Neurodegenerative disorders

Postgraduate Course Human Genetics 13-11-2015

Bart Dermaut

Center for Medical Genetics Ghent

Outline

Triplet repeat disorders

Alzheimer disease (AD)

Introduction

Diseases of mitochondrial DNA (mt DNA)

General introduction

AD: senile plaques, neurofibrillary ‘tangles’ neuronal lossPD: Lewy bodies, depletion of dopamineALS: cellular inclusions, axon swelling of motor neurons HD: nuclear inclusions, loss of striatal neurons

Martin J.B., NEJM 340:1970-1980 (1999)

Chronic and progressive disorders

Progressive and selective loss of neuronsMotor, sensorial and cognitive system

Nosological classification following pattern of neuronal loss and disease specific cellular markers

General introduction

Genetic factors

Environment: toxic or metabolic processes, infection, unknown

-Mendelian inheritance – monogenic: rare familial forms of common disordersclassic monogenic e.g. repeat expansion disorder

-Multifactorial - common disorders: several genes contribute to diseasevariation in age of onset and progression point to

different pathogenetic mechanismse.g. AD

Causes

Outline

Triplet repeat disorders

Alzheimer disease (AD)

Introduction

Diseases of mitochondrial DNA (mt DNA)

Prevalence strongly increases with age

70% are Alzheimer’s disease cases(860,000 cases in France in 2005)

Alzheimer’s disease (AD) => characterised in the brain by :

Neurofibrillary degeneration Amyloid deposition

0

10

20

30

40

60 70 80 90 100

Age (years)

Cas

es /

100

CVD

Dementia

Parkinson

0

10

20

30

40

60 70 80 90 100

Age (years)

Cas

es /

100

CVD

Dementia

Parkinson

Intraneuronal accumulation of hyperphosphorylated Tau

Extracellular accumulation of amyloid peptides

Alzheimer disease

Alzheimer disease

http://www.whonamedit.com/doctor.cfm/177.html

Alois Alzheimer (1864-1915)Described AD in 1907

Alzheimer A. Über eine eigenartige Erkrankung der Hirnrinde Allgemeine Zeitschrift fur Psychiatrie und Psychisch-gerichtliche Medizin. 1907 Jan ; 64:146-8.

“Die Sektion ergab ein gleichmäßig atrophisches Gehirn ohne makroskopische Herde. Die größeren Hirngefäße sind arteriosklerotisch verändert.An Präparaten. die mit der Bielschowskyschen Silbermethode angefertigt sind, zeigen sich sehr merkwürdige Veränderungen der Neurofibrillen. Im Innern einer im übrigen noch normal erscheinenden Zelle treten zunächst eine oder einige Fibrillen durch ihre besondere Dicke und besondere Imprägnierbarkeit stark hervor. Im weiteren Verlauf zeigen sich dann viele nebeneinander verlaufende Fibrillen in der gleichen Weise verändert. Dann legen sie sich zu dichten Bündeln zusammen und treten allmählich an die Oberfläche der Zelle. Schließlich zerfällt der Kern und die Zelle, und nur ein aufgeknäueltes Bündel von Fibrillen zeigt den Ort, an dem früher eine Ganglienzelle gelegen hat.Da sich diese Fibrillen mit anderen Farbstoffen färben lassen als normale Neurofibrillen, muß eine chemische Umwandung der Fibrillensubstanz stattgefunden haben. Diese dürfte wohl die Ursache sein, daß die Fibrillen den Untergang der Zelle uberdauern. Die Umwandlung der Fibrillen scheint Hand in Hand zu gehen mit der Einlagerung eines noch nicht näher erforschten pathologischen Stoffwechselproduktes in die Ganglienzelle. Etwa 1/4 bis 1/3 aller Ganglienzellen der Hirnrinde zeigt solche Veränderungen. Zahlreiche Ganglienzellen, besonders in den oberen Zellschichten. sind ganz verschwunden.”

“Über die ganze Rinde zerstreut, besonders zahlreich in den oberen Schichten, findet man miliare Herdchen, welche durch Einlagerung eines eigenartigen Stoffes in die Hirnrinde bedingt sind. Er läßt sich schon ohne Färbung erkennen. ist aber Färbungen gegenüber sehr refractär.”

Amyloid plaques

Tau tangles

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

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

• post mortem: macroscopic - microscopic

Alzheimer disease

Near and connected to hippocampus 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

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 with AD, but with another 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 senileplaques led to cloning of the -amyloid gene

-amyloid gene (chromosome 21q21.2): • encodes large protein, amyloid precursor protein

(APP) -, - and -secretases (proteases) cleave APP into

smaller fragments A40 (normal) en A42 (toxic -amyloid)

Alzheimer disease - APP

• APP: transmembrain protein -amyloid motif extracellular to the middle of membrane

• mutations cause increased production of specific -amyloid fragment (A42) • APP mutations in less than 1% of all early-onset cases

Alzheimer disease - APP

normal AA

AA substitutions

normal function APP: neuronal survival, neurite outgrowth,synaptic plasticity,cell adhesion

Alzheimer disease - APPmutations in APP influence proteolytic - and -secretases

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• rare missense mutations in PS-2

• new protein family, transmembranary, 450 AA, 8 transmembranary domains

• genomic organization of PS genes is similar (10 exons)

Alzheimer disease - genetics

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

2° lic Biomedische Wetenschappen 2006 - 2007

AD – genetic testing

Other risk genes recently identified

PresenileFamilial AD

SenileFamilial AD

Sporadic AD

Presenilin 1gene

(chr 14)Age: 25-60 y

APPgene

(chr 21)Age: 40-65 y

Presenilin 2gene

(chr 1)Age: 45-84 y

Inheritance of e4 alleleof ApoE

gene(chr 19)

Age: > 50 y

Meta-analysis: 74.000 individuals!Consortium: > 200 authors, >250 collaborators!

Lambert et al, Nat Genet 2013

AD risk loci

Clear! Unclear!

AD risk loci: causal gene, causal variant?

Genetics of Alzheimer’s disease: a paradigm

APP, PS1 PS2

ABCA7, APOE, BIN1, CASS4, CD2AP, CELF1, CLU, CR1, EPHA1, FERMT2, HLA-DRB1, INPP5D, MEF2C, MS4A6A, NME8, PICALM, PTK2B, SLC24A4, SORL, ZCWPW1

TREM2

APOE

Less than 1% of the cases are monogenic forms.

The majority of other forms are defined as without obvious mendelian inheritance. The genetic attributable risk has been estimated between 60 and 80% and to date, 22 loci have been associated with AD risk.

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%

AD genes – therapy

Current (symptomatic) therapy

• cholinergic replacement (cholinesterase inhibitors)

Therapies under development

• inhibit of -secretases (PS1)

• inhibit -secretase

• stimulate -secretase

• Inhibit fibril formation and disaggregate amyloid

-Immunization against -amyloid

Outline

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

Alzheimer disease (AD)

Introduction

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 retardationintronic in Friedreich ataxia (FRDA) putative antisense sequences in SCA8

CAG repeats in coding regions of following disease genes: Huntington diseasedentatorubropallidoluysian atrophy (DRLPA)spinal and bulbar muscular atrophySCA 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 carriersexception: FRAXA, FRDA, MD en SCA8

(*) 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

Disease Inher Triplet(*)

Location Genderbias

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

(premut)

50-2000

Friedreich ataxia

AR AAG intron mat <34 36-100 >100

Unique for humans (repeat stable in transgenic mice)

Triplet repeat disease

Huntington disease (HD)

“On Chorea” 1872 – Huntington’s choreaLatin ‘choreus’ = dance Uncontroled, dance-like movements

Chorea not obligate, a lot of other featuresCorrect name: Huntington disease (HD)

Clinical characteristics progressive disorder with motor, cognitive and psychiatric symptoms (www.genetests.org)

first signs mostly in 4° decademovement- 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

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) – basal ganglia

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 : normal27-35 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

Pathogenesis: toxic gain-of-function due to nuclear inclusions

Gain-of function due to CAG repeat mutations

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 in 4.000 males. 30% of female mutation carriers: mild to moderate retardation (1/8.000)

“fragile site”: chromatin 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) FMR1normal 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)

Myotonic dystrophy (MD1, Steinert disease)

• multisystem disorder

• multi-organ involvement: muscles, eye, heart, endocrine system, CNS

• variable expression: mild to severe (congenital)

mild: cataract and mild myotoniaclassic: muscle weakness and dystrophy,

myotonia, cataract, cardiac conduction abnormalitiescongenital: 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

mild form60 CTG

adult form150 CTG

congenital form2000 CTG

adult form180 CTG

juvenile form500 CTG

CTG expansion in 3’ UTR of DMPK gene: genotype-phenotype correlation

Outline

Triplet repeat disorders

Alzheimer disease (AD)

Introduction

Diseases of mitochondrial DNA (mt DNA)

Diseases of mt DNA

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 energy-dependent: brains, skeletal muscles, eye (retina), ears (inner ear), kidney, heart

mitochondrial genome (mt DNA): circular, ds

- 900 gene products- mtDNA: 37 genes, no introns, very compact, 16.6 kb, 22 tRNAs, 2 rRNAs, 13 subunits of the RC- each cell contains hundreds to thousands of mitochondria, 3-10 genomes per mitochondrion

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

homoplasmy: cell contains same mt DNA-molecules (normal or

mutant mt DNA)

heteroplasmy: cell contains mixed mt DNA-molecules (normal

and mutant)

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

Diseases of mt DNA

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