Transcript mg-1011-neurodisorder-genetics

Lecture 11
Genetics of Neurological Disorders
Parkinson and Alzheimer's
Epidemiology of Parkinson Disease
1-2% of population over age 65 years
 85% sporadic, 10-15% familial clustering and <5%
monogenic inheritance
 Advancing age is important risk factor
 Twin studies report similar concordance of 1020% for monozygotic and dizogtic twins.

The -Synuclein gene 1
Autosomal dominant inheritance. Located on
chromosome 4q21-q23
 -Synuclein is a 140 amino acid length
presynaptic protein found in abundance in the
human brain, particularly in the substantia nigra,
hypothalamus and olfactory neurons.
 It is an unfolded protein in solution but assumes
an alpha helical configuration within lipid
containing vesicles and in high concentrations
may aggregate into beta sheets typical of amyloid
fibrils.

Ubiquitin-proteasome system
-Synuclein
is normally degraded by the
ubiquitin-proteasome system, a pathway that
clears unwanted cytotoxic proteins from neuronal
cells.
 The ubiquitin system consists of three enzymes –
a ubiquitin activating enzyme (E1) that binds
ubiquitin molecules and sequentially transfers
them to a ubiquitin conjugating enzyme (E2) and
a ubiquitin ligase (E3). E3 is attached to a target
protein that in turn becomes polyubiquinated
enabling it to undergo protealysis by a 26S
proteasome.

-Synuclein and Lewy bodies
A possible mechanism of neurotoxicity from
mutations of the -synuclein gene is the
production of proteins that are more prone to
self-aggregation or alternatively the production
misfolded proteins that cannot be degraded.
 -Synuclein is a major constituent of Lewy bodies.
Two opposing theories for a Lewy body: (i) toxic
aggregation of proteins that contributes directly
to neuronal death, and (ii) protective aggregation
that ‘clears’ excess unfolded or misfolded synuclein

-Synuclein gene mutations
 Two mutations have been identified: (i) a
G209A substitution in exon 4 resulting in an
Ala53Thr mutation which has been found in
at least 13 Italian-Greek families including
one Australian family of Greek origin, and
(ii) G88C substitution in exon 3 resulting in
an Ala30Pr mutation found in a single
German family.
Parkin



465 amino acid protein that contains a ubiquitin
homologous domain in its amino-terminus and two
RING finger domains in its carboxy-terminus. Proteins
with RING finger domains have a ubiquitin ligase
function, thus linking parkin to the ubiquitinproteasome system.
It is postulated that parkin interacts with substrate
proteins and by acting as a ubiquitin ligase is involved
in their degradation.
Parkin is postulated to interact with a novel
glycosylated isoform of -Synuclein. Mutated parkin
cannot bind and hence this -Synuclein isoform
accumulates.
Parkin gene mutations
Autosomal recessively inherited mutations of the parkin
gene on chromosome 6q25.2-q27.
 Pathological hallmarks of autosomally recessive earlyonset Parkinson’s disease due to parkin mutations include
loss of nigral neurons and the absence of Lewy body
formation.
 The lack of Lewy bodies is consistent with an inability of
the ubiquitin-proteasome pathway to form ubiquinated
aggregates of -synuclein
 A number of different homozygous point mutations, gene
deletions and multiplications have been detected in
patients with parkin gene mutation.
 There are no major clinical differences between the
different types of mutations, that is, parkin cases do not
represent a phenotypically distinct group.

Ubiquitin carboxy-terminal hydrolase-L1 (UCHL1) gene
mutation
Ubiquitin carboxy-terminal hydrolase-L1 is a deubiquitinating enzyme that hydrolyses the Cterminal of ubiquitin to generate ubiquitin
monomers that can be reutilized for further
“target” protein clearance.
 Gene located on chromosome 4p14-15.1.
 Autosomal dominantly inherited mutation were
identified in 2 German siblings.
 GBA and UCHL1 Very rare.
 beta-glucocerebrosidase

Other genes
Other loci include PARK 3:- chromosome 2p13,
PARK 4:- chromosome 4p14-16.3, PARK 6:–
chromosome 1p35-36, PARK 7:– chromosome
1p35-36, PARK 8: AD - chromosome 12p11.2q13.1
 Linkage to chromosomes 17q and 9q has also
been found in families with late onset Parkinson’s
disease.

Genetic polymorphisms
Many genetic loci have variations at a nucleotide
site in normal individuals.
 A polymorphism is defined as one at which the
most common gene variation (or allele) occur.
 A number of candidate genes have been
investigated for an association with Parkinson’s
disease including cytochrome P450 1A1 and 2D6,
N-acetyltransferase 2 (NAT2), monoamine
oxidase-B (MAO-B), the dopamine transporter
gene (DAT) and glutathione s-transferase M1.

NAT2

NAT2, which maps to chromosome 8p22, is another
polymorphic gene associated with drug and xenobiotic
metabolism.

Approximately 50% of Caucasians are slow acetylators.

Increased frequency of the two most common slow
acetylation gene polymorphisms NAT2*5B and NAT2*6A
in one study of Caucasian patients with early onset (age
<50 years) onset Parkinson’s disease although not
associated in older patients in this study and not
replicated in a Netherlands study.
MAO-B



Monoamine oxidase B (MAO-B) is a candidate gene in PD
by virtue of its role in the metabolism of dopamine and
conversion of MPTP to the active neurotoxic metabolite 1methyl-4-phenylpyridinium ion (MPP+).
MAO-A and MAO-B are two distinct isoforms of MAO
encoded by different genes on the X-chromosome.
An Australian study found an association between PD and
a polymorphic GT repeat sequence (normal range 168 to
190 base pairs) in intron 2 with longer repeat units (186
and 188 base pairs) significantly associated with PD.
DAT – dopamine transporter gene



Located on chromosome 15p15.3. Involved in the
presynaptic uptake of dopamine by dopaminergic
neurons. Can transport toxins, eg MPTP, into substantia
nigra.
The 3’-untranslated region of the gene contains a 40 base
pair variable number tandem repeat and accounts for
approximately 90% of alleles in Chinese subjects and 70%
in Caucasian and black populations.
A rare other copy of allele has been reported to increase
the risk of PD in Caucasians. ? significance as found in only
0.25% of normal Caucasians
Other candidate gene polymorphisms

Glutathione-S-transferases involved in metabolism of
pesticides; dopamine D2 and D4 receptor genes; ACE
gene; Nurrl gene on chromosome 2 which increases
transcription of DAT and tyrosine hydroxylase;
mitochondrial gene tRNA and APOE.
The interaction of gene and environment


Caucasian studies have shown that genetic polymorphism
of MAO-B modifies the association of smoking and PD in
that smoking may increase the risk of association with PD
in one genotype but may reduce the risk in another.
Similarly, glutathione transferase polymorphisms interact
with pesticide in increasing the risk for PD.
Epilepsy
Genetic causes of epilepsy
• Microscopic: strong polygenic dysfunction
• Chromosomal aberations
• Angelman/ Prader-Willi syndrome – 15q11-13 region (UBE3A
gene defect and likely many other genes)
• Submicroscopic: monogenic-plus defects
• small deletions, insertions, point mutations, etc.
Genetic causes of epilepsy:
Submicroscopic genetic defects
1. Defects of neuronal metabolism
2. Defects of network development
3. Defects of membrane and synaptic signaling
Genetic epidemiology of epilepsy
> Genetic mutations are linked to epilepsy
1%: familial: single gene with major effect + genetic
and environmental modifiers
99%: sporadic: polygenic (many genes with variable
degree of effect + other modifiers)
Genetic epidemiology of epilepsy
Febrile seizures
• 3% baseline population prevalence in children 6 months – 6 years
old
• Risk factors for familial recurrence of FS
• Affected sib: 8 – 12% risk (RR = 3-5)
• Multiple affected family members: < 50%
• FS and risk of epilepsy later in life
• 4% at 7 years
• 7% at 25 years
Summary
Genetically identified facts about epilepsy
Genetics is believed to be involved in the majority of cases, either directly or indirectly.
Single gene defect (1–2%); most are due to the interaction of multiple genes and
environmental factors.
Most genes involved affect ion channels such as enzymes, GABA, and G proteincoupled receptors.
Monzoygotic twins (50–60%) chance that the other will also be affected.
Dizygotic twins the risk is 15%.
Between 1 and 10% of those with Down syndrome and 90% of those with Angelman
syndrome have epilepsy.
Alzheimer's Disease
ALZHEIMER’S DISEASE(AD)
AD was first described by Alois Alzheimer in 1906 , by analyzing two
stains section from Augustine Deter’s brain (Alzheimer’s first
patient).
AD is an irreversible, progressive neurodegenerative disorder
characterized by the presence of lesions both at an extracellular
level (the b-amyloid plaques), and at an intracellular levels (the
Neurofibrillary tangles, NFT).
This neuropathological hallmark for the disease correlates with both
reduction in the volume of the brain (as a consequence of the
neurodegenerative phenomena), and with cognitive decline
associated to loss of memory.
TYPES OF ALZHEIMER’S DISEASE
1.
Early-onset AD: Very rare form, diagnosed before age 65 (≤10%), Because they
experience premature aging, people with Down syndrome are particularly at
risk for a form of early onset Alzheimer's disease.
Early-onset Alzheimer's appears to be linked with a genetic defect on
chromosome 14, to which late-onset Alzheimer's is not linked. A condition
called myoclonus -- a form of muscle twitching and spasm -- is also more
commonly seen in early-onset Alzheimer's than in late-onset Alzheimer's.
1.
Late-onset AD: This is the most common form of Alzheimer's disease,
accounting for about 90% of cases, and usually occurs after age 65. Late-onset
Alzheimer's disease strikes almost half of all people over the age of 85. Lateonset dementia is also called sporadic Alzheimer's disease.
2.
Familial AD (FAD). This is a form of Alzheimer's disease that is known to be
entirely inherited. In affected families, members of at least two generations
have had Alzheimer's disease. FAD is extremely rare, accounting for less than
1% of all cases of Alzheimer's disease. It has a much earlier onset (often in the
40s).
PET
MRI
Normal
AD
ANATOMY OF THE ALZHEIMER’S DISEASE BRAIN.
Figure 3: BRAIN CROSS-SECTIONS OF BOTH NORMAL AND ALZHEIMER’S.
Source: Fall, 2010.
NEUROPATHOLOGY AND BIOCHEMISTRY OF AD
 Alzheimer's disease is characterised by loss of neurons and
synapses in the cerebral cortex and certain subcortical
regions.
 This loss results in degeneration in the temporal lobe and
parietal lobe, and parts of the frontal cortex.
 Reductions in the size of specific brain regions in patients as
they progressed from mild cognitive impairment to
Alzheimer's disease
 Enzymes act on the APP (amyloid precursor protein) and cut it
into fragments. The beta-amyloid fragment is crucial in the
formation of senile plaques in AD.
 Plaques are made up of small peptides, 39–43 amino acids,
called beta-amyloid (also written as a-beta or aβ).
Fragment from a larger protein called APP, a
transmembrane protein that penetrates through the
neuron's membrane.
 In Alzheimer's disease, an abnormal aggregation of the tau
protein lead to the disintegration of microtubules in brain cells.
 Tau protein(microtubule-associated protein) stabilizes the
microtubules when phosphorylated.
 In AD, tau becoming hyperphosphorylated; creating
neurofibrillary tangles and disintegrating the neuron's
transport system.
RISK FACTORS OF ALZHEIMER’S DISEASE
The major risk factors of AD are;
1. Ageing
2. Hereditary
 Amyloid Precursor proteins (APP)
 Members of the secretase complex, like Presenilin 1
(PS1) and Presenilin 2 (PS2)
 ApoE e4 allele polymorphism
3. Environmental Factor such as
 head injuries
 hormonal changes
 vascular diseases
 inflammation
 exposure to metals, like Cu++ and Zn++
Genetics of AD
 Depending on the kind of mutation and on the gene that carries it, the
onset of the disease is SIGNIFICANTLY EARLIER than in sporadic cases
(before 65 years of age).
 All the mutations are autosomal dominant, and involve :APP protein,
Members of the g-secretase complex, like Presenilin 1 (PS1) and
Presenilin 2 (PS2) and ApoE e4 allele polymorphism
AMYLOID PRECURSOR PROTEINS(APP)
 Beta-amyloid is a fragment from a larger protein called amyloid
precursor protein (APP), a transmembrane protein that penetrates
through the neuron's membrane.
 APP is critical to neuron growth, survival and post-injury repair. In
Alzheimer's disease, an unknown process causes APP to be divided into
smaller fragments by enzymes through proteolysis.
 One of these fragments gives rise to fibrils of beta-amyloid, which form
clumps that deposit outside neurons in dense formations known as
senile plaques.
When APP is cleaved by βand γ-secretase enzymes,
neurotoxic Aβ peptides are
released, which can
accumulate into oligomer
aggregates.
Small size allows them to
diffuse into the synapse,
impairing synaptic function
between neurons.
Aggregate into insoluble βsheet amyloid fibrils, which
can trigger a local
inflammatory response,
leading to oxidative stress
and biochemical changes
which cause death of
neurons and development of
plaques
Mutations on APP
 Chromosome 21. Relation to Down’s syndrome (Trisomy 21; excess of APP
transcript induces plaque formation already at early age, and progresses with
aging).
 Swedish mutation: K(Lys)670M(Met)/N(Asn)671L(Leu), facilitates the
amyloidogenic processing of APP by increasing the affinity for and the cleavage
by b-secretase of ~100 times.
 Dutch Mutation: E(Glu)693Q(Gln), induces cerebral amyloidosis
 Arctic Mutation:E(Glu)693G(Gly) enhances b-Amyloid protofibril formation
 London Mutation: V(Val)717I(Iso), Affects the cleavage of g-secretase
K670M/N671L
Swedish Mutation
E693G
Arctic Mutation
V717I
London Mutation
AMYLOID PRECURSOR PROTEINS (APP) CONT’D
 AD is also considered a tauopathy due to abnormal aggregation of the tau protein. Every
neuron has a cytoskeleton, an internal support structure partly made up of structures
called microtubules.
 These microtubules act like tracks, guiding nutrients and molecules from the body of the
cell to the ends of the axon and back. A protein called tau stabilizes the microtubules
when phosphorylated, and is therefore called a microtubule-associated protein.
 In AD, tau undergoes chemical changes, becoming hyperphosphorylated; it then begins
to pair with other threads, creating neurofibrillary tangles and disintegrating the
neuron's transport system.
Mutations on Presenilins
PS1 (on chromosome 14) PS2 (on chromosome 1) are ~450 aa long aspartyl proteases
comprised of 7- to 8- transmembrane spanning domains. They are the catalytic part of a
tetrameric protein complex called g-secretase, able to cleave APP at the end of the
sequence for b-Amyloid peptides, generating b-Amyloid species.
More than 70 mutations on PS1 gene account for inherited AD.
Two mutations on PS2 gene account for inherited AD.
APOLIPOPROTEIN E (APOE)
Apolipoprotein E (APOE) found on chromosome 19 appears to be a
predisposing genetic risk factor for the late on-set of AD the most
typical AD.
APOE helps carry cholesterol in the bloodstream.
APOE comes in several different forms, or alleles.
Three forms—APOE ε2, APOE ε3, and APOE ε4—occur most
frequently.
AGING
 This is the major risk factor of the late-onset type of AD.
 Ageing is not adaptation or genetically programmed perhaps by biological timetable
which regulate growth and development. There are two of the various theories of
ageing that fully support the risk factor of ageing leading to AD:
1. Immunological theory stipulates that the immune system is programmed to
decline over time which leads to increased vulnerability to infectious diseases
and then death.
2. Cross-linking theory which states that accumulation of cross-linked proteins
damages cells and tissues, slowing down bodily process.
 Telomerase theory could also be linked to this. Due to inadequate amount of
telomerase in the stem cells, the cell shortens as it divides and with time this could
lead to AD
 This stage of disease is sporadic and is the common form of dementia accounting for
over 60% of the population of Alzheimic patients.
 Dementia is the loss of intellectual functions (such as thinking, remembering, and
reasoning) of sufficient severity to interfere with a person’s daily functioning.
Current Immunotherapy for Alzheimer's Disease
The administration of Aβ antigens (active vaccination) or anti-Aβ antibodies
(passive vaccination) stimulates Aβ clearance from the Alzheimer's disease brain
and represents the most innovative approach of anti-Alzheimer's disease therapy.
However, an active anti-Aβ vaccine (AN1792) has been discontinued because it
caused meningoencephalitis in 6% of Alzheimer's disease patients treated. Among
passive immunotherapeutics, two Phase III clinical trials in mild-to-moderate
Alzheimer's disease patients with bapineuzumab, a humanized monoclonal
antibody directed at the N-terminal sequence of Aβ, were disappointing. Another
antibody, solanezumab, directed at the mid-region of Aβ, failed in two Phase III
clinical trials in mild-to-moderate Alzheimer's disease patients. A third Phase III
study with solanezumab is ongoing in mildly affected Alzheimer's disease patients
based on encouraging results in this subgroup of patients. Second-generation active
Aβ vaccines (ACC-001, CAD106, and Affitope AD02) and new passive anti-Aβ
immunotherapies (gantenerumab and crenezumab) are being tested in Alzheimer's
disease patients, in presymptomatic individuals with Alzheimer's disease-related
mutations, or in asymptomatic individuals at risk of developing Alzheimer's disease
to definitely test the Aβ cascade hypothesis of Alzheimer's disease.
CONCLUSION
The relationship between Aβ protein and Tau Protein is still little
understood.
Scientific research is essential in answering all related questions.
AD is better known in its complexity and new avenues raise real hope
for the eradication of this devastating disease.