+++
Hypoxanthine-Guanine Phosphoribosyltransferase
Deficiency
++
In 1967, Seegmiller and coworkers discovered a complete deficiency
of hypoxanthine-guanine phosphoribosyltransferase (HPRT or HGPRT)
in children with the Lesch-Nyhan syndrome (OMIM 300322), an incapacitating neurological
disorder that, with few exceptions, is limited to males.69 It
is characterized by choreoathetosis, spasticity, compulsive self-mutilation
manifested by biting away lips and tongue and ends of the fingers,
and three- to fourfold normal uric acid levels in plasma and urine.70 That
same year, a partial deficiency of HPRT was described in patients
with milder presentations, sometimes termed Kelley-Seegmiller
syndrome or now more often called Lesch-Nyhan variants (OMIM 300323);
this disorder involves overproduction of uric acid that leads to
severe gouty arthritis in early adult life and involves minor or no
neurological dysfunction.71 This led to the identification
of a continuous spectrum of HPRT deficiencies, ranging from gout
to a devastating neurological syndrome.72
+++
Clinical Presentation
++
At birth, children with complete HPRT deficiency (Lesch-Nyhan
disease) appear entirely normal. The first indication of the disease
may be the presence of brownish to red-orange sand in the diapers,
particularly when the baby becomes dehydrated. Some of the infants
are very irritable, with episodes of screaming, suggesting the possibility
of renal colic. Most infants show normal development during the first
4 to 6 months of life. During the second part of the first year,
impaired motor development becomes evident by the inability to support
the head, generalized muscle hypotonia, and voluntary movements
of both athetoid and choreiform types with spasticity. Recently,
the motor syndrome of complete HPRT deficiency has been reclassified
as a severe action dystonia, superimposed to a baseline hypotonia.73 By
1 year of age, pyramidal symptoms consist of an increase in muscle
tone, an increase in deep tendon reflexes, a sustained ankle clonus, and
a scissoring of the lower extremities, and extensor plantar responses
are usually present. An increased incidence of dislocation of the hips
and club feet may be related to the early hypotonia. Severely affected
children are never able to walk.
++
Over the years, the patients develop a compulsive aggressiveness
and self-mutilation that involves biting away lips, tongue, and ends
of the fingers if given the opportunity. Because it is painful,
children are fearful of the biting. Biting can, in some cases, begin
with the eruption of incisors. In other patients, it may be delayed
until early adolescence. In any given patient, biting can be highly
variable, with patients going through periods when they show extensive
self-mutilation and other periods when this is no longer a problem.
Self-mutilation tends to be correlated, at least in some cases,
with emotional stress. In addition, some older children develop
opisthotonic spasms, which appear to be at least semivoluntary.
An accompanying laryngeal spasm and stridor sometimes produces a
temporary cyanosis. If the patient’s head is in range of
a hard object at the time of a spasm, he or she may injure it. Children
also sometimes throw themselves from the bed if left unattended
or may injure themselves on sharp edges of wheelchairs that are
left unpadded.
++
Aggressive acts against others are also included in the bizarre
behavior of these children. This can take the form of biting, hitting, spitting,
or kicking. Eyeglasses are common targets for their aggression.
Patients often pinch or strike attendants in areas of sexual significance,
and become verbally aggressive, often with a remarkably shocking
vocabulary. Projectile vomiting is also used by older children as
a weapon, especially when the child becomes upset emotionally. Although
this peculiar behavior would seem to alienate them from others,
these children are often favorite patients of ward personnel, because
they are charming and smile and laugh easily. They are very responsive
to their environment and show a remarkably good sense of humor.
They have a characteristic dysarthric speech, although they can
usually make themselves understood to those who are caring for them.
++
The first descriptions of Lesch-Nyhan syndrome included mental
retardation. However, reevaluation with specific tests for their
motor difficulties has shown attention deficits but preservation
of nonverbal intelligence, and some patients display normal intelligence.74-76
++
Partial HPRT deficiency is found in rare patients with gout.
Most of them or normal on neurological examination, but occasionally spasticity,
dysarthria, and a spinocerebellar syndrome are found.77 Some
but not all of the patients with less severe deficiencies of the HPRT
enzyme with resulting minimal neurological dysfunction also show
a compulsive behavior. Whereas most subjects with Lesch-Nyhan syndrome
do not develop gouty arthritis, this finding is common in partial
HPRT deficiency.
+++
Metabolic Derangement
++
HPRT is normally expressed in all cells of the body. Early studies
reported that its specific activity is highest in the basal ganglia,
which correlates with the brain area that can cause analogous movement
disorders. However, later investigations have not confirmed these findings.
HPRT activity is also high in testes, which accords with failure
of sexual maturation and atrophic testes found in some patients with
the most severe enzyme deficit. Alterations in the kinetic properties
of the enzymes in patients comprise, besides reduction of the maximal
velocity up to its complete loss, reduced affinities for phosphoribosyl
pyrophosphate (PRPP) and the purine bases, which have been shown
to correlate with the gene lesions (see below).
++
The considerably excessive production of uric acid in HPRT deficiency
is explained by an enhancement of the purine synthesis rate, which
is caused by the increased amount of PRPP that accumulates from
its underutilization (caused by the HPRT mutation). Fibroblasts
cultured from affected patients show a two- to threefold accumulation
of PRPP, and their erythrocytes show a tenfold accumulation over
normal values. Since PRPP is a rate-limiting, normally not saturating
substrate for the first, presumably rate-determining reaction of
purine biosynthesis de novo catalyzed by the enzyme PRPP glutamine
amidotransferase, its elevation will increase uric acid production.
Further support for this concept comes from the correlation of increased
intracellular PRPP in fibroblasts and excessive rates of purine
synthesis in PRPP synthetase superactivity (see above).
++
Theoretically, the deficiency of HPRT should provoke accumulation
of its substrates (hypoxanthine, guanine, and its deamination product
xanthine) and of PRPP and depletion of its products (GMP and the
other guanine nucleotides and IMP). Owing to the conversion of IMP
into AMP (Fig. 168-1), the synthesis of the
latter and the other adenine nucleotides could also be decreased.
Hypoxanthine and xanthine are elevated in body fluids of HPRT-deficient
patients, but their toxicity is ruled out by the absence of neurological
symptoms in isolated xanthine oxidase deficiency (see above) and
the absence of worsening neurological symptoms upon allopurinol
treatment. Because allopurinol lowers uric acid but does not influence
the neurobehavioral abnormalities, uric acid toxicity is ruled out.
PRPP toxicity is also unlikely, since its elevation in PRPP synthetase superactivity
(see above) is not associated with the neurological symptoms of
HPRT deficiency. Similarly, accumulation of AICAR, an intermediate
of de novo purine biosynthesis, and its further phosphorylation
products (collectively termed Z-nucleotides) is
unlikely to be involved, since it has also been recorded in disorders
without neurobehavioral abnormalities.
++
Depletion of guanine or adenine nucleotides is also proposed
as an explanation for the neurological symptoms of HPRT deficiency.
ATP depletion might result in impairment of energy production, DNA
repair,78 and metabolic defense against oxidant
stress. GTP depletion, which can be shown in HPRT-deficient cells
under certain conditions,79 has been proposed to
interfere with the function of G proteins and the formation of tetrahydrobiopterin
(BH4), a cofactor in dopamine synthesis. However, other studies80 have invalidated
the latter hypothesis, which might have provided an explanation
for the abnormalities of the dopaminergic system observed in Lesch-Nyhan
patients.
++
Multiple studies have shown abnormalities of the dopaminergic
neurotransmitter system in Lesch-Nyhan disease. Autopsy studies81,82 showed
a reduction in the basal ganglia (to 10–30% of
control) of dopamine; its metabolite homovanillic acid; and tyrosine
hydroxylase, the dopamine synthetizing, BH4-dependent enzyme. In
vivo PET studies of the brains of Lesch-Nyhan subjects have revealed large
reductions in dopaminergic terminals not only in the basal ganglia,
but also in other brain areas.83,84 Dopamine deficiency
is also found in HPRT knockout mice.85 How HPRT deficiency
impairs the dopaminergic system remains an open question. One possibility could
be an increased sensitivity to dopamine-associated oxidant stress,
as thought to play a role in Parkinson disease. Dopamine and other catecholamines
are toxic to many cell types, probably through various mechanisms,
including spontaneous and metal-catalyzed oxidation that produces
reactive oxidant species such as superoxide radicals.86 Studies
in HPRT knockout mice provide evidence for an increased oxidant
stress, which is not affected by overexpression of superoxide dismutase
indicating that other mechanisms intervene.85
++
The human hypoxanthine-guanine phosphoribosyltransferase (HPRT)
gene spans approximately 45 kb on the long arm of chromosome X at
Xq26-q27.2 and contains nine exons.87 It encodes
a tetrameric protein composed of four identical subunits of 219
amino acids with a mass of 25 kDa. The three-dimensional structure
of the human enzyme has been resolved by X-ray crystallography.88 A
single restriction fragment length polymorphism with three alleles
is known. A rare histidine-to-arginine mutation at codon 60 reduces
HPRT activity by approximately 40% but does not provoke symptoms89 and
might thus be a rare polymorphism. The HPRT gene on the inactivated
X-chromosome is rendered transcriptionally silent by methylation
of the majority of its CpG clusters.90
++
Over 250 mutations causing HPRT deficiency have been reported,91 and
new mutations are regularly identified.92,93 Although with
some overlap, they can be divided into three categories, based on
clinical phenotype: (1) full-fledged Lesch-Nyhan disease; (2) intermediate
with some neurological dysfunction but no self-injurious behavior;
and (3) mild, with uric acid overproduction without neurological
symptoms. As reviewed by Jinnah and colleagues,91 single
base substitutions account for approximately 65% of all
mutations; deletions account for 25%; and insertions, duplications,
and complex mutations account for the remaining 10%. About
60% of the point mutations causing amino acid substitutions,
nearly all those causing a premature stop, and 80% of those
leading to splicing errors provoke Lesch-Nyhan disease. As expected,
90% of intermediate and mild cases have point mutations
leading to amino acid substitutions. Mutations are found throughout
the HPRT gene, but several hot spots seem to exist. Although some
mutations that alter HPRT kinetics implicate the binding site of
its substrates, most are located outside, which suggests that they
provoke conformational changes in the active site. Overall, location
of the HPRT mutations seems to have a less significant influence
on phenotype than the different amino acid changes that can be found
at the same codon. Structural and functional analysis with modeling
methods has also shown that most mutations can be explained by the
predicted effect on protein structure and function.94 With
rare exceptions,95 genotype/phenotype
correlations are thus quite apparent in HPRT deficiency.
++
Although Lesch-Nyhan disease is an X-linked disorder, a small
number of females with the disorder have been identified.96–99 This
can be explained by the coexistence of a nonsense mutation or deletion
on one of the parents’ alleles, with a nonrandom inactivation
of the other parent’s allele.
++
The excessive excretion of uric acid forms the basis for a relatively
simple screening test in which the ratio of uric acid to creatinine
(both expressed as mg/100 ml in the morning urine sample)
is measured.100 Lesch-Nyhan patients with the full
syndrome show ratios between 2.0 to 5.3, as opposed to 0.2 to 0.6
for normal individuals. However, some overlap with the normal may
be found in the first few months or years of life because of high
ratios observed at birth in normal infants, followed by a marked
age-related decline. Although most patients also show an elevation
of serum urate—up to 18 mg/dl (1 mmol/l),
particularly in later stages of the disease—this measurement
cannot be used to rule out Lesch-Nyhan disease, as around 5% to
10 % may show a normal serum urate, particularly in early
life.
++
Measuring HPRT is most easily performed in red blood cells or
skin fibroblasts. In patients with the most severe enzyme defect, HPRT
activity is virtually absent in erythrocyte lysates. In patients
with less severe deficiencies, HPRT activity may range from less than
0.1 to 10% to 20% of normal. In rare kinetic mutations
(see below), near-normal activities can be measured when saturating substrate
concentrations are used.
++
Carrier detection of Lesch-Nyhan disease has long been accomplished,
most practically by assaying individual hair follicles, which are advantageous
because they are largely clonal in females. Prenatal diagnosis of
Lesch-Nyhan disease can be made by measuring HPRT activity in cultured
amniocytes and chorionic villi. These techniques are now largely
supplanted by molecular diagnosis in families in which the mutation
is known.101
+++
Treatment and
Prognosis
++
Treatment comprises controlling the uric acid overproduction,
the motor syndrome, and the behavioral manifestations. Allopurinol,
at doses up to 20 mg/kg of body weight per day, prevents
the damage to kidneys caused by the excessive amounts of uric acid
excreted in the urine. Although it produces a marked decrease in
both urine and serum uric acid, it fails to decrease de novo purine
synthesis: Hypoxanthine and xanthine merely replace uric acid in urine.
Established renal calculi can thereby be substantially reduced in
size. Further general measures for treatment are those of gout,
including high fluid intake (at least 50 ml/kg per day).
This will diminish the occurrence of urinary concretions composed
of xanthine that have been noted occasionally in children treated
with allopurinol.8 Adequate nutrition should also
be prescribed, since many of these children take a very long time
to eat and may actually be malnourished in an understaffed institution.
++
Although children with the Lesch-Nyhan syndrome seem to be incapable
of learning from punishment, they do respond to positive experiences,
and some success in behavior modification has been achieved by simply turning
away from the child when they display aberrant behavior. Partial
improvement in self-mutilation has also been reported from using
both positive and negative conditioning programs. Patients should
also be made more comfortable by appropriate restraints. Hands can
be kept away from the mouth and yet be left free for use by constructing
loose-fitting wraparound fabric splints for the elbows. Chronic
irritation in the mouth from sharp teeth edges can be eliminated
by lip guards and even tooth extraction.
++
Despite trials with a variety of agents, no successful pharmacological
treatment of the motor syndrome and the behavioral manifestations
has yet been found. Children treated with diazepam (Valium) are
less spastic. Levodopa with or without a decarboxylase inhibitor
has been reported to improve, worsen, or not influence the dystonia.
Hydroxytryptophan has been reported to reduce self-mutilation but
only transiently and is hardly used anymore. Risperdal and carbamazepine
may be useful. Gabapentin has been suggested to improve self-injurious
behavior.102 Disappearance of self-mutilating behavior
with bilateral chronic stimulation of the globus pallidus internus
has been reported.103,104 Recently, local injection
of botulinum toxin has also been advocated.105 Lesch-Nyhan
disease was one of the first disorders proposed for gene therapy
and continues to serve as a model for its development. Bone marrow
transplantation has been performed in a small number of Lesch-Nyhan
patients without obvious benefit and with significant morbidity,
since the majority of the patients died shortly thereafter.
+++
Adenine Phosphoribosyltransferase
Deficiency
++
The deficiency of adenine phosphoribosyltransferase (APRT; OMIM
102600) was first identified in heterozygotes with a partial enzyme defect.106 Later,
Cartier and Hamet107 reported a homozygous 4-year-old
with urinary crystals and stones. Approximately 300 patients have been
diagnosed worldwide, but up to 50% of APRT-deficient subjects
may be asymptomatic.
+++
Clinical Presentation
++
The deficiency may clinically manifest in childhood, even from
birth, but may also remain silent for several decades.108 Symptoms include
urinary passage of gravel, small stones, and crystals, frequently
accompanied by abdominal colic, dysuria, hematuria, and urinary
tract infection. Some patients may present with acute anuric renal
failure, while others have developed chronic renal failure requiring
dialysis and transplantation. The urinary precipitates are composed
of 2,8-dihydroxyadenine (2,8-DHA) and radiotranslucent.
+++
Metabolic Derangement
++
The APRT deficiency results in the loss of adenine salvage (Fig. 168-1), provided by food and by the
polyamine pathway. Consequently, adenine is oxidized by xanthine
oxidase into 2,8-DHA. Symptoms are caused by the very poor solubility
of 2,8-DHA. Its solubility in urine, at pH 5 and 37°C, is about
0.3 mg/dl, considerably less than that of 15 mg/dl
for uric acid.
++
Two types of APRT deficiency are known. Patients with type I
deficiency have no detectable activity in erythrocyte lysates. In
patients with type II deficiency, significant residual activity
is found, reaching 5% to 25% of normal when measurements
are made at supraphysiological, saturating concentrations of PRPP. However,
kinetic studies reveal that the S0.5 value for PRPP, which
is about 3 μM for the normal enzyme, is increased
to 50 to 80 μM. This decreased affinity for PRPP
results in near inactivity under physiological conditions. Consequently,
APRT activity is not detectable in intact cells such as erythrocytes
or fibroblasts. Confirmatory diagnosis of type II deficiency is
performed on viable T cells.109 To date, type II
patients have been found only in Japan, where they account for approximately 80% of
the affected subjects.110 Determining the three-dimensional
structure of human APRT111 has provided insights
in the mechanisms by which many mutations hamper normal enzyme activity.
++
The human adenine phosphoribosyltransferase gene has been mapped
to chromosome 16q24.3. Both in Caucasian and Japanese subjects,
numerous polymorphisms are observed. In type I deficiency, more
than 30 mutations have been identified.108 About
half of these are single base changes. T insertion mutations, resulting
in frameshift, and splicing errors are also frequent. The observation
of several more common mutations suggests founder effects. All the
type II Japanese patients carry the same c.2069T- > C substitution
in exon 5, resulting in an M136T change. Approximately 80% are
homozygous. Another W98X nonsense mutation, and a CCGA insertion
resulting in a frameshift, account for nearly all the other cases.112
++
The presence of brownish spots on diapers or the finding of round,
brownish crystals in urine under a light microscope suggest the presence
of 2,8-DHA. Notwithstanding its very low solubility, 2,8-DHA can
also be identified by HPLC. Confirming its presence requires complex
analyses, including UV and infrared spectrography, mass spectrometry,
X-ray crystallography, and capillary electrophoresis. It is therefore
usually easier to measure adenine phosphoribosyltransferase (APRT)
activity. As said under “Metabolic Derangement,” based
on the level of residual enzyme activity in red blood cell lysates,
two types of APRT deficiency are recognized. In both types, APRT
activity is absent in intact cells.
+++
Treatment and
Prognosis
++
In patients with symptoms, allopurinol should be given (as detailed
under treatment of PRPP synthetase superactivity) to inhibit the
formation of 2,8-DHA. Both in patients with stones and in those
without symptoms, dietary purine restriction and high fluid intake
are recommended. Alkalinization of the urine is, however, not advised:
unlike that of uric acid, the solubility of 2,8-DHA does not increase
up to pH 9. Shock-wave lithotripsy has been beneficial in a small
number of patients.
++
Ultimate prognosis depends on renal function at the time of diagnosis:
late recognition may result in irreversible renal insufficiency requiring
chronic dialysis, and early treatment may result in prevention of
stones. Kidney transplantation has been followed by recurrence of
microcrystalline deposits and subsequent loss of graft function.
+++
Deoxyguanosine Kinase
Deficiency
++
In three large consanguineous kindreds of Druze origin with the
hepatocerebral form of mitochondrial DNA depletion syndrome (characterized
by early progressive liver failure, neurological abnormalities,
hypoglycemia, and increased lactate), homozygosity mapping was used
to search for the causative gene defect.113 It
led to the identification of a mitochondrial deoxyguanosine kinase
deficiency. This enzyme phosphorylates the deoxy counterpart of
guanosine (Fig. 168-1) into deoxy GMP. The
disorder is discussed in Chapter 176.
+++
Thiopurine Methyltransferase Deficiency
++
Thiopurine S-methyltransferase (TPMT) catalyzes the S-methylation
of several synthetic pharmacological purine analogs that contain
a thiol group such as 6-mercaptopurine, 6-thioguanine, and azathioprine
that is converted to 6-mercaptopurine in vivo. These drugs are used
to treat various diseases, including cancers, rheumatoid arthritis,
and other autoimmune disorders, and immunosuppressants after organ
transplantation. They are converted via phosphoribosylation by hypoxanthine-guanine
phosphoribosyltransferase and adenine phosphoribosyltransferase into
active thionucleotides, which exert their therapeutic action by
incorporation into DNA and RNA. Their oxidation by xanthine oxidase
and S-methylation by TPMT results in inactivation.
++
The wide variations in therapeutic response and occurrence of
toxic side effects in some patients receiving thiopurines led to
the identification of TPMT as a determining factor in this variability.114,115Approximately
90% of individuals in various ethnic populations have high TPMT
activity, about 10% have intermediate activity, and 1 in
300 lack activity. Patients with no or less efficient methylation
of thiopurines have more extensive conversion to active thionucleotides
that leads to severe, potentially fatal myelosuppression. Therefore,
determining the TPMT status prior to treatment with thiopurines
is now recommended in predictive pharmacogenetics.116