CHAPTER 110 UREA CYCLE DISORDERS

The urea cycle is a sequence of six enzymatic and two transport steps necessary to metabolize and excrete the nitrogen generated by the breakdown of amino acids in protein and other nitrogen-containing molecules (Fig. 110-1). The diet and the breakdown of endogenous tissues, particularly of skeletal muscle, are important sources of protein. Endogenous protein breakdown during episodes of acute catabolism presents the deficient ureagenic system with an overwhelming burden and results in the hyperammonemia that occurs in acute infections, after parturition, or during the menstrual cycle.¹ The complete urea cycle is found only in the liver, although individual enzymes are present at lesser levels in other organs and may have additional metabolic roles. Severe liver disease with biosynthetic failure may also result in a deficient urea cycle and hyperammonemia. The first three enzymes in this cycle, N-acetylglutamate synthase (NAGS), carbamoyl phosphate synthase I (CPSI), and ornithine transcarbamylase (OTC) function inside mitochondria, and the latter three, argininosuccinic acid synthase, argininosuccinic acid lyase (ASL), and arginase, act in the cytosol. The two transporters are for ornithine and aspartate. Defects in citrin, the transporter for aspartate, causes citrin deficiency, also called citrullinemia type II. Defects in ornithine translocase, the transporter for ornithine, causes ornithine translocase deficiency (ORNT1), also called hyperammonemia, hyperornithinemia, and homocitrullinuria syndrome.

Figure 110-1 The six primary enzymatic steps of the urea cycle are shown in bold capital letters. Function of ornithine transporter (ORNT) and aspartate transporter (CITR) is necessary for the movement of substrates in and out of the mitochondrion. ARG1, arginase; ASL, argininosuccinic acid lyase; ASS, argininosuccinic acid synthase; Co-A, coenzyme A; CP, carbamoyl phosphate; CPS-1, carbamoyl phosphate synthase; NAG, N-acetylglutamate; NAGS, N-acetylglutamate synthase; NO, nitric oxide; NOS, nitric oxide synthase; OTC, ornithine transcarbamylase.

Defects in all six steps of the urea cycle and in the transporters are known. Any deficiency of these proteins may result in the accumulation of excess ammonia in the body. Ammonia is toxic to the central nervous system, and any continuous or intermittent elevation of ammonia can result in encephalopathy and neurological damage. This damage can lead to seizures, psychosis, mental retardation, and death. The essential genetic characteristics of the eight disorders are summarized in Table 110-1.

TABLE 110-1 Disorders of the Urea Cycle^*

DEFINITION

The diagnosis of a urea cycle disorder is based on clinical examination and on biochemical, enzymatic, and molecular analyses. A urea cycle defect is first suspected in an infant with anorexia, alterations in respiratory function and thermoregulation, lethargy, seizures, and deteriorating neurological status or in a child with decreased appetite, vomiting, lethargy, behavioral abnormalities, and an altered finding on neurological examination. In the affected older child or adult, blood ammonia determination should be part of the evaluation of any acute encephalopathy or recurrent late-onset psychosis or somnolence.² The diagnosis is supported by an elevated plasma ammonia concentration with a normal anion gap and a normal serum glucose concentration (Fig. 110-2). An encephalopathic electroencephalographic pattern during an episode of hyperammonemia and evidence of brain atrophy on magnetic resonance imaging, although nonspecific, provide further support for the diagnosis of a urea cycle defect.

Figure 110-2 Sequence of tests used in the differential diagnosis of hyperammonemia and urea cycle disorders. ASL, argininosuccinic acid lyase; ASS, argininosuccinic acid synthase; CPSI, carbamoyl phosphate synthase I; NAGS, N-acetylglutamate synthase; OTC, ornithine transcarbamylase.

(Modified from Summar ML, Tuchman M: Urea Cycle Disorders Overview. In GeneReviews at GeneTests. Seattle: University of Washington, Seattle, April 2003.)

Plasma quantitative amino acid analysis can be used to aid in the delineation of the specific urea cycle disorder. Plasma amino acid analysis reveals reduced levels of arginine in all urea cycle disorders except arginase deficiency, in which arginine levels are elevated. Citrulline levels can also aid in discriminating the various urea cycle defects. Citrulline is produced by the first three enzymes, NAGS, CPSI, and OTC, and decreased levels are found when the level of any of these enzymes is deficient. In contrast, citrulline levels are increased with deficiencies of argininosuccinic acid synthase and ASL, because citrulline serves as a substrate for these more distal reactions. Urinary orotic acid levels are also used to differentiate CPSI and NAGS deficiency from OTC deficiencies. In the former, orotic acid levels are normal or reduced, whereas in the latter, they are elevated. A definitive diagnosis is made through measurement of enzyme activity, often from a liver tissue sample. If liver biopsy is not possible, diagnosis can be based on family history, clinical presentation, amino acid and orotic acid testing, and molecular genetic testing. These laboratory studies are carried out in highly specialized laboratories, which can be found on the GeneTests website (www.genetests.org).

EPIDEMIOLOGY

Seven of the eight urea cycle disorders—NAGS, CPSI, argininosuccinic acid synthase, ASL, arginase, ORNT1, and citrin deficiencies—are inherited as autosomal recessive traits. OTC deficiency is an X-linked disorder. Because of the absence of nationwide neonatal screening for these disorders, the true incidence of the individual urea cycle defects is not known. The estimated combined incidence for all urea cycle defects is 1 per 8200. OTC deficiency has the highest incidence, and arginase and NAGS deficiencies, the lowest.

CLINICAL FEATURES

Severity of symptoms and age at onset are related, at least partially, to the position of the deficient enzyme in the pathway and to the degree of the enzymatic defect. Urea cycle disorders usually manifest either in the neonatal period or later in childhood. Manifestation in the neonatal period results from severe deficiency one of the first five enzymes in the cycle: NAGS, CPSI, OTC, argininosuccinic acid synthase, or ASL. Clinical manifestation after the neonatal period usually results from milder or partial defects of these enzymes, from arginase deficiency, or from disorders of one of the two transporters.

Severe deficiency of any of the urea cycle enzymes except arginase results in the accumulation of ammonia and other intermediate metabolites during the first few days of life. Unlike these disorders, arginase, ORNT1, and citrin deficiencies infrequently result in symptomatic elevation of plasma ammonia in the neonatal period and are the mildest of the eight urea cycle disorders. In patients with partial defects of these enzymes, a metabolic crisis with ammonia accumulation may be triggered by intercurrent illnesses or by stress at almost any time in life. Although these disorders share common symptoms, the severity and age at first manifestation can vary a great deal between and within the specific disorders.

Many newborns with a severe enzyme deficiency initially appear well but rapidly develop hyperammonemia and lethargy, anorexia, abnormal respiratory patterns, hypothermia, seizures, abnormal posturing, and deterioration into coma. This process is accompanied by cerebral edema. Severe deficiency of NAGS, CPSI, OTC, argininosuccinic acid synthase, or ASL, the first five enzymes in the cycle, almost invariably manifests within the first few days after birth and has a high mortality rate. Children with arginase, ORNT1 and citrin deficiencies can present in childhood, but episodes of symptomatic hyperammonemia are uncommon. In partial urea cycle enzyme deficiencies, individuals do well until an intercurrent illness or other stress results in a metabolic crisis with ammonia accumulation. In these individuals, the first recognized clinical episode may be delayed for months or years, and these patients typically have serial and milder elevations of plasma ammonia concentration throughout their lives. These individuals may have only recurrent abdominal pain, and the first indication of an inborn error may be developmental delay resulting from ammonia intoxication.

Although the clinical signs and symptoms of the specific urea cycle disorders vary to a degree, a typical hyperammonemic episode is marked by loss of appetite, vomiting, lethargy, and behavioral abnormalities. The episode can be quite subtle and nonspecific. These initial symptoms progress to coma if there is no therapeutic intervention. Abnormal posturing and encephalopathy are often related to the degree of central nervous system swelling and pressure on the brainstem. Seizures are common with severe hyperammonemia and are present in about half of affected patients. Respiratory alkalosis secondary to the hyperventilation caused by cerebral edema is a common early finding. Hypoventilation and respiratory arrest can occur as pressure on the brainstem increases.

Deficiency of the sixth enzyme, arginase, typically manifests in childhood with growth failure, developmental delay, and/or school failure and affects primarily the central nervous system. Episodic hyperammonemia of variable degree can occur but is rarely severe enough to be life-threatening. Typically, birth and early childhood are normal. At the age of 1 to 3 years, there is growth failure, and spasticity begins to develop. Soon, development, previously normal, slows or stops, and the child begins to lose previously achieved developmental milestones. If untreated, arginase deficiency progresses to severe spasticity with joint contractures, loss of ambulation, and severe mental retardation. Seizures are common and can usually be well controlled.

Neurotoxicity

A common manifestation of all urea cycle disorders is episodic encephalopathy associated with hyperammonemia. Although ammonia is a well-recognized neurotoxin, the nature and specific effect that hyperammonemia may have on the central nervous system is not well understood. During a crisis, hyperammonemia causes increased blood-brain barrier permeability, depletion of intermediates of energy metabolism, and disaggregation of microtubules. Ammonia is toxic to the central nervous system even when levels are only mildly elevated, as during long-term therapy. Mildly elevated ammonia levels may cause alterations of axonal development and alterations in brain amino acid and neurotransmitter levels.

Glutamine, an amino acid usually in equilibrium with ammonia and present in much higher levels in the blood, is also a likely proximate toxin. The elevated levels of glutamine in blood are mirrored in the cerebrospinal fluid and have been associated with astrocyte swelling and cerebral hypercirculation. Although cerebrospinal fluid glutamine is not usually monitored, instances of patients with neurological symptoms disproportionate to plasma ammonia levels have been associated with higher elevations of cerebrospinal fluid glutamine levels. Elevated glutamine levels may also cause neurotransmitter abnormalities. Chronically elevated glutamine levels stimulate the transport of large neutral amino acids, including tryptophan. Elevated amounts of tryptophan are converted to serotonin and quinolinic acid, both levels of which are elevated in the brains of OTC-deficient patients. These changes in serotonin metabolism may contribute to the behavioral, sleep and feeding problems seen in patients with urea cycle disorders. Clinicians’ ability to measure brain glutamine by magnetic resonance spectroscopy is improving, and these studies may become an essential part of the evaluation of any patient suspected of having a urea cycle disorder. Therapy could then focus on lowering the brain glutamine levels as an endpoint.

Neuroimaging Abnormalities and Neuropathology

Head ultrasonography performed on neonates during initial presentations has revealed cerebral edema with largely obliterated ventricles. In surviving patients, the edema recedes with normalization of ammonia level, and the ventricles often become enlarged, as one manifestation of the cerebral atrophy that has occurred (Fig. 110-3). Hyperammonemic episodes at later ages are also accompanied by cerebral edema, which becomes even more dangerous once cranial sutures have fused. After hyperammonemic coma, magnetic resonance imaging and computed tomography also demonstrate increased sulcal markings, bilateral symmetrical low-density white matter lesions, and diffuse atrophy, sparing the cerebellum. In patients who have died during a hyperammonemic crisis, neuropathological changes have included intracerebral hemorrhage, prominent cerebral edema, swelling of type II astrocytes, and generalized neuronal cell loss (Fig. 110-4). Neuropathology in older children has included ulegyria, cortical atrophy with ventriculomegaly, and prominent cortical neuronal loss.