What is Ornithine Transcarbamylase Deficiency?
 

Ornithine Transcarbamylase Deficiency (OTCD), also known as hyperammonemia type II, is an X-linked inherited metabolic disorder caused by mutations in the ornithine transcarbamylase (OTC) gene, leading to reduced or absent activity of the OTC enzyme, thereby interrupting the urea cycle. It is the most common type of urea cycle disorder (UCD), accounting for approximately 50%-67% of all cases, with an estimated incidence of 1/80,000 to 1/56,500.
 

Since the OTCgene is located on the X chromosome (Xp11.4), the disease follows an X-linked incomplete dominant inheritance pattern. Male hemizygous patients typically have severe symptoms, while female heterozygous carriers can present with a wide clinical spectrum ranging from asymptomatic to severe hyperammonemic crisis, due to varying degrees of X-chromosome inactivation (Lyonization).
 

Pathogenesis
 

The pathogenesis of Ornithine Transcarbamylase Deficiency stems from mutations in the OTCgene, leading to dysfunction of the encoded ornithine transcarbamylase enzyme, which interrupts a critical step in the urea cycle.
 

The urea cycle primarily occurs in the mitochondria and cytoplasm of hepatocytes. Within the mitochondria, carbamoyl phosphate synthetase I (CPS1) catalyzes the synthesis of carbamoyl phosphate. OTC further catalyzes the condensation of carbamoyl phosphate with ornithine to form citrulline. Citrulline then enters the cytoplasm, where it is sequentially catalyzed by argininosuccinate synthetase (ASS), argininosuccinate lyase (ASL), and arginase (ARG) to ultimately generate urea and regenerate ornithine, completing the cycle. Dysfunction of OTC directly blocks citrulline synthesis, causing the entire urea cycle to stall. This leads to massive accumulation of its precursor substrates, ammonia and carbamoyl phosphate, in the body.
 

Accumulated ammonia enters the bloodstream, causing hyperammonemia. Excess ammonia enters the central nervous system, combines with glutamate to form glutamine, leading to astrocyte swelling, cerebral edema, energy metabolism disorders (e.g., inhibition of the tricarboxylic acid cycle, ATP depletion), and imbalance of neurotransmitters (glutamate/GABA disturbances). This ultimately results in acute or chronic encephalopathy, cognitive impairment, epilepsy, and other neurological damage. Meanwhile, accumulated carbamoyl phosphate is shunted into the pyrimidine biosynthesis pathway, where it combines with aspartate to form orotic acid, which is excreted in large amounts in urine, forming the characteristic orotic aciduria.
 

The OTCgene is located at Xp11.4, spans 68 kb, contains 10 exons and 9 introns, and encodes 354 amino acids. It is primarily expressed in the liver, with a small amount in intestinal mucosal cells. Over 400 pathogenic mutations in the OTCgene have been reported globally, with missense mutations being the most frequent. In the Chinese population, the most common mutations include:
 

p.Arg40His (R40H):​ Located in exon 2, accounting for about 6-8%. This mutation causes the 40th arginine to be replaced by histidine, affecting the enzyme's active site or protein folding, often leading to severe early-onset hyperammonemia.
 

p.Arg129His (R129H):​ Located in exon 4, a common international mutation accounting for 3-4%, often presenting as late-onset or mild-to-moderate phenotypes.
 

p.Arg277Trp (R277W):​ Located in exon 7, accounting for about 6-8%. This mutation occurs at a CpG dinucleotide site, prone to deamination, causing a change in the hydrophobicity of the arginine side chain, often leading to severe loss of enzyme activity.
 

p.Tyr345Thrfs50*:​ Located in exon 9, causing a frameshift mutation and producing a truncated protein. It is usually a pathogenic variant (PVS1) and often results in complete enzyme inactivation.

Image source: Frequency and Pathophysiology of Acute Liver Failure in Ornithine Transcarbamylase Deficiency (OTCD)

Mouse Models
 

OTC KO Mice:​ Complete knockout of the OTCgene, primarily used to model severe, often neonatal lethal forms of human OTCD. Suitable for studying acute metabolic disturbances, neurotoxicity, and early lethality mechanisms.
 

OTC p. R129H Mice:​ Carry the c.386G>A point mutation in exon 4 of the OTCgene, leading to the substitution of histidine for arginine at position 129. Male hemizygous mice of this model exhibit sparse hair, growth retardation, retain about 5%-10% of OTC enzyme activity, and are prone to inducing acute hyperammonemia under high-protein diet or metabolic stress.
 

Liver-Specific OTC Knockout Mice:​ Specific knockout of the OTCgene in hepatocytes, modeling late-onset or adult-onset human OTCD. Used to study chronic liver injury and carcinogenesis mechanisms.
 

Supporting Gene Therapy
 

Gene therapy offers hope for rare diseases, but its development and validation rely heavily on animal model support. MingCeler Biotech, leveraging its self-developed TurboMice™ technology, has developed multiple rare disease mouse models. TurboMice™ technology overcomes the technical challenges of long mouse model generation cycles and low success rates for complex models, enabling editing at almost any target gene locus. Complete homozygous gene-edited mouse models can be prepared directly from embryonic stem cells in as little as two months.
 

MingCeler Biotech​ can customize various OTCD mouse models according to client needs, such as OTC KO mice, OTC p. R129H mice, and liver-specific OTC knockout mice. Inquiries are welcome.
 

References:
 

1.Wei Jingya, Ren Xianhui. Research Progress in the Treatment of Ornithine Transcarbamylase Deficiency [J]. Advances in Clinical Medicine, 2022, 12(9): 8235-8242.

2.Division of Genetics and Metabolism, Child Diseases and Health Care Branch, Chinese Association for Maternal and Child Health. [Consensus on diagnosis and treatment of ornithine trans-carbamylase deficiency]. Zhejiang Da Xue Xue Bao Yi Xue Ban. 2020 Oct 25;49(5):539-547. Chinese. doi: 10.3785/j.issn.1008-9292.2020.04.11. PMID: 33210478; PMCID: PMC8800749.

Kozel BA, Barak B, Kim CA, Mervis CB, Osborne LR, Porter M, Pober BR. Williams syndrome. Nat Rev Dis Primers. 2021 Jun 17;7(1):42. doi: 10.1038/s41572-021-00276-z. PMID: 34140529; PMCID: PMC9437774.