Neurodegenerative disorders
Bart Dermaut
Center for Medical Genetics Ghent
Postgraduate Course Human Genetics 12-12-2013
Outline
Introduction
Alzheimer disease (AD)
Triplet repeat disorders
Diseases of mitochondrial DNA (mt DNA)
General introduction
Chronic and progressive disorders
Selective and symmetric loss of neurons
Motor, sensorial and cognitive system
Nosological classification following pattern of
neuronal loss and disease specific cellular markers
AD:
PD:
ALS:
HD:
senile plaques, neurofibrillar ‘tangles’
neuronal loss, acetylcholine deficiency
Lewy bodies, depletion of dopamine
cellular inclusions, axon swelling of
motor neurons
loss of striatal neurons
Martin J.B., NEJM 340:1970-1980 (1999)
General introduction
Causes
Genetic factors
-Mendelian inheritance – monogenic:
rare familial forms of common disorders
classic monogenic e.g. repeat expansion disorder
-Multifactorial - common disorders:
several genes contribute to disease
variation in age of onset and progression point to
different pathogenetic mechanisms
e.g. AD
Environment: toxic or metabolic processes, infection, unknown
Outline
Introduction
Alzheimer disease (AD)
Triplet repeat disorders
Diseases of mitochondrial DNA (mt DNA)
Alzheimer disease
Alois Alzheimer (1864-1915)
Described AD in 1907
http://www.whonamedit.com/doctor.cfm/177.html
2° lic Biomedische Wetenschappen 2006 - 2007
Alzheimer disease
Disease characteristics
• adult-onset slow progressive dementia (memory,
cognition, personality)
• most frequent form of dementia
• >60 y: 5-10%, >85 y: 45%
• 4 mill/y, 100.000 +/y in US, cost 60 miljard US dollar
• 25% of cases familial
- mostly late onset
- < 2% early-onset familial AD (EOFAD)
symptoms always < 65 y
2° lic Biomedische Wetenschappen 2006 - 2007
Alzheimer disease
Clinical features
• dementia, typically begins with subtle and poorly recognized
failure of memory
• slowly becomes more severe and, eventually, incapacitating
• other common symptoms: anxiety, confusion, poor judgment,
language disturbance, agitation, withdrawal, and hallucinations
• occasional symptoms: seizures, Parkinsonian features, increased
muscle tone, myoclonus, incontinence, mutism
• death usually results from general inanition, malnutrition,
pneumonia
• typical clinical duration of the disease: 8-10 yrs range: 1- 25 yrs
2° lic
Biomedischemacroscopic
Wetenschappen 2006
- 2007
• post
mortem:
- microscopic
Alzheimer disease
Near and connected to hippocampus
2° lic Biomedische Wetenschappen 2006 - 2007
Learning processes, short term
memory and conversion to long
term memory in other parts
(olfactory bulb, amygdala,
nucleus basalis)
Alzheimer disease - neuropathology
Diagnosis/testing
histological findings (neuropathological hallmarks) in brain
tissue of ß-amyloid (senile) plaques and intraneuronal
neurofibrillary tangles
2° lic Biomedische Wetenschappen 2006 - 2007
Alzheimer disease - tau
Although tau neurofibrillary tangles appear to be one of the causes of
the neuronal degeneration in AD, mutations in the tau gene are
associated
not withWetenschappen
AD, but with2006
another
2° lic Biomedische
- 2007 autosomal dominant dementia,
FTD
Alzheimer disease - genetics
mid-80s:
• older patients with Down syndrome have
neurofibrillary tangles and senile plaques
• suggests that extra copies of a gene on chromosome
21 induce the pathologic spectrum of AD
finding of a protein fragment, -amyloid, in senile
plaques led to cloning of the -amyloid gene
-amyloid gene (chromosome 21q21.2):
• encodes large protein, amyloid precursor protein
(APP)
• -, - and -secretases (proteases) split APP in
smaller fragments A40 (normal) en A42 (toxic amyloid)
2° lic Biomedische Wetenschappen 2006 - 2007
Alzheimer disease - APP
•
APP: transmembranary protein
-amyloid motif extracellular to the middle of membrane
•
mutations cause increased production of specific
-amyloid fragment (A42)
•
APP mutations in less than 1% of all early-onset cases
2° lic Biomedische Wetenschappen 2006 - 2007
Alzheimer disease - APP
normal AA
AA substitutions
normal function APP:
neuronal survival,
neurite outgrowth,
synaptic plasticity,
cell adhesion
2° lic Biomedische Wetenschappen 2006 - 2007
Alzheimer disease - APP
mutations in APP influence proteolytic - and -secretases
2° lic Biomedische Wetenschappen 2006 - 2007
Alzheimer disease - genetics
• < 1% of EOFAD caused by APP mutations, other genes ???
• linkage to locus 14q in 50% of all EOFAD!!
• positional cloning PS-1 on 14q24.3
• identification PS-2 on 1q31-q42
through sequence homology with PS-1
• mostly missense mutations in PS-1
• two missense mutations in PS-2
• new protein family, transmembranary, 450 AA, 8
transmembranary domains
• genomic organization of PS genes is similar (10 exons)
2° lic Biomedische Wetenschappen 2006 - 2007
APOE gene
apolipoprotein E variant 4
•
•
•
•
•
•
•
4th locus on 19q
involved in late-onset AD
homozygotes develop AD 10-20 years earlier than carriers of 2, 3
mainly produced in astrocytes
uptake in neurons through LDLR
part of LDL particle
apolipoprotein E4 increases deposition of -amyloid
second modifier on chromosome 10
2° lic Biomedische Wetenschappen 2006 - 2007
2° lic Biomedische Wetenschappen 2006 - 2007
CLU and PICALM associated with AD
-CLU (Cx 8): amyloid clearance
-PICALM (Cx 11): function in
synapses
2° lic Biomedische Wetenschappen 2006 - 2007
CLU and CR1 associated with AD
-CLU (Cx 8): amyloid clearance
-CR1 (Cx 1): inflammation
2° lic Biomedische Wetenschappen 2006 - 2007
AD – genetic testing
Presenile
Familial AD
Presenilin 1
gene
(chr 14)
Age: 25-60 y
Senile
Familial AD
Presenilin 2
gene
(chr 1)
Age: 45-84 y
Sporadic
AD
Inheritance of e4 allele
of ApoE
gene
(chr 19)
Age: > 50 y
APP
gene
(chr 21)
Age: 40-65 y
2° lic Biomedische Wetenschappen
2006
- 2007
Other genes
yet
to
be identified
AD – genetic counseling
Genetic counseling
• first degree relatives of individuals with sporadic AD
have about a 20% lifetime risk of developing AD
• presumably, when several individuals in a family have
AD, the risk is further increased
• EOFAD is inherited in an autosomal dominant manner
The risk to offspring of individuals with EOFAD is
50%
2° lic Biomedische Wetenschappen 2006 - 2007
AD genes – therapy
Current (symptomatic) therapy
• cholinergic replacement (anticholinesterase)
Therapies under development
• inhibit of -secretases (PS1)
• inhibit -secretase
• stimulate -secretase
• Block immune response
• Inhibit fibril formation and disaggegate amyloid
- Proteoglycans to inhibit amyloid formation
-2°Immunization
against -amyloid
lic Biomedische Wetenschappen
2006 - 2007
Outline
Introduction
Alzheimer disease (AD)
Triplet repeat disorders
- Class I: non-coding repeat, loss of protein function
- Class II: non-coding repeat, novel protein function
- Class III: coding repeat expansions
Diseases of mitochondrial DNA (mt DNA)
Triplet repeat expansions
5’UTR
coding
3’UTR
Dynamic mutations in NDD
10 hereditary NDD have polyglutamine expansions
different genes but specific neuropathological characteristics: same
pathogenesis ?
new paradigm for genetic disease - anticipation
repeats in non-coding regions of disease genes
3’ UTR in myotonic dystrophy,
5’ in fragile X mental retardation
intronic in Friedreich ataxia (FRDA)
putative antisense sequences in SCA8
CAG repeats in coding regions of following disease genes:
Huntington disease
dentatorubropallidoluysian atrophy (DRLPA)
spinal and bulbar muscular atrophy
SCA 1,2,3,6,7,12 and 17
Dynamic mutations in NDA
often autosomal dominant or X-linked inheritance
FRDA: autosomal recessive
somatic and germline instability
rather prone to expansions than contractions during transmission
increase in severity phenotype associated with repeat expansion
parental origin can influence degree of anticipation
increased risk for anticipation in male carriers
exception: FRAXA, FRDA, MD en SCA8
Triplet repeat disease
Unique for humans (repeat stable in transgenic mice)
Disease
Inher
Triplet
(*)
Location
Gender
bias
Nl
Unstable
Affected
Huntington AD
CAG
exon
pat
<36
27-35
>35
Fragile X
XL
CGG
5’UTR
mat
<60
59-200
(premut)
>200
Myotonic
dystrophy
AD
CTG
3’UTR
mat
<38
38-49
50-2000
(premut)
Friedreich
ataxia
AR
AAG
intron
mat
<34
36-100
>100
(*) 64 possible trinucleotide sequences; however only 10 different ones if you take into
account permutations (CAG)n=(AGC)n=(GCA)n and reading from either strand
5’(CAG)n=5’(CTG)n
Huntington disease (HD)
“On Chorea” 1872 – Huntington’s chorea
Latin ‘choreus’ = dance
Uncontroled, dance-like movements
Chorea not obligate, a lot of other features
Correct name: Huntington disease (HD)
Clinical characteristics
progressive disorder with motor,
cognitive and psychiatric symptoms (www.genetests.org)
first signs mostly in 4° decade
movement- and behavioural changes
progressive chorea, abnormal eye movements
Huntington disease (HD)
Clinical characteristics
end stage: dementia, personality changes,
irritability, depression, hearing loss
death due to swallowing problems, infection, suicide…
(15-18 y after ‘onset’)
juvenile form, first symptoms before age of 20
Huntington disease (HD) – basal ganglia
1. Gyrus cinguli
2. Gyrus frontalis superior
3. Gyrus frontalis medius
4. Gyrus frontalis inferior
5. Corpus callosum
6. Cornu frontale ventriculi
lateralis
7. Nucleus caudatus
8. Capsula interna
9. Putamen
10. Polus temporalis
11. Septum pellucidum
12. Corpus striatum
13. Arteria cerebri media
14. Gyri orbitales
http://www.neuropat.dote.hu/anastru/anastru.htm
Huntington disease (HD)
From disease to gene: a long journey…
Linkage
IT15 gene
HD – IT15 gene
CAG triplet repeat expansion in coding region, exon 1
10-26 CAGs : normal
27-35 CAGs : intermediary allele, carrier normal, next generation
‘at risk’
36-39 CAGs : ‘reduced penetrance’, check family history
> 40: 40-150 glutamine residues: HD
>70 repeats: juvenile HD (children, adolescents)
protein: 348 kDa, huntingtin, ubiquitously expressed
anticipation: increase of repeats and severity disease
in successive generations
HD – genetic testing
Slipped mispairing mechanism
Gain-of function due to CAG repeat mutations
Pathogenesis: toxic gain-of-function due to nuclear inclusions
Fragile-X syndrome
frequent cause of X-linked mental retardation (males)
clinical diagnosis: long face with prominent ears, chin,
ogival palate; macroorchidism, mild joint hyperlaxity.
Behavioural changes in children. MR milder in women
than men
1 op 4.000 males. 30% of female mutation carriers: mild
to moderate retardation (1/8.000)
“fragile site”: chromatine does not condense adequately
during meiosis (visible when X chromosome cultured in
folium-deficient medium)
Fragile-X syndrome
FMR1-gene (Fragile-X Mental Retardation-1) (Xq27.3)
expanding CGG-repeat in 5’ UTR (promotor region) FMR1
normal alleles: 6-43 CGG
premutation alleles: 59-200 CGG.
-premutation carriers: normal intelligence
full mutation: > 200 CGG
-hypermethylation of FMR1 promotor, leading to loss of function
-male mutation carriers: Fragile-X syndrome
-females: 30% mild to moderate mental retardation and 50%-70% IQ <
85
deletion or point mutation in FMR1, also leading to loss of function
FXTAS: Fragile X associated Tremor-Ataxia syndrome
FXTAS
-25-30% of male carriers premutation: FXTAS
-> 50 years
-intentional tremor
-ataxia
-Parkinson-like manifestations
-MRI: white matter lesions in cerebellum
Fragile-X syndrome vs FXTAS
Fragile-X syndrome
meiotic instability: maternal
transmission
premutation > mutation: in female
premutation carriers
risk of expansion premutation to
mutation: ~ number CGG-repeats
male premutation carriers: transmission
premutation to daughters without
expansion (normal transmitting males or
NTM)
Sherman paradox (anticipation)
Fragile X syndrome – genetic testing
CGG-repeats meiotically and mitotically unstable
Myotonic dystrophy (MD1, Steinert disease)
• multisystem disorder
• multi-organ involvement: muscles, eye, heart, endocrine
system, CNS
• variable expression: mild to severe (congenital)
mild:
classic:
cataract and mild myotonia
muscle weakness and dystrophy,
myotonia, cataract, cardiac conduction
abnormalities
congenital: severe hypotonia and muscle weakness
neonatal respiratory insufficiency
with early death
mental retardation
Myotonic dystrophy
CTG expansion in 3’ UTR of DMPK gene: genotype-phenotype correlation
Myotonic dystrophy
CTG expansion in 3’ UTR of DMPK gene: genotype-phenotype correlation
mild form
60 CTG
adult form
150 CTG
adult form
180 CTG
congenital form
2000 CTG
juvenile form
500 CTG
Myotonic dystrophy- genetic testing
Small expansions: PCR; large expansions: Southern blot
Outline
Introduction
Alzheimer disease (AD)
Triplet repeat disorders
Diseases of mitochondrial DNA (mt DNA)
Diseases of mt DNA
mitochondrial genome (mt DNA): circular, ds
16.5 kb, intronless, 37 genes, transcription
in mitochondria
-13 encode enzymes < 5 enzyme complexes
involved in oxydative phosphorylation and
apoptosis
-22 encode tRNAs and 2 rRNA’s necessary for
synthesis of these enzymes
energy supply (ATP) of nearly all systems
dependent of oxydative phosphorylation
-mutations in mitochondrial genome: a wide
variety of symptoms (pleiotropy)
-affect mainly organs that are highly energydependent: brains, skeletal muscles, eye (retina),
ears (inner ear), kidney, heart
Diseases of mt DNA
one mitochondrion: several mt DNA-molecules; most cells: more than
1000 mt DNA-molecules divided over 100 mt; in mature oocytes:
number is higher
mt DNA: higher mutation frequency than nuclear genome (10 x)
cause: no repair mechanisms
inheritance: maternal (mt DNA exclusively maternally transmitted)
- female with mutation in mt DNA: transmitted to 100% of offspring
- male: no transmission of mt DNA mutations
Diseases of mt DNA
mt DNA mutation: in 1 mt DNA-molecule
mitochondrial division: replication of each mt DNA-molecule. At random
division of new mt DNA-molecules over new organels
cell division: at random division of mitochondria over daughter cells
homoplasmy: cell contains same mt DNA-molecules (normal or
mutant mt DNA)
heteroplasmy: cell contains mixed mt DNA-molecules (normal
and mutant)
proportion normal/mutant underlies phenotypic expression
and variability of mt DNA disorders
Genotype-phenotype correlations
Mutations in tRNA genes: MELAS
Mitochondrial Encephalopathy Lactic Acidosis and Stroke-like lesions
Mutation in mitochondrial tRNALeu (3243)
Mutations in tRNA genes: MELAS and MERFF
mutations leading to MELAS
mutation leading to MERFF
AD genes – therapy