Mutations and polymorphisms in the human ornithine transcarbamylase (OTC) gene

Perioperative management in pediatric domino liver transplantation for metabolic disorders: A narrative review

Domino liver transplantation and domino-auxiliary partial orthotopic liver transplantation are emerging techniques that can expand the liver donor pool and provide hope for children with liver disease. The innovative technique of domino liver transplantation has emerged as a pioneering strategy, capitalizing on structurally preserved livers from donors exhibiting single enzymatic defects within a morphologically normal context, effectively broadening the donor pool. Concurrently, the increasingly prevalent domino-auxiliary partial orthotopic liver transplantation method assumes a critical role in bolstering available donor resources. These advanced transplantation methods present a unique opportunity for pediatric patients who, despite having structurally and functionally intact livers and lacking early signs of portal hypertension or extrahepatic involvement, do not attain priority on conventional transplant lists. Utilizing optimal clinical conditions enhances posttransplant outcomes, benefiting patients who would otherwise endure extended waiting periods for traditional transplantation. The perioperative management of children undergoing these procedures is complex and requires careful consideration of some factors, including clinical and metabolic conditions of the specific metabolic disorder, and the need for tailored perioperative management planning. Furthermore, the prudent consideration of de novo disease development in the recipient assumes paramount significance when selecting suitable donors for domino liver transplantation, as it profoundly influences prognosis, mortality, and morbidity. This narrative review of domino liver transplantation will discuss the pathophysiology, clinical evaluation, perioperative management, and prognostic expectations, focusing on perioperative anesthetic considerations for children undergoing domino liver transplantation.

Clinical characteristics and molecular genetic analysis of ten cases of ornithine carbamoyltransferase deficiency in southeastern China

BACKGROUND

This study aimed to investigate the clinical and molecular genetic characteristics of ten children with ornithine carbamoyltransferase deficiency (OTCD) in southeastern China, as well as the correlation between the genotype and phenotype of OTCD.

METHODS

A retrospective analysis was performed on the clinical manifestations, laboratory testing, and genetic test findings of ten children with OTCD admitted between August 2015 and October 2021 at Quanzhou Maternity and Children's Hospital of Fujian Province in China.

RESULTS

Five boys presented with early-onset symptoms, including poor appetite, drowsiness, groaning, seizures, and liver failure. In contrast, five patients (one boy and four girls) had late-onset gastrointestinal symptoms as the primary clinical manifestation, all presenting with hepatic impairment, and four with hepatic failure.Nine distinct variants of the OTC gene were identified, including two novel mutations: c.1033del(p.Y345Tfs*50) and c.167T > A(p.M56K). Of seven patients who died, five had early-onset disease despite active treatment. Three patients survived, and two of them underwent liver transplantation.

CONCLUSIONS

The clinical manifestations of OTCD lack specificity. However, elevated blood ammonia levels serve as a crucial diagnostic clue for OTCD. Genetic testing aids in more accurate diagnosis and prognosis assessment by clinicians. In addition, we identified two novel pathogenic variants and expand the mutational spectrum of the gene OTC, which may contribute to a better understanding of the clinical and genetic characteristics of OTCD patients.

Father-to-daughter transmission in late-onset OTC deficiency: an underestimated mechanism of inheritance of an X-linked disease

BACKGROUND

Ornithine Transcarbamylase Deficiency (OTCD) is an X-linked urea cycle disorder characterized by acute hyperammonemic episodes. Hemizygous males are usually affected by a severe/fatal neonatal-onset form or, less frequently, by a late-onset form with milder disease course, depending on the residual enzymatic activity. Hyperammonemia can occur any time during life and patients could remain non- or mis-diagnosed due to unspecific symptoms. In heterozygous females, clinical presentation varies based on the extent of X chromosome inactivation. Maternal transmission in X-linked disease is the rule, but in late-onset OTCD, due to the milder phenotype of affected males, paternal transmission to the females is possible. So far, father-to-daughter transmission of OTCD has been reported only in 4 Japanese families.

RESULTS

We identified in 2 Caucasian families, paternal transmission of late-onset OTCD with severe/fatal outcome in affected males and 1 heterozygous female. Furthermore, we have reassessed the pedigrees of other published reports in 7 additional families with evidence of father-to-daughter inheritance of OTCD, identifying and listing the family members for which this transmission occurred.

CONCLUSIONS

Our study highlights how the diagnosis and pedigree analysis of late-onset OTCD may represent a real challenge for clinicians. Therefore, the occurrence of paternal transmission in OTCD should not be underestimated, due to the relevant implications for disease inheritance and risk of recurrence.

Induced pluripotent stem cell technology as diagnostic tool in patients with suspected ornithine transcarbamylase deficiency lacking genetic confirmation

Ornithine transcarbamylase (OTC) deficiency (OTCD) is an X-linked urea cycle disorder. In females - undergoing random X chromosomal inactivation (XCI) - disease severity depends on the XCI pattern. Hence, female OTCD subjects with favorable XCI display normal OTC expression and activity and are healthy carriers. Whereas females undergoing less favorable XCI may suffer from severe and fatal OTCD. In approximately 20% of patients with biochemical evidence of OTCD, no mutation can be identified hampering definitive diagnosis and adequate treatment.Here, we describe a female patient with high suspicion of OTCD in whom molecular genetic work-up did not reveal pathogenic variants in the OTC gene. In her case, this was particularly challenging, since she was awaiting liver transplantation due to metabolic instability. In order to substantiate the suspected diagnosis of OTCD, we applied our previously reported in vitro OTCD liver disease model. Patient-derived skin fibroblasts were reprogrammed into human induced pluripotent stem cells (hiPSCs) followed by differentiation into hepatocytes (hiPSC-Heps). Among five randomly selected hiPSC clones - differentiated into hiPSC-Heps - one clone expressed OTC protein, while the four remaining clones lacked OTC expression, supporting the patient's suspected diagnosis of OTCD.To conclude, we demonstrate that hiPSC technology is a powerful diagnostic tool to substantiate the suspected diagnosis of OTCD in patients lacking genetic confirmation. Furthermore, selecting clones that exclusively express the wild-type OTC protein, could be used strategically as cellular therapy in future. Ultimately, this approach might be applicable to virtually any X-linked disease.

Synopsis

Induced pluripotent stem cell technology is a powerful diagnostic tool to substantiate the suspected diagnosis of OTCD in patients lacking genetic confirmation.

Datamining approaches for examining the low prevalence of N-acetylglutamate synthase deficiency and understanding transcriptional regulation of urea cycle genes

Ammonia, which is toxic to the brain, is converted into non-toxic urea, through a pathway of six enzymatically catalyzed steps known as the urea cycle. In this pathway, N-acetylglutamate synthase (NAGS, EC 2.3.1.1) catalyzes the formation of N-acetylglutamate (NAG) from glutamate and acetyl coenzyme A. NAGS deficiency (NAGSD) is the rarest of the urea cycle disorders, yet is unique in that ureagenesis can be restored with the drug N-carbamylglutamate (NCG). We investigated whether the rarity of NAGSD could be due to low sequence variation in the NAGS genomic region, high NAGS tolerance for amino acid replacements, and alternative sources of NAG and NCG in the body. We also evaluated whether the small genomic footprint of the NAGS catalytic domain might play a role. The small number of patients diagnosed with NAGSD could result from the absence of specific disease biomarkers and/or short NAGS catalytic domain. We screened for sequence variants in NAGS regulatory regions in patients suspected of having NAGSD and found a novel NAGS regulatory element in the first intron of the NAGS gene. We applied the same datamining approach to identify regulatory elements in the remaining urea cycle genes. In addition to the known promoters and enhancers of each gene, we identified several novel regulatory elements in their upstream regions and first introns. The identification of cis-regulatory elements of urea cycle genes and their associated transcription factors holds promise for uncovering shared mechanisms governing urea cycle gene expression and potentially leading to new treatments for urea cycle disorders.

Disruption of proteome by an oncogenic fusion kinase alters metabolism in fibrolamellar hepatocellular carcinoma

Fibrolamellar hepatocellular carcinoma (FLC) is a usually lethal primary liver cancer driven by a somatic dysregulation of protein kinase A. We show that the proteome of FLC tumors is distinct from that of adjacent nontransformed tissue. These changes can account for some of the cell biological and pathological alterations in FLC cells, including their drug sensitivity and glycolysis. Hyperammonemic encephalopathy is a recurrent problem in these patients, and established treatments based on the assumption of liver failure are unsuccessful. We show that many of the enzymes that produce ammonia are increased and those that consume ammonia are decreased. We also demonstrate that the metabolites of these enzymes change as expected. Thus, hyperammonemic encephalopathy in FLC may require alternative therapeutics.

Challenges of managing ornithine transcarbamylase deficiency in female heterozygotes

Urea cycle disorders (UCDs) are a group of rare inherited metabolic conditions caused by enzyme deficiency within the hepatic ammonia detoxification pathway. Ornithine transcarbamylase (OTC) deficiency, the most frequently occurring UCD, is an X-linked condition known to yield a vastly heterogeneous phenotype, with variable onset and presentation across the lifespan. Here, we introduce a series of 4 original cases, published as part of this special supplement, that illustrate learnings for the care of heterozygous females with OTC deficiency, including challenges with diagnosis, potential triggers of hyperammonemia, cognitive effects, and approaches to disease management, including peripartum care.

Pathogenic variants of ornithine transcarbamylase deficiency: Nation-wide study in Japan and literature review

Ornithine transcarbamylase deficiency (OTCD) is an X-linked disorder. Several male patients with OTCD suffer from severe hyperammonemic crisis in the neonatal period, whereas others develop late-onset manifestations, including hyperammonemic coma. Females with heterozygous pathogenic variants in the OTC gene may develop a variety of clinical manifestations, ranging from asymptomatic conditions to severe hyperammonemic attacks, owing to skewed lyonization. We reported the variants of CPS1, ASS, ASL and OTC detected in the patients with urea cycle disorders through a nation-wide survey in Japan. In this study, we updated the variant data of OTC in Japanese patients and acquired information regarding genetic variants of OTC from patients with OTCD through an extensive literature review. The 523 variants included 386 substitution (330 missense, 53 nonsense, and 3 silent), eight deletion, two duplication, one deletion-insertion, 55 frame shift, two extension, and 69 no category (1 regulatory and 68 splice site error) mutations. We observed a genotype-phenotype relation between the onset time (neonatal onset or late onset), the severity, and genetic mutation in male OTCD patients because the level of deactivation of OTC significantly depends on the pathogenic OTC variants. In conclusion, genetic information about OTC may help to predict long-term outcomes and determine specific treatment strategies, such as liver transplantation, in patients with OTCD.

Liver transplantation in rare late-onset ornithine transcarbamylase deficiency with central nervous system injury: A case report and review of the literature

BACKGROUND

Ornithine transcarbamylase deficiency (OTCD) is a genetic metabolic disease. Its clinical manifestations are mainly central nervous system dysfunction caused by high blood ammonia. Late-onset OTCD combined with central nervous system injury has a poor therapeutic response, which is one of the main factors affecting the prognosis and quality of life of patients. liver transplantation (LT) has gradually become a radical treatment for OTCD, which has achieved good results. However, there is no consensus on the timing of LT and problems of nervous system damage and repair.

METHODS

We report the development of late-onset OTCD with central nervous system injury in an 11-year-old child who received liver transplantation at our transplant center. His first symptoms were nonprojectile vomiting, followed by irritability and disturbance of consciousness, after which the disease progressed rapidly and finally resulted in a coma. After liver transplantation, the child's consciousness returned to normal, muscle strength of the limbs gradually recovered from grade 0 to grade 4, and muscle tone gradually recovered from grade 4 to grade 1, suggesting that the motor nerves had gradually recovered. However, the child is currently mentally retarded, and the language center has not yet fully recovered.At the same time, we made a literature review of OTCD.

CONCLUSION

For OTCD patients with central nervous system injury, liver transplantation can fundamentally solve the problem of ammonia metabolism in the liver and avoids further damage to the central nervous system caused by hyperammonemia. At the same time, children's nervous systems are in the developmental stage when neuroplasticity is greatest. If liver transplantation is performed as soon as possible, nerve repair is still possible.

Psychological Stress Triggers a Hyperammonemia Episode in Patient with Ornithine Transcarbamylase Deficiency

An 18-year-old male motorcycle racer, who was a participant in the FIM Road Racing World Championship and had a history of Ornithine Transcarbamylase deficiency, developed nausea and dizziness while driving his motorcycle and became unconscious right after he stopped at the box. He was rapidly attended to by the medical personnel of the circuit, and once he recovered consciousness, he was taken to the local hospital where the blood analysis showed hyperammonemia (307 μg/dL) and excess alkalosis. The patient was properly following the prescribed treatment, and there were no environmental stressors. Hence, psychological stress and its somatization due to the risky task that the patient was performing could have triggered the episode. Stress must be considered as a potential cause, triggering strenuous metabolic stress that leads to hyperammonemia.

A complex case of delayed diagnosis of ornithine transcarbamylase deficiency in an adult patient with multiple comorbidities

We report the case of a medically complex African American adult female with ornithine transcarbamylase (OTC) deficiency diagnosed after lifelong protein aversion and new onset of chronic vomiting and abdominal pain with intermittent lethargy and confusion. Symptomatology was crucial to diagnosis as genetic testing did not identify any pathogenic variants in OTC; however, the patient's diagnosis was delayed despite her having longstanding symptoms of a urea cycle disorder (UCD). Her symptoms improved after treatment with a modified protein-restricted diet, long-term nitrogen-scavenger therapy, and supplemental L-citrulline. Adherence to her UCD management regimen remained a challenge due to her underlying frailty and other medical conditions, which included primary renal impairment (further exasperated by type 2 diabetes mellitus) and decreased left-ventricular function. She passed away 3 years after her OTC deficiency diagnosis due to complications of congestive heart failure. Her OTC deficiency did not have a major impact on her final illness, and appropriate OTC deficiency management was provided until the decision was made to withdraw medical care.

Glutaminase 2 knockdown reduces hyperammonemia and associated lethality of urea cycle disorder mouse model

Amino acids, the building blocks of proteins in the cells and tissues, are of fundamental importance for cell survival, maintenance, and proliferation. The liver plays a critical role in amino acid metabolism and detoxication of byproducts such as ammonia. Urea cycle disorders with hyperammonemia remain difficult to treat and eventually necessitate liver transplantation. In this study, ornithine transcarbamylase deficient (Otc^spf-ash ) mouse model was used to test whether knockdown of a key glutamine metabolism enzyme glutaminase 2 (GLS2, gene name: Gls2) or glutamate dehydrogenase 1 (GLUD1, gene name: Glud1) could rescue the hyperammonemia and associated lethality induced by a high protein diet. We found that reduced hepatic expression of Gls2 but not Glud1 by AAV8-mediated delivery of a short hairpin RNA in Otc^spf-ash mice diminished hyperammonemia and reduced lethality. Knockdown of Gls2 but not Glud1 in Otc^spf-ash mice exhibited reduced body weight loss and increased plasma glutamine concentration. These data suggest that Gls2 hepatic knockdown could potentially help alleviate risk for hyperammonemia and other clinical manifestations of patients suffering from defects in the urea cycle.

Corticosteroid suppresses urea-cycle-related gene expressions in ornithine transcarbamylase deficiency

Background

Ornithine transcarbamylase deficiency (OTCD) is most common among urea cycle disorders (UCDs), defined by defects in enzymes associated with ureagenesis. Corticosteroid administration to UCD patients, including OTCD patients, is suggested to be avoided, as it may induce life-threatening hyperammonemia. The mechanism has been considered nitrogen overload due to the catabolic effect of corticosteroids; however, the pathophysiological process is unclear.

Methods

To elucidate the mechanism of hyperammonemia induced by corticosteroid administration in OTCD patients, we analyzed a mouse model by administering corticosteroids to OTC^spf−ash mice deficient in the OTC gene. Dexamethasone (DEX; 20 mg/kg) was administered to the OTC^spf−ash and wild-type (WT) mice at 0 and 24 h, and the serum ammonia concentrations, the levels of the hepatic metabolites, and the gene expressions related with ammonia metabolism in the livers and muscles were analyzed.

Results

The ammonia levels in Otc^spf−ash mice that were administered DEX tended to increase at 24 h and increased significantly at 48 h. The metabolomic analysis showed that the levels of citrulline, arginine, and ornithine did not differ significantly between Otc^spf−ash mice that were administered DEX and normal saline; however, the level of aspartate was increased drastically in Otc^spf−ash mice owing to DEX administration (P < 0.01). Among the enzymes associated with the urea cycle, mRNA expressions of carbamoyl-phosphate synthase 1, ornithine transcarbamylase, arginosuccinate synthase 1, and arginosuccinate lyase in the livers were significantly downregulated by DEX administration in both the Otc^spf−ash and WT mice (P < 0.01). Among the enzymes associated with catabolism, mRNA expression of Muscle RING-finger protein-1 in the muscles was significantly upregulated in the muscles of WT mice by DEX administration (P < 0.05).

Conclusions

We elucidated that corticosteroid administration induced hyperammonemia in Otc^spf−ash mice by not only muscle catabolism but also suppressing urea-cycle-related gene expressions. Since the urea cycle intermediate amino acids, such as arginine, might not be effective because of the suppressed expression of urea-cycle-related genes by corticosteroid administration, we should consider an early intervention by renal replacement therapy in cases of UCD patients induced by corticosteroids to avoid brain injuries or fatal outcomes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12876-022-02213-0.

Therapeutic enzyme engineering using a generative neural network

Enhancing the potency of mRNA therapeutics is an important objective for treating rare diseases, since it may enable lower and less-frequent dosing. Enzyme engineering can increase potency of mRNA therapeutics by improving the expression, half-life, and catalytic efficiency of the mRNA-encoded enzymes. However, sequence space is incomprehensibly vast, and methods to map sequence to function (computationally or experimentally) are inaccurate or time-/labor-intensive. Here, we present a novel, broadly applicable engineering method that combines deep latent variable modelling of sequence co-evolution with automated protein library design and construction to rapidly identify metabolic enzyme variants that are both more thermally stable and more catalytically active. We apply this approach to improve the potency of ornithine transcarbamylase (OTC), a urea cycle enzyme for which loss of catalytic activity causes a rare but serious metabolic disease.

OTC intron 4 variations mediate pathogenic splicing patterns caused by the c.386G>A mutation in humans and spfash mice, and govern susceptibility to RNA-based therapies

Background

Aberrant splicing is a common outcome in the presence of exonic or intronic variants that might hamper the intricate network of interactions defining an exon in a specific gene context. Therefore, the evaluation of the functional, and potentially pathological, role of nucleotide changes remains one of the major challenges in the modern genomic era. This aspect has also to be taken into account during the pre-clinical evaluation of innovative therapeutic approaches in animal models of human diseases. This is of particular relevance when developing therapeutics acting on splicing, an intriguing and expanding research area for several disorders. Here, we addressed species-specific splicing mechanisms triggered by the OTC c.386G>A mutation, relatively frequent in humans, leading to Ornithine TransCarbamylase Deficiency (OTCD) in patients and spf^ash mice, and its differential susceptibility to RNA therapeutics based on engineered U1snRNA.

Methods

Creation and co-expression of engineered U1snRNAs with human and mouse minigenes, either wild-type or harbouring different nucleotide changes, in human (HepG2) and mouse (Hepa1-6) hepatoma cells followed by analysis of splicing pattern. RNA pulldown studies to evaluate binding of specific splicing factors.

Results

Comparative nucleotide analysis suggested a role for the intronic +10-11 nucleotides, and pull-down assays showed that they confer preferential binding to the TIA1 splicing factor in the mouse context, where TIA1 overexpression further increases correct splicing. Consistently, the splicing profile of the human minigene with mouse +10-11 nucleotides overlapped that of mouse minigene, and restored responsiveness to TIA1 overexpression and to compensatory U1snRNA. Swapping the human +10-11 nucleotides into the mouse context had opposite effects.

Moreover, the interplay between the authentic and the adjacent cryptic 5′ss in the human OTC dictates pathogenic mechanisms of several OTCD-causing 5′ss mutations, and only the c.386+5G>A change, abrogating the cryptic 5′ss, was rescuable by engineered U1snRNA.

Conclusions

Subtle intronic variations explain species-specific OTC splicing patterns driven by the c.386G>A mutation, and the responsiveness to engineered U1snRNAs, which suggests careful elucidation of molecular mechanisms before proposing translation of tailored therapeutics from animal models to humans.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-021-00418-9.

Ornithine Transcarbamylase - From Structure to Metabolism: An Update

Ornithine transcarbamylase (OTC; EC 2.1.3.3) is a ubiquitous enzyme found in almost all organisms, including vertebrates, microorganisms, and plants. Anabolic, mostly trimeric OTCs catalyze the production of L-citrulline from L-ornithine which is a part of the urea cycle. In eukaryotes, such OTC localizes to the mitochondrial matrix, partially bound to the mitochondrial inner membrane and part of channeling multi-enzyme assemblies. In mammals, mainly two organs express OTC: the liver, where it is an integral part of the urea cycle, and the intestine, where it synthesizes citrulline for export and plays a major role in amino acid homeostasis, particularly of L-glutamine and L-arginine. Here, we give an overview on OTC genes and proteins, their tissue distribution, regulation, and physiological function, emphasizing the importance of OTC and urea cycle enzymes for metabolic regulation in human health and disease. Finally, we summarize the current knowledge of OTC deficiency, a rare X-linked human genetic disorder, and its emerging role in various chronic pathologies.

Metabolomics in Clinical Practice: Improving Diagnosis and Informing Management

BACKGROUND

Metabolomics is the study of small molecules to simultaneously identify multiple low molecular weight molecules in a system. Broadly speaking, metabolomics can be subdivided into targeted and untargeted types of analysis, each type having advantages and drawbacks. Targeted metabolomics can quantify analytes but only looks for known or expected analytes related to particular disease(s), whereas untargeted metabolomics is typically nonquantitative but can detect thousands of analytes from an agnostic or nonhypothesis driven perspective, allowing for novel discoveries.

CONTENT

One application of metabolomics is the study of inborn errors of metabolism (IEM). The biochemical hallmark of IEMs is decreased concentrations of analytes distal to the enzymatic defect and buildup of analytes proximal to the defect. Metabolomics can detect these changes with one test and is effective in screening for and diagnosis of IEMs. Metabolomics has also been used to study many nonmetabolic diseases such as autism spectrum disorder, various cancers, and multiple congenital anomalies syndromes. Metabolomics has led to the discovery of many novel biomarkers of disease. Recent publications demonstrate how metabolomics can be useful clinically in the diagnosis and management of patients, as well as for research and clinical discovery.

SUMMARY

Metabolomics has proved to be a useful tool clinically for screening and diagnostic purposes and from a research perspective for the detection of novel biomarkers. In the future, metabolomics will likely become a routine part of the evaluation for many diseases as either a supplementary test or it may simply replace historical analyses that require several individual tests and sample types.

Chemical rescue of mutant proteins in living Saccharomyces cerevisiae cells by naturally occurring small molecules

Intracellular proteins function in a complex milieu wherein small molecules influence protein folding and act as essential cofactors for enzymatic reactions. Thus protein function depends not only on amino acid sequence but also on the concentrations of such molecules, which are subject to wide variation between organisms, metabolic states, and environmental conditions. We previously found evidence that exogenous guanidine reverses the phenotypes of specific budding yeast septin mutants by binding to a WT septin at the former site of an Arg side chain that was lost during fungal evolution. Here, we used a combination of targeted and unbiased approaches to look for other cases of "chemical rescue" by naturally occurring small molecules. We report in vivo rescue of hundreds of Saccharomyces cerevisiae mutants representing a variety of genes, including likely examples of Arg or Lys side chain replacement by the guanidinium ion. Failed rescue of targeted mutants highlight features required for rescue, as well as key differences between the in vitro and in vivo environments. Some non-Arg mutants rescued by guanidine likely result from "off-target" effects on specific cellular processes in WT cells. Molecules isosteric to guanidine and known to influence protein folding had a range of effects, from essentially none for urea, to rescue of a few mutants by DMSO. Strikingly, the osmolyte trimethylamine-N-oxide rescued ∼20% of the mutants we tested, likely reflecting combinations of direct and indirect effects on mutant protein function. Our findings illustrate the potential of natural small molecules as therapeutic interventions and drivers of evolution.

OTC deficiency in females: Phenotype-genotype correlation based on a 130-family cohort

OTC deficiency, an inherited urea cycle disorder, is caused by mutations in the X-linked OTC gene. Phenotype-genotype correlations are well understood in males but still poorly known in females. Taking advantage of a cohort of 130 families (289 females), we assessed the relative contribution of OTC enzyme activity, X chromosome inactivation, and OTC gene sequencing to genetic counseling in heterozygous females. Twenty two percent of the heterozygous females were clinically affected, with episodic (11%), chronic (7.5%), or neonatal forms of the disease (3.5%). Overall mortality rate was 4%. OTC activity, ranging from 0% to 60%, did not correlate with phenotype at the individual level. Analysis of multiple samples from 4 mutant livers showed intra-hepatic variability of OTC activity and X inactivation profile (range of variability: 30% and 20%, respectively) without correlation between both parameters for 3 of the 4 livers. Ninety disease-causing variants were found, 27 of which were novel. Mutations were classified as "mild" or "severe," based on male phenotypes and/or in silico prediction. In our cohort, a serious disease occurred in 32% of females with a severe mutation, compared to 4% in females with a mild mutation (odds ratio = 1.365; P = 1.6e-06). These data should help prenatal diagnosis for heterozygous females and genetic counseling after fortuitous findings of OTC variants in pangenomic sequencing.

Common polymorphic OTC variants can act as genetic modifiers of enzymatic activity

Understanding the role of common polymorphisms in modulating the clinical phenotype when they co‐occur with a disease‐causing lesion is of critical importance in medical genetics. We explored the impact of apparently neutral common polymorphisms, using the gene encoding the urea cycle enzyme, ornithine transcarbamylase (OTC), as a model system. Distinct combinations of genetic backgrounds embracing two missense polymorphisms were created in cis with the pathogenic p.Arg40His replacement. In vitro enzymatic assays revealed that the polymorphic variants were able to modulate OTC activity both in the presence or absence of the pathogenic lesion. First, we found that the combination of the minor alleles of polymorphisms p.Lys46Arg and p.Gln270Arg significantly enhanced enzymatic activity in the wild‐type protein. Second, enzymatic assays revealed that the minor allele of the p.Gln270Arg polymorphism was capable of ameliorating OTC activity when combined in cis with the pathogenic p.Arg40His replacement. Structural analysis predicted that the minor allele of the p.Gln270Arg polymorphism would serve to stabilize the OTC wild‐type protein, thereby corroborating the results of the experimental assays. Our findings demonstrate the potential importance of cis‐interactions between common polymorphic variants and pathogenic missense mutations and illustrate how standing genetic variation can modulate protein function.

The role of orotic acid measurement in routine newborn screening for urea cycle disorders

Urea cycle disorders (UCDs), including OTC deficiency (OTCD), are life-threatening diseases with a broad clinical spectrum. Early diagnosis and initiation of treatment based on a newborn screening (NBS) test for OTCD with high specificity and sensitivity may contribute to reduction of the significant complications and high mortality. The efficacy of incorporating orotic acid determination into routine NBS was evaluated. Combined measurement of orotic acid and citrulline in archived dried blood spots from newborns with urea cycle disorders and normal controls was used to develop an algorithm for routine NBS for OTCD in Israel. Clinical information and genetic confirmation results were obtained from the follow-up care providers. About 1147986 newborns underwent routine NBS including orotic acid determination, 25 of whom were ultimately diagnosed with a UCD. Of 11 newborns with OTCD, orotate was elevated in seven but normal in two males with early-onset and two males with late-onset disease. Orotate was also elevated in archived dried blood spots of all seven retrospectively tested historical OTCD patients, only three of whom had originally been identified by NBS with low citrulline and elevated glutamine. Among the other UCDs emerge, three CPS1D cases and additional three retrospective CPS1D cases otherwise reported as a very rare condition. Combined levels of orotic acid and citrulline in routine NBS can enhance the detection of UCD, especially increasing the screening sensitivity for OTCD and differentiate it from CPS1D. Our data and the negligible extra cost for orotic acid determination might contribute to the discussion on screening for proximal UCDs in routine NBS.

Clinical and structural insights into potential dominant negative triggers of proximal urea cycle disorders

Despite biochemical and genetic testing being the golden standards for identification of proximal urea cycle disorders (UCDs), genotype-phenotype correlations are often unclear. Co-occurring partial defects affecting more than one gene have not been demonstrated so far in proximal UCDs. Here, we analyzed the mutational spectrum of 557 suspected proximal UCD individuals. We probed oligomerizing forms of NAGS, CPS1 and OTC, and evaluated the surface exposure of residues mutated in heterozygously affected individuals. BN-PAGE and gel-filtration chromatography were employed to discover protein-protein interactions within recombinant enzymes. From a total of 281 confirmed patients, only 15 were identified as "heterozygous-only" candidates (i.e. single defective allele). Within these cases, the only missense variants to potentially qualify as dominant negative triggers were CPS1 p.Gly401Arg and NAGS p.Thr181Ala and p.Tyr512Cys, as assessed by residue oligomerization capacity and surface exposure. However, all three candidates seem to participate in critical intramolecular functions, thus, unlikely to facilitate protein-protein interactions. This interpretation is further supported by BN-PAGE and gel-filtration analyses revealing no multiprotein proximal urea cycle complex formation. Collectively, genetic analysis, structural considerations and in vitro experiments point against a prominent role of dominant negative effects in human proximal UCDs.

Clinical and molecular characteristics of 69 Chinese patients with ornithine transcarbamylase deficiency

Background

This study aimed to describe the clinical and biochemical features of Chinese patients with ornithine transcarbamylase deficiency (OTCD), and to investigate the mutation spectrum of OTC gene and their potential correlation with phenotype.

Methods

Sixty-nine patients with OTCD were enrolled between 2004 and 2019. Clinical and laboratory data were reviewed retrospectively from medical records.

Results

Fifteen cases (13 males, 2 females) presented with early onset; 53 cases (21 males, 32 females) had late onset, and one female was asymptomatic. The median onset age was 1.5 years (range 1 day–56 years). Urine orotic acid levels were increased in all patients tested, while only 47.6% of patients showed decreased serum levels of citrulline. The peak plasma ammonia levels were higher in early-onset patients than in late-onset patients (P < 0.01). Fifty-four different mutations of OTC gene were identified and 18 of them were novel. R277W (10.6%) was the most common mutation, followed by G195R (4.6%) and A209V (3.0%). By June 2019, 41 patients had survived, 24 were deceased, and 4 were lost to follow-up. Among the survivors, 13 patients had received liver transplantation at a median age of 3 years, with a one-year survival rate of 100%. The mortality of OTCD is extremely high among patients with early onset (80.0% versus 24.5% in patients with late onset).

Conclusions

The evaluation of serum citrulline level is of limited value in diagnosis of OTCD, while urine orotic acid detection and genetic testing are more helpful.

Genomic Sequencing for Newborn Screening: Results of the NC NEXUS Project

Newborn screening (NBS) was established as a public health program in the 1960s and is crucial for facilitating detection of certain medical conditions in which early intervention can prevent serious, life-threatening health problems. Genomic sequencing can potentially expand the screening for rare hereditary disorders, but many questions surround its possible use for this purpose. We examined the use of exome sequencing (ES) for NBS in the North Carolina Newborn Exome Sequencing for Universal Screening (NC NEXUS) project, comparing the yield from ES used in a screening versus a diagnostic context. We enrolled healthy newborns and children with metabolic diseases or hearing loss (106 participants total). ES confirmed the participant's underlying diagnosis in 15 out of 17 (88%) children with metabolic disorders and in 5 out of 28 (∼18%) children with hearing loss. We discovered actionable findings in four participants that would not have been detected by standard NBS. A subset of parents was eligible to receive additional information for their child about childhood-onset conditions with low or no clinical actionability, clinically actionable adult-onset conditions, and carrier status for autosomal-recessive conditions. We found pathogenic variants associated with hereditary breast and/or ovarian cancer in two children, a likely pathogenic variant in the gene associated with Lowe syndrome in one child, and an average of 1.8 reportable variants per child for carrier results. These results highlight the benefits and limitations of using genomic sequencing for NBS and the challenges of using such technology in future precision medicine approaches.

CRISPR Start-Loss: A Novel and Practical Alternative for Gene Silencing through Base-Editing-Induced Start Codon Mutations

CRISPR-Cas9-mediated gene knockout and base-editing-associated induction of STOP codons (iSTOP) have been widely used to exterminate the function of a coding gene, while they have been reported to exhibit side effects. In this study, we propose a novel and practical alternative method referred to as CRISPR Start-Loss (CRISPR-SL), which eliminates gene expression by utilizing both adenine base editors (ABEs) and cytidine base editors (CBEs) to disrupt the initiation codon (ATG). CRISPR-SL has been verified to be a feasible strategy on the cellular and embryonic levels (mean editing efficiencies up to 30.67% and 73.50%, respectively) and in two rabbit models mimicking Otc deficiency (Otc gene) and long hair economic traits (Fgf5 gene).

The Application of Next-Generation Sequencing (NGS) in Neonatal-Onset Urea Cycle Disorders (UCDs): Clinical Course, Metabolomic Profiling, and Genetic Findings in Nine Chinese Hyperammonemia Patients

During Jan. 2016–Dec. 2019, nine Chinese patients from eight unrelated families were diagnosed with neonatal-onset UCDs by targeted panel sequencing or whole-exome sequencing (WES). Their clinical manifestations, biochemical features, 180-day-age outcomes, and molecular genetic characteristics were reviewed retrospectively. NGS-based tests revealed 7 patients diagnosed with ornithine transcarbamylase deficiency (OTCD) and 2 with carbamoylphosphate synthetase I deficiency (CPS1D). The spectrum of the clinical presentation of nine affected individuals progressed from unspecific symptoms like poor feeding to somnolence, coma, and death. All patients presented with an acute hyperammonemia. The most robust metabolic pattern in OTCD was hyperglutaminemic hyperammonemia with high concentration of urine orotic acid, and it was reported in six patients. Of ten variants found on the OTC gene and CPS1 gene, 3 were novel: (c.176T>C (p.L59P)) in the OTC gene, c.2938G>A (p.G980S) and c.3734T>A (p.L1245H) in the CPS1 gene. There was a high mortality rate of 77.78% (7/9) for all the defects combined. An OTC-deficient male and a CPS1-deficient female survived from episodes of hyperammonemia. Although prompt recognition of UCD and the use of alternative pathway therapy in addition to provision of appropriate nutrition and dialysis improved survival, the overall outcomes for the neonatal-onset type are poor in China.

Expanding Role of Proton Magnetic Resonance Spectroscopy: Timely Diagnosis and Treatment Initiation in Partial Ornithine Transcarbamylase Deficiency

We report the case of a 3-year-old male patient who presented with a 3-day history of altered mental status, emesis, and abdominal pain in the setting of a viral illness. A rapid screening revealed a high ammonia level and after reviewing his proton magnetic resonance spectroscopy (1H MRS) which showed the classic triad of high glutamate, low choline, and myoinositol, a diagnosis of ornithine transcarbamylase deficiency (OTCD) was made within 6 hours of presentation. Therapy with sodium phenylbutyrate and sodium benzoate was initiated and patient was discharged after 3 days with no neurologic disability. Biochemical and molecular testing eventually confirmed the diagnosis. 1H MRS is a practical and fast neuroimaging modality that can aid in diagnosis of OTCD and enables faster initiation of treatment in acute settings.

Probing remote residues important for catalysis in Escherichia coli ornithine transcarbamoylase

Understanding how enzymes achieve their tremendous catalytic power is a major question in biochemistry. Greater understanding is also needed for enzyme engineering applications. In many cases, enzyme efficiency and specificity depend on residues not in direct contact with the substrate, termed remote residues. This work focuses on Escherichia coli ornithine transcarbamoylase (OTC), which plays a central role in amino acid metabolism. OTC has been reported to undergo an induced-fit conformational change upon binding its first substrate, carbamoyl phosphate (CP), and several residues important for activity have been identified. Using computational methods based on the computed chemical properties from theoretical titration curves, sequence-based scores derived from evolutionary history, and protein surface topology, residues important for catalytic activity were predicted. The roles of these residues in OTC activity were tested by constructing mutations at predicted positions, followed by steady-state kinetics assays and substrate binding studies with the variants. First-layer mutations R57A and D231A, second-layer mutation H272L, and third-layer mutation E299Q, result in 57- to 450-fold reductions in k_cat/K_M with respect to CP and 44- to 580-fold reductions with respect to ornithine. Second-layer mutations D140N and Y160S also reduce activity with respect to ornithine. Most variants had decreased stability relative to wild-type OTC, with variants H272L, H272N, and E299Q having the greatest decreases. Variants H272L, E299Q, and R57A also show compromised CP binding. In addition to direct effects on catalytic activity, effects on overall protein stability and substrate binding were observed that reveal the intricacies of how these residues contribute to catalysis.

Late-Onset Ornithine Transcarbamylase Deficiency and Variable Phenotypes in Vietnamese Females With OTC Mutations

Background: Ornithine transcarbamylase deficiency (OTCD) is an X- linked recessive disorder and the most common error of the urea cycle, caused by the mutations in the OTC gene. Due to X-inactivation, 15-20% of female carriers present symptoms of OTCD at late onset. Early diagnosis of OTCD by molecular analysis in females is highly desirable. The aim of the study was to identify the mutations in two unrelated Vietnamese girls suspected with OTCD and the carriers in their families for definitive diagnosis and proper counseling. Case Presentation: Two patients presented with an acute encephalopathy at the first admission. Biochemical tests revealed hyperammonemia, hyperlactatemia, elevated glutamine level, elevated transaminase, elevated urinary orotic and uracil acid levels, and disorder of prothrombin time. Brain magnetic resonance imaging indicated cerebral edema. Based on the clinical and laboratory results, the two patients were diagnosed with urea cycle disorders. Therefore, the two patients were managed by stopping feeding, with infused glucose, l-carnitine, l-arginine, and sodium benzoate, and with hemofiltration. The two patients were alert and recovered with normal blood ammonia levels after 72 h of treatment. The family history of patient 1 showed that her brother died at 4 days of age due to a coma and dyspnea, while her parents were asymptomatic. Variable phenotypes were observed in three generations of the patient 2's family, including asymptomatic (mother), affected female adults dying at the first symptom (grandmother and aunt), and affected males dying in the first week of life (uncle, cousin, and siblings). Whole-exome sequencing showed two mutations in the OTC gene, including one novel missense mutation, c.365A>T, in the patient 1 and one previously reported splicing mutation, c.717+1G>A, in the patient 2. The two mutations are evaluated as likely pathogenic and pathogenic, respectively, according to the recommendations of the American College of Medical Genetics and Genomics (ACMG). Genetic analyses in the families indicated the mothers were heterozygous. Conclusion: Clinical, biochemical, and molecular findings accurately diagnosed the two patients with late-onset OTCD. Our results explained the genetic causes and proposed the risk in the patients' families, which could be useful for genetic counseling and monitoring in prenatal diagnosis.

Suggested guidelines for the diagnosis and management of urea cycle disorders: First revision

In 2012, we published guidelines summarizing and evaluating late 2011 evidence for diagnosis and therapy of urea cycle disorders (UCDs). With 1:35 000 estimated incidence, UCDs cause hyperammonemia of neonatal (~50%) or late onset that can lead to intellectual disability or death, even while effective therapies do exist. In the 7 years that have elapsed since the first guideline was published, abundant novel information has accumulated, experience on newborn screening for some UCDs has widened, a novel hyperammonemia-causing genetic disorder has been reported, glycerol phenylbutyrate has been introduced as a treatment, and novel promising therapeutic avenues (including gene therapy) have been opened. Several factors including the impact of the first edition of these guidelines (frequently read and quoted) may have increased awareness among health professionals and patient families. However, under-recognition and delayed diagnosis of UCDs still appear widespread. It was therefore necessary to revise the original guidelines to ensure an up-to-date frame of reference for professionals and patients as well as for awareness campaigns. This was accomplished by keeping the original spirit of providing a trans-European consensus based on robust evidence (scored with GRADE methodology), involving professionals on UCDs from nine countries in preparing this consensus. We believe this revised guideline, which has been reviewed by several societies that are involved in the management of UCDs, will have a positive impact on the outcomes of patients by establishing common standards, and spreading and harmonizing good practices. It may also promote the identification of knowledge voids to be filled by future research.

Challenges in diagnosing and managing adult patients with urea cycle disorders

Urea cycle disorders (UCD) are a group of rare inherited metabolic conditions of amino acid catabolism caused by an enzyme deficiency within the hepatic ammonia detoxification pathway. The presentation of these disorders ranges from life-threatening intoxication in the neonate to asymptomatic status in adults. Late-onset UCDs can present for the first time in adulthood and may mimic other causes of acute confusion or psychiatric diseases, and are often associated with neurological symptoms. Late-onset UCDs may become apparent during periods of metabolic stress such as rapid weight loss, gastric bypass surgery, chronic starvation or the postpartum period. Early diagnosis is critical for effective treatment and to prevent long-term complications of hyperammonemia. The challenges of management of adults include for example: (a) poor compliance to dietary and medical treatment which can result in recurrent hospital admissions; (b) severe neurological dysfunction; (c) the management of pregnancy and the postpartum period; and (d) access to multidisciplinary care peri-operatively. In this review, we highlight a number of challenges in the diagnosis and management of adult patient with late-onset UCDs and suggest a systematic management approach.

Clinical and cranial MRI features of female patients with ornithine transcarbamylase deficiency: Two case reports

INTRODUCTION

Ornithine transcarbamylase deficiency (OTCD) is a common metabolic disease of urea circulation disorder. We reported the clinical, brain imaging and genetic characteristics of 2 cases with OTCD. The patients' clinical features, novel gene mutations, cranial MR specific imaging changes and blood tandem mass spectrometry, and urine gas chromatography-mass spectrometry were, retrospectively, analyzed.

PATIENT CONCERNS

Patient 1 was a 1.6-year-old female. She was admitted to the hospital with 2-months history of general irritability and disturbance of consciousness for a day. Patient 2 was a 3.7-year-old female. She was admitted to the hospital due to decline of language ability and irritability for 5 days. Blood tandem mass spectrometry and urine gas chromatography-mass spectrometry showed uracil and orotate increased significantly in urine while amino acids in the urea cycle ring were in the normal range. The features of brain MRI are consistent with those of urea circulatory disorders. Gene detection showed 1 novel mutation in the OTC gene (c.658C>T) in patient 1 and, 1 novel mutation (c.298+2T>G) in the OTC gene in patient 2.

DIAGNOSIS

Combined with metabolic screening and gene detection, both patients were diagnosed with OTCD.

INTERVENTIONS

The patients' condition improved after following a low protein diet and receiving treatments for decreasing blood ammonia, energy supplement, correcting acid-base imbalance, and other symptomatic treatments.

OUTCOMES

After prompt symptomatic treatment, the consciousness and cognition of the children improved. Besides, liver function also improved significantly.

CONCLUSIONS

For patients with neurological symptoms and unexplained increase in transaminase and ammonia, OTCD should be considered as a possible diagnosis. Brain MRI can help the diagnosis of genetic metabolic encephalopathy and reflect the level of brain injury. Metabolic screening and genetic detection are helpful to make a confirmed diagnosis.

Genomic study via chromosomal microarray analysis in a group of Romanian patients with obesity and developmental disability/intellectual disability

Background Obesity with developmental disability/intellectual disability (DD/ID) is the most common association in syndromic obesity. Genomic analysis studies have allowed the decipherment of disease aetiology, both in cases of syndromic obesity as well as in cases of isolated or syndromic DD/ID. However, more data are needed to further elucidate the link between the two. The aim of this pangenomic study was to use single nucleotide polymorphism (SNP) array technology to determine the copy number variant (CNV) type and frequency associated with both obesity and DD/ID. Methods Thirty-six patients were recruited from the Clinical Emergency Hospital for Children, in Cluj-Napoca, Romania during the period 2015-2017. The main inclusion criterion was a diagnosis that included both obesity and DD/ID. Genomic analysis via SNP array technology was performed. Results Out of the 36 patients, 12 (33%) presented CNVs with a higher degree of pathogenicity (A group) and 24 (66%) presented benign CNVs (B group). The SNP array results for the A group were as follows: pathogenic CNVs in 8/12 patients (67%); variants of unknown significance (VOUS) in 2/12 patients (16%); and uniparental disomy (UPD) in 2/12 patients (16%). Conclusions Some of these CNVs have already been observed in patients with both obesity and DD/ID, but the others were noticed only in DD/ID patients and have not been described until now in association with obesity.

Comparative Hepatotoxicity of Novel PFOA Alternatives (Perfluoropolyether Carboxylic Acids) on Male Mice

As novel alternatives to perfluorooctanoic acid (PFOA), perfluoropolyether carboxylic acids (multiether PFECAs, CF₃(OCF₂) _nCOO^-, n = 2-4) have been detected in various environmental matrices; however, public information regarding their toxicities remains unavailable. To compare the hepatotoxicity of multiether PFECAs (e.g., PFO2HxA, PFO3OA, and PFO4DA) with PFOA, male mice were exposed to 0.4, 2, or 10 mg/kg/d of each chemical for 28 d, respectively. Results demonstrated that PFO2HxA and PFO3OA exposure did not induce marked increases in relative liver weight; whereas 2 and 10 mg/kg/d of PFO4DA significantly increased relative liver weight. Furthermore, PFO2HxA and PFO3OA demonstrated almost no accumulation in the liver or serum; whereas PFO4DA was accumulated but with weaker potential than PFOA. Exposure to 10 mg/kg/d of PFO4DA led to 198 differentially expressed liver genes (56 down-regulated, 142 up-regulated), with bioinformatics analysis highlighting the urea cycle disorder. Like PFOA, 10 mg/kg/d of PFO4DA decreased the urea cycle-related enzyme protein levels (e.g., carbamoyl phosphate synthetase 1) and serum ammonia content in a dose-dependent manner. Both PFOA and PFO4DA treatment (highest concentration) caused a decrease in glutamate content and increase in both glutamine synthetase activity and aquaporin protein levels in the brain. Thus, we concluded that PFO4DA caused hepatotoxicity, as indicated by hepatomegaly and karyolysis, though to a lesser degree than PFOA, and induced urea cycle disorder, which may contribute to the observed toxic effects.

Hyperammonemia in a Woman with Late-onset Ornithine Transcarbamylase Deficiency

A 52-year-old woman developed vomiting and disturbance of consciousness after consuming raw fish and sushi on a trip. A blood test showed hyperammonemia (310 μg/dL) with a normal liver function. She fell into a deep coma, and her serum ammonia level increased to 684 μg/dL. L-arginine was administered as a diagnostic treatment for urea cycle disorder (UCD) and serum ammonia, and her consciousness levels improved. She was diagnosed with ornithine transcarbamylase deficiency (OTCD) by analyses of plasma amino acids, urinary orotic acid, and the OTC gene mutation. UCD should be considered for patients with hyperammonemia without severe liver function abnormalities.

Untargeted metabolomic profiling reveals multiple pathway perturbations and new clinical biomarkers in urea cycle disorders

Purpose:

Untargeted metabolomic analysis is increasingly being used in the screening and management of individuals with inborn errors of metabolism (IEM). We aimed to test whether untargeted metabolomic analysis in plasma might be useful for monitoring the disease course and management of urea cycle disorders (UCDs).

Methods:

Untargeted mass spectrometry-based metabolomic analysis was used to generate z-scores for more than 900 metabolites in plasma from 48 individuals with various UCDs. Pathway analysis was used to identify common pathways that were perturbed in each UCD.

Results:

Our metabolomic analysis in plasma identified multiple potentially neurotoxic metabolites of arginine in arginase deficiency and, thus, may have utility in monitoring the efficacy of treatment in arginase deficiency. In addition, we were also able to detect multiple biochemical perturbations in all UCDs that likely reflect clinical management, including metabolite alterations secondary to dietary and medication management.

Conclusions:

In addition to utility in screening for IEM, our results suggest that untargeted metabolomic analysis in plasma may be beneficial for monitoring efficacy of clinical management and off-target effects of medications in UCDs and potentially other IEM.

Adult Presentation of Ornithine Transcarbamylase Deficiency: 2 Illustrative Cases of Phenotypic Variability and Literature Review

Ornithine transcarbamylase (OTC) deficiency is an X-linked recessive disorder that usually presents in the neonatal period. Late-onset presentation of OTC can cause mild to severe symptoms. We describe laboratory and clinical findings of late-onset presentations of OTC deficiency. We conducted a literature search using search terms "ornithine transcarbamylase deficiency," "late onset presentation," and "hyperammonemia" from January 1, 1987, to December 31, 2016, was performed. Only papers published in English were included. We searched on PubMed, MEDLINE, and Google Scholar. We also present 2 OTC deficiency cases. A total of 30 adult cases had late-onset presentation of OTC deficiency reported. The majority were women (57%) with a median age of 37 years. The median level of ammonia was 308 mmol/L and the mortality rate was 30%. Our case 1 was a 40-year-old woman who succumbed to neurologic complications after a hyperammonemia crisis following an increased protein intake. Our case 2 was a 43-year-old woman with seizures associated with increased ammonia levels. Our 2 case reports show the wide phenotypic variability and severity in late-onset presentation of OTC ranging from seizures to cerebral herniation. Our literature review is the first to detail published laboratory and neurologic sequelae of late-onset OTC deficiency.

Gene Mutation Analysis and Prenatal Diagnosis of the Ornithine Transcarbamylase (OTC) Gene in Two Families with Ornithine Transcarbamylase Deficiency

Background

The aim of this study was to perform gene detection in 2 clinical cases of highly suspected ornithine transcarbamylase deficiency (OTCD) pediatric patients by first-generation sequencing technology in order to confirm the pathogenic genetic factors of their families and allow the families to undergo genetic counselling and prenatal diagnosis.

Material/Methods

The peripheral DNA samples of 2 children with highly suspected OTCD (the probands) and their parents were collected. DNA fragments corresponding to exons 1–10 of the OTC gene from the samples were amplified using polymerase chain reaction (PCR), and then subjected to Sanger sequencing to confirm the pathogenic mutation sites.

Results

The probands were both confirmed to have OTCD. The proband in Family 1 was a male carrying a c.867+1G>C mutation at a splice site within the OTC gene. The gene detection results of amniotic fluid cells at 16 weeks of pregnancy showed that the fetus was a male who also carried the c.867+1G>C mutation. The proband in Family 2 was a male carrying a c.782T>C(p. I261T) mutation in the OTC gene. The gene detection results of amniotic fluid cells at 18 weeks showed that the fetus was a male without pathogenic mutations in the OTC gene. The gene detection results of peripheral blood from the fetus after birth were consistent with those obtained from amniotic fluid cells.

Conclusions

Pediatric children who are clinically suspected of OTCD can receive a definitive diagnosis through OTC gene detection.

Urea cycle disorders in India: clinical course, biochemical and genetic investigations, and prenatal testing

Background

Urea cycle disorders (UCDs) are inherited metabolic disorders that present with hyperammonemia, and cause significant mortality and morbidity in infants and children. These disorders are not well reported in the Indian population, due to lack of a thorough study of the clinical and molecular profile.

Results

We present data from two major metabolic centres in India, including 123 cases of various UCDs. The majority of them (72/123, 58%) presented in the neonatal period (before 30 days of age) with 88% on or before day 7 of life (classical presentation), and had a high mortality (64/72, 88%). Citrullinemia type 1 was the most common UCD, observed in 61/123 patients. Ornithine transcarbamylase (OTC) deficiency was the next most common, seen in 24 cases. Argininosuccinic aciduria was diagnosed in 20 cases. Deficiencies of arginase, N-acetylglutamate synthase, carbamoyl phosphate synthetase, citrin, and lysinuric protein intolerance were also observed.

Molecular genetic analysis revealed two common ASS1 mutations: c.470G > A (p.Arg157His) and c.1168G > A (p.Gly390Arg) (36 of 55 tested patients). In addition, few recurrent point mutations in ASL gene, and a deletion of the whole OTC gene were also noted. A total of 24 novel mutations were observed in the various genes studied. We observed a poor clinical outcome with an overall all time mortality of 63% (70/110 cases with a known follow-up), and disability in 70% (28/40) among the survivors. Prenatal diagnosis was performed in 30 pregnancies in 25 families, including one pre-implantation genetic diagnosis.

Conclusions

We report the occurrence of UCDs in India and the spectrum that may be different from the rest of the world. Citrullinemia type 1 was the most common UCD observed in the cohort. Increasing awareness amongst clinicians will improve outcomes through early diagnosis and timely treatment. Genetic diagnosis in the proband will enable prenatal/pre-implantation diagnosis in subsequent pregnancies.

Electronic supplementary material

The online version of this article (10.1186/s13023-018-0908-1) contains supplementary material, which is available to authorized users.

The phenotypic and mutational spectrum of Thai female patients with ornithine transcarbamylase deficiency

Ornithine transcarbamylase deficiency (OTCD) is an X-linked urea cycle disorder affecting both males and females. Hemizygous males commonly present with severe hyperammonemic encephalopathy during the neonatal period. Heterozygous females have great phenotypic variability. The majority of female patients can manifest later in life or have unrecognized symptoms, making the diagnosis of OTCD in females very challenging. Here we report on three unrelated Thai female cases with OTCD presenting with different manifestations including aggressive behavior, acute liver failure and severe encephalopathy. Whole exome sequencing successfully identified disease-causing mutations in all three cases including two novel ones: the c.209_210delAA (p.Lys70Argfs*17) and the c.850T>A (p.Tyr284Asn). This study affirms variable symptoms in female patients with OTCD and emphasizes the importance of early recognition and prompt management for favorable outcomes. In addition, identification of two novel causative variants expands the genotypic spectrum of OTC.

Mutation Study of Malaysian Patients with Ornithine Transcarbamylase Deficiency: Clinical, Molecular, and Bioinformatics Analyses of Two Novel Missense Mutations of the OTC Gene

Ornithine transcarbamylase deficiency (OTCD), an X-linked disorder that results from mutations in the OTC gene, causes hyperammonemia and leads to various clinical manifestations. Mutations occurring close to the catalytic site of OTCase can cause severe OTCD phenotypes compared with those caused by mutations occurring on the surface of this protein. In this study, we report two novel OTC missense mutations, Q171H and N199H, found in Malaysian patients. Q171H and N199H caused neonatal onset OTCD in a male and late OTCD in a female, respectively. In silico predictions and molecular docking were performed to examine the effect of these novel mutations, and the results were compared with other 30 known OTC mutations. In silico servers predicted that Q171H and N199H, as well as 30 known missense mutations, led to the development of OTCD. Docking analysis indicated that N-(phosphonoacetyl)-L-ornithine (PALO) was bound to the catalytic site of OTCase mutant structure with minimal conformational changes. However, the mutations disrupted interatomic interactions in the catalytic site. Therefore, depending on the severity of disruption occurring at the catalytic site, the mutation may affect the efficiency of mechanism and functions of OTCase.

Late-onset ornithine transcarbamylase deficiency caused by a somatic mosaic mutation

An 18-month-old boy was diagnosed with late-onset ornithine transcarbamylase deficiency. Genetic analysis revealed a mosaic frameshift mutation (p.Q279fs) in the OTC gene. Despite the presence of a null mutation, he exhibited a milder phenotype, suggesting that the wild-type allele could rescue the function of OTC. The presence of mosaicism has great effects on the clinical phenotype and recurrence-risk assessment, which should be taken into consideration for genetic counseling.

Highly efficient RNA-guided base editing in rabbit

Cytidine base editors (CBEs) and adenine base editors (ABEs), composed of a cytidine deaminase or an evolved adenine deaminase fused to Cas9 nickase, enable the conversion of C·G to T·A or A·T to G·C base pair in organisms, respectively. Here, we show that BE3 and ABE7.10 systems can achieve a targeted mutation efficiency of 53-88% and 44-100%, respectively, in both blastocysts and Founder (F0) rabbits. Meanwhile, this strategy can be used to precisely mimic human pathologies by efficiently inducing nonsense or missense mutations as well as RNA mis-splicing in rabbit. In addition, the reduced frequencies of indels with higher product purity are also determined in rabbit blastocysts by BE4-Gam, which is an updated version of the BE3 system. Collectively, this work provides a simple and efficient method for targeted point mutations and generation of disease models in rabbit.

Case 252: Acute Hyperammonemic Encephalopathy Resulting from Late-Onset Ornithine Transcarbamylase Deficiency

History A 19-year-old woman with no pertinent medical history was brought to the emergency department after being found unconscious on her bathroom floor by her roommate. In the preceding weeks, she had reported intractable nausea and vomiting, for which she had been taking ondansetron. No other medications had been prescribed. The day prior to presentation, she had contacted her mother and described increasing confusion. Glasgow coma scale score on arrival in the emergency department was 4. Intravenous naloxone was administered, without immediate response. Initial blood glucose level was 232 mg/dL (12.8 mmol/L) (normal range, 79-140 mg/dL [4.4- 7.7 mmol/L]), and other routine laboratory test results were normal. Urine toxicology results were negative. Cerebrospinal fluid evaluation revealed levels were within normal limits. Neurologic examination revealed dilated pupils, which showed a sluggish response to light, and left lower extremity rigidity with intermittent tremors. Initial unenhanced cranial computed tomographic (CT) findings were negative. Magnetic resonance (MR) imaging of the brain was performed. The patient's condition deteriorated, with increasing cerebral edema over the next week, and she was declared brain dead. Her liver was transplanted into an adult recipient, who subsequently developed cerebral edema and elevated plasma ammonia levels, resulting in death in the immediate postoperative period.

Clinical utility of exome sequencing in individuals with large homozygous regions detected by chromosomal microarray analysis

BACKGROUND

Chromosomal microarray analysis (CMA) is recommended as the first-tier clinical diagnostic test for individuals with developmental disabilities. In addition to detecting copy number variations, CMA platforms with single nucleotide polymorphism probes can detect large homozygous regions within the genome, which represent potential risk for recessively inherited disorders.

METHODS

To determine the frequency in which pathogenic or likely pathogenic variants can be detected in these regions of homozygosity, we performed whole exome sequencing (WES) in 53 individuals where homozygosity was detected by CMA. These patients were referred to our clinical laboratory for a variety of neurodevelopmental conditions including autism spectrum disorder, developmental delay, epilepsy, intellectual disability and microcephaly.

RESULTS

In 11.3% (6/53) of cases, the analysis of homozygous variants revealed pathogenic or likely pathogenic variants in GJB2, TPP1, SLC25A15, TYR, PCCB, and NDUFV2 which are implicated in a variety of diseases. The evaluation of heterozygous variants with autosomal dominant inheritance, compound heterozygotes and variants with X-linked inheritance revealed pathogenic or likely pathogenic variants in PNPLA4, CADM1, HBB, SOS1, SFTPC, OTC and ASMT in 15.1% (8/53) of cases. Two of these patients harbored both homozygous and heterozygous variants relevant to their phenotypes (TPP1 and OTC; GJB2 and ASMT).

CONCLUSIONS

Our study highlights the clinical utility of WES in individuals whose CMA uncovers homozygosity. Importantly, we show that when the phenotype is complex and homozygosity levels are high, WES can identify a significant number of relevant variants that explain neurodevelopmental phenotypes, and these mutations may lie outside of the regions of homozygosity, suggesting that the appropriate follow up test is WES rather than targeted sequencing.

Modeling lethal X-linked genetic disorders in pigs with ensured fertility

The development of therapies for rare and intractable genetic disorders represents a significant unmet medical need. Disease model pigs characterized by physiological, anatomical, and pathogenetic similarities to humans allow translational studies to be performed, yielding valuable data that can be extrapolated to patients. The establishment of an efficient reproduction system is a key element in the practical application of disease model pigs, which often suffer from reproductive inability due to severe symptoms. Here, we showed that the valuable trait of genetically modified disease model pigs can be maximized by generating unique chimeric boars composed of mutant and normal cells.

Genetically engineered pigs play an indispensable role in the study of rare monogenic diseases. Pigs harboring a gene responsible for a specific disease can be efficiently generated via somatic cell cloning. The generation of somatic cell-cloned pigs from male cells with mutation(s) in an X chromosomal gene is a reliable and straightforward method for reproducing X-linked genetic diseases (XLGDs) in pigs. However, the severe symptoms of XLGDs are often accompanied by impaired growth and reproductive disorders, which hinder the reproduction of these valuable model animals. Here, we generated unique chimeric boars composed of mutant cells harboring a lethal XLGD and normal cells. The chimeric boars exhibited the cured phenotype with fertility while carrying and transmitting the genotype of the XLGD. This unique reproduction system permits routine production of XLGD model pigs through the male-based breeding, thereby opening an avenue for translational research using disease model pigs.

Alternative Start Sites Downstream of Non-Sense Mutations Drive Antigen Presentation and Tolerance Induction to C-Terminal Epitopes

CTL responses to the transgene product remain an active area of concern for the gene therapy field. A patient's underlying genetic mutation may influence the qualitative nature of these potentially destructive T cell responses. Individuals with a mutation that introduces a premature termination codon (PTC) that prevents synthesis of the full-length peptide are considered more likely to mount a transgene-specific T cell response because of a lack of immune tolerance to C-terminal epitopes as a consequence of absent endogenous Ag presentation. In this article, we demonstrate that a human ornithine transcarbamylase gene containing various PTC-inducing non-sense mutations is able to generate and present epitopes downstream of the termination codon. Generation of these epitopes occurs primarily from alternative translation start sites downstream of the stop codon. Furthermore, we show that expression of these genes from adeno-associated virus vectors in C57BL/6 mice is able to induce peripheral tolerance to epitopes downstream of the PTC. These results suggest that, despite the lack of full-length endogenous protein, patients with PTC-inducing non-sense mutations may still present T cell epitopes downstream of the premature termination site that may render the subject tolerant to wild-type transgene products.

Late-onset ornithine transcarbamylase deficiency associated with hyperammonemia

The urea cycle converts ammonia and produces urea. One form of urea cycle abnormality is ornithine transcarbamylase (OTC) deficiency. This hereditary disorder is associated with hyperammonemia. OTC deficiency commonly appears during neonatal and early childhood life and is rare in adults. We report a 69-year-old man who presented at the local hospital with 3-day loss of appetite, early morning vomiting, and state of confusion. Blood ammonia was 293 μg/dl. At 2-3 h after admission, the patient went into a deep coma. He was intubated and admitted immediately to the intensive care unit. Treatment, including sustained hemodialysis, failed to lower blood ammonia level. His grandchild died of OTC deficiency at 6 year of age. Computed tomography, magnetic resonance imaging and esophagogastroduodenoscopy showed no abnormalities. On admission to our hospital, he complained of vomiting and disturbance of consciousness, hyperammonemia, and normal anion gap. Genetic analysis showed A208T mutation. The deceased grandchild with OTC deficiency also had the same mutation. Long-term hemodialysis coupled with administration of L-arginine and lactulose resulted in improvement of blood ammonia level. Early diagnosis and treatment of adult-onset OTC deficiency are essential to avoid serious complications.