Djamel Grid

Homozygous Defects in LMNA, Encoding Lamin A/C Nuclear-Envelope Proteins, Cause Autosomal Recessive Axonal Neuropathy in Human (Charcot-Marie-Tooth Disorder Type 2) and Mouse

The Charcot-Marie-Tooth (CMT) disorders comprise a group of clinically and genetically heterogeneous hereditary motor and sensory neuropathies, which are mainly characterized by muscle weakness and wasting, foot deformities, and electrophysiological, as well as histological, changes. A subtype, CMT2, is defined by a slight or absent reduction of nerve-conduction velocities together with the loss of large myelinated fibers and axonal degeneration. CMT2 phenotypes are also characterized by a large genetic heterogeneity, although only two genes---NF-L and KIF1Bbeta---have been identified to date. Homozygosity mapping in inbred Algerian families with autosomal recessive CMT2 (AR-CMT2) provided evidence of linkage to chromosome 1q21.2-q21.3 in two families (Zmax=4.14). All patients shared a common homozygous ancestral haplotype that was suggestive of a founder mutation as the cause of the phenotype. A unique homozygous mutation in LMNA (which encodes lamin A/C, a component of the nuclear envelope) was identified in all affected members and in additional patients with CMT2 from a third, unrelated family. Ultrastructural exploration of sciatic nerves of LMNA null (i.e., -/-) mice was performed and revealed a strong reduction of axon density, axonal enlargement, and the presence of nonmyelinated axons, all of which were highly similar to the phenotypes of human peripheral axonopathies. The finding of site-specific amino acid substitutions in limb-girdle muscular dystrophy type 1B, autosomal dominant Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy type 1A, autosomal dominant partial lipodystrophy, and, now, AR-CMT2 suggests the existence of distinct functional domains in lamin A/C that are essential for the maintenance and integrity of different cell lineages. To our knowledge, this report constitutes the first evidence of the recessive inheritance of a mutation that causes CMT2; additionally, we suggest that mutations in LMNA may also be the cause of the genetically overlapping disorder CMT2B1.

Mutations in SPG11 , encoding spatacsin, are a major cause of spastic paraplegia with thin corpus callosum

Autosomal recessive hereditary spastic paraplegia (ARHSP) with thin corpus callosum (TCC) is a common and clinically distinct form of familial spastic paraplegia that is linked to the SPG11 locus on chromosome 15 in most affected families. We analyzed 12 ARHSP-TCC families, refined the SPG11 candidate interval and identified ten mutations in a previously unidentified gene expressed ubiquitously in the nervous system but most prominently in the cerebellum, cerebral cortex, hippocampus and pineal gland. The mutations were either nonsense or insertions and deletions leading to a frameshift, suggesting a loss-of-function mechanism. The identification of the function of the gene will provide insight into the mechanisms leading to the degeneration of the corticospinal tract and other brain structures in this frequent form of ARHSP.

Mutations in FGD4 encoding the Rho GDP/GTP exchange factor FRABIN cause autosomal recessive Charcot-Marie-Tooth type 4H.

Charcot-Marie-Tooth (CMT) disorders are a clinically and genetically heterogeneous group of hereditary motor and sensory neuropathies characterized by muscle weakness and wasting, foot and hand deformities, and electrophysiological changes. The CMT4H subtype is an autosomal recessive demyelinating form of CMT that was recently mapped to a 15.8-Mb region at chromosome 12p11.21-q13.11, in two consanguineous families of Mediterranean origin, by homozygosity mapping. We report here the identification of mutations in FGD4, encoding FGD4 or FRABIN (FGD1-related F-actin binding protein), in both families. FRABIN is a GDP/GTP nucleotide exchange factor (GEF), specific to Cdc42, a member of the Rho family of small guanosine triphosphate (GTP)-binding proteins (Rho GTPases). Rho GTPases play a key role in regulating signal-transduction pathways in eukaryotes. In particular, they have a pivotal role in mediating actin cytoskeleton changes during cell migration, morphogenesis, polarization, and division. Consistent with these reported functions, expression of truncated FRABIN mutants in rat primary motoneurons and rat Schwann cells induced significantly fewer microspikes than expression of wild-type FRABIN. To our knowledge, this is the first report of mutations in a Rho GEF protein being involved in CMT.

Mutation analysis of the paraplegin gene (SPG7) in patients with hereditary spastic paraplegia

Background: Mutations in the SPG7 gene, which encodes paraplegin, are responsible for an autosomal recessive hereditary spastic paraplegia (HSP). Objective: To screen the SPG7 gene in a large population of HSP families compatible with autosomal recessive transmission. Methods: The authors analyzed 136 probands with pure or complex HSP for mutations in the SPG7 using denaturation high-performance liquid chromatography and direct sequencing. Results: The authors identified 47 variants including 6 mutations, 27 polymorphisms, and 14 changes with unknown effects. In one family from Morocco, compound c.850_851delTTinsC and c.1742_1744delTGG heterozygous mutations were shown to be causative. This family had complex HSP with cerebellar impairment. Progression of the disease was rapid, resulting in a severe disease after 8 years of duration. Also detected were 20 families with one heterozygous mutation that was not found in a large control population. The mutations produced highly defective proteins in four of these families, suggesting that they were probably causative. Direct sequencing of all exons and reverse transcription PCR experiments demonstrated the absence of a second mutation. However, the p.Ala510Val missense substitution previously described as a polymorphism was shown to be significantly associated with HSP, suggesting that it had a functional effect. Conclusion: SPG7 mutations account for less than 5% of hereditary spastic paraplegia (HSP) families compatible with autosomal recessive inheritance. Cerebellar signs or cerebellar atrophy on brain imaging were the most frequent additional features in patients with SPG7 HSP. Rare nucleotide variants in SPG7 are frequent, complicating routine diagnosis.

Spine deformities in Charcot-Marie-Tooth 4C caused by SH3TC2 gene mutations.

Background: Charcot-Marie-Tooth (CMT) disease is a heterogeneous group of inherited peripheral motor and sensory neuropathies with several modes of inheritance: autosomal dominant, X-linked, and autosomal recessive (AR) CMT. A locus responsible for the demyelinating form of ARCMT was assigned to the 5q23-q33 region (CMT4C) by homozygosity mapping. Recently, 11 mutations were identified in the SH3TC2 (KIAA1985) gene in 12 families with demyelinating ARCMT from Turkish, Iranian, Greek, Italian, or German origin. Objective: To identify mutations in the SH3TC2 gene. Methods: The authors searched for SH3TC2 gene mutations in 10 consanguineous CMT families putatively linked to the CMT4C locus on the basis of haplotype segregation and linkage analysis. Results: Ten families had mutations, eight of which were new and one, R954X, recurrent. Six of the 10 mutations were in exon 11. Onset occurred between ages 2 and 10. Scoliosis or kyphoscoliosis and foot deformities were found in almost all patients and were often inaugural. The median motor nerve conduction velocity values (≤34 m/s) were not correlated with disease duration. The functional disability score was ≤3, indicating that the patients could walk without help. Unexpectedly, typical giant axons were observed on biopsies from a large Algerian family. Conclusions: Charcot-Marie-Tooth type 4C (CMT4C) is less severe than other autosomal recessive (AR) CMT. Intrafamilial variability is important, making phenotype-genotype correlations difficult, but spine deformities are clearly a hallmark of CMT4C. In the presence of scoliosis, a neurologic examination is recommended. Giant axons on biopsies are also suggestive of CMT4C. For genetic analysis, the R954X mutation should be looked for before systematic sequencing of exon 11.

A clinical, electrophysiologic, neuropathologic, and genetic study of two large algerian families with an autosomal recessive demyelinating form of Charcot-Marie-Tooth disease

The hereditary sensory and motor neuropathies form a clinically heterogenous group of disorders, the most frequent of which is Charcot-Marie-Tooth disease (CMT). The autosomal dominant forms of CMT are well characterized, but the nosology of autosomal recessive CMT is still controversial. We report two large consanguineous Algerian families with an autosomal recessive demyelinating CMT and similar clinical manifestations. The clinical, electrophysiologic, and neuropathologic features resemble those of autosomal dominant CMT1, but the early onset and rapid progression of deformities are specific. We excluded by linkage analysis the three loci CMT1A (17p11.2), CMT1B (1q22–231, and CMT4A (Sq11–21.1) responsible for demyelinating forms of CMT. These findings suggest a subtype of autosomal recessive neuropathy, the locus of which is undetermined.

Ataxia with oculomotor apraxia type 2: A clinical and genetic study of 19 patients

Ataxia with oculo-motor apraxia type 2 (AOA2) is a recently described autosomal recessive cerebellar ataxia (ARCA) caused by mutations in the senataxin gene (SETX). We analysed the phenotypic spectrum of 19 AOA2 patients with mutations in SETX, which seems to be the third most frequent form of ARCA in Algeria after Freidreich ataxia and Ataxia with vitamin E deficiency. In AOA2 patients, the mean age at onset for all families was in the second decade. Cerebellar ataxia was progressive, slowly leading to disability which was aggravated by axonal polyneuropathy present in almost all the patients. Mean disease duration until wheelchair was around 20 years. Oculo-motor apraxia (OMA) was present in 32% of the patients while convergent strabismus was present in 37%. Strabismus is therefore also very suggestive of AOA2 when associated with ataxia and polyneuropathy even in the absence of OMA. Cerebellar atrophy was more severe in the eldest patients; however it may also be an early sign since it was present in the youngest and paucisymptomatic patients. The initial sign was gait ataxia in all but two patients who presented with head tremor and writer cramp, respectively. Serum alpha-fetoprotein, which was elevated in all tested patients, was a good marker to suggest molecular studies of the SETX gene.

Homozygosity mapping of autosomal recessive demyelinating Charcot-Marie-Tooth neuropathy (CMT4H) to a novel locus on chromosome 12p11.21-q13.11

Hereditary motor and sensory neuropathies, commonly referred to as Charcot-Marie-Tooth disease (CMT), are among the most common inherited neurological diseases, with an overall prevalence of about 1–4/10 000.1 Clinically, the hereditary motor and sensory neuropathies are characterised by progressive muscular and sensory loss in the distal extremities with chronic distal weakness, deformation of the feet (pes cavus), and loss of deep tendon reflexes.2 Two main subgroups have been defined on the basis of electrophysiological and histopathological characteristics: the demyelinating form (CMT1) and the axonal form (CMT2). CMT1 can be distinguished from CMT2 by measuring motor nerve conduction velocities in the median nerve: patients affected by CMT1 show reduced velocities (<38 m/s), whereas those affected by CMT2 show velocities of ⩾38 m/s, the normal value being ⩾48 m/s. Recently, a new group of CMT has been described, referred to as intermediate CMT3,4; in this, nerve conduction velocities overlap CMT1 and CMT2, and nerve biopsies present characteristics of both demyelination and axonal loss. CMT is also characterised by great genetic heterogeneity, with more than 30 loci and 19 genes identified to date5,6 (inherited peripheral neuropathy mutation database, IPNMDB, http://www.molgen.ua.ac.be/CMTMutations). All modes of inheritance have been reported: autosomal dominant, autosomal recessive, and X linked. Autosomal recessive demyelinating forms (CMT4) are, in most cases, less frequent, of earlier onset, and more severe than the autosomal dominant CMT forms (CMT1), with a fast progression to severe disability leading to a higher frequency of wheelchair dependency early in life.7 To date, at least seven demyelinating forms with autosomal recessive inheritance have been identified:

Phenotypic variability in giant axonal neuropathy

Giant axonal neuropathy (GAN), a severe childhood disorder affecting both the peripheral nerves and the central nervous system, is due to mutations in the GAN gene encoding gigaxonin, a protein implicated in the cytoskeletal functions and dynamics. In the majority of the GAN series reported to date, patients had the classical clinical phenotype characterized by a severe axonal neuropathy with kinky hair and early onset CNS involvement including cerebellar and pyramidal signs. We present 12 patients (6 families) with GAN mutations and different clinical phenotypes. Four families were harbouring an identical homozygous nonsense mutation but with different severe clinical phenotypes, one patient had a novel missense homozygous mutation with a peculiar moderate phenotype and prominent skeletal deformations. The last family (4 patients) harbouring a homozygous missense mutation had the mildest form of the disease. In contrast with recent reported series of patients with typical GAN clinical features, the present series demonstrate obvious clinical heterogeneity.

Phenotypic and genetic exploration of severe demyelinating and secondary axonal neuropathies resulting from GDAP1 nonsense and splicing mutations

We identified two novel GDAP1 homozygous mutations in children affected with severe demyelinating peripheral neuropathies and born to consanguineous parents. A 9 year old Lebanese girl carried a nonsense mutation in exon 5 and two Algerian brothers aged 10 and 8 years carried a mutation at the intron 3 acceptor splicing site. The clinical, electrophysiological, and neuropathological explorations showed common features consistent with a severe demyelinating peripheral neuropathy associated with loss of major fibres. Our findings, supported by the first GDAP1 expression study in patients, show further evidence that mutations in this gene cause an autsomal recessive severe demyelinating peripheral phenotype (CMT4A) associated with axon loss. Charcot-Marie-Tooth disease (CMT) (also called hereditary motor and sensory neuropathy (HMSN)) is clinically, electrophysiologically, and genetically extremely heterogeneous with more than 40 loci and 16 genes identified to date.1 The most common inherited peripheral neuropathies are CMT type 1 and CMT type 2, which are characterised by progressive weakness and atrophy, initially of the peroneal muscles and later of the distal muscles of the arms. Myelination is mainly affected in CMT1, with neuropathological findings of demyelination and remyelination with onion bulb formation, clusters of regeneration, and Schwann cell proliferation. CMT2 is an axonal neuropathy characterised by the reduction of fibre density in the absence of clusters of myelin regeneration or proliferation of Schwann cells. Very often, neuropathological analysis of patients carrying demyelinating neuropathies shows concomitant axon fibre loss.2 A motor nerve conduction velocity (MNCV) threshold value of 38 ms−1 at the median nerve is used to classify patients with CMT as being either CMT1 (MNCV 38 ms−1). CMT4A defines a particularly severe polyneuropathy of the demyelinating type, associated with distal weakness and atrophy of the limbs, with early onset. The muscular atrophy rapidly progresses, extending proximally …