Nature Publishing Group
Yüklə 386,38 Kb. Pdf görüntüsü
|
- Bu səhifədəki naviqasiya:
- Superoxide dismutase 1.
- ALS disease type Gene Chromosome Inheritance Clinical features
- NEUROSCIENCE
- VAMP-associated protein B.
- Mitochondrial gene defects.
- Complex genetics in ALS.
- Aberrant chemistry and oxidative stress.
|
©
200 6
(ALS; also known as Lou Gehrig’s disease and motor neuron disease) is a progressive, lethal, degenerative disorder of motor neurons. The hallmark of this disease is the selective death of motor neurons in the brain and spinal cord, leading to paralysis of voluntary muscles 1 . The paralysis begins focally and disseminates in a pattern that sug- gests that degeneration is spreading among contiguous pools of motor neurons. Currently, there are approxi- mately 25,000 patients with ALS in the USA, with a median age of onset of 55 years. The incidence and prevalence of ALS are 1–2 and 4–6 per 100,000 each year, respectively, with a lifetime ALS risk of 1/600 to 1/1,000
(REFS 2,3) . ALS is therefore an orphan disease , although its uniform lethality imparts an importance out of proportion to its prevalence. Although most cases are classed as sporadic ALS (SALS) 4 , 10% of cases are inherited (known as familial ALS; FALS). Age and gender are documented SALS risk factors 5 (the male: female ratio is 3:2). In both SALS and FALS, there are progressive manifestations of dysfunction of lower motor neurons (atrophy, cramps and fasciculations ) and cortical motor neurons ( spasticity and pathological reflexes) in the absence of sensory symptoms 1,3 . However, muscles that control eye movements and the urinary sphincters are spared 3 . Respiratory failure causes death, which typically occurs within five years of developing this debilitating condition. The pathological hallmark of ALS is the atrophy of dying motor neurons. Swelling of the perikarya and proximal axons is also observed, as is the accumulation of phosphorylated neurofilaments, Bunina bodies and Lewy body-like inclusions, and the deposition of inclusions (spheroids) and strands of ubiquitinated material 6 in these axons. In addition, the activation and proliferation of astrocytes and micro- glia
6 are also common in ALS. Regrettably, there is no primary therapy for this disorder, and the single drug approved for use in ALS, riluzole, only slightly prolongs survival. Symptomatic measures (for example, feed- ing tube and respiratory support) are the mainstay of management of this disorder. The causes of most cases of ALS are as yet undefined. Investigations have identified multiple perturbations of cellular function in ALS motor neurons, incriminating excessive excitatory tone, protein misfolding, impaired energy production, abnormal calcium metabolism, altered axonal transport and activation of proteases and nucleases 7,8 . Several factors are proposed to insti- gate these phenomena, including latent infections by viral and non-viral agents 9,10 , toxins (for example, insecticides and pesticides) and autoimmune reac- tions
8 . Recent studies of inherited ALS have extended the understanding of the pathophysiology of this dis- ease and approaches to its treatment. Here, we discuss the broad physiological implications of Mendelian, mitochondrial and complex genetic defects in ALS, and present an overview of how new knowledge of this disease can generate new strategies in ALS therapy. Mendelian genetics of ALS Five Mendelian gene defects have been reported to cause ALS (TABLE 1) . The protein products of these mutated genes are cytosolic Cu/Zn superoxide dismutase ( SOD1
) 11 , alsin 12,13 , senataxin ( SETX )
, synaptobrevin/ VAMP (vesicle-associated membrane protein)-associa- ted protein B ( VAPB
) 16 and dynactin 17 . Additionally, loci have been identified for ALS (on chromosomes 15, 16, 18, 20 and X) 18–23 and for ALS with frontotemporal dementia (ALS-FTD) 24,25
. Day Neuromuscular Research Laboratory, Massachusetts General Hospital, Room 3125, Building 114, 16th Street, Navy Yard, Charlestown, Massachusetts 02429, USA. Correspondence to P.P. or R.H.B. e-mails: ppasinelli@partners.org; rhbrown@partners.org doi:10.1038/nrn1971 Orphan disease A condition that affects fewer than 200,000 people nationwide. Fasciculation A muscle contraction visible under the skin that represents the spontaneous firing of a single motor neuron and, as a result, of all the muscle fibres it innervates. Spasticity Persistent muscle contraction that causes stiffness and interferes with gait, movements and speech. Bunina bodies Characteristic proteinacious inclusions in ALS motor neurons. Molecular biology of amyotrophic lateral sclerosis: insights from genetics Piera Pasinelli and Robert H. Brown Abstract | Amyotrophic lateral sclerosis (ALS) is a paralytic disorder caused by motor neuron degeneration. Mutations in more than 50 human genes cause diverse types of motor neuron pathology. Moreover, defects in five Mendelian genes lead to motor neuron disease, with two mutations reproducing the ALS phenotype. Analyses of these genetic effects have generated new insights into the diverse molecular pathways involved in ALS pathogenesis. Here, we present an overview of the mechanisms for motor neuron death and of the role of non-neuronal cells in ALS. R E V I E W S 710
| SEPTEMBER 2006 | VOLUME 7 www.nature.com/reviews/neuro ©
200 6
A ubiquitous protein present in all eukaryotes (but absent from prokaryotes). As part of the ubiquitin–proteasome complex, ubiquitin binds and labels proteins to be proteolytically digested and removed from the cell. The ubiquitin–proteasome system is essential for many cellular processes, including cell cycling, signal transduction and the regulation of gene expression. Guanine nucleotide exchange factor (GEF). A protein that mediates the exchange of GDP to GTP, catalysed by a GTP-binding protein.
GTPases A large family of enzymes that bind and hydrolyse GTP.
antly cytosolic protein consisting of 153 amino-acids that functions as a homodimer. Each subunit of SOD1 binds one zinc and one copper atom. Through cyclical reduction and oxidation (dismutation) of copper, SOD1 converts the superoxide anion, a by-product of oxida- tive phosphorylation in the mitochondrion, to hydrogen peroxide. About 20–25% of all FALS cases arise because of mutations in SOD1, the protein product of which accounts for 0.1–0.2% of the cellular proteins in the CNS. More than 125 mutations have been identified, span- ning all five exons of SOD1; 114 cause disease, whereas six silent mutations and five intronic variants do not. Although most mutations are missense, 12 are nonsense or deletion mutations that produce a truncated protein 26
(see ALS Online Database in Online links box). These mutations are specific for FALS, and are infrequently found in SALS (~1% of cases). Although most mutations reduce dismutation activity, others retain full catalytic function. There is no clear correlation between enzyme activity, clinical progression and disease phenotype, although the duration of the disease is similar for any given mutation 27–29 .
D90A substitutions in the protein sequence produce distinctive phenotypes. The A4V mutation (the most common SOD1 mutation in North America) is associa- ted with short survival (a mean of about one year) and limited upper motor neuron involvement 30 . In northern Scandinavia, 2–3% of the population is heterozygous for the D90A mutation, which is a benign polymorphism in that population 31 . However, individuals in that region who are homozygous for D90A develop slowly progres- sive motor neuron disease with prominent corticospinal signs and prolonged survival of more than a decade. By contrast, patients who are D90A heterozygotes outside the northern Sweden gene pool develop classic ALS with survival times of 3–5 years. It is proposed that the milder D90A/D90A phenotype in northern Scandinavia reflects the presence either of alleles that are tightly linked to the D90A variant that blunt the toxicity of this mutation, or of neuroprotective alleles throughout the genome in that population. The latter proposal is supported by the observation that another dominantly inherited neuro- degenerative disorder, amyloidotic polyneuropathy, which is caused by the mutation of transthyretin, is less aggressive in Sweden than in other European or Japanese populations 32 .
ALS2 gene comprises 34 exons that encode alsin, a 184 kDa protein 13 . Alsin contains three puta- tive guanine nucleotide exchange factor (GEF) domains, involving Ras, Rab and Ran motifs. GTPases of the
Ras subfamily regulate cellular signalling that couples extracellular signals to intracellular responses regulat- ing vesicle transport and microtubule assembly. Alsin is ubiquitously expressed and is abundant in neurons, where it localizes to the cytosolic portion of the endo- somal membrane 33 .
is known that it acts as an exchange factor for Rab5a in vitro, which regulates endosomal trafficking and Rac1 activity 34,35
. Interestingly, alsin suppresses mutant SOD1-mediated toxicity in immortalized motor neuron cell lines (NSC34) by binding to SOD1 through the RhoGEF domain 35 . Multiple different mutations have been identified in ALS2, including a recently found homozygous missense mutation 36 . Most mutations are Table 1 | Genes that predispose to ALS ALS disease type Gene Chromosome Inheritance Clinical features Mendelian genes ALS1 SOD1
21q22 AD Typical ALS ALS2 ALS2
2q33 AR Juvenile onset, slowly progressive, predominantly corticospinal ALS4
SETX 9q34
AD Adult onset, slowly progressive ALS8 VAPB
20q13 AD Typical ALS ALS Dynactin
2p13 AD Adult onset, slowly progressive, early vocal cord paralysis Mendelian loci ALS5
? 15q15–21
AR Juvenile onset, slowly progressive ALS6 ?
AD Typical ALS ALS7 ?
AD Typical ALS ALS-FTD ?
AD ALS, frontotemporal dementia ALS-FTD ?
AD ALS, frontotemporal dementia ALS-X ?
Typical ALS
Mitochondrial genes ALS-M
COX1 mtDNA
Maternal Single case, predominantly corticospinal ALS-M IARS2
mtDNA Maternal
Single case, predominantly lower motor neuron Variants of the genes encoding for angiogenin, neurofilament light subunit, survival motor neuron and vascular endothelial growth factor have been implicated in sporadic ALS. AD, autosomal dominant; ALS, amyotrophic lateral sclerosis; AR, autosomal recessive; ALS2, the gene that encodes alsin; COX1, cyclooxygenase 1; FTD, frontotemporal dementia; IARS2, mitochondrial isoleucine tRNA synthetase; mtDNA, mitochondrial DNA; SETX, senataxin; SOD1, superoxide dismutase 1; VAPB, vesicle-associated membrane protein-associated protein B; Xcen, centromere on the X chromosome; XD, X-linked dominant. R E V I E W S NATURE REVIEWS |
VOLUME 7 | SEPTEMBER 2006 | 711
©
200 6
A membrane-bound, intracellular oganelle. Endocytotic vesicles derived from the plasma membrane are actively transported to fuse with endosomes; endosomes also fuse with vesicles of the endoplasmic reticulum that contain newly expressed proteins. Dynein A motor protein that converts the chemical energy of ATP into mechanical energy for movement. Dynein transports several cellular cargos along the microtubules. predicted to truncate the protein 12,13 , with the extent of alsin truncation varying with phenotype — alsin is less truncated in patients with milder phenotypes 12,13 . The
findings that all alsin mutants are unstable 37 and that most patients are homozygous for the mutations indi- cate that this form of ALS is caused by a loss of function of alsin. Loss of alsin in mice does not trigger motor neuron degeneration and disease, but does predispose to oxidative stress 38 , and causes age-dependent neurological defects and altered vesicle and endosome
trafficking 39 . Senataxin. Defects in SETX cause an autosomal dominant, juvenile onset motor neuron disease with distal muscle weakness and atrophy, normal sensation, pyramidal signs and a normal life-span. These individu- als have missense mutations in SETX, which encodes a 303 kDa DNA/RNA helicase domain with homology to human RENT1 and IGHMBP2 — two genes that encode proteins involved in RNA processing 15,40 . Altered RNA processing is implicated in two other inherited motor neu- ron diseases — spinal muscular atrophy (with mutations in the survival motor neuron gene) or a severe, infantile, distal spinal muscular atrophy with prominent respira- tory dysfunction ( SMARD
; spinal muscular atrophy with respiratory distress, with mutations in IGHMBP2). VAMP-associated protein B. Defects in this gene cause adult-onset, autosomal dominant ALS and atypical ALS (slowly progressive with tremors) but not frontotempo- ral dementia (for an example, see REF. 41 ). VAPB has six exons that encode a ubiquitously expressed 27.2 kDa homodimer, which belongs to a family of intracellular vesicle-associa ted/membrane-bound proteins that are presumed to regulate vesicle transport. Dynactin. Recently, dominantly transmitted mutations causing adult-onset, slow progressive, atypical motor neuron disorder have been identified in the p150 sub- unit of dynactin, a component of the dynein complex that comprises the major axonal retrograde motor 17 . The mutations in the p150 subunit appear to affect the binding of the dynactin–dynein motor to microtubules. The onset of this form of motor neuron degeneration is often heralded by vocal cord paralysis. Mitochondrial and complex genetics in ALS
drial genes cause motor neuron disorders with clinical features that are suggestive of ALS (TABLE 1) . A 5 bp deletion in the mitochondrial gene cyclooxygenase 1 ( COX1
) results in early adult onset of corticospinal motor neuron loss 42 . By contrast, a 4272T>C mutation in mitochondrial transfer RNA (isoleucine) causes a late onset, slowly progressive, predominantly lower motor neuron disease 43 . Complex genetics in ALS. There have been few stud- ies of genetic variants that modify ALS susceptibility or phenotype. In DNA sets from Sweden, Belgium and Birmingham (UK) 44 , but not London (UK) 44 , Utrecht (The Netherlands) 45 or Boston (USA) (R.H.B., unpublished data), promoter polymorphisms that reduce the expression of vascular endothelial growth factor ( VEGF
) are associated with an increased risk of disease. Variants predicted to reduce the activity of another vascularizing factor, angiogenin ( ANG
), increase ALS risk selectively in Irish and Scottish DNA sets 46 . These
observations support the view that vascularization and, indirectly, blood supply and cellular oxygen pressure are important determinants of motor neuron viability, or that VEGF and ANG exert direct neurotrophic influ- ences on motor neurons. Polymorphisms in the genes that encode the neurofilament heavy subunit 47–49 and the survival motor neuron protein 50,51
are also incriminated as risk factors for development of ALS. Insights into ALS pathogenesis from genetics Our understanding of the pathobiology of ALS is predicated largely on studies of ALS-associated gene mutations. Because the clinical and pathological pro- files of sporadic and familial ALS are similar, it can be predicted that insights from studies of ALS-caus- ing gene mutations apply to sporadic ALS. Most data on ALS pathogenesis are derived from studies of cell death initiated by mutant SOD1 protein. Mutations in SOD1 and, as a result, its protein product initiate motor neuron disease through one or more toxic properties. This is consistent with observations that the inactiva- tion of SOD1 does not cause fulminant motor neuron disease in mice 52 , whereas transgenic mice that overex- press ALS-associated, mutant SOD1 proteins develop motor neuron disease despite having normal or elevated SOD1 activity 53 . Finally, neither age of onset nor rapid- ity of disease progression correlates with SOD1 activity in ALS patients 54 . Two broad hypotheses explain the adverse function of the mutant SOD1 protein (FIG. 1)
: either the mutant protein perturbs oxygen metabo- lism, or the primary problem is mutation-induced conformational instability and misfolding of the mutant peptide. In either case, perturbations of the biophysical properties of the SOD1 protein induce diverse pathogenic phenomena (FIG. 2)
. Aberrant chemistry and oxidative stress.
The first hypothesis above proposes that mutations in SOD1 alter enzyme activities through either aberrant copper catalysis or improper metal binding. This is presumed to be a consequence of alterations in the configura tion of the active channel that allow atypical substrates to interact promiscuously with copper. Mutant SOD1 might accept peroxynitrite 55 (formed by the spontaneous combination of superoxide and nitric oxide) or hydrogen peroxide 56
(the normal product of the first step of the dismutase reaction catalysed by SOD1) as a substrate, and thereby catalyse the nitration of tyrosine residues of SOD1 and hydroxyradicals 55,56 .
protein fails to bind zinc properly, allowing the rapid reduction of the SOD1-bound copper which, in its reduced state, catalyses the formation of superoxide anion rather than dismutation (so-called ‘backward catalysis’) 57 . Diminished metal binding by mutant SOD1 R E V I E W S 712
| SEPTEMBER 2006 | VOLUME 7 www.nature.com/reviews/neuro ©
200 6
1. Decreased chaperone and/or proteasome activity 2. Aberrant protein–protein interactions 2. Tyrosine nitration 3. Reverse catalysis 4. Cu/Zn release H 2
2 OH . SOD1 – Cu + ONOO
– NO-Tyr
SOD1 – Cu + O 2 O 2 . – ΔZn Insufficient clearance of intracellular proteins
Protein toxicity Aberrant redox chemistry Aberrant binding Small protein aggregate Large aggregate SOD1
dimer Unstable
monomer Wild-type SOD1 Mutant SOD1 ∆Zn
Hydrophobic loops
Yüklə 386,38 Kb. Dostları ilə paylaş:
|