Huntington’s Disease
Parmvir Singh
4/14/2010
What is Huntington’s Disease?
• Huntington’s Chorea or Huntington’s Disorder
• A genetic disorder[5]
• 1-3% of patients have no familial history of the
disease[7]
• Progressive neuro-degenerative disorder
• 30,000 impacted Americans
• Age of onset varies: usually 30-50 yrs
• Before age 20: Juvenile(10%)[5]
• Very “individualized” disorder (10-20yrs)
What Causes Huntington’s
Disease?
• Autosomal dominant mutation on chromosome 4
with complete penetrance[5]
• Does not skip generations
• Huntingtin gene encodes huntingtin protein
• Normal huntingtin protein becomes mutant
huntingtin protein
• PolyQ expansion from CAG repeats[5]
• Genetically programmed death (apoptosis) of
neurons in the brain
Symptoms of Huntington’s
Disease
• Emotional instability
• Personality change
• Loss of intellectual capabilities (learning,
memory, etc.)
• Uncontrolled movements (chorea)
• Dementia
• Loss of motor skills
• Anxiety
• Paranoia
• Etc.[5][7]
How is Huntington’s Disease
Diagnosed?
• Pre-natal testing is available (chronic villus)
• Look at family history to determine who is at risk
• Physical and neurological exam after symptoms start
to occur[3]
• Cannot predict when onset will occur
• A blood test
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DNA from blood is analyzed to determine # of repeats
<30 repeats will not develop disease
35-40 may or may not develop
>70 likely to develop juvenile-onset form of the disease[3]
Social Impact of Huntington’s
Disease
• Costs associated with care-giving
• Decision to get tested
– Inevitability
– What do you think when you see someone with a
neuro-degenerative disorder?
• Risk of having kids
– Coin toss
– Age of onset
Nancy Wexler and Discovery of
CAG repeats
http://ethosgeek.files.wordpress.com/2
009/04/f_r31_nancywexler1.jpg
• Mother died of disease
• Clinical psychology, 1 formal
course in biology
• 1969: becomes head of
Hereditary Disease Foundation,
founded by her father
• Maracaibo, Venezuela for 20
years
• Currently researching for a cure
• 64 years old[13]
• Mapping Fate: A Memoir of
Family, Risk, and Genetic
Research by Alice Wexler[1]
Woodrow Wilson Guthrie
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7/14/1912 – 10/3/1967
Most famous person to ever suffer from the disease
Wrote over 1400 songs
Mother died at age 41
Two of his daughters have died from the disease as well
(6 children)
“Huntington’s Chorea Blues”
“I got this thing called chorea in my head
wanna walk but I fall down instead
folks say "Woody, he's just drunk again"
but I haven't had a drink since I don't know when
besides...I only drink when I'm alone...or with somebody
My arms felt funny moving all the time
and sometimes my head didn't feel like mine
kept telling myself it was the Ballantine Ale
and them jugs of wine on the writing trail
I prefer a disease you can sober up from”[4]
Huntingtin
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Normally a 350 kDa cytoplasmic protein
Exact function is unknown but may be involved in signaling, transportation,
protein-protein interactions and prevention of apoptosis[9]
PolyQ expansion >36 near amino terminus causes Huntington’s (exon 1)
Age of onset and severity depend on # of repeats
At least 9 neuro-degenerative diseases are known to be caused by polyQ
repeats[10]
Gain of function mutation
– Aggregation, altered gene transcription, apoptosis, proteosome function,
ubiquitination, axonal transport, endocytosis, AP transmission, and Ca2+
signaling[9]
What conformation does the expanded polyQ region adopt?
– Beta sheet (aggregates), alpha helix (not involved in many protein
interactions), random coil (relatively inert, but many weak interactions,
polyQ believed to increase random coil length and lead to abnormal protein
interactions and aggregation)
– Conformation flexibility and amyloid diseases (Alzheimer’s, Parkinson’s, Prion
disease)
– Length of poly Q is one of the determinants of conformation
Amino-terminal Region of Huntingtin (PDB ID
3IOT: Maltose Binding Fusion Protein
Figure 1: The three
chains of maltose
binding fusion protein
are shown in in cyan,
red, and magenta.
Calcium ions are
shown in yellow and
zinc ions are shown in
blue.
Amino Terminal Region of Huntingtin
Figure 2: Location of
the amino terminal
region that is bound
with maltose binding
fusion protein.
Huntingtin amino
terminal region is
shown in red. MBFP
is shown in cyan.
Calcium and zinc ions
have been omitted for
clarity.
9
A Closer Look at the Amino Terminal
Region Figure 3: Ribbon structure of the amino terminal region of
Huntingtin. The variable N-terminus region (residues 371-387)
is shown in cyan. The Poly Q region (residues 388-404,
equilibrates between a-helix and random coil but sometimes
goes into the extended conformation as in the last few polyQ
residues) is shown in red. The Poly P region (residues 405-415,
induces the polyQ into extended configuration and might
stabilize aggregates) is shown in yellow. The P, Q region
(residues 416-419 here, but actually 15 AA) is shown in blue.
Figure 4: Stick structure of the amino
terminal region of Huntingtin. Residue 371
M (residue 1 in Huntingtin) is shown in
orange. Residue 387 (residue 17 in
Huntingtin) is shown in Magenta. The polyP
region can also exist as a-helix, random coil,
or extended conformations.
Huntingtin Interacting Protein
1- PDB ID 2QA7
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Protein family is important in endocytosis
Marker for colon, prostate, and brain cancer
“Proapoptotic” protein, leads to death of medium spiny neurons
Huntington’s disease occurs when mutant Huntingtin does not
interact with HIP 1 (>36 glutamine greatly reduces interaction)
Normal huntingtin locks the HIP1 into a “rigid, extended
conformation” [12].
HIP1 protein interactor (HIPPI) recruited to form a complex
with HIP1
The two proteins interact at their pseudo-death factor domains
(pDED)
Recruits procaspase-8, which activates caspase3apoptosis[12]
HIP1 Asymmetric Unit and
Biological Assembly
Figure 5: Ribbon structure of the
asymmetric unit of the HIP1 coiled coil at
2.8 angstrom. Each dimer is made up of 2
parallel helices. Notice the anti-parallel
configuration of the dimers. Residues 371481 are shown.
Figure 6: Biological assembly of HIP1.
Notice the 2 right handed helices twisted
into a left-handed coiled coil. Helix 1 is
shown in red. Helix 2 is shown in cyan.
Residues 441-481 are not visible in Helix
2.
The Opened Region of the
Coiled Coil
Figure 7: The biological assembly showing the opened
region near the C-terminus. The helix becomes slightly
distorted due to the excessive negative charge. This
region is conserved in HIP1 and HIP1 related. It is
also conserved in many closely related species such as
rabbits and mice. The repulsive forces are thought to
make the protein more flexible[9].
Figure 8: A close-up of the opened region of HIP1.
The residues E445, D446, E448, E456, E458, and
E465 are thought to be crucial in this region. This
region is highly negatively charged at physiological
pH. This creates significant repulsive tension in the
highly acidic region. The unfavorable juxtaposition of
like charges in a coiled-coil motif is an important
feature for kinesin activity[9].
Disulfide bond at Cys 430
Figure 9: A close up view of the disulfide bond between the Cys 430(yellow) of Helix
1(red) and Helix 2(cyan). It brings together repulsive forces and distorts HIP1 [9].
HIP1 and HIPPI Interaction
Region
Figure 10: Biological assembly of HIP1
showing the region that interacts with
HIPPI in yellow. The residues that interact
with HIPPI are F432-Y474, but the tyrosine
was not available in the structure. The
surface is 58 angstroms in length. It is
highly positively charged and highly
basic[9].
Figure 11: Zoomed in view of
Phenylalanine 432(yellow). When F432 is
replaced with glycine, the cell toxicity is
reducedThis residue is conserved in the
death effector domain of other proteins as
well. K474 is unique to HIP1 and HIPPI. .
K474, which was not shown in the structure
is important for recruiting HIPPI [9].
Caspase-3 (PDB ID 1QX3)
• Recall: Huntingtin with >36 Q fails to bind with
HIP1This allows HIP1 and HIPPI to form a
complex HIP1/HIPPI complex recruits
procaspase-8 to activate caspase-3Apoptosis
• Cysteine proteases; proteolytic cleavage of
cellular proteins(zymogen) leading to apoptosis
• Implicated in cancers and neurodegenerative
disorders[11]
Caspase- 3
Figure 12: The biological assembly of caspase-3. The protease is composed of 2 identical catalytic domains. Each
catalytic domain is a heterodimer. The 1st subunit of each heterodimer is shown in cyan. The 2nd subunit of each
heterodimer is shown in red. The two catalytic domains are anti-parallel to each other. There are 2 active sites,
located at opposite ends of the 2 catalytic domains. [11]
Active Site of Caspase-3
Figure 13: The active site of caspase-3, delineated by binding with Ac-DEVD-CHO, is shown. The side chains of the active site make direct
contact with proteins. The side chains of Tyr204, Phe256 and Trp206 delineate a hydrophobic pocket within the narrow binding cleft
at the surface of caspase-3. The active site:Cys 163 (yellow), His 121 (cyan), Ser 205 (blue), Gln 160 (pink), Arg 64 (white), Arg 207
(forest green), Ser 209 (grey), Phe 250 (gold), Asn 208 (sky blue), Lys 210 (salmon), Asp 211 (green) [2]
Tetrabenazine
Current treatments
http://dailymed.nlm.nih.gov/dailymed/archives/i
mage.cfm?archiveid=9363&type=img&name=xen
azine-figure-01.jpg
Pridopidine
• No treatment or cure
• Drugs only to help symptoms
• Most drugs have side-effects like fatigue,
restlessness, and over-exicatability[7]
• People who exercise more during their
lifetime tend to fight better [7]
• Tetrabenazine
– 1st FDA approved drug for Huntington’s
– 8/15/2008
– Fights symptoms of Chorea[8]
• Huntexil
– 2/3/10
– Improves motor function[6]
http://upload.wikimedia.org/wikipedia/commons
/3/35/Pridopidine.png
Antibody MW1 (PDB ID
2GSG): Hope for a Cure?
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MW1 Fv recognizes poly Q of all lengths and binds to it
Soluble polyQ are a potential target for therapeutic drugs
MW1 is a heterodimer
Multimeric MW1 has higher affinity for longer polyQ regions than shorter
polyQ regions because the longer polyQ regions contain more binding
sites(epitopes)
– Study comparing binding with 10, 16, 25,39,46 Gln
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Antibody can only bind to proteins in their pre-aggregation state
A dimeric MW1 binds 46Q with 27X greater affinity than monomeric
A tetrameric MW1 binds a 46 Q region with 39-52 x greater affinity than a
monomeric MW1mutant huntingtin can have many more repeats than this
– No significant increase in binding affinity for 10 and16 Q regions
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A tetrameric MW1 binds shorter polyQ repeats with much lower affinity than
it binds longer poly Q repeats helpful in distinguishing normal polyQ from
mutated polyQ[10]
A Look at Antibody MW1
Figure 14: A look at the asymmetric unit of
MW1, an antibody for polyQ. Chain A of
each heterodimer is shown in cyan. Chain B
of each heterodimer is shown in red. MW1
recognizes polyQ via hydrogen bonds and
van der waals interactions. [10]
Figure 15: The biological assembly of MW1.
Chain A is shown in blue. Chain B is shown in
yellow. The affinity for poly Q increases as the
length of the polyQ segment increases. [10]
There is Hope
http://www.brain.riken.jp/labs/cagrds/JPEG/polyQ_e.jpg
References
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