Prenatal monitoring in a family at high risk for ornithine transcarbamylase (OTC) deficiency: a new mutation of an A-to-C transversion in position +4 of intron 1 of the OTC gene that is likely to abolish enzyme activity

A deep intronic variant is a common cause of OTC deficiency in individuals with previously negative genetic testing

Pathogenic variants in non-coding regions of genes encoding enzymes or transporters of the urea cycle can lead to urea cycle disorders (UCDs). However, not all commercially available testing platforms interrogate these regions. Here, we used a gene panel based on massively parallel sequencing (MPS) in 10 individuals with clinical or pedigree-based evidence of a proximal UCD but without a molecular confirmation of the diagnosis. We identified causal variant(s) in 5 of 10 individuals, including in 3 of 7 individuals in whom prior molecular testing was unrevealing. We show that a deep-intronic pathogenic variant in OTC, c.540+265G>A, is an important cause of ornithine transcarbamylase (OTC) deficiency.

Differentiation Effects by the Combination of Spheroid Formation and Sodium Butyrate Treatment in Human Hepatoblastoma Cell Line (Hep G2): A Possible Cell Source for Hybrid Artificial Liver

The aim of this study was to investigate the feasibility of human hepatoblastoma cell line (Hep G2), which differentiates by spheroid formation, and treatment with sodium butyrate (SB) as a cell source for hybrid artificial liver (HAL). Hep G2 spontaneously formed spheroids in polyurethane foam (PUF) within 3 days of culture and restored weak ammonia removal activity. Treatment with SB, which is a histone deacetylase inhibitor, further increased the ammonia removal activity of Hep G2 spheroids in a concentration-dependent manner. The activation of ornithine transcarbamylase—a urea cycle enzyme—was significantly related to the upregulation of ammonia removal by spheroid formation, but scarcely contributed to the further upregulation following SB treatment. In contrast with ammonia removal, treatment with SB reduced the albumin secretion of Hep G2 spheroids in a concentration-dependent manner. In the PUF-HAL module in a circulation culture, the ammonia removal rate and albumin secretion rate (per unit volume of the module) of Hep G2 spheroids treated with 5 mM SB were almost the same as those of primary porcine hepatocyte spheroids. These results suggest that simultaneous use of spheroid formation and SB treatment in Hep G2 is beneficial in enhancing the functions of human hepatocytes with potential applications in regenerative medicine and drug screening.

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

Ornithine transcarbamylase (OTC) deficiency is the most common inherited disorder of the urea cycle and is transmitted as an X-linked trait. Defects in the OTC gene cause a block in ureagenesis resulting in hyperammonemia, which can lead to brain damage and death. Three previous mutation updates for the OTC gene have been published, in 1993, 1995, and 2002. The most recent comprehensive update, in 2002, contained 244 mutations including 13 nondisease-causing mutations and polymorphisms. This current update reports 341 mutations, of which 93 have not been previously reported, and an additional 29 nondisease-causing mutations and polymorphisms. Out of the 341 mutations, 149 were associated with neonatal onset of hyperammonemia (within the first week of life), 70 were seen in male patients with later onset of hyperammonemia, and 121 were found in heterozygous females (one unknown). Along with the reported mutations, residual enzyme activities and other pertinent clinical information are included whenever available. Most mutations in the OTC gene are specific to a particular family ("private" mutations). They are distributed throughout the gene, with a significant paucity of mutations in the 32 first codons encoding the "leader" peptide (exon 1 and the beginning of exon 2). Almost all mutations in consensus splice sites confer a neonatal onset phenotype. Using the current molecular screening methods, mutations are found in about 80% of the patients. The remaining patients may have mutations in regulatory domains or mutations deep in the introns, which constitute 98.5% of the genomic sequence. In addition, a phenocopy of OTC deficiency caused by mutations in another unknown gene cannot be excluded.

Mutations and polymorphisms in the human ornithine transcarbamylase gene

Ornithine transcarbamylase (OTC) deficiency, an X-linked, semidominant disorder, is the most common inherited defect in ureagenesis resulting in hyperammonemia. The previous two mutation updates for the OTC gene were published in 1993 and 1995 and included 36 and 30 mutations respectively. This comprehensive update contains a compilation of 244 mutations including 13 polymorphisms. Twenty-four of the mutations are reported here for the first time. Forty-two percent of the disease-causing mutations are associated with acute neonatal hyperammonemia; 21% were found in patients with late onset disease and approximately 37% were found in manifesting heterozygous females, most of which are presumed to confer a neonatal phenotype in hemizygous males. Also included are residual enzyme activities and residual incorporation of ammonium nitrogen into urea and results of expression studies for a small proportion of the mutations. Most mutations in the OTC gene are "private" and are distributed throughout the gene with paucity of mutation in the sequence encoding the leader peptide (exon 1 and beginning of exon 2) and in exon 7. Almost all mutations in consensus splicing sites confer a neonatal phenotype. Thirteen polymorphisms have been found, several of which are useful for allele tracking in patients in whom the mutation can't be found. Even with sequencing of the entire reading frame and exon/intron boundaries, only about 80% of the mutations are detected in patients with proven OTC deficiency. The remaining probably occur within the introns or in regulatory domains.

Augmentation of urea-synthetic capacity by inhibition of nitric oxide synthesis in butyrate-induced differentiated human hepatocytes

We have recently developed an in vitro differentiation model of immortalized non-transformed human hepatocytes using butyrate, and observed the induction of inducible NO synthase (iNOS). In this study, we analyzed the effect of NO on the urea-synthetic capacity of these cells. The inhibition of iNOS during butyrate treatment significantly increased the urea-synthetic capacity as compared to that of butyrate treatment alone, possibly through the further induction of ornithine transcarbamylase expression. Therefore, the inhibition of NO production might be useful for obtaining more differentiated hepatocytes in the process of in vitro induction of hepatocyte-specific differentiation.