Collette K. Hand

A gene encoding a putative GTPase regulator is mutated in familial amyotrophic lateral sclerosis 2

Amyotrophic lateral sclerosis 2 (ALS2) is an autosomal recessive form of juvenile ALS and has been mapped to human chromosome 2q33. Here we report the identification of two independent deletion mutations linked to ALS2 in the coding exons of the new gene ALS2. These deletion mutations result in frameshifts that generate premature stop codons. ALS2 is expressed in various tissues and cells, including neurons throughout the brain and spinal cord, and encodes a protein containing multiple domains that have homology to RanGEF as well as RhoGEF. Deletion mutations are predicted to cause a loss of protein function, providing strong evidence that ALS2 is the causative gene underlying this form of ALS.

Familial amyotrophic lateral sclerosis.

The increasing complexity of the pathways implicated in the pathogenesis of familial amyotrophic lateral sclerosis (ALS) has stimulated intensive research in many directions. Genetic analysis of familial ALS has yielded six loci and one disease gene (SOD1), initially suggesting a role for free radicals in the disease process, although the mechanisms through which the mutant exerts toxicity and results in selective motor neuron death remain uncertain. Numerous studies have focused on structural elements of the affected cell, emphasizing the role of neurofilaments and peripherin and their functional disruption in disease. Other topics examined include cellular homeostasis of copper and calcium, particularly in the context of oxidative stress and the processes of protein aggregation, glutamate excitotoxicity, and apoptosis. It has become evident that there is considerable interplay between these mechanisms and, as the role of each is established, a common picture may emerge, enabling the development of more targeted therapies. This study discusses the main areas of investigation and reviews the findings.

A Novel Locus for Familial Amyotrophic Lateral Sclerosis, on Chromosome 18q

Amyotrophic lateral sclerosis (ALS) is an adult-onset degenerative disorder characterized by the death of motor neurons in the cortex, brain stem, and spinal cord. Despite intensive research the basic pathophysiology of ALS remains unclear. Although most cases are sporadic, approximately 10% of ALS cases are familial (FALS). Mutations in the Cu/Zn superoxide dismutase (SOD1) gene cause approximately 20% of FALS. The gene(s) responsible for the remaining 80% of FALS remain to be found. Using a large European kindred without SOD1 mutation and with classic autosomal dominant adult-onset ALS, we have identified a novel locus by performing a genome scan and linkage analysis. The maximum LOD score is 4.5 at recombination fraction 0.0, for polymorphism D18S39. Haplotype analysis has identified a 7.5-cM, 8-Mb region of chromosome 18q21, flanked by markers D18S846 and D18S1109, as a novel FALS locus.

An ALS2 gene mutation causes hereditary spastic paraplegia in a Pakistani kindred

Hereditary spastic paraplegia (HSP) and amyotrophic lateral sclerosis (ALS) are genetically heterogeneous progressive neurodegenerative disorders distinguished by differential motor neuron involvement. In ALS, both upper and lower motor neurons are affected, whereas in HSP only upper motor neuron function is affected, resulting in a less severe disease. Recently, juvenile forms of ALS (ALS2) and primary lateral sclerosis (PLS) were shown to be caused by mutations in the ALS2 gene, which encodes a putative GTPase regulator. The ALS2 gene has 34 exons with at least two splice variants. The long variant (6,394 nucleotides) is expressed in various tissues with highest expression in the brain. The protein, alsin, shows similarity to three domains (RCC1, Pleckstrin-DB1, VPS9) and to membrane occupation and recognition nexus repeat motifs, which are characteristic of various guanine exchange factors (Ran, Rho, and Rab, respectively). We report the identification of a novel ALS2 mutation in a large consanguineous Pakistani kindred with infantileonset autosomal recessive complicated HSP. The proband initially presented with gait disturbance and hyperreflexia at 18 months. Currently, at age 12 years, she is anarthric and confined to a wheelchair. Family history indicates that the disease slowly progresses to tetraplegia and death by the fourth decade of life, with relatively preserved intellect. The clinical picture is similar to that recently reported by Eymard-Pierre and colleagues. Because our family is clinically related to previously reported ALS2 families and the fact that an HSP locus (SPG13) initially overlapped with the ALS2 locus, the proband was analyzed for mutations in the ALS2 gene using the denaturing high performance liquid chromatography (DHPLC)–WAVE system (Transgenomics, Mountain View, CA). Sequencing of a DHPLC variant in exon 32 showed a 1bp deletion (4844delT; Fig, a). Cosegregation of the mutation and the disease in the family was confirmed, and the mutation was absent in 155 control individuals. The deletion occurs at the beginning of the VPS9 domain and adds 43 unique residues to the truncated protein (see Fig, b). Our results show that absence of a functional VPS9 domain of alsin is sufficient to cause neurodegeneration. The yeast VPS9 protein and its mammalian homolog RABEX-5 are guanine nucleotide exchange factors for specific proteins thought to be involved in vacuolar endocytic transport. Disruption of intracellular trafficking has long been suggested to cause selective degeneration of the long axons of the pyramidal tract in HSP, but identification of additional mutations, functional studies, and animal models are necessary to further understand the pathogenesis resulting from ALS2 mutations.

Compound heterozygous D90A and D96N SOD1 mutations in a recessive amyotrophic lateral sclerosis family

We describe a French amyotrophic lateral sclerosis (ALS) family with two distinct mutations in the Cu/Zn superoxide dismutase (SOD1) gene. The D90A mutation has been well described and clearly shown to cause recessive ALS. In this family, affected individuals are heterozygous for the D90A mutation and also carry a single copy of a novel SOD1 mutation, D96N. We propose that in this family both mutations are required for the development of disease. Ann Neurol 2001;49:267–271

Novel RPGR mutations with distinct retinitis pigmentosa phenotypes in French-Canadian families

PURPOSE To characterize the molecular defects in two x-linked retinitis pigmentosa (RP) families. We hypothesized that different RPGR mutations result in distinct RP phenotypes. DESIGN Observational case series. METHODS Fifteen members in family I and three members in family II were evaluated. Full ophthalmic evaluations were done. Linkage analyses were performed and likelihood of odds scores (LOD score) were calculated. For mutation analyses, we used dHPLC and automated sequencing. RESULTS Two novel RPGR mutations were identified in the two families; a Glu 414 (2-bp del) frameshift mutation in family I and an IVS 2-1 (g to a) splice site mutation in family II. All male family members in family I were severely affected by RP but maintained central visual acuities until their 50s and did not develop a bulls eye maculopathy. The female phenotype was highly variable. Some of the carriers exhibited a severe phenotype, one female displayed an asymmetric phenotype, and other carriers were asymptomatic. All members with the RPGR frameshift mutation exhibited rod-cone electroretinograms abnormalities, whereas five members had hearing loss. Male members of family II were severely affected, with early visual acuity loss, central scotomas, and bulls eye maculopathy. The female family members were asymptomatic but displayed cone-rod electroretinograms changes. There was no hearing loss. CONCLUSIONS Different RPGR mutations lead to distinct RP phenotypes, with a highly variable inter- and intrafamilial phenotypic spectrum of disease that is associated with the type of mutation in RPGR and nonrandom X chromosome inactivation, respectively.

Homozygosity mapping and linkage analysis demonstrate that autosomal recessive congenital hereditary endothelial dystrophy (CHED) and autosomal dominant CHED are genetically distinct

BACKGROUND Congenital hereditary endothelial dystrophy (CHED) is a corneal dystrophy characterised by diffuse bilateral corneal clouding resulting in impaired vision. It is inherited in either an autosomal dominant (AD) or autosomal recessive (AR) manner. The AD form of CHED has been mapped to the pericentromeric region of chromosome 20. Another endothelial dystrophy, posterior polymorphous dystrophy (PPM), has been linked to a larger but overlapping region on chromosome 20. A large, Irish, consanguineous family with AR CHED was investigated to determine if there was linkage to this region. METHODS The technique of linkage analysis with polymorphic microsatellite markers amplified by polymerase chain reaction (PCR) was used. In addition, a DNA pooling approach to homozygosity mapping was employed to demonstrate the efficiency of this method. RESULTS Conventional genetic analysis in addition to a pooled DNA strategy excludes linkage of AR CHED to the AD CHED and larger PPMD loci. CONCLUSION This demonstrates that AR CHED is genetically distinct from AD CHED and PPMD.

A Locus for Autosomal Dominant Hereditary Spastic Ataxia, SAX1,Maps to Chromosome 12p13

The hereditary spastic ataxias (HSA) are a group of clinically heterogeneous neurodegenerative disorders characterized by lower-limb spasticity and generalized ataxia. HSA was diagnosed in three unrelated autosomal dominant families from Newfoundland, who presented mainly with severe leg spasticity, dysarthria, dysphagia, and ocular-movement abnormalities. A genomewide scan was performed on one family, and linkage to a novel locus for HSA on chromosome 12p13, which contains the as-yet-unidentified gene locus SAX1, was identified. Fine mapping confirmed linkage in the two large families, and the third, smaller family showed LOD scores suggestive of linkage. Haplotype construction by use of 13 polymorphic markers revealed that all three families share a disease haplotype, which key recombinants and overlapping haplotypes refine to about 5 cM, flanked by markers D12S93 and GATA151H05. SAX1 is the first locus mapped for autosomal dominant HSA.

A Novel ATP1A2 Gene Mutation in an Irish Familial Hemiplegic Migraine Kindred

Objective.— We studied a large Irish Caucasian pedigree with familial hemiplegic migraine (FHM) with the aim of finding the causative gene mutation.

Les anomalies du gène superoxyde dismutase 1 dans la sclérose latérale amyotrophique familiale : corrélations phénotype/génotype et implications pratiques. L’expérience française et revue de la littérature

Resume La sclerose laterale amyotrophique familiale (SLAF) represente 20 p. 100 des cas de SLA. Dans 15 p. 100 des SLAF, une anomalie du gene codant pour la superoxyde dismutase 1 (SOD1) est retrouvee. Pour la majorite d’entre elles, il s’agit d’une mutation ponctuelle, avec remplacement d’un nucleotide par un autre. La transmission est autosomique dominante pour toutes les anomalies genetiques sauf deux, qui sont recessives. A ce jour, plus de 100 anomalies genetiques differentes sont rapportees. Le profil clinique caracteristique des SLAF associees aux mutations SOD1 est homogene lorsque le lien entre la mutation et la maladie est clairement etabli : âge moyen de debut a 42 ans, debut aux membres, evolution lente. En dehors d’un caractere familial dominant (plusieurs sujets atteints sur plusieurs generations), la causalite de la mutation est souvent discutable, ne justifiant pas une recherche systematique de mutation SOD1 devant une SLA atypique (debut tres jeune, evolution tres longue, par exemple). Hormis dans le cas des quelques mutations frequentes et prouvees causales (moins de 15 sur plus de 90), la multiplicite des mutations, la variabilite de leur mode de transmission et le caractere souvent discutable de leur responsabilite dans la maladie, justifient l’etude la plus complete possible d’une famille afin d’etablir la causalite de l’anomalie genetique retrouvee ainsi que sa transmission.