Neurologic Damage and
`Neurocognitive Dysfunction in Urea Cycle Disorders
`Gregory M. Enns, MB, ChB
`
`Although the survival of patients who have urea cycle disorders has improved with the use
`of modalities such as alternative pathway therapy and hemodialysis, neurologic outcome is
`suboptimal. Patients often manifest with a variety of neurologic abnormalities, including
`cerebral edema, seizures, cognitive impairment, and psychiatric illness. Current hypothe-
`ses of the pathogenesis underlying brain dysfunction in these patients have focused on
`several lines of investigation, including the role of glutamine in causing cerebral edema,
`mitochondrial dysfunction leading to energy failure and the production of free radicals, and
`altered neurotransmitter metabolism. Advances in understanding the pathogenetic mech-
`anisms underlying brain impairment in urea cycle disorders may lead to the development
`of therapies designed to interfere with the molecular cascade that ultimately leads to
`cerebral edema and other brain pathological findings.
`Semin Pediatr Neurol 15:132–139 © 2008 Elsevier Inc. All rights reserved.
`
`The urea cycle consists of 6 enzymes (N-acetyl-glutamate
`
`synthetase, carbamyl phosphate synthetase [CPS], orni-
`thine transcarbamylase [OTC], argininosuccinate synthetase
`[AS], argininosuccinate lyase [AL], and arginase) that are re-
`sponsible for eliminating nitrogenous waste. Partial or com-
`plete inactivity of any of these enzymes secondary to an in-
`herited urea cycle disorder (UCD) can predispose patients to
`episodic, life-threatening hyperammonemia. UCD patients
`often manifest neurologic problems, especially cognitive im-
`pairment, if they survive the acute hyperammonemic epi-
`sode. Acute hyperammonemia is associated with anorexia,
`vomiting, and altered mental status. Lethargy, progressing to
`coma, and possibly death if therapy is not instituted quickly
`may ensue. Seizures, ataxia, asterixis, slurred speech, trem-
`ors, weakness, muscle tone abnormalities, and hypothermia
`are other common manifestations. Patients who have a partial
`enzyme deficiency typically manifest outside the neonatal
`period. Clinical features may be subtle in such late-onset
`cases, leading to delays in diagnosis. Psychiatric symptoms in
`late-onset UCDs, including hyperactive behavior, mood dis-
`turbances, self-injurious behavior, and psychosis, may occur.
`A tendency to avoid dietary protein is common.1 Although
`
`From the Department of Pediatrics, Division of Medical Genetics, Lucile
`Packard Children’s Hospital, Stanford University, Stanford, CA.
`Dr Enns reports serving on the Scientific Advisory Board and being on the
`speaker’s bureau for Hyperion Therapeutics, Inc.
`Address reprint requests to Gregory M. Enns, MB, ChB, Division of Medical
`Genetics, Stanford University, 300 Pasteur Drive, H-315, Stanford, CA
`94305. E-mail: greg.enns@stanford.edu
`
`132
`
`1071-9091/08/$-see front matter © 2008 Elsevier Inc. All rights reserved.
`doi:10.1016/j.spen.2008.05.007
`
`disease manifestations tend to be milder in patients with
`late-onset disease, devastating sequelae, including significant
`neurologic damage and death, may occur if the diagnosis is
`not suspected, therefore delaying appropriate therapy.
`Acute hyperammonemia affects the brain white matter se-
`lectively and causes astrocyte swelling and global cerebral
`edema. Changes involving the deep insular and perirolandic
`sulci may be reversible.1 However, in cases of severe hyper-
`ammonemia, permanent changes occur. Neuropathological
`findings in patients who have neonatal-onset proximal UCDs
`consist of gross cerebral atrophy, ventriculomegaly, delayed
`myelination, and the appearance of Alzheimer type II astro-
`cytes, ulegyria, and spongiform degeneration of the cortex,
`gray-white matter junction, and deep gray nuclei, including
`the basal ganglia and thalamus.2-4 This review concentrates
`on describing current hypotheses on the pathophysiology of
`brain damage in UCDs, after first reviewing historical sur-
`vival data and developmental outcomes, as well as electroen-
`cephalography and brain-imaging findings in these condi-
`tions.
`
`Survival
`Historically, most children born with a severe UCD enzyme
`deficiency died as neonates, and few survived infancy.5 How-
`ever, pioneering work by Brusilow et al6 resulted in the de-
`velopment of alternative pathway medications (intravenous
`sodium phenylacetate plus sodium benzoate and intravenous
`arginine hydrochloride) for treating acute hyperammonemic
`episodes. Prolonged survival and improved clinical outcome
`
`Horizon Exhibit 2039
`Par v. Horizon
`IPR2017-01768
`
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`Urea cycle disorders
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`133
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`in UCD patients were noted after the initial use of alternative
`pathway therapy.7 Recently, a retrospective study of 299
`UCD patients treated with alternative pathway therapy who
`had undergone a total of 1,181 episodes of hyperammonemia
`showed an overall survival rate of 84% during the 25-year
`period of study. Patients with late-onset disease were more
`likely to survive an episode of hyperammonemia than neo-
`nates (98% v 73%, P ⬍ 0.001). Dialysis was used in conjunc-
`tion with alternative pathway therapy in 60% of neonatal and
`7% of late-onset episodes.8 In contrast, a study of 217 UCD
`patients who were not treated with alternative pathway med-
`ications showed a survival rate of only 16% in patients with
`neonatal-onset disease and 72% in those with late-onset dis-
`ease.9
`
`Neurodevelopmental Outcome
`Although alternative pathway therapy and other therapies,
`especially hemodialysis, for UCDs has led to improved pa-
`tient survival, cognitive impairment remains a common find-
`ing, especially in those who have neonatal-onset disease.10
`However, the age at which the first symptom is noted is not
`necessarily predictive of outcome in individual cases because
`patients who have neonatal-onset disease may still have a
`normal long-term outcome.11 In a study of 26 children who
`survived neonatal hyperammonemia, the overwhelming ma-
`jority (79%) had 1 or more developmental disabilities at 12 to
`74 months of age. Interestingly, IQ correlated with the depth
`of coma but not the peak plasma ammonium level over a
`range of 351 ␮mol/L to 1800 ␮mol/L.12 Other studies have
`found correlation between the peak plasma ammonium level
`and cognitive outcome.10,11,13 When the concentration of
`plasma ammonium exceeded 350 ␮mol/L at the time of the
`first episode of hyperammonemia, patients either died or had
`severe neurologic deficits in a Japanese study of 108 UCD
`patients.10 This study was performed by questionnaire, and
`no mention was made of any specific therapeutic interven-
`tions. The surprisingly low level of peak plasma ammonium
`associated with catastrophic outcome may be related to the
`lack of availability of intravenous sodium phenylacetate and
`sodium benzoate (Ammonul, Ucyclyd Pharma, Inc. Scotts-
`dale, AZ) and arginine HCl or other factors. In a European
`questionnaire study, no surviving UCD patients with an ini-
`tial plasma ammonium level ⬎300 ␮mol/L or a peak plasma
`ammonium level ⬎480 ␮mol/L had normal psychomotor
`development.11 Neonatal-onset OTC deficiency is particu-
`larly devastating with respect to neurologic outcome.13,14
`However, prospective neonatal therapy, after prenatal diag-
`nosis by DNA or biochemical analysis, decreases the risk of
`neonatal hyperammonemia and may lead to a more favorable
`neurologic outcome in OTC deficiency and other UCDs.15
`Even in late-onset UCDs, the incidence of neurodevelop-
`mental impairment remains high. Despite a relatively low
`number of overall hyperammonemia episodes (mean of 4),
`14 children who had late-onset disease showed significant
`impairment. The majority (56%) had mental retardation
`(mean IQ 56; range, 10-103), 33% had multiple neurodevel-
`opmental disabilities, 33% had signs of cerebral palsy, 20%
`
`had seizures, and 13% were cortically blind.16 A large retro-
`spective study of 217 UCD patients showed that 43% of the
`late-onset UCD cohort (n ⫽ 96) had moderate to severe
`neurologic impairment.9 On the other hand, some patients
`appear cognitively normal for years and even decades until
`the first presentation of illness. Altered mental status after the
`physiologic stress of illness or fasting, surgery, sodium val-
`proate use, or related to pregnancy or the postpartum period
`has been associated with late-onset forms of UCDs.1,17
`Heterozygote OTC females have variable clinical features
`related to random X-inactivation and allelic heterogeneity.18
`An estimated 15% of OTC females (designated “manifesting
`heterozygotes”) display symptoms such as protein intoler-
`ance, cyclical vomiting, behavioral and neurologic abnormal-
`ities, and even episodic hyperammonemic coma.19 In a recent
`study of 19 mildly symptomatic and asymptomatic women
`heterozygous for OTC deficiency, comprehensive neuropsy-
`chological testing showed significant weaknesses in fine-mo-
`tor dexterity/speed and nonsignificant weaknesses in nonver-
`bal intelligence, visual memory, attention/executive skills,
`and math, despite overall normal IQs.20 When the patients
`were divided into symptomatic and asymptomatic groups,
`the asymptomatic cohort outperformed those who had
`symptoms in all tested neuropsychological functioning do-
`mains. Overall, these findings were considered to support the
`presence of a nonverbal learning disability in OTC heterozy-
`gote females, consistent with selective vulnerability of white
`matter and better preservation of gray matter.20
`
`Electroencephalography
`Electroencephalography (EEG) most commonly shows changes
`of a nonspecific diffuse encephalopathy, although a variety of
`patterns of abnormality have been described in UCD patients,
`including multifocal independent spike- and sharp-wave dis-
`charges, repetitive paroxysmal activity, unusually low-volt-
`age fast activity, and findings consistent with complex partial
`seizures.21-23 EEG may not correlate with plasma ammonium
`level but still has the potential to provide useful clinical in-
`formation. For example, the reversal of an initially flat en-
`cephalogram may represent an encouraging prognostic factor
`that could lead the clinician to pursue an aggressive plan of
`treatment.1 EEG may also provide information that is com-
`plementary to physical examination, biochemical evalua-
`tions, and neuroimaging studies. Such an assessment may be
`particularly helpful in cases of severe coma in which neuro-
`logic examination has limited utility.1 There is a lag in the
`normalization of the encephalographic tracing after the re-
`turn of plasma ammonium levels to normal.21 In late-onset
`UCD cases, EEG may show continuous semirhythmic activity
`with sharp components, leading to diagnosis of complex par-
`tial status epilepticus.23
`
`Neuroimaging
`The type of brain injury detected in UCDs varies, but 4 broad
`patterns may be discerned.24 Type 1 is characterized by dif-
`fuse severe cerebral edema followed by diffuse atrophy; type
`
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`134
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`Gregory M. Enns
`
`2 by extensive infarct-like abnormality25; type 3 by presum-
`ably ischemic lesions in cerebral
`intravascular boundary
`zones; and type 4 by reversible symmetric cortical involve-
`ment of the cingulate gyri, temporal lobes, and insular cortex
`with sparing of the perirolandic cortex.24 These types roughly
`correspond to disease severity and age on onset as follows:
`type 1, neonatal period or infancy; types 2 and 3, infancy to
`childhood; and type 4, adulthood.24 Neonatal hyperam-
`monemic encephalopathy may also be associated with injury
`to the lentiform nuclei and deep sulci of the insular and
`perirolandic regions.24 In the acute phase, cerebral edema
`affecting both gray and white matter with T1 shortening of
`the gray matter may resemble hypoxic-ischemic encephalop-
`athy.1,26 Chronic changes include persistent focal abnormal-
`ities in the cerebral hemispheres, diffuse cortical and subcor-
`tical atrophy, and development of subcortical cysts and
`ulegyria.13,27,28
`Magnetic resonance spectroscopy (MRS) has shown eleva-
`tions in brain glutamine in hyperammonemic animal models
`and in UCD patients, consistent with the hypothesis that
`accumulation of intracerebral glutamine contributes to en-
`cephalopathy in UCD patients (see later).29-33 A study of 6
`OTC patients undergoing proton MRS showed myoinositol
`depletion and elevated brain glutamine plus glutamate con-
`centrations that increased in proportion to the clinical stage
`of disease in 5 symptomatic patients. In addition, choline
`depletion was detected in 2 severely affected patients.34 One
`OTC patient showed normalization of all metabolites on MRS
`evaluation after liver transplantation.34 These studies suggest
`that it may be possible to monitor metabolic control of UCD
`patients using MRS.1
`
`The Pathogenesis of
`Cerebral Dysfunction
`The precise pathogenic mechanisms involved in causing ce-
`rebral dysfunction in UCDs are unknown. However, research
`has focused on several key areas, especially the role of glu-
`tamine in causing cerebral edema, mitochondrial dysfunc-
`tion leading to energy failure and the production of free rad-
`icals, and altered neurotransmitter metabolism.1
`
`Cerebral Edema: The Glutamine Hypothesis
`Hyperammonemia is known to cause increased cerebral cor-
`tical glutamine content, activation of astrocytic glutamine
`synthetase, and astrocyte swelling.35 Ammonia diffuses freely
`across the blood-brain barrier and is rapidly incorporated
`into glutamine via glutamine synthetase. Glutamine syn-
`thetase, a cytosolic enzyme primarily localized to the astro-
`cyte in the brain, catalyzes the following reaction: NH3 ⫹
`L-Glutamate ⫹ ATP ¡ L-Glutamine ⫹ ADP ⫹ Pi.
`This reaction, therefore, represents a short-term means of
`buffering excess plasma ammonium. However, glutamine
`has been considered to be an organic osmolyte that increases
`intracellular osmolarity. This leads to increased cellular vol-
`ume as water enters the astrocyte and subsequent cytotoxic
`cerebral edema.35 During hyperammonemia, astrocyte glu-
`
`tamine accumulation may represent an increase in osmolality
`of as much as 30 mOsm per kg.35
`In vivo nuclear magnetic resonance spectroscopy has been
`a useful technique to evaluate cerebral glutamine and gluta-
`mate concentrations, the rate of glutamine synthesis, energy
`metabolism, and intracellular pH in rats undergoing intrave-
`nous ammonium infusion.29 In the hyperammonemic rat
`model, the degree of hyperammonemia correlates with glu-
`tamine synthesis activity up to a point of enzymatic satura-
`tion, after which ammonia accumulates in the brain and en-
`cephalopathy worsens.30,36 Neuronal presynaptic terminal
`glutaminase activity is also increased during hyperammone-
`mia, as is cycling of glutamate between neurons and glia,
`lending evidence to a neuronal-glial neurotransmitter cy-
`cle.31,32,37 The transport of glutamine from the glia to the
`extracellular fluid and the uptake of extracellular glutamine
`into neurons are essential components of this cycle. This
`cycle couples glial glutamine production to the synthesis of
`neuronal glutamate (Fig 1). Current evidence supports a pre-
`dominant role for the sodium-coupled amino-acid trans-
`porter as a mediator of neuronal glutamine uptake from the
`extracellular fluid in adult rat brain in vivo.38
`The central importance of brain glutamine metabolism in
`the pathogenesis of cerebral edema has been shown by the
`inhibition of glutamate synthetase by L-methionine sulfoxi-
`mine (MSO) in rat models of hyperammonemia. Pretreat-
`ment with MSO prevents the increase in brain glutamine
`levels and water content despite elevated plasma ammonium
`levels.39 Hyperammonemia also increases the production of
`reactive astroglial cytoskeletal components, including the in-
`termediate filament glial fibrillary acidic protein (GFAP) and
`the gap junction protein connexin-43. The number of GFAP
`immunopositive cells in the cerebral cortex was decreased by
`pretreatment with MSO, although no change in connexin-43
`was observed.40 Therefore, glutamine synthetase inhibition
`reduces astrocyte edema and ameliorates some of the reactive
`astroglial cytoskeletal changes but does not seem to be in-
`volved with gap junction alterations.40
`However, several lines of evidence are not compatible with
`glutamine causing cerebral edema because of its action as an
`osmolyte, including (1) in portacaval-shunted rats infused
`with ammonia, the initial rise in brain water preceded gluta-
`mate accumulation; (2) in the same animal model, mild hy-
`pothermia prevented the development of brain edema but
`had no effect on lowering elevated brain glutamine concen-
`tration; (3) some nuclear magnetic resonance studies showed
`that hypothermia normalized brain water content but did not
`prevent brain glutamine accumulation; (4) in rats with acute
`liver failure, glutamine levels did not correlate with degree of
`cerebra edema; (5) although various compounds reduced
`ammonia-induced swelling in brain slices, this effect was not
`correlated to a comparable decrease in glutamine concentra-
`tion; and (6) ammonia-induced astrocyte swelling was de-
`layed with respect to the rise of cytoplasmic glutamine levels,
`and maximal cell swelling occurred when glutamine concen-
`tration was low.41,42
`
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`Urea cycle disorders
`
`135
`
`Pre-Synaptic Neuron
`
`nlG
`GNase
`Glu
`
`Glu
`
`Ca2+
`
`cGMP
`
`NMDAR
`
`CM-Ca2+
`+
`
`NO
`
`+
`
`NOS
`
`cGMP
`
`GC
`
`L-Arg
`Post-Synaptic Neuron
`
`nlG
`
`GS
`
`Glu
`
`GLT-1
`
`NH4
`+
`
`Blood
`
`Astrocyte
`
`Figure 1
`The glutamine-glutamate cycle and NMDA receptor–mediated nitric oxide and cGMP production. The conver-
`sion of glutamate to glutamine in astrocytes by glutamine synthetase constitutes the primary mechanism for detoxification of
`ammonia in the brain (see text). Ammonia removes the NMDA receptor blockade by Mg2⫹ and allows glutamate to activate
`the receptor. This causes increased calcium flux into the postsynaptic neuron and, in conjunction with calmodulin, stimu-
`lation of nitric oxide synthetase. Nitric oxide then activates the soluble form of guanylyl cyclase, which increases cGMP
`production. Part of the generated cGMP is released into the extracellular matrix.41,56,59 CM ⫽ calmodulin; Gln ⫽ glutamine;
`GLT-1 ⫽ glutamate transporter; Glu ⫽ glutamate; GS ⫽ glutamate synthetase; GNase ⫽ glutaminase; L-Arg ⫽ L-arginine;
`NMDAR ⫽ N-methyl-D-aspartate receptor; NO ⫽ nitric oxide; NOS ⫽ nitric oxide synthetase; cGMP ⫽ cyclic guanosine
`monophosphate; GC ⫽ guanylyl cyclase. (Color version of figure is available online.)
`
`Mitochondrial Energy Failure and Oxidative Stress
`Although the “glutamine hypothesis” has been a leading ex-
`planation for the development of cerebral edema, recent in-
`terest has focused on glutamine-independent mechanisms to
`explain the pathogenesis of hyperammonemic encephalopa-
`thy. One area of active investigation is the role of impaired
`brain oxidative metabolism in causing cerebral dysfunction
`associated with hyperammonemia.41-46 Acute hyperammone-
`mia causes a decrease in brain metabolic rate and decreased
`high-energy phosphate concentration.43 Increased production
`of toxic reactive oxygen species (ROS) by brain mitochondria
`also occurs, likely secondary to increased formation of these
`intermediates by the respiratory chain and by xanthine and al-
`dehyde oxidases, as well as decreased activities of free radical-
`scavenging enzymes, including glutathione peroxidase, super-
`oxide dismutase, and catalase.44 Ammonium ions also directly
`inhibit ␣-ketoglutarate dehydrogenase, which may lead to de-
`creased cerebral energy production.43 “Peripheral-type” benzo-
`diazepine receptors (PTBRs) are localized to the outer mito-
`chondrial membrane in brain astrocytes and are upregulated
`during acute hyperammonemia. Drugs that bind to the PTBR
`cause a significant decrease in the mitochondrial respiratory
`
`control ratio so PTBRs may also play a role in the cellular
`energy deficit observed during hyperammonemia.47
`Although glutamine has primarily been considered an os-
`motically active compound that draws water into the cell
`when present in high concentration, glutamine also interacts
`directly with mitochondria. Glutamine enters mitochondria
`through a histidine-sensitive carrier, a process that is poten-
`tiated by ammonia.42 Phosphate-activated glutaminase is lo-
`cated in the inner mitochondrial membrane and cleaves glu-
`tamine into glutamate and ammonia. Because of
`this
`localized production of ammonia, intramitochondrial am-
`monia levels may potentially become very high, leading to
`the induction of mitochondrial permeability transition
`(MPT), increased oxidative and nitrosative stress, and astro-
`glial dysfunction.42,46 The production of ROS and reactive
`nitrogen species and MPT have been hypothesized to initiate
`a cascade of events that includes activation of mitogen-acti-
`vated protein kinases and resultant failure of astrocytes to
`regulate their intracellular volume.42 Both treatment with an-
`tioxidants and cyclosporine A, an inhibitor of MPT, block
`ammonia-induced astrocyte swelling.48,49 Interestingly, the
`treatment of cultured astrocytes with L-histidine, an inhibitor
`
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`Gregory M. Enns
`
`of mitochondrial glutamine transport, completely blocks or
`significantly attenuates ammonia-induced ROS production,
`cell swelling, MPT, and loss of adenosine triphosphate.50
`Such findings confirm the importance of glutamine in the
`production of astrocyte swelling, although its ability to or-
`chestrate an increase in cellular water content may not de-
`pend on altered intracellular osmolality. Finally, integral
`membrane proteins that mediate transmembrane water
`movement (aquaporins) have also been implicated in ammo-
`nia-induced astrocyte swelling. Aquaporin-4 is the most
`abundant aquaporin in the brain. Cultured astrocytes ex-
`posed to high levels of ammonia upregulate aquaporin-4,
`and this increase precedes the development of astrocyte
`swelling.51
`
`Neurotransmitter Abnormalities
`Ammonium ions have a multitude of effects on mammalian
`neurotransmitters, including systems involving cholinergic,
`serotonergic, and glutamatergic neurotransmission.52 In-
`creased brain concentrations of the excitatory amino acid
`neurotransmitters glutamate and aspartate are present in the
`sparse-fur (spf) mouse, a model of OTC deficiency, which
`may explain the seizure predisposition in OTC patients.53
`Tryptophan, a precursor of serotonin, and quinolinic acid, an
`N-methyl-D-aspartate (NMDA) receptor agonist known to
`produce selective striatal cell loss, are also increased in spf
`mice and in rats after portacaval anastomosis.54,55 In addition,
`brain pathology in spf mice is characterized by a significant
`loss of medium spiny neurons and increased numbers of
`reactive oligodendroglia and microglia in the striatum. Am-
`monia also inhibits high-affinity transport of glutamate in
`astrocytes, which results in increased extracellular concen-
`tration of glutamate.56 These biochemical and pathological
`features are suggestive of NMDA-mediated excitotoxic brain
`injury.54 The description of an OTC patient who had “stroke-
`like” episodes is consistent with this theory of neurotransmit-
`ter excitotoxicity.57 Furthermore, in mice exposed to hyper-
`ammonemia, either by the induction of acute liver failure or
`by treatment with ammonium acetate, PTBR upregulation
`leads to elevated levels of pregnenolone-derived neuros-
`teroids, which have the potential to enhance ␥-aminobutyric
`acid-ergic (GABAergic) neurotransmission.58 Ammonium
`ions also depress postsynaptic ␣-amino-3-hydroxy-5-methyl-
`4-isoxazolepropionic acid receptor–mediated currents.56
`␣-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid re-
`ceptors mediate fast synaptic transmission and are involved
`with learning and memory.
`Acute hyperammonemia results in the activation of NMDA
`receptors, which causes increased flux of Na⫹ and Ca2⫹ into
`the postsynaptic neuron. Ca2⫹ then activates calcineurin,
`which in turn dephosphorylates and activates Na⫹-K⫹-aden-
`osine triphosphatase, resulting in Na⫹ extrusion, consump-
`tion of adenosine triphosphate, and increased synthesis of
`nitric oxide and cGMP (Fig 1).43,56,59 Excess Ca2⫹ is primarily
`taken up by the mitochondria, which leads to increased pro-
`duction of ROS and further energy depletion.43 NMDA re-
`ceptor antagonists prevent the oxidative stress induced by
`hyperammonemia by preserving the function of important
`
`enzymes that participate in free radical scavenging, including
`superoxide dismutase, glutathione peroxidase, and cata-
`lase.60 In addition, NMDA receptor blockade prevents brain
`adenosine triphosphate depletion and increases glutamine
`synthetase activity, with resulting increase in brain glu-
`tamine. These data suggest that ammonia toxicity may be
`directly related to excessive activation of NMDA receptors,
`not increased glutamine synthesis per se.59
`In contrast, chronic hyperammonemia results in a loss of
`NMDA receptor densities and increased uptake of trypto-
`phan, a precursor of serotonin, into the brain by activation of
`the L-system carrier.52,55,56,61 The uptake of tryptophan is
`further enhanced if plasma levels of branched-chain amino
`acids are low, as is common in UCD patients who are re-
`stricted in dietary protein intake.61 Serotoninergic symp-
`toms, such as anorexia, altered sleep patterns, and disorders
`of motor coordination, may be related to the increased brain
`turnover of
`serotonin observed in hyperammonemic
`states.1,55 The adaptive changes in NMDA receptors that oc-
`cur in chronic hyperammonemia result in a decrease in exci-
`tatory neurotransmission and impaired production of nitric
`oxide and cyclic guanosine monophosphate (cGMP).59,62 De-
`creased cGMP production may inhibit long-term potentia-
`tion in the hippocampus. Because long-term potentiation is a
`long-lasting enhancement of synaptic transmission efficacy,
`considered to be the basis for some forms of learning and
`memory, this effect of hyperammonemia may be involved in
`the abnormal cognitive function observed in patients who
`have UCDs.62,63 Abnormal axonal growth, accompanied by
`decreased creatine and phosphocreatine levels (creatine is
`essential for axonal elongation), and alteration of brain cy-
`toskeletal elements are also observed in hyperammonemia.61
`Glial fibrillary acidic protein (GFAP) is reduced and micro-
`tubule-associated protein-2 and neurofilament protein ex-
`hibit decreased phosphorylation, possibly through an effect
`of ammonia on mitogen-activated kinase function.61 Al-
`though the precise interrelationship between these proposed
`pathogenetic mechanisms is unclear, it is reasonable to hy-
`pothesize that all play at least some role in the mental impair-
`ment observed in UCD patients.
`
`Arginase Deficiency
`Arginase deficiency has clinical and pathophysiological fea-
`tures that differ from other UCDs and, hence, deserves spe-
`cial mention. Hyperammonemia is rare or, if present, usually
`mild. The classic neonatal hyperammonemic crisis has been
`described only rarely in arginase deficiency.64 Affected pa-
`tients typically develop signs, including cognitive defects,
`seizures, spastic paraparesis or paraplegia (more prominent
`in the lower extremities), brisk tendon reflexes, scissoring,
`ataxia, and toe walking in later infancy and early childhood.65
`Not surprisingly, a misdiagnosis of cerebral palsy is common.
`Although elevated plasma arginine levels often point to the
`diagnosis, the extent of elevation may be mild. Head imaging
`shows mild cerebral and cerebellar atrophy as well as T2
`hyperintensity in the posterior putamen, periventricular
`white matter, and insular cortex.65,66 MRS may show an ele-
`vated choline/creatinine ratio, suggesting demyelination or
`
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`Urea cycle disorders
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`137
`
`inflammation, as well as an abnormal peak at 3.8 ppm, likely
`representing arginine accumulation.66
`Because hyperammonemia occurs only rarely in arginase
`deficiency, the pathogenesis of brain disease is unclear. Ele-
`vated arginine may be directly neurotoxic, but some patients
`with only minimal plasma arginine elevation have shown
`significant developmental delay. In such cases, it is possible
`that the blood arginine concentration does not reflect levels
`in the brain. Alternatively, other compounds may cause neu-
`rologic damage in arginase deficiency. In particular, gua-
`nidino compounds derived directly from arginine, including
`N-acetylarginine, homoarginine, and argininic acid, are at-
`tractive candidates for causing toxicity.67 In theory, gua-
`nidino compound accumulation may affect GABAergic neu-
`rotransmission, decrease plasma membrane fluidity, or
`inhibit the activity of Na⫹, K⫹-adenosine triphosphatase,
`which could result in seizure activity.65,67 In addition, gua-
`nidino compounds have been shown to inhibit key enzymes
`involved in free radical scavenging, including catalase, super-
`oxide dismutase, and glutathione peroxidase, in the cerebral
`cortex of a rat model.67 Such inhibition would be expected to
`result in stimulation of oxidative stress, which could result in
`brain damage.67 Elevated argininic acid, ␣-N-acetylarginine,
`and ␣-keto-␦-guanidinovaleric acid were found to be ele-
`vated in the brain tissue of an arginase knockout mouse
`model, lending further support to the role of these com-
`pounds in the pathogenesis of brain disease in arginase defi-
`ciency.68 In addition, chronically increased arginine concen-
`tration has the potential to alter intracellular nitric oxide
`production because arginine is a substrate for nitric oxide
`synthetase. Although the precise role of arginine in modulat-
`ing neuronal survival is unknown, it is tempting to speculate
`that altered nitric oxide metabolism also might play a role in
`neurologic dysfunction in arginase deficiency.69
`
`Treatment of UCDs:
`The Present and Future
`Current therapeutic strategies include reducing the produc-
`tion of nitrogenous waste by low dietary protein intake and
`preventing endogenous catabolism through the provision of
`adequate nutrition. The exploitation of alternative pathways
`for excretion of waste nitrogen has played a critical role in the
`management of UCDs since Brusilow et al6 first suggested
`using endogenous biosynthetic pathways to eliminate nonu-
`rea waste nitrogen as a substitute for aberrant urea synthesis.
`One method of enhancing nonurea nitrogenous waste excre-
`tion consists of supplementing the amino acid arginine to
`drive the urea cycle to produce excess citrulline (for treating
`AS deficiency) or argininosuccinate (for treating AL defi-
`ciency). Citrulline or argininosuccinate are then excreted in
`the urine carrying waste nitrogen atoms within their chemi-
`cal structures. Another approach makes use of endogenous
`liver and kidney glycine- and glutamine-specific N-acyltrans-
`ferases by infusing sodium phenylacetate and sodium benzo-
`ate. These N-acyltransferases conjugate glycine to benzoate,
`forming hippurate, and glutamine to phenylacetate, forming
`
`phenylacetylglutamine. Hippurate and phenylacetylglutamine
`are both eliminated in urine and contain either 1 or 2 waste
`nitrogen atoms, respectively. As mentioned earlier, the use of
`these “alternative pathway” medications, in conjunction with
`other therapies such as hemodialysis, has improved patient
`survival.8 Lastly, although arginine therapy has an additional
`role in AS and AL deficiencies (ie, to maintain arginine pool
`sizes) citrulline is used in the proximal urea cycle disorders,
`CPS and OTC deficiencies, to prevent arginine depletion in
`liver and other tissues.70
`Despite improved survival, developmental outcomes re-
`main suboptimal in most cases. Nevertheless, normal intelli-
`gence is at least possible after a hyperammonemic event and
`appears to depend on the duration of coma and the extent of
`brain damage.7,12,16 The importance of rapid identification
`and institution of therapy, especially alternative pathway
`therapy and hemodialysis or hemofiltration if needed, to
`lower blood ammonium levels as rapidly as possible to pro-
`tect the brain cannot be overemphasized. The establishment
`of a network of specialized centers with expertise in provid-
`ing state-of-the-art treatment for UCDs has clear potential for
`improving neurologic outcomes.11 The sponsorship of a Rare
`Disease Clinical Research Center Network for UCDs by the
`National Institutes of Health is a step in this direction (http://
`rarediseasesnetwork.epi.usf. edu/ucdc/index.htm).
`Expanded newborn screening using tandem mass spec-
`trometry (MS/MS) is becoming the standard in developed
`countries. However, MS/MS screening does not detect prox-
`imal UCDs, including OTC and CPS deficiencies. In addi-
`tion, neonates with severe forms of disease would likely ex-
`hibit symptoms before the results of newborn screening are
`available so MS/MS screening may not necessarily lead to the
`ability to institute therapy in a timely fashion. Therefore,
`increased awareness of signs and symptoms of UCDs and a
`high index of suspi