431e: Disorders of Purine and Pyrimidine Metabolism

Purines (adenine and guanine) and pyrimidines (cytosine, thymine, uracil) serve fundamental roles in the replication of genetic material, gene transcription, protein synthesis, and cellular metabolism. Disorders that involve abnormalities of nucleotide metabolism range from relatively common diseases such as hyperuricemia and gout, in which there is increased production or impaired excretion of a metabolic end product of purine metabolism (uric acid), to rare enzyme deficiencies that affect purine and pyrimidine synthesis or degradation. Understanding these biochemical pathways has led, in some instances, to the development of specific forms of treatment, such as the use of allopurinol and febuxostat to reduce uric acid production.

Uric acid is the final breakdown product of purine degradation in humans. It is a weak diprotic acid with pK_a values of 5.75 and 10.3. Urates, the ionized forms of uric acid, predominate in plasma, extracellular fluid, and synovial fluid, with ~98% existing as monosodium urate at pH 7.4.

Plasma is saturated with monosodium urate at a concentration of 405 μmol/L (6.8 mg/dL) at 37°C. At higher concentrations, plasma is therefore supersaturated—a situation that creates the potential for urate crystal precipitation. However, plasma urate concentrations can reach 4800 μmol/L (80 mg/dL) without precipitation, perhaps because of the presence of solubilizing substances.

The pH of urine greatly influences the solubility of uric acid. At pH 5.0, urine is saturated with uric acid at concentrations ranging from 360 to 900 μmol/L (6–15 mg/dL). At pH 7, saturation is reached at concentrations from 9840 to 12,000 μmol/L (158–200 mg/dL). Ionized forms of uric acid in urine include monosodium, disodium, potassium, ammonium, and calcium urates.

Although purine nucleotides are synthesized and degraded in all tissues, urate is produced only in tissues that contain xanthine oxidase, primarily the liver and small intestine. Urate production varies with the purine content of the diet and with rates of purine biosynthesis, degradation, and salvage (Fig. 431e-1). Normally, two-thirds to three-fourths of urate is excreted by the kidneys, and most of the remainder is eliminated through the intestines.

The kidneys clear urate from the plasma and maintain physiologic balance by utilizing specific organic anion transporters (OATs), including urate transporter 1 (URAT1, SLC22A12) (Fig. 431e-2). In humans, OAT1 (SLC22A6), OAT2 (SLC22A7), and OAT3 (SLC22A8) are located on the basolateral membrane of renal proximal tubule cells. OAT4 (SLC22A11), OAT10 (SLC22A13), and URAT1 are located on the apical brush-border membrane of these cells. The latter transporters carry urate and other organic anions into the tubular cells from the lumen in exchange for intracellular organic anions. Once inside the cell, urate must pass to the basolateral side of the lumen in a process controlled by voltage-dependent carriers, including glucose transporter 9 (GLUT9, SLC2A9). Uricosuric compounds (Table 431e-1) directly inhibit URAT1 on the apical side of the tubular cell (so-called cis-inhibition). In contrast, antiuricosuric compounds (those that promote hyperuricemia), such as nicotinate, pyrazinoate, lactate, and other aromatic organic acids, serve as the exchange anion inside the cell, thereby stimulating anion exchange and urate reabsorption (trans-stimulation). The activities of URAT1, other OATs, and sodium anion transporters result in excretion of 8–12% of the filtered urate as uric acid.

Most children have serum urate concentrations of 180–240 μmol/L (3–4 mg/dL). Levels begin to rise in males during puberty but remain low in females until menopause. The most recent mean serum urate values for men and premenopausal women in the United States are 415 and 360 μmol/L (6.14 and 4.87 mg/dL), respectively, according to National Health and Nutrition Evaluation Survey (NHANES) data for 2007–2008. After menopause, values for women increase to approximately those for men. In adulthood, concentrations rise steadily over time and vary with height, body weight, blood pressure, renal function, and alcohol intake.

Hyperuricemia can result from increased production or decreased excretion of uric acid or from a combination of the two processes. Sustained hyperuricemia predisposes some individuals to develop clinical manifestations including gouty arthritis (Chap. 395), urolithiasis, and renal dysfunction (see below).

In general, hyperuricemia is defined as a plasma (or serum) urate concentration >405 μmol/L (>6.8 mg/dL). The risk of developing gouty arthritis or urolithiasis increases with higher urate levels and escalates in proportion to the degree of elevation. The prevalence of hyperuricemia is increasing among ambulatory adults and even more markedly among hospitalized patients. The prevalence of gout in the United States more than doubled between the 1960s and the 1990s. Based on NHANES data from 2007–2008, these trends continue, with an approximate prevalence of gout among men of 5.9% (6.1 million) and among women of 2.0% (2.2 million). Mean serum urate levels rose to 6.14 mg/dL among men and 4.87 mg/dL among women, with consequent hyperuricemia prevalences of 21.2% and 21.6%, respectively (with hyperuricemia defined as a serum urate level of >7.0 mg/dL [415 μmol/L] for men and >5.7 mg/dL [340 μmol/L] for women). These numbers represent a 1.2% increase in the prevalence of gout, a 0.15-mg/dL increase in the serum urate level, and a 3.2% increase in the prevalence of hyperuricemia over figures reported in NHANES-III (1988–1994). These rises are thought to be driven by increased obesity and hypertension and perhaps also by better medical care and increased longevity.

CAUSES OF HYPERURICEMIA

Hyperuricemia may be classified as primary or secondary, depending on whether the cause is innate or an acquired disorder. However, it is more useful to classify hyperuricemia in relation to the underlying pathophysiology—i.e., whether it results from increased production, decreased excretion, or a combination of the two (Fig. 431e-1, Table 431e-2).

Increased Urate Production

Diet contributes to the serum urate concentration in proportion to its purine content. Strict restriction of purine intake reduces the mean serum urate level by ~60 μmol/L (~1 mg/dL) and urinary uric acid excretion by ~1.2 mmol/d (~200 mg/d). Foods high in nucleic acid content include liver, “sweetbreads” (i.e., thymus and pancreas), kidney, and anchovy.

Endogenous sources of purine production also influence the serum urate level (Fig. 431e-3). De novo purine biosynthesis is a multistep process that forms inosine monophosphate (IMP). The rates of purine biosynthesis and urate production are predominantly determined by amidophosphoribosyltransferase (amidoPRT), which combines phosphoribosylpyrophosphate (PRPP) and glutamine. A secondary regulatory pathway is the salvage of purine bases by hypoxanthine phosphoribosyltransferase (HPRT). HPRT catalyzes the combination of the purine bases hypoxanthine and guanine with PRPP to form the respective ribonucleotides IMP and guanosine monophosphate (GMP).

Serum urate levels are closely coupled to the rates of de novo purine biosynthesis, which is driven in part by the level of PRPP, as evidenced by two X-linked inborn errors of purine metabolism (Table 431e-3). Both increased PRPP synthetase activity and HPRT deficiency are associated with overproduction of purines, hyperuricemia, and hyperuricaciduria (see below for clinical descriptions).

Accelerated purine nucleotide degradation can also cause hyperuricemia—i.e., with conditions of rapid cell turnover, proliferation, or cell death, as in leukemic blast crises, cytotoxic therapy for malignancy, hemolysis, or rhabdomyolysis. Hyperuricemia can result from excessive degradation of skeletal muscle ATP after strenuous physical exercise or status epilepticus and in glycogen storage disease types III, V, and VII (Chap. 433e). The hyperuricemia of myocardial infarction, smoke inhalation, and acute respiratory failure may also be related to accelerated breakdown of ATP.

Decreased Uric Acid Excretion

More than 90% of individuals with sustained hyperuricemia have a defect in the renal handling of uric acid. For any given plasma urate concentration, patients who have gout excrete ~40% less uric acid than those who do not. When plasma urate levels are raised by purine ingestion or infusion, uric acid excretion increases in patients with and without gout; however, in those with gout, plasma urate concentrations must be 60–120 μmol/L (1–2 mg/dL) higher than normal to achieve equivalent uric acid excretion rates.

Diminished uric acid excretion could theoretically result from decreased glomerular filtration, decreased tubular secretion, or enhanced tubular reabsorption. Decreased urate filtration does not appear to cause primary hyperuricemia but does contribute to the hyperuricemia of renal insufficiency. Although hyperuricemia is invariably present in chronic renal disease, the correlation among serum creatinine, urea nitrogen, and urate concentrations is poor. Extrarenal clearance of uric acid increases as renal damage becomes more severe.

Many agents that cause hyperuricemia exert their effects by stimulating reabsorption rather than inhibiting secretion. This stimulation appears to occur through a process of “priming” renal urate reabsorption through the sodium-dependent loading of proximal tubular epithelial cells with anions capable of trans-stimulating urate reabsorption. The sodium-coupled monocarboxyl transporters SMCT1 and 2 (SLC5A8, SLC5A12) in the brush border of the proximal tubular cells mediate sodium-dependent loading of these cells with monocarboxylates. A similar transporter, SLC13A3, mediates sodium-dependent influx of dicarboxylates into the epithelial cell from the basolateral membrane. Some of these carboxylates are well known to cause hyperuricemia, including pyrazinoate (from pyrazinamide treatment), nicotinate (from niacin therapy), and the organic acids lactate, β-hydroxybutyrate, and acetoacetate. The mono- and divalent anions then become substrates for URAT1 and OAT4, respectively, and are exchanged for uric acid from the proximal tubule. Increased blood levels of these anions result in their increased glomerular filtration and greater reabsorption by proximal tubular cells. The increased intraepithelial cell concentrations lead to increased uric acid reabsorption by promoting URAT1-, OAT4-, and OAT10-dependent anion exchange. Low doses of salicylates also promote hyperuricemia by this mechanism. Sodium loading of proximal tubular cells also provokes urate retention by reducing extracellular fluid volume and increasing angiotensin II, insulin, and parathyroid hormone release. Additional organic anion transporters OAT1, OAT2, and OAT3 are involved in the movement of uric acid through the basolateral membrane, although the detailed mechanisms are still being elucidated.

GLUT9 (SLC2A9) is an electrogenic hexose transporter with splicing variants that mediate co-reabsorption of uric acid along with glucose and fructose at the apical membrane (GLUT9ΔN/SLC2A9v2) as well as through the basolateral membrane (SLC2A9v1) and thus into the circulation. GLUT9 has recently been identified as a high-capacity urate transporter, with rates 45–60 times faster than its glucose/fructose transport activity. GLUT9 may be responsible for the observed association of the consumption of fructose-sweetened soft drinks with an increased risk of hyperuricemia and gout. Genome-wide association scanning suggests that polymorphisms in SLC2A9 may play an important role in susceptibility to gout in the Caucasian population. The presence of one predisposing variant allele increases the relative risk of developing gout by 30–70%, most likely by increasing expression of the shorter isoform, SLC2A9v2 (GLUT9ΔN). Notably, though, genetic polymorphisms explain only ~6% of the differences in serum uric acid levels in Caucasians. Clearly, gout is polygenic and complex, and at this time the utility of genetic testing for relevant polymorphisms remains investigational and of no clinical utility.

Alcohol promotes hyperuricemia because of increased urate production and decreased uric acid excretion. Excessive alcohol consumption accelerates hepatic breakdown of ATP to increase urate production. Alcohol consumption can also induce hyperlacticacidemia, which blocks uric acid secretion. The higher purine content in some alcoholic beverages may also be a factor. Consumption of beer confers a greater risk of gout than liquor, and moderate wine intake does not increase gout risk. Intake of red meat and fructose increases the risk of gout, whereas intake of low-fat dairy products, purine-rich vegetables, whole grains, nuts and legumes, less sugary fruits, coffee, and vitamin C reduces the risk.

EVALUATION

Hyperuricemia does not necessarily represent a disease, nor is it a specific indication for therapy. The decision to treat depends on the cause and the potential consequences of hyperuricemia in each individual.

Quantification of uric acid excretion can be used to determine whether hyperuricemia is caused by overproduction or decreased excretion. On a purine-free diet, men with normal renal function excrete <3.6 mmol/d (600 mg/d). Thus, the hyperuricemia of individuals who excrete uric acid above this level while on a purine-free diet is due to purine overproduction; for those who excrete lower amounts on the purine-free diet, it is due to decreased excretion. If the assessment is performed while the patient is on a regular diet, the level of 4.2 mmol/d (800 mg/d) can be used as the discriminating value.

COMPLICATIONS

The most recognized complication of hyperuricemia is gouty arthritis. NHANES 2007–2008 found a prevalence of gout among U.S. adults of 3.9%, with figures of ~6% for men and ~2% for women. The higher the serum urate level, the more likely an individual is to develop gout. In one study, the incidence of gout was 4.9% among individuals with serum urate concentrations >540 μmol/L (>9.0 mg/dL) as opposed to only 0.5% among those with values between 415 and 535 μmol/L (7.0 and 8.9 mg/dL). The complications of gout correlate with both the duration and the severity of hyperuricemia. For further discussion of gout, see Chap. 395.

Hyperuricemia also causes several renal problems: (1) nephrolithiasis; (2) urate nephropathy, a rare cause of renal insufficiency attributed to monosodium urate crystal deposition in the renal interstitium; and (3) uric acid nephropathy, a reversible cause of acute renal failure resulting from deposition of large amounts of uric acid crystals in the renal collecting ducts, pelvis, and ureters.

Nephrolithiasis

Uric acid nephrolithiasis occurs most commonly, but not exclusively, in individuals with gout. In gout, the prevalence of nephrolithiasis correlates with the serum and urinary uric acid levels, reaching ~50% with serum urate levels of 770 μmol/L (13 mg/dL) or urinary uric acid excretion >6.5 mmol/d (1100 mg/d).

Uric acid stones can develop in individuals with no evidence of arthritis, only 20% of whom are hyperuricemic. Uric acid can also play a role in other types of kidney stones. Some individuals who do not have gout but have calcium oxalate or calcium phosphate stones have hyperuricemia or hyperuricaciduria. Uric acid may act as a nidus on which calcium oxalate can precipitate or lower the formation product for calcium oxalate crystallization.

Urate Nephropathy

Urate nephropathy, sometimes referred to as urate nephrosis, is a late manifestation of severe gout and is characterized histologically by deposits of monosodium urate crystals surrounded by a giant-cell inflammatory reaction in the medullary interstitium and pyramids. The disorder is now rare and cannot be diagnosed in the absence of gouty arthritis. The lesions may be clinically silent or cause proteinuria, hypertension, and renal insufficiency.

Uric Acid Nephropathy

This reversible cause of acute renal failure is due to precipitation of uric acid in renal tubules and collecting ducts that obstructs urine flow. Uric acid nephropathy develops following sudden urate overproduction and marked hyperuricaciduria. Factors that favor uric acid crystal formation include dehydration and acidosis. This form of acute renal failure occurs most often during an aggressive “blastic” phase of leukemia or lymphoma prior to or coincident with cytolytic therapy but has also been observed in individuals with other neoplasms, following epileptic seizures, and after vigorous exercise with heat stress. Autopsy studies have demonstrated intraluminal precipitates of uric acid, dilated proximal tubules, and normal glomeruli. The initial pathogenic events are believed to include obstruction of collecting ducts with uric acid and obstruction of the distal renal vasculature.

If recognized, uric acid nephropathy is potentially reversible. Appropriate therapy has reduced the mortality rate from ~50% to practically nil. Serum levels cannot be relied on for diagnosis because this condition has developed in the presence of urate concentrations varying from 720 to 4800 μmol/L (12–80 mg/dL). The distinctive feature is the urinary uric acid concentration. In most forms of acute renal failure with decreased urine output, urinary uric acid content is either normal or reduced, and the ratio of uric acid to creatinine is <1. In acute uric acid nephropathy, the ratio of uric acid to creatinine in a random urine sample or a 24-h specimen is >1, and a value that high is essentially diagnostic.

HYPERURICEMIA AND METABOLIC SYNDROME

Metabolic syndrome (Chap. 422) is characterized by abdominal obesity with visceral adiposity, impaired glucose tolerance due to insulin resistance with hyperinsulinemia, hypertriglyceridemia, increased low-density lipoprotein cholesterol, decreased high-density lipoprotein cholesterol, and hyperuricemia. Hyperinsulinemia reduces the renal excretion of uric acid and sodium. Not surprisingly, hyperuricemia resulting from euglycemic hyperinsulinemia may precede the onset of type 2 diabetes, hypertension, coronary artery disease, and gout in individuals with metabolic syndrome.

TREATMENT

Hypouricemia, defined as a serum urate concentration <120 μmol/L (<2.0 mg/dL), can result from decreased production of urate, increased excretion of uric acid, or a combination of both mechanisms. This condition occurs in <0.2% of the general population and <0.8% of hospitalized individuals. Hypouricemia causes no symptoms or pathology and therefore requires no therapy.

Most hypouricemia results from increased renal uric acid excretion. The finding of normal amounts of uric acid in a 24-h urine collection from an individual with hypouricemia is evidence for a renal cause. Medications with uricosuric properties (Table 431e-1) include aspirin (at doses >2.0 g/d), losartan, fenofibrate, x-ray contrast materials, and glyceryl guaiacolate. Total parenteral hyperalimentation can also cause hypouricemia, possibly a result of the high glycine content of the infusion formula. Other causes of increased urate clearance include conditions such as neoplastic disease, hepatic cirrhosis, diabetes mellitus, and inappropriate secretion of vasopressin; defects in renal tubular transport such as primary Fanconi syndrome and Fanconi syndromes caused by Wilson’s disease, cystinosis, multiple myeloma, and heavy metal toxicity; and isolated congenital defects in the bidirectional transport of uric acid. Hypouricemia can be a familial disorder that is generally inherited in an autosomal recessive manner. Most cases are caused by a loss of function mutation in SLC22A12, the gene that encodes URAT-1, resulting in increased renal urate clearance. Individuals with normal SLC22A12 most likely have a defect in other urate transporters. Although hypouricemia is usually asymptomatic, some patients suffer from urate nephrolithiasis or exercise-induced renal failure.

(See also Table 431e-3, Table 431e-4, Fig. 431e-3, and Fig. 431e-4) More than 30 defects in human purine and pyrimidine metabolic pathways have been identified thus far. Many are benign, but about half are associated with clinical manifestations, some causing major morbidity and mortality. Advances in genetics, along with high-performance liquid chromatography and tandem mass spectrometry, have facilitated diagnosis.

PURINE DISORDERS

HPRT Deficiency

The HPRT gene is located on the X chromosome. Affected males are hemizygous for the mutant gene; carrier females are asymptomatic. A complete deficiency of HPRT, the Lesch-Nyhan syndrome, is characterized by hyperuricemia, self-mutilative behavior, choreoathetosis, spasticity, and mental retardation. A partial deficiency of HPRT, the Kelley-Seegmiller syndrome, is associated with hyperuricemia but no central nervous system manifestations. In both disorders, the hyperuricemia results from urate overproduction and can cause uric acid crystalluria, nephrolithiasis, obstructive uropathy, and gouty arthritis. Early diagnosis and appropriate therapy with allopurinol can prevent or eliminate all the problems attributable to hyperuricemia without affecting behavioral or neurologic abnormalities.

Increased PRPP Synthetase Activity

Like the HPRT deficiency states, PRPP synthetase overactivity is X-linked and results in gouty arthritis and uric acid nephrolithiasis. Nerve deafness occurs in some families.

Adenine Phosphoribosyltransferase (APRT) Deficiency

APRT deficiency is inherited as an autosomal recessive trait. Affected individuals develop kidney stones composed of 2,8-dihydroxyadenine. Caucasians with the disorder have a complete deficiency (type I), whereas Japanese individuals have some measurable enzyme activity (type II). Expression of the defect is similar in the two populations, as is the frequency of the heterozygous state (0.4–1.1 per 100). Allopurinol treatment prevents stone formation.

Hereditary Xanthinuria

A deficiency of xanthine oxidase causes all purine in the urine to occur in the form of hypoxanthine and xanthine. About two-thirds of deficient individuals are asymptomatic. The remainder develop kidney stones composed of xanthine.

Myoadenylate Deaminase Deficiency

Primary (inherited) and secondary (acquired) forms of myoadenylate deaminase deficiency have been described. The primary form is inherited as an autosomal recessive trait. Clinically, some persons may have relatively mild myopathic symptoms with exercise or other triggers, but most individuals with this defect are asymptomatic. Therefore, another explanation for the myopathy should be sought in symptomatic patients with this deficiency. The acquired deficiency occurs in association with a wide variety of neuromuscular diseases, including muscular dystrophies, neuropathies, inflammatory myopathies, and collagen vascular diseases.

Adenylosuccinate Lyase Deficiency

Deficiency of this enzyme is due to an autosomal recessive trait and causes profound psychomotor retardation, seizures, and other movement disorders. All individuals with this deficiency are mentally retarded, and most are autistic.

Adenosine Deaminase Deficiency and Purine Nucleoside Phosphorylase Deficiency

See Chap. 374.

PYRIMIDINE DISORDERS

The pyrimidine cytidine is found in both DNA and RNA; it is a complementary base pair for guanine. Thymidine is found only in DNA, where it is paired with adenine. Uridine is found only in RNA and can pair with either adenine or guanine in RNA secondary structures. Pyrimidines can be synthesized by a de novo pathway (Fig. 431e-4) or reused in a salvage pathway. Although more than 25 different enzymes are involved in pyrimidine metabolism, disorders of these pathways are rare. Seven disorders of pyrimidine metabolism have been discovered (Table 431e-4), three of which are discussed below.

Orotic Aciduria

Hereditary orotic aciduria is caused by mutations in a bifunctional enzyme, uridine-5′-monophosphate (UMP) synthase, which converts orotic acid to UMP in the de novo synthesis pathway (Fig. 431e-4). The disorder is characterized by hypochromic megaloblastic anemia that is unresponsive to vitamin B₁₂ and folic acid, growth retardation, and neurologic abnormalities. Increased excretion of orotic acid causes crystalluria and obstructive uropathy. Replacement of uridine (100–200 mg/kg per day) corrects anemia, reduces orotic acid excretion, and improves the other sequelae of the disorder.

Pyrimidine 5′-nucleotidase Deficiency

Pyrimidine 5′-nucleotidase catalyzes the removal of the phosphate group from pyrimidine ribonucleoside monophosphates (cytidine-5′-monophosphate or UMP) (Fig. 431e-4). An inherited deficiency of this enzyme causes hemolytic anemia with prominent basophilic stippling of erythrocytes. The accumulation of pyrimidines or cytidine diphosphate choline is thought to induce hemolysis. There is no specific treatment. Acquired pyrimidine 5′-nucleotidase deficiency has been reported in lead poisoning and in thalassemia.

Dihydropyrimidine Dehydrogenase Deficiency

Dihydropyrimidine dehydrogenase is the rate-limiting enzyme in the pathway of uracil and thymine degradation (Fig. 431e-4). Deficiency of this enzyme causes excessive urinary excretion of uracil and thymine. In addition, this deficiency causes nonspecific cerebral dysfunction with convulsive disorders, motor retardation, and mental retardation. No specific treatment is available.

Medication Effect on Pyrimidine Metabolism

A variety of medications can influence pyrimidine metabolism. The anticancer agents fluorodeoxyuridine and 5-fluorouracil and the antimicrobial agent fluorocytosine cause cytotoxicity when converted to fluorodeoxyuridylate, a specific suicide inhibitor of thymidylate synthase. Fluorocytosine must be converted to 5-fluorouracil to be effective. This conversion is catalyzed by cytosine deaminase activity. Fluorocytosine’s action is selective because cytosine deaminase is present in bacteria and fungi but not in human cells. Dihydropyrimidine dehydrogenase is involved in the degradation of 5-fluorouracil. Consequently, deficiency of this enzyme is associated with 5-fluorouracil neurotoxicity.

Leflunomide, which is used to treat rheumatoid arthritis, inhibits de novo pyrimidine synthesis by inhibiting dihydroorotate dehydrogenase, resulting in an antiproliferative effect on T cells. Allopurinol, which inhibits xanthine oxidase in the purine metabolic pathway, also inhibits the activity of orotidine-5′-phosphate decarboxylase, a step in UMP synthesis. Consequently, allopurinol use is associated with increased excretion of orotidine and orotic acid. There are no known clinical effects of this inhibition.