A new mouse model of mild ornithine transcarbamylase deficiency (spf-j) displays cerebral amino acid perturbations at baseline and upon systemic immune activation.

Ornithine transcarbamylase deficiency (OTCD, OMIM# 311250) is an inherited X-linked urea cycle disorder that is characterized by hyperammonemia and orotic aciduria. In this report, we describe a new animal model of OTCD caused by a spontaneous mutation in the mouse Otc gene (c.240T>A, p.K80N). This transversion in exon 3 of ornithine transcarbamylase leads to normal levels of mRNA with low levels of mature protein and is homologous to a mutation that has also been described in a single patient affected with late-onset OTCD. With higher residual enzyme activity, spf-J were found to have normal plasma ammonia and orotate. Baseline plasma amino acid profiles were consistent with mild OTCD: elevated glutamine, and lower citrulline and arginine. In contrast to WT, spf-J displayed baseline elevations in cerebral amino acids with depletion following immune challenge with polyinosinic:polycytidylic acid. Our results indicate that the mild spf-J mutation constitutes a new mouse model that is suitable for mechanistic studies of mild OTCD and the exploration of cerebral pathophysiology during acute decompensation that characterizes proximal urea cycle dysfunction in humans

Urea cycle perturbations in <i>Spf-J</i> males.

<p><i>Spf-J</i> (8 weeks) were maintained on a high protein chow diet (70% protein) and euthanized after 2 weeks (N = 6 / group). Blood was collected by retro-orbital bleeding with plasma separated and stored at −80°C until use. WT – wild-type, NP – normal protein, HP – high protein. A) Plasma ammonia and B) Plasma orotate for <i>spf-j</i> and WT. For immune challenge, 8-week-old <i>spf-J</i> (N = 6) and WT (N = 4) received an intraperitoneal injection of poly I:C (100 μg) once a day for 3 days. Tissues harvested and snap frozen and stored at −80°C until use. C) Plasma ammonia. D) Liver transaminases. Hatched bar indicates P < 0.05. WT – wild-type.</p

Changes in amino acids from baseline during immune challenge.

<p>Spf-J (N = 5) and WT (N = 5) received an intraperitoneal injection of poly I:C (100 μg) once a day for 3 days. Tissues harvested and snap frozen and stored at −80°C until use. Heat maps were constructed as described in <b>Methods</b> to represent the change in amino acid concentrations from baseline for plasma (left panel), liver (middle panel) and cerebrum (right panel). P-value < 0.05 was set as the threshold for significance.</p

Identification of the molecular lesion in <i>spf-J</i>.

<p>A) PCR amplification of gDNA from <i>spf-J</i> revealed a transversion mutation in exon 3 leading to an asparagine replacement of a conserved lysine residue at position 80 (K80N). B) Genotyping assay of female <i>spf-J</i> with PCR amplification followed by digestion with EcoRI. Hem – hemizygote, Het – heterozygote.</p