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| {{short description|Urea cycle disorder}}
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| {{Infobox medical condition (new) | | {{Infobox medical condition |
| | name = Ornithine transcarbamylase deficiency | | | name = Ornithine transcarbamylase deficiency |
| | synonyms = OTC deficiency | | | image = <!-- Image removed --> |
| | image = File:Urea cycle.svg | | | caption = <!-- Caption removed --> |
| | caption = The [[urea cycle]]. The enzyme OTC, labeled prominently in the center of the mitochondria, is deficient in patients with this disorder. | | | field = [[Genetics]] |
| | pronounce =
| | | symptoms = [[Hyperammonemia]], [[lethargy]], [[vomiting]], [[seizures]] |
| | field =
| | | onset = [[Neonatal]] or later in life |
| | symptoms = | | | causes = [[Genetic mutation]] in the [[OTC gene]] |
| | complications =
| | | diagnosis = [[Genetic testing]], [[ammonia levels]] |
| | onset = | | | treatment = [[Low-protein diet]], [[ammonia scavenging drugs]], [[liver transplant]] |
| | duration =
| | | prognosis = Variable, depending on severity and treatment |
| | types =
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| | causes = | |
| | risks =
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| | diagnosis = | |
| | differential = [[Orotic aciduria]]; other urea cycle disorders
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| | prevention =
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| | treatment = Low protein diet; dialysis; liver transplant | |
| | medication = Sodium benzoate
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| | prognosis = In severe cases, death may occur within one week of birth. In mild cases, diagnosis may not be made until middle age.<ref name=Wraith/> | |
| | frequency = 1:60,000 to 1:72,000
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| | deaths =
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| }} | | }} |
| '''Ornithine transcarbamylase deficiency''' is the most common [[urea cycle disorder]] in humans. It is an inherited disorder which causes toxic levels of [[ammonia]] to build up in the blood.<ref name=GeneticsHomeReference>{{cite web|url=https://ghr.nlm.nih.gov/condition/ornithine-transcarbamylase-deficiency|title=Ornithing transcarbamylase deficiency |date=22 May 2018 |accessdate=25 May 2018 |publisher=[[United States National Library of Medicine]]}}</ref>
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| [[Ornithine transcarbamylase]], the defective [[enzyme]] in this disorder, is the final enzyme in the proximal portion of the [[urea cycle]]. It is responsible for converting [[carbamoyl phosphate]] and [[ornithine]] into [[citrulline]]. OTC deficiency is inherited in an [[X-linked recessive]] manner, meaning males are more commonly affected than females.
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| In severely affected individuals, [[hyperammonemia|ammonia concentrations]] increase rapidly, causing [[ataxia]], [[lethargy]], and death without rapid intervention. OTC deficiency is diagnosed using a combination of clinical findings and biochemical testing, while confirmation is often done using molecular genetics techniques.
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| Once an individual has been diagnosed, the treatment goal is to avoid precipitating episodes that can cause an increased ammonia concentration. The most common treatment combines a low protein diet with nitrogen scavenging agents. Liver transplant is considered curative for this disease. Experimental trials of [[gene therapy]] using adenoviral vectors resulted in the death of one participant, [[Jesse Gelsinger]], and have been discontinued.
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| ==Signs and symptoms==
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| OTC deficiency can become apparent at any age. Early-onset OTC deficiency is most commonly found in males. Later in life, the disease may present in both males and females.
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| In the classic presentation, a male infant appears well initially, but by the second day of life becomes irritable, lethargic, and stops feeding. Infants may have poorly-controlled body temperature and respiratory rates, and may experience [[seizure]]s.<ref name=GeneticsHomeReference/> Without urgent intervention, a metabolic [[encephalopathy]] develops; this can progress to [[coma]] and death within the first week of life.<ref name=Wraith /> High levels of ammonia cause preferential damage to the brain, leading to devastating consequences.<ref name=Walker>{{Cite journal | last1 = Walker | first1 = V. | title = Ammonia toxicity and its prevention in inherited defects of the urea cycle | doi = 10.1111/j.1463-1326.2009.01054.x | journal = Diabetes, Obesity and Metabolism | volume = 11 | issue = 9 | pages = 823–835 | year = 2009 | pmid = 19531057 | pmc = }}</ref>
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| Later onset forms of OTC deficiency can have variable presentations. Although late onset forms of the disease are often considered milder than the classic infantile presentation, any affected individual is at risk for an episode of hyperammonemia that could still be life-threatening if presented with the appropriate stressors.<ref name=genereviews>{{Cite journal | last1 = Lichter-Konecki | first1 = U. | last2 = Caldovic | first2 = L. | last3 = Morizono | first3 = H. | last4 = Simpson | first4 = K. | last5 = Pagon | first5 = R. A. | last6 = Adam | first6 = M. P. | last7 = Bird | first7 = T. D. | last8 = Dolan | first8 = C. R. | last9 = Fong | first9 = C. T. | last10 = Stephens | first10 = K. | title = Ornithine Transcarbamylase Deficiency | year = 1993 | pmid = 24006547}}</ref> These patients will often present with headaches, nausea, vomiting, delirium, erratic behavior, or seizures.<ref name = GeneticsHomeReference /> A detailed dietary history of an affected individual with undiagnosed OTC deficiency will often reveal a history of protein avoidance.<ref name = genereviews />
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| The prognosis of a patient with severe OTC deficiency is well correlated with the length of the hyperammonemic period rather than the degree of hyperammonemia or the presence of other symptoms, such as seizures.<ref name=genereviews /> Even for patients with late onset forms of the disease, their overall clinical picture is dependent on the extent of hyperammonemia they have experienced, even if it has remained unrecognized.<ref name=Walker />
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| ==Genetics==
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| [[Image:X-linked recessive.svg|thumb|right|OTC deficiency is inherited in a X-linked recessive manner]]
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| OTC deficiency is caused by mutations in the ''[[Ornithine transcarbamylase|OTC]]'' gene, which is located on the [[X chromosome]].<ref name=omim>{{cite web|title=#311250 - Ornithine Transcarbamylase Deficiency, Hyperammonemia Due To | url = http://omim.org/entry/311250 | publisher = [[Johns Hopkins University]] | accessdate = 2014-01-01}}</ref> ''OTC'' codes for the [[mitochondria]]l enzyme [[ornithine transcarbamylase]], which is expressed only in liver. The functional enzyme consists of three identical subunits.<ref name=genbank>{{cite web|title = Human ornithine transcarbamylase (OTC) mRNA, complete coding sequence | publisher = US National Library of Medicine }}</ref> OTC is the last enzyme in the proximal portion of the urea cycle, which consists of the reactions that take place in the mitochondria. The [[substrate (biochemistry)|substrates]] of the reaction catalyzed by ornithine transcarbamylase are [[ornithine]] and carbamyl phosphate, while the product is [[citrulline]].<ref name=Walker />
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| There are no common mutations that cause disease, however 10 - 15% of disease causing mutations are deletions.<ref name=omim /> It is inherited in an [[X-linked recessive]] manner, meaning males are more commonly affected than females. Females who carry a defective copy of the gene can be severely affected or asymptomatic, largely depending on the random nature of [[X-inactivation]].<ref name = omim /> There is some degree of genotype - phenotype correlation with OTC deficiency, but this depends on a number of situations. Individuals with milder mutations, often associated with late onset disease can still present with severe illness when exposed to sufficient metabolic stress. Correlations are more difficult to ascertain in females, since the residual activity of OTC in the liver is impacted not only by the nature of the mutation, but also by the random pattern of X-inactivation.<ref name=genereviews /> OTC deficiency is estimated to be the most common [[urea cycle disorder]].<ref name=genereviews /> An exact incidence is difficult to calculate, due to the varying clinical presentations of later onset forms of the disease. Early estimates of the incidence were as high as 1:14,000 live births, however later studies have decreased these estimates to approximately 1:60,000 - 1:72,000.<ref name = genereviews />
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| ==Diagnosis==
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| In individuals with marked hyperammonemia, a urea cycle disorder is usually high on the list of possible causes. While the immediate focus is lowering the patient's ammonia concentrations, identifying the specific cause of increased ammonia levels is key as well.
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| Diagnostic testing for OTC deficiency, or any individual with hyperammonemia involves plasma and urine amino acid analysis, urine organic acid analysis (to identify the presence or absence of [[orotic acid]], as well as rule out an [[organic acidemia]]) and plasma acylcarnitines (will be normal in OTC deficiency, but can identify some other causes of hyperammonemia). An individual with untreated OTC deficiency will show decreased [[citrulline]] and [[arginine]] concentrations (because the enzyme block is proximal to these intermediates) and increased orotic acid. The increased orotic acid concentrations result from the buildup of [[carbamoyl phosphate]]. This biochemical phenotype (increased ammonia, low citrulline and increased orotic acid) is classic for OTC deficiency, but can also be seen in neonatal presentations of [[ornithine aminotransferase deficiency]].<ref name= Wraith /> Only severely affected males consistently demonstrate this classic biochemical phenotype.
| | '''Ornithine transcarbamylase deficiency''' (OTC deficiency) is a rare [[genetic disorder]] that affects the [[urea cycle]], a series of biochemical reactions that occur in the [[liver]] to remove [[ammonia]] from the bloodstream. It is the most common [[urea cycle disorder]] and is inherited in an [[X-linked recessive]] pattern. |
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| Heterozygous females can be difficult to diagnose. With the rise of sequencing techniques, molecular testing has become preferred, particularly when the disease causing mutations in the family are known.<ref name=omim /> Historically, heterozygous females were often diagnosed using an [[allopurinol]] challenge. In a female with reduced enzyme activity, an oral dose of allopurinol would be metabolized to oxypurinol ribonucleotide, which blocks the [[pyrimidine]] biosynthetic pathway. When this induced enzymatic block is combined with reduced physiologic enzyme activity as seen in heterozygotes, the elevation of orotic acid could be used to differentiate heterozygotes from unaffected individuals. This test was not universally effective, as it had both [[false negative]] and [[false positive]] results.<ref name=Walker />
| | == Signs and Symptoms == |
| | Individuals with OTC deficiency may present with symptoms of [[hyperammonemia]], which can include [[lethargy]], [[vomiting]], [[seizures]], and [[coma]]. In severe cases, especially in [[neonates]], it can lead to [[brain damage]] or [[death]] if not treated promptly. Milder forms may present later in life with episodes of [[confusion]], [[headache]], and [[behavioral changes]]. |
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| [[Ornithine transcarbamylase]] is only expressed in the liver, thus performing an enzyme assay to confirm the diagnosis requires a liver biopsy. Before molecular genetic testing was commonly available, this was one of the only methods for confirmation of a suspected diagnosis. In cases where [[prenatal diagnosis]] was requested, a fetal liver biopsy used to be required to confirm if a fetus was affected.<ref name = Wraith/> Modern molecular techniques have eliminated this need, and gene sequencing is now the preferred method of diagnosis in asymptomatic family members after the diagnosis has been confirmed in a [[proband]].<ref name=genereviews /><ref name=omim /> | | == Genetics == |
| | OTC deficiency is caused by mutations in the [[OTC gene]], which provides instructions for making the enzyme [[ornithine transcarbamylase]]. This enzyme is critical for the proper function of the urea cycle. Mutations in the OTC gene lead to a deficiency or absence of the enzyme, resulting in the accumulation of ammonia in the blood. |
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| ==Treatment== | | == Diagnosis == |
| The treatment goal for individuals affected with OTC deficiency is the avoidance of hyperammonemia. This can be accomplished through a strictly controlled low-protein diet, as well as preventive treatment with nitrogen scavenging agents such as [[sodium benzoate]]. The goal is to minimize the nitrogen intake while allowing waste nitrogen to be excreted by alternate pathways.<ref name=omim /> [[Arginine]] is typically supplemented as well, in an effort to improve the overall function of the urea cycle.<ref name=omim /> If a hyperammonemic episode occurs, the aim of treatment is to reduce the individual's ammonia levels as soon as possible. In extreme cases, this can involve [[hemodialysis]].<ref name=Wraith>{{Cite journal | last1 = Wraith | first1 = J. E. | title = Ornithine carbamoyltransferase deficiency | journal = Archives of Disease in Childhood | volume = 84 | issue = 1 | pages = 84–88 | year = 2001 | pmid = 11124797 | pmc = 1718609 | doi = 10.1136/adc.84.1.84}}</ref>
| | Diagnosis of OTC deficiency is based on clinical symptoms, elevated [[ammonia levels]] in the blood, and confirmation through [[genetic testing]]. Newborn screening may detect elevated levels of certain metabolites that suggest a urea cycle disorder. |
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| [[Gene therapy]] had been considered a possibility for curative treatment for OTC deficiency, and [[clinical trials]] were taking place at the [[University of Pennsylvania]] in the late 1990s. These were halted after the death of [[Jesse Gelsinger]], a young man taking part in a phase I trial using an [[adenovirus]] vector.<ref name=Deakin2009>{{Cite journal | last1 = Deakin | first1 = C. T. | last2 = Alexander | first2 = I. E. | last3 = Kerridge | first3 = I. | doi = 10.1038/mt.2009.223 | title = Accepting Risk in Clinical Research: Is the Gene Therapy Field Becoming Too Risk-averse? | journal = Molecular Therapy | volume = 17 | issue = 11 | pages = 1842–1848 | year = 2009 | pmid = 19773741 | pmc =2835028 }}</ref> Currently, the only option for curing OTC deficiency is a [[liver transplant]], which restores normal enzyme activity.<ref name=Morioka2005>{{Cite journal | last1 = Morioka | first1 = D. | last2 = Kasahara | first2 = M. | last3 = Takada | first3 = Y. | last4 = Shirouzu | first4 = Y. | last5 = Taira | first5 = K. | last6 = Sakamoto | first6 = S. | last7 = Uryuhara | first7 = K. | last8 = Egawa | first8 = H. | last9 = Shimada | first9 = H. | last10 = Tanaka | first10 = K. | doi = 10.1002/lt.20587 | title = Current role of liver transplantation for the treatment of urea cycle disorders: A review of the worldwide English literature and 13 cases at Kyoto University | journal = Liver Transplantation | volume = 11 | issue = 11 | pages = 1332–1342 | year = 2005 | pmid = 16237708 | pmc = }}</ref> A 2005 review of 51 patients with OTC deficiency who underwent liver transplant estimated 5-year survival rates of greater than 90%.<ref name=Morioka2005 /> Severe cases of OTC deficiency are typically evaluated for liver transplant by 6 months of age.<ref name=genereviews />
| | == Treatment == |
| | Management of OTC deficiency involves reducing ammonia levels in the blood. This can be achieved through a [[low-protein diet]], the use of [[ammonia scavenging drugs]] such as [[sodium phenylbutyrate]] or [[sodium benzoate]], and in some cases, [[liver transplant]]. Early diagnosis and treatment are crucial to prevent [[neurological damage]]. |
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| == Prognosis == | | == Prognosis == |
| A 1999 retrospective study of 74 cases of neonatal onset found that 32 (43%) patients died during their first hyperammonemic episode. Of those who survived, less than 20% survived to age 14. Few of these patients received liver transplants.<ref>{{Cite journal|last=Maestri|first=N. E.|last2=Clissold|first2=D.|last3=Brusilow|first3=S. W.|date=1999-03-01|title=Neonatal onset ornithine transcarbamylase deficiency: A retrospective analysis|journal=The Journal of Pediatrics|volume=134|issue=3|pages=268–272|issn=0022-3476|pmid=10064660|doi=10.1016/s0022-3476(99)70448-8}}</ref>
| | The prognosis for individuals with OTC deficiency varies depending on the severity of the enzyme deficiency and the timeliness of treatment. With appropriate management, individuals can lead relatively normal lives, although they may need to adhere to dietary restrictions and medication regimens. |
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| Even with proper identification and treatment, the majority of patients who present in the neonatal period have severe neurological and intellectual impairments. Liver transplantation cannot cure brain damage which has already occurred, but it will prevent future hyperammonemic episodes and prevent further damage.<ref name=Wraith/>
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| == In popular culture ==
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| *[[Mob Rules (House)]]
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| ==References==
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| {{Reflist}}
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| == External links == | | == See Also == |
| http://www.otcdeficiency.com/<nowiki/>{{Medical resources
| | * [[Urea cycle disorder]] |
| | DiseasesDB = 9286
| | * [[Hyperammonemia]] |
| | ICD10 = {{ICD10|E|72|4|e|72}}
| | * [[Genetic disorder]] |
| | ICD9 = {{ICD9|270.6}}
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| | ICDO =
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| | OMIM = 311250
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| | MedlinePlus = 000372
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| | eMedicineSubj = ped
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| | eMedicineTopic = 2744
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| | MeshID = D020163
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| | Orphanet = 664
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| }}
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| | == References == |
| | {{reflist}} |
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| {{Amino acid metabolic pathology}}
| | == External Links == |
| {{X-linked disorders}}
| | * [https://www.nord.org/ National Organization for Rare Disorders] |
| | * [https://www.genome.gov/ Genetic and Rare Diseases Information Center] |
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| {{DEFAULTSORT:Ornithine Transcarbamylase Deficiency}}
| | [[Category:Genetic disorders]] |
| [[Category:Amino acid metabolism disorders]] | | [[Category:Metabolic disorders]] |
| [[Category:X-linked recessive disorders]] | |
| [[Category:Genetic diseases and disorders]]
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| [[Category:Urea cycle]]
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| [[Category:Rare diseases]] | | [[Category:Rare diseases]] |
| {{dictionary-stub1}}
| | [[Category:Urea cycle disorders]] |
Ornithine transcarbamylase deficiency (OTC deficiency) is a rare genetic disorder that affects the urea cycle, a series of biochemical reactions that occur in the liver to remove ammonia from the bloodstream. It is the most common urea cycle disorder and is inherited in an X-linked recessive pattern.
Signs and Symptoms
Individuals with OTC deficiency may present with symptoms of hyperammonemia, which can include lethargy, vomiting, seizures, and coma. In severe cases, especially in neonates, it can lead to brain damage or death if not treated promptly. Milder forms may present later in life with episodes of confusion, headache, and behavioral changes.
Genetics
OTC deficiency is caused by mutations in the OTC gene, which provides instructions for making the enzyme ornithine transcarbamylase. This enzyme is critical for the proper function of the urea cycle. Mutations in the OTC gene lead to a deficiency or absence of the enzyme, resulting in the accumulation of ammonia in the blood.
Diagnosis
Diagnosis of OTC deficiency is based on clinical symptoms, elevated ammonia levels in the blood, and confirmation through genetic testing. Newborn screening may detect elevated levels of certain metabolites that suggest a urea cycle disorder.
Treatment
Management of OTC deficiency involves reducing ammonia levels in the blood. This can be achieved through a low-protein diet, the use of ammonia scavenging drugs such as sodium phenylbutyrate or sodium benzoate, and in some cases, liver transplant. Early diagnosis and treatment are crucial to prevent neurological damage.
Prognosis
The prognosis for individuals with OTC deficiency varies depending on the severity of the enzyme deficiency and the timeliness of treatment. With appropriate management, individuals can lead relatively normal lives, although they may need to adhere to dietary restrictions and medication regimens.
See Also
References
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External Links
