Molecular basis of ornithine transcarbamylase deficiency lacking enzyme protein

A patient with propionic acidemia managed with continuous insulin infusion and total parenteral nutrition

An infant newly diagnosed with propionic acidemic coma was managed successfully with total parenteral nutrition (TPN) and continuous infusion of insulin. The urinary excretion of 3-hydroxypropionic acid was reduced to 3% of the admission value in 4 days, gradually decreasing to 1.5% in 16 days. The treatment did not prevent a prolonged episode of thrombocytopenia. The infant tolerated TPN well, except for continued hyper-lactic acidemia (2 to 4 times normal). Metabolic acidosis and mild hyperammonemia recurred only when the patient had sepsis secondary to Candida albicans and Staphylococcus aureus infection.

An arginine to glutamine mutation in residue 109 of human ornithine transcarbamylase completely abolishes enzymatic activity in Cos1 cells

Ornithine transcarbamylase (OTC) is an important enzyme in the detoxification of ammonia to urea, and its deficiency is the most common inborn error of ureagenesis in humans. Among 24 cases of OTC deficiency previously examined, three unrelated individuals all showed loss of a Taq I site in the OTC gene corresponding to codon 109, suggesting that this Taq I site may be prone to mutation. Two of these patients demonstrated the same C----T transition (in antisense strand) converting Arg109 to Gln. Although these studies implied a strong association between the missense mutation and OTC-deficient phenotype, a causal relationship could not be firmly established. We have investigated this relationship by reconstructing the mutation in vitro. A full-length human OTC cDNA was cloned into an SV40-based expression vector and has been reproducibly expressed at high levels in the cell line Cos1. By site-directed mutagenesis of this wild type sequence, we constructed a missense mutation which contains the C----T transition. Electroporation and transient assay in Cos1 indicated that the specific activity of mutant OTC was 100-fold lower than that of wild type. This result confirms that the Taq I alteration leading to the Gln missense is responsible for the OTC deficiency affecting the above patients.

Expression of the mutant gene for L-gulono-gamma-lactone oxidase in scurvy-prone rats

A mutant strain of Wistar rats with L-gulono-gamma-lactone oxidase deficiency has recently been established. To investigate this deficiency by DNA and RNA blot hybridization analyses, a fragment of a previously cloned cDNA encoding rat L-gulono-gamma-lactone oxidase was used as a probe. When genomic DNA of the mutant rat was digested with several restriction enzymes, the probe hybridized to fragments of the same sizes as those produced from DNA of normal rats. Poly(A)+RNA from the liver of the mutant rat was found to contain an L-gulono-gamma-lactone oxidase-specific mRNA of a normal size at a comparable level to that of normal rats. An in vitro translation experiment revealed that the mRNA programmed the synthesis of an enzyme protein which had the same molecular weight as that of the translational product of the normal mRNA, although the amount synthesized was markedly reduced as compared with that synthesized with the normal mRNA. In accordance with this observation, a very low but definite degree of L-gulono-gamma-lactone oxidase activity was detected in the microsomes of the mutant rat by a newly developed, highly sensitive method.

Characterization of point mutations in the same arginine codon in three unrelated patients with ornithine transcarbamylase deficiency

Point mutations in the X-linked ornithine transcarbamylase (OTC) gene have been detected at the same Taq I restriction site in 3 of 24 unrelated probands with OTC deficiency. A de novo mutation could be traced in all three families to an individual in a prior generation, confirming independent recurrence. The DNA sequence in the region of the altered Taq I site was determined in the three probands. In two unrelated male probands with neonatal onset of severe OTC deficiency, a guanine (G) to adenine (A) mutation on the sense strand (antisense cytosine [C] to thymine [T]) was found, resulting in glutamine for arginine at amino acid 109 of the mature polypeptide. In the third case, where the proband was a symptomatic female, C to T (sense strand) transition converted residue 109 to a premature stop. These results support the observation that Taq I restriction sites, which contain an internal CG, are particularly susceptible to C to T transition mutation due to deamination of a methylated C in either the sense or antisense strand. The OTC gene seems especially sensitive to C to T transition mutation at arginine codon 109 because either a nonsense mutation or an extremely deleterious missense mutation will result.

Ornithine transcarbamylase deficiency: a case with a truncated enzyme precursor and a case with undetectable mRNA activity

The cell-free translation of ornithine transcarbamylase (OTC) mRNA from the livers of two heterozygous patients (from different families) with OTC deficiency was performed. The enzyme activities and the immunoreactive proteins in both patients were about 5% of those in controls. Immunoblotting assay of liver extracts from both patients showed decreased amounts of the OTC protein. The mRNA from the liver of patient 1 directed the synthesis of a very small amount of OTC precursor of normal subunit size (40,000 Da), whereas that from patient 2 directed the synthesis of small amounts of two distinct in vitro products; one was 40,000 Da and the other was about 30,000 Da. The in vitro product of normal precursor synthesized with mRNA from patient 2 was converted to mature-sized OTC by isolated rat liver mitochondria, whereas the smaller product was degraded during the incubation with the mitochondria. These results indicate that in both patients the translatable level of mRNA for active OTC from liver cells was much lower than that in the controls. The results also suggest that in patient 2, the smaller product presumably derived from an abnormal gene could not be transferred to the mitochondria.

Level of translatable messenger RNA coding for argininosuccinate synthetase in the liver of the patients with quantitative-type citrullinemia

The translation activity of mRNA coding for argininosuccinate synthetase in total RNA extracted from the liver of three patients with quantitative-type citrullinemia was determined using a cell-free translation system. In two patients, the hepatic content of the enzyme was about 20% of the control value, whereas translatable mRNA level for the enzyme was similar to or slightly lower than those of control livers. In the third patient, the enzyme content was about 50% of the control value, and mRNA activity for the enzyme was low normal. These results indicate that at least in the first two patients, the decrease in the enzyme protein is due either to increased degradation of the enzyme or to decreased translation in the patient's liver.