What is it?
Deficiency in enzyme hypoxanthine-guanine phosphoribosyl transferase (HGPRT)
Error in purine metabolism
Inheritable chromosomal mutation on X-chromosome
How is it transmitted?
Familial transmission is possible; X-linked recessive
Mutation can arise spontaneously
How common is LNS?
Very rare in males; virtually absent in females
Is LNS curable?
NO
Treatments do exist
Short life expectancy
kidney failure
muscle loss
What are the Symptoms of LNS?
Uric Acid Overproduction
-Gouty Arthritis
-Kidney Stones/Renal Failure
Neurological Disability
-Motor Disability
-Involuntary Writhing
How is it recognized?
Characteristic Behavior
Self-mutilation
Biochemical Distinction
HGPRT Deficiency
Behavioral Abnormality
-Self Mutilation
-Lack of Speech
How is it diagnosed?
Physical and behavioral abnormalities present from infancy
Biochemical confirmation- Azathioprine testing
Drug interacts with HGPRT to lower levels of uric acid in urine
LNS patients will not demonstrate this change
Pre-natal testing available
What is gout?
Accumulation of uric acid crystals- especially in joints
Who first documented instances of gout?
Egyptians (2640BCE) -> Hippocrates (400sBCE) -> van Leeuwenhoek (1600sCE)
What lifestyles exaggerate instances of gout?
Diets rich in purines (i.e. meats, fish)
What is the relation between gout and LNS?
Gout is a symptom of LNS resulting from the absence of HGPRT
~1960 LNS was ‘discovered’ as a specific enzyme deficiency associated with gout
What are the proposed treatments for gout?
Change in diet – reduction of purine intake
drug therapy – e.g. allopurinol
"Diego does not have his teeth anymore. He has his arms in splints.
Everything around him is padded. He underwent plastic surgery on his nose.
He wears a glove to defend one hand from the other. Nevertheless I can say
that he has never lost his will to live and his hope to recover. Almost
everyday he says: - If I could walk...I would do...I would go...I would study …”
http://www.lesch-nyhan.eu/en/experiences.html
Biochemical Role:
Purine Salvage Pathway
-recycle hypoxanthine and guanine nucleotides back to DNA synthesis
-relief from de novo synthesis
* failure of this system results in unused nucleotides
purines are broken down into uric acid
Deficiency in LNS
Two categories of severity:
<1% normal function
1-50% normal function
*Note: this group of patients does not exhibit the
characteristic LNS nervous system abnormalities
Genetic Cause
-mutation ‘hot spots’ where Arg -> STOP
--> unstable, truncated HGPRT proteins
Structure
Homotetrameric
Catalytic Activity
-Hypoxanthine/Guanine + PRPP --> IMP/GMP
Catalytic Loop
Blocks active site from solvent
Prevents hydrolysis of PRPP once bound
Order of Substrate Binding
1. PRPP
2. Base
PRPP Binding
H-bonding with Serine 103
Displacement of catalytic loop from active site
H-bonds with Aspartate 137, Threonine 138, Glycine 138, Serine 141
Disruption of interactions of free enzyme
Purine Binding
-Binds to Lysine 165
Disturbs H-bond between
K165 & K185 of free HGPRT
-Specificity of K165 for
hypoxanthine and guanine
-Pi bonds to Phe 186
Catalysis of Transfer
-Propogated when H+ removed
from the purine ring
Syndrome Hallmarks
-Uric Acid Overproduction
Gouty Arthritis
Kidney Stones
Renal Failure
treatments
-Neurological Disability
Generous hydration
Allopurinol
Xanthine Oxidoreductase
(XOR)
Motor Disability
Involuntary Writhing
-Behavior Problems
Self Mutilation
treatments
Lack of Speech
Impulsive Aggressiveness
DOPA Decarboxylase
(DDC)
Physical Restraints
Carbidopa
Target Enzyme
-Xanthine Oxidoreductase (XOR) **
Desired Effect
-Decrease production of uric acid
-Reduce gouty arthritis induced pain
*effective, common, but must be taken frequently
*no effect on neurology or behavior
Method of Inhibition
-inhibit XOR from catalyzing catabolism of
hypoxanthine and guanine to uric acid
-Allopurinol is a hypoxanthine analog
** Note: both xanthine oxidoreductase and xanthine dehydrogenase will be
examined in order to illustrate the binding of desired substrates. The two are
overwhelmingly similar in binding, catalysis, and inhibition by drugs that they
are considered highly identical. (Dietzel, et al.)
Biochemical Role:
Purine Catabolism
-breakdown of un-recycled hypoxanthine & guanine
for excretion
-exacerbated by absence of HGPRT
Structure
-heterotetrameric
-each of the 4 monomers has 2 subunits
-each monomer has one α chain and one β chain
*Note:
XOR bound to
Hypoxanthine
Reaction Specifics
-Glu 730
deprotonation of Molybdenum
-Molybdenum
nucleophilic attack on hypoxanthine
-Glu 232
binding & stability
H-bonds hypoxanthine
-Arg310
lowers Ea through stabilization via
electrostatic interactions
H-bonds hypoxanthine
-Thr460
positioning
H-bonds hypoxanthine
-Phe 459 & 344
Van derWaals interactions with
hypoxanthine
form opposing walls of active site
Inhibtion Specifics
-upon binding, allopurinol is
hydroxylated to form oxypurinol
-oxypurinol: competitive inhibition
-covalently bound to molybdenum
-H-bonding with residues
Glu 802
Arg 880
Glu 1261
-time dependence
Spontaneous oxidation of
molybdenum cofactor
--> inhibited XOR reactivated
t1/2 = 300 minutes
Promising New Inhibitors
-longer-lasting inhibitory
complexes
How does HGPRT deficiency
affect nervous system?
Link: Dopaminergic Dysfunction
70-90% redution of dopamine in LNS patients
Dopamine
-regulate movement
-inhibit tendency to make unwanted decisions
What is the said dopaminergic dysfunction?
Testing with fluorodopa (F-18) revealed:
-F-18 activity low in dopaminergic regions of LN brain
-Deficit of dopamine is stable, not degenerative
Testing of TH enzyme revealed:
-decreased TH activity
-no loss of dopaminergic fibers, but reduction of
projections into dopaminergic regions
-dopaminergic consequences proposed to be
secondary effect of HGPRT deficit
Testing of Norepinephrine levels revealed:
-low levels in LNS patients
-low activity of DβH (due to substrate limitation)
Target Enzyme
-Dopamine Decarboxylase (DDC)
Desired Effect
-Decrease PNS [dopamine]
-Increase CNS [L-dopa] --> increase CNS [dopamine]
-Bypass PNS production/storage of dopamine
dopaminergic vesicles in which to store the
NT are limited in LNS
Method of Inhibition
-Inhibit DDC in PNS
-Increased levels of L-DOPA cross BBB
-Increased L-DOPA in brain --> Dopamine in brain
-carbiDOPA is an analog of L-DOPA
Biochemical Role:
Dopamine Formation
-decarboxylation of L-DOPA
Structure
-dimeric
-each of the 2 monomers has 3 domains
PLP binding site
C-terminus
N-terminus
Function of PLP
-co-enzymatic prosthetic group
-assists catalytic function
PLP Binding
-H-bonding with Lys 303
- Salt bridge formation with Asp 271
Active Site
-Ile101 & Phe103
Van derWaals with carbiDOPA
Active Site cont’d..
-Thr 82
bottom of binding pocket
H-bonding with carbiDOPA
-His 192
H-bonding with carbiDOPA
-carbiDOPA sits deep within site
aids inhibitory activity
Flexible Loop
-residues 328-339
-at dimer interface
-conserved residues
Tyr 332 & 334(Lys or Arg)
LNS is incurable as we know it
-Further investigation of potential drugs to ameliorate neurological and
behavioral symptoms
-Closer examination for links between HGPRT shortage and dopaminergic
deficiency
-> is HGPRT stored in dopamine containing fibers?
Wherever the search for a cure leads, may the
scientists driving the research not let the science
of Lesch-Nyhan Syndrome overshadow the faces
Of the disease.
Resources
Breese, George R., et al. “Behavioral Differences Between Neonatal and Adult 6-hydroxydopamine-treated Rates to
Dopamine Agonists: Relevance to Neurological Symptoms in Clinical Syndromes with Reduced Brain Dopamine.”
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http://jpet.aspetjournals.org/content/231/2/343.long
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http://www.nature.com/nsmb/journal/v8/n11/abs/nsb1101-963.html
Dietzel, Uwe, et al. “Mechanism of Substrate and Inhibitor Binding of Rhodobacter capsulatus Xanthine Dehydrogenase.” J
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Harris, James C. “Social Neuroscience, Empathy, Brian Integration, and Neurodevelopment
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http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T0P49569GX1&_user=10&_coverDate=08%2F31%2F2003&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&vie
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Jinnah, H. A. "Lesch-Nyhan Syndrome.“ www.lesch-nyhan.org. Lesch-Nyhan Disease International Study Group, 2 Oct.
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Jinnah, H.A., et al. “Dopamine Deficiency in a Genetic Mouse Model of Lesch-Nyhan Disease.” The Journal of
Neuroscience. 14.3 (1994): 1164-1175. The Journal of Neuroscience. 31 March 2010.
http://www.jneurosci.org/cgi/reprint/14/3/1164
Resources cont’d..
Keough, Dianne T., et al. “The Crystal Structure of Free Human Hypoxanthine-Guanine
Phosphoribosyltransferase Reveals Extensive Conformational Plasticity Throughout the Catalytic Cycle.” Journal of
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http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WK7-4GD4PS92&_user=10&_coverDate=08%2F05%2F2005&_rdoc=1&_fmt=
high&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050221
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Lake, Raymond C. and Ziegler, Michael G. “Lesch-Nyhan Syndrome: Low Dopamine-B-Hydroxylase Activity and
Diminished Sympathetic Response to Stress and Posture.” Science. 196.4292 (1977): 905-906. Jstor. 31 March 2010.
http://www.jstor.org/pss/1744110
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and Therapy. 8.1 (2006). Biomedcentral. 1 April 2010. http://www.biomedcentral.com/content/pdf/ar1906.pdf
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