Disorders of Purine and Pyrimidine Metabolism

Genetics

The HPRT gene has been localized to the long arm of the X chromosome (q26-q27). The complete amino acid sequence for HPRT is known and is encoded by the HPRT1 gene (approximately 44 kb; 9 exons). The disorder appears in males; occurrence in females is extremely rare and ascribed to nonrandom inactivation of the normal X chromosome. Absence of HPRT activity results in a failure to salvage hypoxanthine, which is degraded to uric acid. Failure to consume phosphoribosylpyrophosphate in the salvage reaction results in an increase in this metabolite, which drives de novo purine synthesis, leading to the overproduction of uric acid. Excessive uric acid production manifests as gout, necessitating specific drug treatment (allopurinol). Because of the enzyme deficiency, hypoxanthine accumulates in the cerebrospinal fluid (CSF), but uric acid does not; uric acid is not produced in the brain and does not cross the blood-brain barrier. The behavior disorder is not caused by hyperuricemia or excess hypoxanthine because patients with partial HPRT deficiency, the variants with hyperuricemia, do not self-injure, and infants with isolated hyperuricemia from birth do not develop self-injurious behavior.

The prevalence of the classic LND has been estimated at 1 in 100,000 to 1 in 380,000 persons based on the number of known cases in the United States. The incidence of partial variants is not known. Those with the classic syndrome rarely survive the 3rd decade because of renal or respiratory compromise. The life span may be normal for patients with partial HPRT deficiency without severe renal involvement.

Pathology

No specific brain abnormality is documented after detailed histopathology and electron microscopy of affected brain regions. MRI has documented reductions in the volume of basal ganglia nuclei. Abnormalities in neurotransmitter metabolism have been identified in 3 autopsied cases. All 3 patients had very low HPRT levels (<1% in striatal tissue and 1–2% of control in thalamus cortex). There was a functional loss of 65–90% of the nigrostriatal and mesolimbic dopamine terminals, although the cells of origin in the substantia nigra did not show dopamine reduction. The brain regions primarily involved were the caudate nucleus, putamen, and nucleus accumbens. It is proposed that the neurochemical changes may be linked to functional abnormalities, possibly resulting from a diminution of arborization or branching of dendrites rather than cell loss. A neurotransmitter abnormality is demonstrated by changes in CSF neurotransmitters and their metabolites and is confirmed by positron emission tomography (PET) scans of dopamine function. In vivo reductions in the presynaptic dopamine transporter have been documented in the caudate and putamen of 6 individuals.

The mechanism whereby HPRT leads to the neurologic and behavioral symptoms is unknown. However, both hypoxanthine and guanine metabolism are affected, and GTP and adenosine have substantial effects on neural tissues. The functional link between purine nucleotides and the dopamine system is through salvage of guanine by HPRT to form GTP; this is essential for GTP cyclohydrolase activity, the first step in the synthesis of pterins and dopamine. In a controlled study that sought correlations between HPRT and GPRT and behavior, GPRT was more highly correlated than HPRT on 13 of 14 measures of the clinical phenotype; these included severity of dystonia, cognitive impairment, and behavioral abnormalities. These findings suggest that loss of guanine recycling might be more closely linked to the LND/LNV phenotype than loss of hypoxanthine recycling. Moreover, patients with inherited GTP cyclohydrolase deficiency show clinical features in common with LND.

Dopamine reduction in brain is documented in HPRT-deficient strains of mutant mice. Dopamine binding to its receptor results in either an activation (D1 receptor) or an inhibition (D2 receptor) of adenylcyclase. Both receptor effects are mediated by G proteins (GTP-binding proteins) dependent on guanine diphosphate in the guanine diphosphate/GTP exchange for cellular activation. Dopamine and adenosine systems are also linked through the role of adenosine as a neuroprotective agent in preventing neurotoxicity. Adenosine derives from AMP, which depends on hypoxanthine salvage in the brain by HPRT. Adenosine agonists mimic the biochemical and behavioral actions of dopamine antagonists, whereas adenosine receptor antagonists act as functional dopamine agonists. LND can thus be seen as arising ultimately from nucleotide depletion specifically in the brain, which relies on the HPRT salvage pathway, leading to dopamine and adenosine depletions.

Clinical Manifestations

At birth, infants with LND have no apparent neurologic dysfunction. After several months, developmental delay and neurologic signs become apparent. Before age 4 mo, hypotonic, recurrent vomiting, and difficulty with secretions may be noted. By 8-12 mo, extrapyramidal signs appear, primarily dystonic movements. In some patients, spasticity may become apparent at this time; in others it becomes apparent later in life.

Cognitive function is usually reported to be in the mild to moderate range of intellectual disability, although some individuals test in the low-normal range. Because test scores may be influenced by difficulty in testing caused by movement disorder and dysarthric speech, overall intelligence may be underestimated.

The age of onset of self-injury may be as early as 1 yr and occasionally as late as the teens. Self-injury occurs, although all sensory modalities, including pain, are intact. The self-injurious behavior (SIB) usually begins with self-biting, although other SIB patterns emerge with time. Most characteristically, the fingers, mouth, and buccal mucosa are mutilated. Self-biting is intense and causes tissue damage and may result in the amputation of fingers and substantial loss of tissue around the lips. Extraction of primary teeth may be required. The biting pattern can be asymmetric, with preferential mutilation of the left or right side of the body. The type of behavior is different from that seen in other intellectual disability syndromes involving self-injury. Self-hitting and head-banging are the most common initial presentations in other syndromes. The intensity of SIB generally requires that the patient be restrained. When restraints are removed, the patient with LND may appear terrified, and stereotypically place a finger in the mouth. The patient may ask for restraints to prevent elbow movement; when the restraints are placed, or replaced, he may appear relaxed and cheerful. Dysarthric speech may cause interpersonal communication problems; however, the higher-functioning children can express themselves fully and participate in verbal therapy.

The self-mutilation presents as a compulsive behavior that the child tries to control but frequently is unable to resist. Older individuals may enlist the help of others and notify them when they are comfortable enough to have restraints removed. In some cases the behavior may lead to deliberate self-harm. The LND patient may also show compulsive aggression and inflict injury to others through pinching, grabbing, or hitting or by using verbal forms of aggression. Afterward he may apologize, stating that this behavior was out of his control. Other maladaptive behaviors include head- or limb-banging, eye-poking, and psychogenic vomiting.

Diagnosis

The presence of dystonia along with self-mutilation of the mouth and fingers suggests LND. With partial HPRT deficiency, recognition is linked to either hyperuricemia alone or hyperuricemia and a dystonic movement disorder. Serum levels of uric acid that exceed 4-5 mg uric acid/dL and a urine uric acid:creatinine ratio of ≥3-4 : 1 are highly suggestive of HPRT deficiency, particularly when associated with neurologic symptoms. The definitive diagnosis requires an analysis of the HPRT enzyme. This is assayed in an erythrocyte lysate. Individuals with classic LND have near 0% enzyme activity, and those with partial variants show values between 1.5% and 60%. The intact cell HPRT assay in skin fibroblasts offers a good correlation between enzyme activity and severity of disease. Molecular techniques are used for gene sequencing and identification of carriers.

Differential diagnosis includes other causes of infantile hypotonia and dystonia. Children with LND are often initially incorrectly diagnosed as having athetoid cerebral palsy. When a diagnosis of cerebral palsy is suspected in an infant with a normal prenatal, perinatal, and postnatal course, LND should be considered. Partial HPRT deficiency may be associated with acute renal failure in infancy; therefore clinical awareness of partial HPRT deficiency is of particular importance. The simplest test to exclude LND or partial deficiency is the urinary uric acid:creatinine ratio.

An understanding of the molecular disorder has led to effective drug treatment for uric acid accumulation and arthritic tophi, renal stones, and neuropathy. However, reduction in uric acid alone does not influence the neurologic and behavioral aspects of LND. Despite treatment from birth for uric acid elevation, behavioral and neurologic symptoms are unaffected. The most significant complications of LND are renal failure and self-mutilation.

Treatment

Medical management of LND focuses on prevention of renal failure by pharmacologic treatment of hyperuricemia, with high fluid intake along with alkalization and allopurinol (or more often febuxostat). A low-purine diet and reduced fructose intake are desirable.

Allopurinol treatment must be monitored because urinary oxypurine excretion with all overproduction disorders is sensitive to allopurinol, resulting in an increased urine concentration of xanthine, which is extremely insoluble. Self-mutilation is reduced through behavior management and the use of restraints and/or removal of teeth. Pharmacologic approaches to decrease anxiety and spasticity with medication have mixed results. Drug therapy focuses on symptomatic management of anticipatory anxiety, mood stabilization, and reduction of self-injurious behavior. Although there is no standard drug treatment, diazepam may be helpful for anxiety symptoms, risperidone for aggressive behavior, and carbamazepine or gabapentin for mood stabilization. Each of these medications may reduce SIB by helping to reduce anxiety and stabilize mood. S -adenosylmethionine ( SAMe ), which is thought to act by countering nucleotide depletion in the brain, has been reported specifically to reduce the rate of self-injury in some cases. Animal studies have suggested that D1-dopamine receptor antagonists such as ecopipam may suppress SIB. Despite limited data, the drug appears to reduce SIB in most patients, suggesting further study to establish an appropriate dosing regimen and assess toxicity.

Several patients have received bone marrow transplantation (BMT), based on the hypothesis that the central nervous system (CNS) damage is produced by a circulating toxin. There is no evidence that BMT is a beneficial treatment approach; it remains an experimental and potentially dangerous therapy. Two patients received partial exchange transfusions every 2 mo for 3-4 yr. Erythrocyte HPRT activity was 10–70% of normal during this period, but no reduction of neurologic or behavioral symptoms was apparent. Successful preimplantation genetic diagnosis and in vitro fertilization for LND has been reported with the birth of an unaffected male infant.

Both the motivation for self-injury and its biologic basis must be addressed in treatment programs. However, behavioral techniques alone, using operant conditioning approaches, have not proved to be an adequate general treatment. Although behavioral procedures have had some selective success in reducing self-injury, difficulty with generalization outside the experimental setting limits this approach, and patients under stress may revert to their previous SIB. Behavioral approaches may also focus on reducing SIB through treatment of phobic anxiety associated with being unrestrained. The most common techniques are systematic desensitization, extinction, and differential reinforcement of other (competing) behavior. Stress management has been recommended to assist patients to develop more effective coping mechanisms. Individuals with LND do not respond to contingent electric shock or similar aversive behavioral measures. An increase in self-injury may be observed when aversive methods are used.

Restraint (day and night) and dental procedures are common means to prevent self-injury. The time in restraints is linked to the age of onset of self-injury. Children with LND can participate in making decisions regarding restraints and the type of restraints. The time in restraints may potentially be reduced with systematic behavior treatment programs. Many patients have teeth extracted to prevent self-injury. Others use a protective mouth guard designed by a dentist. Most parents suggest that stress reduction and awareness of the patient's needs are the most effective in reducing self-injury. Positive behavioral techniques of reinforcing appropriate behavior are rated effective by almost half the families.

Deep brain stimulation to the anteroventral internal globus pallidus is a procedure that has successfully treated self-injury and lessened dystonia in several case reports.

Genetics

The disorder is an autosomal recessive trait with considerable clinical heterogeneity. The APRT gene is located on chromosome 16q (16q24.3) and encompasses 2.8 kb of genomic DNA.

Clinical Manifestations

These include urinary calculus formation with crystalluria, urinary tract infections, hematuria, renal colic, dysuria, and acute renal failure. Brownish spots on the infant's diaper or yellow-brown crystals in the urine suggest the diagnosis. The 2,8-dihydroxyadenine is cleared efficiently by the kidneys and so does not accumulate in plasma, but precipitates readily in the renal lumen.

Laboratory Findings

Urinary levels of adenine, 8-hydroxyadenine, and 2,8-dihydroxyadenine are elevated, whereas plasma uric acid is normal. The deficiency may be complete ( type I ) or partial ( type II ); the partial deficiency is reported in Japan. The diagnosis is made based on the level of residual enzyme in erythrocyte lysates. The renal calculi, composed of 2,8-dihydroxyadenine, are radiolucent, soft, and easily crushed. These stones are not distinguishable from uric acid stones by routine tests but require high-performance liquid chromatography (HPLC), ultraviolet (UV) light, infrared light, mass spectrometry, x-ray crystallography, or capillary electrophoresis for diagnosis, particularly to distinguish from stones in HPRT deficiency.

Treatment

Treatment includes high fluid intake, dietary purine restriction, and allopurinol, which inhibits the conversion of adenine to its metabolites and prevents further stone formation. Alkalinization of the urine is to be avoided, because unlike that of uric acid, the solubility of 2,8-dihydroxyadenine does not increase up to pH 9. Shock wave lithotripsy has been reported to be successful. The prognosis depends on renal function at diagnosis. Early treatment is critical in the prevention of stones because severe renal insufficiency may accompany late recognition.

Genetics

Three distinct complementary DNAs for PRPS have been cloned and sequenced. Two forms, PRPS-1 and PRPS-2, are X-linked to Xq22-q24 and Xp22.2-p.22.3 (escapes X inactivation), respectively, and are widely expressed. The 3rd locus maps to human chromosome 7 and appears to be transcribed only in the testes. PRPS-1 defects are thus inherited as X-linked traits and present with varying degrees of severity. The late-onset form of superactivity arises from increased transcription of normal messenger RNA; the cause of this has not been discovered. The early-onset form of superactivity arises from mutations affecting allosteric regulation of the protein that controls feedback inhibition by inorganic phosphate and dinucleotides. At the same time, these mutations destabilize the protein, so that in slow or nonreplicating cells, such as neurons and red blood cells (RBCs), the enzyme becomes inactive. In contrast, the deficiency phenotypes of PRPS-1 are produced by mutations directly affecting enzyme function, usually in the substrate binding site. Even though the defect is X-linked, it should be considered in a child or young adult of either sex with hyperuricemia and/or hyperuricosuria and normal HPRT activity in lysed RBCs.

Clinical Manifestations

Affected hemizygous males with early-onset superactivity show signs of uric acid overproduction that are apparent in infancy or early childhood, as well as psychomotor delay and sensorineural deafness. Hypotonia, ataxia, and autistic-like behavior have been described. Heterozygous female carriers may also develop gout and hearing impairment. The late-onset type is found in males who show only hyperuricemia and hyperuricosuria, but no neurologic signs. The mildest form of PRPS-1 deficiency manifests as progressive postlingual hearing loss in X-linked deafness-2 (DFN2). More severe mutations constitute the Charcot-Marie-Tooth disease X-linked-5 phenotype, which includes peripheral neuropathy, hearing impairment, and optic atrophy. The most severe PRPS-1 mutations occur in patients with Arts syndrome , who also have central neuropathy and an impaired immune system. Females appear to be unaffected, but hemizygous males have usually not survived beyond the 1st decade, typically succumbing to lung disease. SAMe therapy has prolonged survival, although the neurologic deficits, including the deafness, do not appear to be responsive.

A mechanism for the neurologic symptoms is not yet known, but it can be hypothesized that nucleotide depletion is present in neural tissues, including the brain. Abnormalities of hearing and vision are typical of PRPS-1 deficiency, where the absence of this enzyme presumably compromises these highly energy-dependent neural functions. The high transcript level of PRPS-1 in lung and bone marrow also suggests that its absence may be causal for the recurrent lung infections that characterize Arts syndrome.

Laboratory Findings

For PRPS-1 “superactivity” (both juvenile and adult presentations), serum uric acid may be grossly raised and the urinary excretion of uric acid increased. For PRPS “deficiency,” uric acid is normal, not low, probably because PRPS-2 provides the major uric acid–forming activity in liver and other major organs. Diagnosis requires that PRPS-1 activity be measured in erythrocytes and cultured fibroblasts. The adult superactivity disorder must be differentiated from partial HPRT deficiency involving the salvage pathway, which also presents with mild or absent neurologic traits accompanied by hyperuricemia.

Treatment

Treatment of PRPS deficiency, specifically Arts syndrome, has involved mainly experimental therapy with SAMe, as a dietary supplement to correct the depletion of purines. Dietary purines are usually not absorbed into the body but are degraded to uric acid by the gut. SAMe supplementation (beginning at 20 mg/kg/day orally) has been effective in greatly reducing the acute hospitalization episodes of 2 brothers with Arts syndrome, over a period of 10 yr. Treatment of PRPS superactivity is aimed at controlling the hyperuricemia with allopurinol, which inhibits xanthine oxidase, the last enzyme of the purine catabolic pathway. Uric acid production is reduced and is replaced by hypoxanthine, which is more soluble, and xanthine. The initial dose of allopurinol is 10-20 mg/kg/24 hr in children and is adjusted to maintain normal uric acid levels in plasma. The risk of xanthine stone formation is similar to that described for LND. A low-purine diet (free of organ meats, dried beans, and sardines), high fluid intake, and alkalinization of the urine to establish a urinary pH of 6.0-6.5 are necessary. These measures control the hyperuricemia and urate nephropathy but do not affect the neurologic symptoms. There is no known treatment for the neurologic complications.

Genetics

Succinylpurinuria is an autosomal recessive disorder; the gene has been mapped to chromosome 22q13.1-q13.2, and approximately 50 gene mutations have been identified. Laboratory investigations show grossly raised succinylpurines in urine and CSF, which are normally undetectable.

Clinical Manifestations

The fatal neonatal form presents with lethal encephalopathy. Clinical manifestations include varying degrees of psychomotor retardation, generally accompanied by a seizure disorder and autistic-like behaviors (poor eye contact and repetitive behaviors). Neonatal seizures and a severe infantile epileptic encephalopathy are often the first manifestations of this disorder. Others demonstrate moderate to severe intellectual disability, sometimes associated with growth retardation and muscle hypotonia. One female tested in the mild range of intellectual disability. The form of adenylosuccinase lyase deficiency with profound intellectual disability has been designated type I and the variant case with mild intellectual disability type II . Other patients have an intermediate clinical symptom pattern with moderately delayed psychomotor development, seizures, stereotypies, and agitation.

Pathology

CT and MRI of the brain may show hypotrophy or hypoplasia of the cerebellum, particularly the vermis. It is proposed that rather than being caused by purine nucleotide depletion, the symptoms are from the neurotoxic effects of accumulating succinylpurines. The S-Ado: SAICAr ratio has been linked to phenotype severity, suggesting that SAICAr is the more toxic compound and that S-Ado might be neuroprotective.

The laboratory diagnosis is based on the presence in urine and CSF of SAICAr and S-Ado, both normally undetectable.

Treatment

No successful treatment has been demonstrated for adenylosuccinase lyase deficiency. SAMe supplementation therapy was tested for 6 mo for an infant diagnosed in the early postnatal period, but no amelioration of symptoms was noted, providing further evidence that the disorder arises from nucleotide toxicity rather than depletion. Prenatal diagnosis has been reported. Systematic screening is suggested in infants and children with unexplained psychomotor retardation or seizure disorder.

Genetics

This inborn error of purine biosynthesis is caused by a mutation of the ATIC gene effecting AICAR transformylase activity. In a single reported case, AICAR transformylase was profoundly deficient, whereas the IMP cyclohydrolase level was 40% of normal.

Clinical Features

The disorder is described in a female infant with profound intellectual disability, epilepsy, dysmorphic features (prominent forehead and metopic suture, brachycephaly, wide mouth with thin upper lip, low-set ears, and prominent clitoris because of fused labia minora), and congenital blindness.

Laboratory Findings

Urinary screening with the Bratton-Marshall test to detect AICA resulted in the identification of this disorder. The transformylase was found to be deficient in fibroblasts, confirming diagnosis of ATIC deficiency.

Treatment

No successful treatment is described.

Clinical Manifestations

Clinical manifestations are typically isolated muscle weakness, fatigue, myalgias following moderate to vigorous exercise, or cramps. Myalgia may be associated with an increased serum creatine kinase level and detectable electromyelographic abnormalities. Muscle wasting or histologic changes on biopsy are absent. The age of onset may be as early 8 mo, with approximately 25% of cases recognized between 2 and 12 yr. The enzyme defect has been identified in asymptomatic family members. Secondary forms of muscle AMP deaminase deficiency have been identified in Werdnig-Hoffmann disease, Kugelberg-Welander syndrome, polyneuropathies, and amyotrophic lateral sclerosis (see Chapter 630.2 ). The metabolic disorder involves the purine nucleotide cycle. As shown in Fig. 108.2 , the enzymes involved in this cycle are AMP deaminase, S-AMP synthetase, and S-AMP lyase. It is proposed that muscle dysfunction in AMP deaminase deficiency results from impaired energy production during muscle contraction. It is unclear how individuals may carry the deficit and be asymptomatic. In addition to muscle dysfunction, a mutation of liver AMP deaminase has been proposed as a cause of primary gout, leading to overproduction of uric acid.