84 Arch Dis Child 2001;84:84–88

CURRENT TOPIC Arch Dis Child: first published as 10.1136/adc.84.1.84 on 1 January 2001. Downloaded from

Ornithine carbamoyltransferase deficiency

J E Wraith

The death of Jesse Gelsinger on 17 September Although our understanding of OCTD has 1999 had major eVects on the gene therapy increased greatly over recent years our abilities community. It brought to a halt a gene therapy to treat the severe variants of this disorder clinical trial at the Institute of Human Gene remain limited. Therapy, University of Pennsylvania, USA and brought to a wider audience the potential clini- The cycle and OCT cal problems associated with this technology.1 Figure 1 shows a simplified version of the urea In addition a number of clinicians became cycle. aware of Jesse’s genetic disorder, car- is an extremely toxic molecule and bamoyltransferase deficiency (OCTD, McKu- organisms have evolved a number of diVerent sick 311250), for the first time. OCTD is the ways of excreting this waste product of protein most common disorder of ureagenesis (preva- . Where water is abundant (for lence 1/40 000) and is inherited as an X linked example, fish) ammonia is directly excreted via trait. The OCT gene is located on the short the gills. Where water intake is often very arm of the X chromosome (Xp21.1) and over scarce (for example, birds and terrestrial 150 mutations have been found in OCTD reptiles) ammonia is first converted to families. There are no common mutations and and excreted via the kidneys, producing in the defects include gross deletions, missense and case of birds, the white crystalline droppings splicing mutations, as well as small insertions which are a common feature of urban architec- ture! In mammals, by a series of chemical reac- and deletions.23 The importance of detecting tions, ammonia is converted to a less toxic, mutations within families lies primarily with Willink Biochemical more water soluble molecule, urea, via the urea accurate carrier detection and prenatal diagno- Genetics Unit, Royal cycle. Manchester Children’s sis, as biochemical and enzymatic methods of In addition to the important excretory func-

Hospital, Manchester http://adc.bmj.com/ detection are less reliable. In addition, as tion, in mammals and some lower organisms, M27 4HA, UK enzyme activity is not expressed in either J E Wraith the is the route of de novo synthesis amniocytes or chorion villus biopsy material, of the , a role that assumes Correspondence to: prenatal testing had to rely on invasive fetal great importance when discussing therapy for Dr Wraith liver biopsy until DNA methods became avail- [email protected] urea cycle disorders. able. In certain families knowledge of the As can be seen from fig 1 the urea cycle takes 4 Accepted 31 August 2000 mutation may help with disease prediction. place partly within the cytosol and partly within the mitochondria. Each turn of the cycle on September 25, 2021 by guest. Protected copyright. Ammonium consumes two molecules of and one molecule of carbon dioxide. In return one mol- CPS ecule of urea is created and one molecule of ornithine is regenerated to stimulate another Carbamoyl turn of the cycle. Beginning and ending with Mitochondrion ornithine, the reactions of the cycle consume OTC three equivalents of ATP and a total of four high energy ; urea is the only new compound generated by the cycle. OCT is present within the mitochondrial Ornithine matrix and has no regulatory significance. Ornithine arising in the cytosol is transported to the where OCT Citrulline Ornithine catalyses the condensation of ornithine with Aspartate AS A to produce the amino Urea acid citrulline. The energy for the reaction is Arginosuccinate Arginine provided by the high energy anhydride of AL carbamoyl phosphate. The product citrulline is transported to the cytosol where the remainder of the reactions of the urea cycle take place. Cytoplasm Fumarate In patients with severe, neonatal onset OCTD, plasma citrulline concentrations are Figure 1 The urea cycle. CPS, carbamyl phosphate synthetase; OTC, ornithine transcarbamylase; AS, synthetase; AL, argininosuccinic acid lyase; A, very low and often undetectable. The concen- arginase; *waste nitrogen molecules. trations in less severe, late presenting or female

www.archdischild.com Ornithine carbamoyltransferase deficiency 85 Arch Dis Child: first published as 10.1136/adc.84.1.84 on 1 January 2001. Downloaded from

ATP + HCO3 + glutamine NH3 + HCO3 carbamoyl phosphate CPS I Mitochondria synthetase I Carbamoyl Carbamoyl phosphate phosphate aspartate carbamoyl ornithine OCT Carbamoyl aspartate Citrulline dihydroorotase

Dihydroorotate dihydroorotase dehydrogenase Allopurinol

Orotate () oxidase Oxipurinol orotate PRPP transferase

Orotidine -5-phosphate Oxipurinol ribonucleotide OMP decarboxylase -5-phosphate Figure 2 synthetic pathway.

heterozygotes are more variable and often not symptoms which are dependent on the age of of diagnostic significance. In addition, accumu- the patient as well as the duration and severity lating carbamoyl phosphate spills into the of the hyperammonaemia. A number of diVer- cytosol stimulating pyrimidine (fig ent mechanisms have been proposed to explain 2). As a consequence phosphoribosylpyrophos- this potent toxic eVect. These include a direct phate (PRPP) may be depleted, limiting flux eVect on neurotransmission, interference with through the orotate PRPP transferase reaction, neuronal energy metabolism, and a direct resulting in orotate (orotic acid) accumulation eVect on astrocyte function. Irrespective of the and excretion into the urine. The combination underlying cause the end result is a clinical dis- of hyperammonaemia with low plasma citrul- order which will range from mild personality line and increased urinary orotic acid concen- dysfunction to seizures, coma, and death.

trations is thought to be pathognomic of http://adc.bmj.com/ OCTD, but an identical biochemical picture Clinical presentation of OCTD occurs in ornithine aminotransferase defi- The hallmark of the severe form of OCTD in ciency presenting in the newborn period. In an aVected male infant is a rapidly progressive addition to helping with the primary diagnosis, metabolic encephalopathy presenting very levels of pyrimidine biosynthesis in OCTD can soon after birth. The infant usually appears be used to try to detect female heterozygotes by well at birth but on the second day of life the inhibition of orotidine monophosphate develops irritability, feed refusal, and becomes on September 25, 2021 by guest. Protected copyright. decarboxylase (OMP decarboxylase) with al- increasingly lethargic. Blood gas analysis will lopurinol (fig 2). In female heterozygotes there often show a respiratory alkalosis at this stage. may be no ostensible biochemical abnormali- Clouding of consciousness increases and the ties despite having an increased rate of pyrimi- baby becomes comatose and has an irregular dine biosynthesis within their OCTD hepato- respiratory pattern. Respiratory arrest and sei- cytes. When OMP decarboxylase is inhibited zures may occur and without urgent resuscita- by oxipurinol ribonucleotide (produced by tion and treatment the infant will die before the allopurinol metabolism), OMP accumulates end of the first week of life. Although this pat- which leads to orotidine accumulation and tern of disease progression is very familiar to orotidinuria, the degree of which serves to dis- paediatricians managing metabolic patients it tinguish normal women from OCTD heterozy- 5 is still usually misdiagnosed as “sepsis” by gotes. This test is much safer than protein neonatologists delaying the onset of treatment.9 loading which can no longer be recommended Like most X linked disorders OCTD is a as a screen for OCTD heterozygotes. The test, very heterogeneous disorder and male hemizy- however, can give both false positive6 and 7 gotes can present with a milder phenotype, negative results, and wherever possible hetero- presumably as a result of carrying less damag- zygosity should be confirmed by mutation ing genetic mutations. In these patients presen- analysis. tation can be in the first year of life or as late as middle age.10 In older, aVected boys, behav- Hyperammonaemia and the brain ioural disturbance often dominates,11 but many The eVects of hyperammonaemia on cerebral patients will also have recurrent episodes of function have recently been reviewed.8 Acute vomiting and failure to thrive. Despite our and chronic elevations in plasma ammonia are increasing knowledge of this mode of presenta- capable of producing devastating neurological tion the variant continues to be under recog-

www.archdischild.com 86 Wraith

nised because of this remarkable variation in lower values may be significant if the patient Arch Dis Child: first published as 10.1136/adc.84.1.84 on 1 January 2001. Downloaded from clinical signs and symptoms.12 The initial has been on a low protein diet or intravenous diVerential diagnostic list after admission to fluids for several days. This can occur in hospital often includes psychiatric illness, patients with late presenting OCTD who may cerebellar ataxia, drug ingestion, “encephali- initially be thought to have a gastrointestinal tis”, cyclical vomiting, and various food illness. In most male patients with milder vari- intolerances/allergies. ants or symptomatic females heterozygous for Female heterozygotes are even more diYcult OCTD, plasma ammonia concentrations will to diagnose, especially if there has been no pre- be above 150 µmol/l during episodes of illness vious positive family history. Considerable but may be normal at other times. diagnostic confusion and delay can occur in In OCTD orotic acid concentrations during this group of patients who may have protein acute episodes will be above the metabolic aversion as their only apparent symptom. The laboratories’ reference range (normal <5 µmol/ phenotypic variability seen in these patients mmol creatinine), and in severe neonatal reflects both genetic heterogeneity and the ran- patients plasma citrulline concentrations will dom pattern of X inactivation in their hepato- be low (normal 20–60 mmol/l depending on cytes. These patients may remain completely age). The diagnosis can be confirmed by normal throughout life or develop the same enzyme assay on a liver biopsy specimen, but degree of profound neurological impairment as many clinicians proceed directly to DNA seen in the male hemizygote with a severe neo- mutation studies now that these are readily natal onset. In large studies up to 20% of available. female heterozygotes will experience encepha- Diagnosis by enzyme or DNA analysis is lopathic episodes and as the diagnosis is often essential if prenatal diagnosis is to be oVered in not considered, a significant percentage (80%) the future. will die.13 Parenteral nutrition,14 pregnancy,15 and initiation of sodium valproate therapy16 are specific risk factors in this group. The treatment of OCTD Irrespective of age any individual with an The outcome in severely aVected males is so encephalopathic illness should have an urgent poor that I think it is reasonable to question blood ammonia estimation. If this is increased whether or not we should regard this variant as in the presence of normal liver function, a treatable disorder. Although earlier diagnosis OCTD should be high on the list of possible may be expected to give a better outcome even diagnoses and further investigation should be in those in whom the diagnosis is anticipated undertaken. At the same time urgent measures and who are managed prospectively, the results should be taken to reduce the ammonia after severe neonatal presentation of OCTD concentration down to the normal range as are very poor with almost every survivor show- quickly as possible. ing severe neurological impairment.917 In OCTD infants presenting early in the neonatal Making the diagnosis period, who have been comatose for over 24 http://adc.bmj.com/ The most important step is suspicion, but this hours, and who have a blood ammonia above needs to be quickly followed by urgent plasma 1000 µmol/l, it may be kinder to let nature run ammonia estimation. At the same time samples its course rather than embarking on heroic for urine orotic acid and plasma amino acid therapy which will result in a survivor with very concentrations are collected, but treatment is poor neurological and intellectual functions. dictated by the ammonia concentration. In patients presenting with less severe In the neonatal period a healthy, term variants, aggressive therapy is indicated with on September 25, 2021 by guest. Protected copyright. neonate will have a plasma ammonia concen- the aim of reducing the concentration of blood tration of less than 50 µmol/l, but in growth ammonia as quickly as possible. This approach retarded or premature infants a modest eleva- should also be undertaken in the management tion up to 80 µmol/l may be seen in otherwise of metabolic decompensation in known pa- healthy infants. In OCTD deficiency present- tients as well as with the diagnostic episode. ing in the newborn period ammonia concentra- The standard long term therapy for OCTD is tions are usually greater than 300 µmol/l at directed at reducing the requirement for urea presentation and often rise quickly thereafter. biosynthesis by using a low protein diet and by Confusion often occurs when moderately increasing waste nitrogen excretion by alterna- increased plasma ammonia concentrations (for tive pathway therapy.18 As arginine becomes an example, 90–150 µmol/l) are received back essential amino acid in this disorder a supple- from the laboratory. Any illness can be associ- ment is required to replace that which is not ated with a raised plasma ammonia but the synthesised (see Appendix). In the emergency concentration in these circumstances is usually situation this approach has to be augmented by less than 170 µmol/l. If in doubt, the estimation other means of rapid ammonia removal. There should be repeated; if the values rises above have been no controlled studies of the various 200 µmol/l treatment should be instituted and modalities employed to manage hyperammo- further investigation pursued with vigour. naemia. Published studies are usually single In older children plasma ammonia concen- centre and involve very small numbers of trations depend partly on protein intake. In patients.19–25 Although often technically more normal children an upper limit of 50 µmol/l is diYcult, the evidence that is available suggests usual with an elevation up to 80 µmol/l with that haemofiltration or haemodialysis may be non-specific illness. Concentrations above 100 more eVective than peritoneal dialysis in the µmol/l require further investigation although acute stages. Because of the intensity of therapy

www.archdischild.com Ornithine carbamoyltransferase deficiency 87

that is needed to manage these patients, early Table A1 Intravenous medication Arch Dis Child: first published as 10.1136/adc.84.1.84 on 1 January 2001. Downloaded from transfer to a major centre with good metabolic back up is advocated. Sodium Sodium Arginine 10% benzoate phenylbutyrate (mg/kg In those patients who survive the initial (mg/kg) (mg/kg) (ml/kg)) hyperammonaemic insult, liver transplantation 26–28 Priming 250 250 200 (2) becomes an option. While this will not infusion* improve central nervous system damage pre- dating transplantation, it will prevent further Maintenance† 250 250 200 (2) episodes of hyperammonaemia. Successful *Make up in 30 ml/kg of 10% dextrose and give over 90 min. transplantation leads to a reduction of daily †Make up in 30 ml/kg of 10% dextrose and give as continuous medication and the ability to consume an infusion over 24 hours. unrestricted diet. 1 Editorial. Gene therapy—a loss of innocence. Nat Med 2000;6:1. The outcome 2 Anonymous. Ornithine carbamoyltransferase (ornithine transcarbamylase). The Human Gene Mutation Database As mentioned, large studies of outcome for the CardiV. http://www.uwcm.ac.uk/ukcm/mg/search/119468. severe neonatal form of OCTD make depress- html. 3 Tuchman M, Morizono H, Rajagopal BS, Plante RJ, ing reading. Survival is much better in those Allewell NM. The biochemical and molecular spectrum of male infants who present outside the neonatal ornithine transcarbamylase deficiency. J Inher Metab Dis 1998;21(suppl 1):40–58. period (presumed milder variants) and females 4 Tuchman M, Morizono H, Reish O, Yuan X, Allewell NM. (heterozygotes). In patients with a neonatal The molecular basis of ornithine transcarbamylase deficiency: modelling the human enzyme and the eVects of onset the usual outcome is death or severe mutations. J Med Genet 1995;32:680–8. developmental delay. The only way to improve 5 Sebesta I, Fairbanks LD, Davies PM, Simmonds HA, Leon- ard JV. The allopurinol loading test for identification of this would be by early diagnosis, aggressive carriers for ornithine carbamoyl transferase deficiency: management of the hyperammonaemia, and studies in a healthy control population and females at risk. Clin Chim Acta 1994;224:45–54. early liver transplantation in survivors with 6 Bonham JR, Guthrie P, Downing M, et al. The allopurinol good intellectual function. load test lacks specificity for primary urea cycle defects but may indicate unrecognized mitochondrial disease. J Inher In female heterozygotes, alternative pathway Metab Dis 1999;22:174–84. therapy (see Appendix) improves prognosis by 7 Spada M, Guardamagna O, Rabler D, et al. Recurrent episodes of bizarre behaviour in a boy with ornithine tran- reducing hyperammonaemic episodes and thus scarbamylase deficiency: diagnostic failure of protein load- preventing further cognititive decline.29 ing and allopurinol challenge tests. J Pediatr 1994;125:249– 51. 8 Butterworth RF. EVects of hyperammonaemia on brain function. JInherMetabDis1998;21(suppl 1):6–20. The future 9 Maestri NE, Clissold D, Brusilow SW. Neonatal onset orni- At first sight OCTD would appear to be an thine transcarbamylase deficiency: a retrospective analysis. J Pediatr 1999;134:268–72. attractive candidate for a gene therapy ap- 10 Finkelstein JE, Hauser ER, Leonard CO, Brusilow SW. proach. The gene has been cloned, it is a rela- Late-onset ornithine transcarbamylase deficiency in male patients. J Pediatr 1990;117:897–902. tively common disorder, there is a very good 11 Drogari E, Leonard JV. Late onset ornithine carbamoyl trans- animal model (the sparse fur, spf/Y mouse) and ferase deficiency in males. Arch Dis Child 1988;63:1363–7.

12 Schultz REH, Salo MK. Under recognition of late onset http://adc.bmj.com/ current therapy is unsatisfactory. In addition ornithine transcarbamylase deficiency. Arch Dis Child the evidence that liver transplantation normal- 2000;82:390–1. 13 Batshaw ML, Msall M, Beaudet AL, Trojak J. Risk of illness ises the metabolic dysfunction suggests that a in heterozygotes for ornithine transcarbamylase deficiency. liver based gene transfer approach could be J Pediatr 1986;108:236–41. 14 Felig DM, Brusilow SW, Boyer JL. Hyperammonaemic successful; this was confirmed in animal coma due to parenteral nutrition in a woman with hetero- studies.30 Initial experience in humans has zygous ornithine transcarbamylase deficiency. Gastroenter- ology 1995;109:282–4. seemingly ended in disaster, but lessons will 15 Schimanski U, Krieger D, Horn M, Stremmel W, Wermuth B, Thielman L. A novel two-nucleotide deletion in the

undoubtedly be learned from Jesse Gelsinger’s on September 25, 2021 by guest. Protected copyright. ornithine transcarbamylase gene causing fatal hyperammo- death; it must not be forgotten that gene nia in early pregnancy. Hepatology 1996;24:1413–15. therapy is a treatment in its infancy and is cer- 16 Oechsner M, Steen C, Sturenburg HJ, Kohlschutter A. Hyperammonaemic encephalopathy after initiation of val- tain to improve. OCTD can be a devastating proate therapy in unrecognised ornithine transcarbamylase disorder and an innovative approach to therapy deficiency. J Neurol Neurosurg Psychiatry 1998;64:680–2. 17 Nagata N, Matsuda I, Matsuura T, et al. Retrospective sur- will be necessary if prognosis is to improve. vey of urea cycle disorders: Part 2. Neurological outcome in forty-nine Japanese patients with urea cycle enzymopa- thies. Am J Med Genet 1991;40:477–81. 18 Feillet F, Leonard JV. Alternative pathway therapy for urea Appendix cycle disorders. JInherMetabDis1998;21(suppl 1):101–11. Acute management of hyperammonaemia in OCTD 19 Herrin JT, McCredie DA. Peritoneal dialysis in the with alternative pathway therapy and toxin removal via reduction of blood ammonia levels in a case of hyperammo- naemia. Arch Dis Child 1969;44:149–51. dialysis 20 Donn SM, Swartz RD, Theone JG. Comparison of 1) Stop all exogenous protein. exchange transfusion, peritoneal dialysis and hemodialysis 2) Inhibit endogenous catabolism: for the treatment of hyperammonaemia in an anuric correct acid base balance newborn infant. J Pediatr 1979;95:67–70. + 21 Siegel NJ, Brown RS. Peritoneal clearance of ammonia and + give maintenance fluids as 10–15% glucose (aim creatinine in a neonate. J Pediatr 1990;82:1044–6. for blood glucose values of 7–10 mmol/l). 22 Elshihabi I, Brzowski A, Kaye C, Kearon P. EYciency of Combine with continuous insulin infusion at haemodialysis therapy for a urea cycle defect in a neonate. Clin Nephrol 1995;43:208–9. 0.05–0.1U/kg/h. 23 Klee KM, Greenleaf K, Fouser L, Watkins SL. Continuous 3) Intravenous medication (table A1). Start if ammonia venovenous hemofiltration with and without dialysis in >180 µmol/l. pediatric patients. ANNA J 1996;23:35–9. Monitor NH +, sodium, potassium, acid–base, and 24 Wong KY, Wong SN, Lam SY, Tam S, Tsoi NS. Ammonia 4 clearance by peritoneal dialysis and continuous hemodiafil- glucose every four hours. tration. Pediatr Nephrol 1998;12:589–91. 4) Dialysis for ammonia concentrations >400 µmol/l. 25 Schaefer F, Straube E, Oh J, Mehls O, Mayatepek E. Dialy- Haemodialysis is preferable. sis in neonates with inborn errors of metabolism. Nephrol 5) Oral medication in same dosage with low protein Dial Transplant 1999;14:910–18. 26 Whittington PF, Alonso EM, Boyle JT, et al. Liver diet is the basis of long term treatment after the acute transplantation for the treatment of urea cycle disorders. J episode. Inher Metab Dis 1998;21(suppl 1):112–18.

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27 Busuttil AA, Goss JA, Seu P, et al. The role of orthotopic 29 Maestri NE, Brusilow SW, Clissold DB, Bassett SS. liver transplantation in the treatment of ornithine Long-term treatment of girls with ornithine transcarbamy- Arch Dis Child: first published as 10.1136/adc.84.1.84 on 1 January 2001. Downloaded from transcarbamylase deficiency. Liver Transpl Surg 1998;4: lase deficiency. N Engl J Med 1996;335:855–9. 350–4. 30 Raper SE, Wilson JM, YudkoV M, Robinson MB, Ye X, 28 Saudubray JM, Touati G, Delonlay P, et al. Liver transplan- Batshaw ML. Developing adenoviral-mediated in vivo gene tation in urea cycle disorders. Eur J Pediatr 1999;158(suppl therapy for ornithine transcarbamylaase deficiency. J Inher 2):S55–9. Metab Dis 1998;21(suppl 1):119–37. http://adc.bmj.com/ on September 25, 2021 by guest. Protected copyright.

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