebellum 1 (ZIC1) gene.

Joubert syndrome (JS) is a rare autosomal recessive malformation syndrome, involving dysgenesis of the cerebellar vermis with accompanying brainstem malformations (comprising the molar tooth sign). JS is characterized by hypotonia, developmental delay, intermittent hyperpnea and apnea, and abnormal eye movements. A single locus for JS was previously identified on 9q34 in a consanguineous family of Arabian origin. However, linkage to this locus has subsequently been shown to be rare. We have ascertained 35 JS pedigrees for haplotype segregation analysis of genetic loci for genes with a putative role in cerebellar development. We examined the ZIC1 gene as a functional candidate for JS as Zic1(-/-) null mice have a phenotype reminiscent of JS. We undertook mutational analysis of ZIC1 by standard mutational analysis (dideoxy-fingerprinting (ddf)) of 47 JS probands, and fully sequenced the coding region in five of these probands. By these means, ZIC1 was excluded from playing a causal role in most cases of JS as no disease-associated mutations were identified. Further, linkage to the ZIC1 genetic locus (3q24) was excluded in 21 of 35 pedigrees by haplotype segregation analysis of closely spaced markers. The remaining 14 of 35 pedigrees were consistent with linkage. However, this number does not significantly depart from that expected by random chance (16.5) for this cohort. Therefore, this systematic approach has been validated as a means to prioritize functional candidate genes and enables us to confine mutational analysis to only those probands whose segregation is consistent with linkage to any given locus.

Mapping of Charcot-Marie-Tooth disease type 1C to chromosome 16p identifies a novel locus for demyelinating neuropathies.

Charcot-Marie-Tooth (CMT) neuropathy represents a genetically heterogeneous group of diseases affecting the peripheral nervous system. We report genetic mapping of the disease to chromosome 16p13.1-p12.3, in two families with autosomal dominant CMT type 1C (CMT1C). Affected individuals in these families manifest characteristic CMT symptoms, including high-arched feet, distal muscle weakness and atrophy, depressed deep-tendon reflexes, sensory impairment, slow nerve conduction velocities, and nerve demyelination. A maximal combined LOD score of 14.25 was obtained with marker D16S500. The combined haplotype analysis in these two families localizes the CMT1C gene within a 9-cM interval flanked by markers D16S519 and D16S764. The disease-linked haplotypes in these two pedigrees are not conserved, suggesting that the gene mutation underlying the disease in each family arose independently. The epithelial membrane protein 2 gene (EMP2), which maps to chromosome 16p13.2, was evaluated as a candidate gene for CMT1C.