Download Lesch-Nyhan Syndrome

Lesch-Nyhan Syndrome
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Nucleic acid turnover (synthesis and degradation) is an ongoing metabolic
process in most cells. Messenger RNA in particular is actively synthesized
and degraded. These degradative processes can lead to the release of free
purines in the form of adenine, guanine, and hypoxanthine (the base in
IMP). These substances represent a metabolic investment by cells. Socalled salvage pathways exist to recover them in useful form. Salvage
reactions involve resynthesis of nucleotides from bases via
phosphoribosyltransferases.
Base + PRPP -----nucleoside-5'-phosphate + PPi
The subsequent hydrolysis of PPi to inorganic phosphate by
pyrophosphatases renders the phosphoribosyltransferase reaction
effectively irreversible.
The purine phosphoribosyltransferases are adenine
phosphoribosyltransferase (APRT), which mediates AMP formation, and
hypoxanthine-guanine phosphoribosyltransferase (HGPRT), which can
act on either hypoxanthine to form IMP or guanine to form GMP
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Lesch-Nyhan Syndrome: HGPRT Deficiency Leads to Severe Clinical Disorder
The symptoms of Lesch-Nyhan syndrome are tragic: a crippling gouty arthritis due to
excessive uric acid accumulation and, worse, severe malfunctions in the nervous
system that lead to mental retardation, spasticity, aggressive behavior, and selfmutilation. Lesch-Nyhan syndrome results from a complete deficiency in HGPRT
activity. The structural gene for HGPRT is located on the X chromosome, and the
disease is a congenital, recessive, sex-linked trait manifested only in males. The
severe consequences of HGPRT deficiency argue that purine salvage has greater
metabolic importance than simply the energy-saving recovery of bases. Although
HGPRT might seem to play a minor role in purine metabolism, its absence has
profound consequences: de novo purine biosynthesis is dramatically increased and
uric acid levels in the blood are elevated. Presumably, these changes ensue because
lack of consumption of PRPP by HGPRT elevates its availability for glutamine-PRPP
amidotransferase, enhancing overall de novo purine synthesis and, ultimately, uric
acid production (Figure 27.8). Despite these explanations, it remains unclear why
deficiency in this single enzyme leads to the particular neurological aberrations
characteristic of the syndrome. Fortunately, deficiencies in HGPRT activity in fetal
cells can be detected following amniocentesis.
• Crystal structures have been determined
for free Escherichia coli hypoxanthine
phosphoribosyltransferase (HPRT) (2.9 Å
resolution) and for the enzyme in complex
with the reaction products, inosine 5`monophosphate (IMP) and guanosine 5`monophosphate (GMP) (2.8 Å resolution).
(A) Two orthogonal views of the structure of subunit A from the E. coli HPRT-GMP complex. The βstrands, shown as direction arrows are yellow in the core domain and pink in the hood domain. The
mobile loop, which includes residues 73–82, is not observed in the crystal structure. To complete the
structure, a hypothetical mobile loop has been modeled in and depicted as white coil. The GMP
molecule is drawn as solid spheres and the atoms colored green for carbon, blue for nitrogen, red for
oxygen, and pink for phosphorous. (B) The structure of the E. coli HPRT-GMP tetramer viewed down
the crystallographic twofold axes. (C) The active site of subunit A of E. coli HPRT. (Top) The IMP
complex. (Middle) The GMP complex. (Bottom) Free enzyme showing bound water molecules (red)
and Mg2+ (pink) as solid spheres.
• Data reported to GenBankTM indicate that the HPRTs of
distantly related organisms share extensive primary
sequence homology. For example, there is 41% identity
for amino acids in the human and a bacterial HPRT.
However, among well over 20 HPRT sequences reported
there are only 9 invariant amino acids and all but the
HPRT of Giardia lamblia also are invariant at Glu-133
and Asp-134 (Table ). In general, conserved residues of
HPRTs and bacterial XPRTs differ at positions
homologous with human Leu-67 and Glu-133 (Ser-36
and Asp-88 in the XPRT of Escherichia coli). Solutions
for the crystal structures of HPRTs reveal that the 11
conserved residues immediately flank or are very near
the active site of HPRTs.
• If the crystal structures of all purine PRTs are
analyzed together with the amino acid
sequences reported to GenBankTM, there are
only 2 residues (corresponding with human Gly69 and Asp-134) that are clearly invariant. A
Gly69Glu mutation virtually inactivates the
human HPRT, resulting in Lesch-Nyhan
syndrome, whereas a D134G mutation partially
inactivates the enzyme, resulting in gouty
arthritis . The invariant glycine may be essential
for the formation of a tight turn and an unusual
non-proline cis-peptide in active site.
• An aspartate at position 193 of HPRTs has been shown to
participate indirectly in binding pyrophosphate and purines via the
formation of a direct protein metal bond with a magnesium ion
designated M2. Asp-193 also forms hydrogen bonds with two water
molecules coordinated by the metal, and the metal forms
coordinated interactions with two oxygens of PRPP or PPi. A third
coordinated water molecule forms another hydrogen bond with the
N-3 atom of purine substrates.
• An invariant arginine at position 199 of HPRTs participates directly in
binding pyrophosphate and may contribute to positioning both
substrates by being close enough to the carboxyl group of Asp-193
to affect its position. Together, these interactions help to position
both substrates for in-line nucleophilic attack at the C1′ carbon of
PRPP or a nucleotide. A Asp193Asn mutation virtually inactivates
the human enzyme resulting in Lesch-Nyhan's syndrome .
La sindrome è determinata da diversi tipi di
mutazione che ne determinano anche la gravità
• Different HPRT1 mutations result in varied
levels of residual HGprt enzyme activity as
well as a spectrum of disease
characteristics. Classic features of LND
include hyperuricemia and its sequelae
(gout, nephrolithiasis,), motor disability
(dystonia, chorea, and spasticity),
intellectual impairment, and self-injurious
behavior
• We identiWed ten patients from eight diVerent
families all with the c.143G>A mutation. All
exhibited signs of uric acid overproduction. Nine
were classiWed as HND because of evidence
for motor or cognitive impairment, while one was
classiWed as HRH. The age at presentation
varied from infancy to 28 years.
• The biochemical properties of the 143G>A
mutation leading to arg48his were compared
with normal human
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Although early studies suggested
that mutations encoding the active site of the enzyme would
be associated most closely with disease, subsequent studies
demonstrated that mutations were spread throughout the
gene and upstream regulatory sequences (Jinnah et al.
2000, Jinnah et al. 2004). Presumably, mutations distant
from the active site aVect enzyme activity by interfering
with protein expression, stability, dimerization or by causing
other deleterious conformational changes (Duan et al.
2004). While there is no obvious correlation between the
location of gene mutations and the clinical phenotype, the
predicted consequence of the mutation does correlate with
clinical phenotype (de Gemmis et al. 2010; Jinnah et al.
2000, 2004; Jurecka et al. 2008)