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Amyotrophic lateral sclerosis



 (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

)

14,15



, 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

 

Nature Publishing Group

 

 

Ubiquitin



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.

Superoxide dismutase 1. SOD1 is a ubiquitous, predomin-

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

.

The missense mutations that lead to the A4V and 



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

.

Alsin. The 



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

.

The function of alsin is not fully understood, but it 



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

?

16q12



AD

Typical ALS

ALS7

?

20p13



AD

Typical ALS

ALS-FTD

?

9q21–22



AD

ALS, frontotemporal dementia

ALS-FTD

?

9p



AD

ALS, frontotemporal dementia

ALS-X

?

Xcen XD



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 

|

 

NEUROSCIENCE



 

 VOLUME 7 

|

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|

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Endosome



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

Mitochondrial gene defects. Defects in two mitochon-

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

.

The second hypothesis proposes that mutant SOD1 



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



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Nature Publishing Group

 

 

1.  Peroxidation



1.  Decreased chaperone 

and/or proteasome 

activity

 2.  Aberrant protein–protein

 interactions

2.  Tyrosine

nitration

3.  Reverse

catalysis

4.  Cu/Zn release

H

2

O



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



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