Mouse Model for Human Arginase Deficiency

Abstract

Deficiency of liver arginase (AI) causes hyperargininemia (OMIM 207800), a disorder characterized by progressive mental impairment, growth retardation, and spasticity and punctuated by sometimes fatal episodes of hyperammonemia. We constructed a knockout mouse strain carrying a nonfunctional AI gene by homologous recombination. Arginase AI knockout mice completely lacked liver arginase (AI) activity, exhibited severe symptoms of hyperammonemia, and died between postnatal days 10 and 14. During hyperammonemic crisis, plasma ammonia levels of these mice increased >10-fold compared to those for normal animals. Livers of AI-deficient animals showed hepatocyte abnormalities, including cell swelling and inclusions. Plasma amino acid analysis showed the mean arginine level in knockouts to be approximately fourfold greater than that for the wild type and threefold greater than that for heterozygotes; the mean proline level was approximately one-third and the ornithine level was one-half of the proline and ornithine levels, respectively, for wild-type or heterozygote mice-understandable biochemical consequences of arginase deficiency. Glutamic acid, citrulline, and histidine levels were about 1.5-fold higher than those seen in the phenotypically normal animals. Concentrations of the branched-chain amino acids valine, isoleucine, and leucine were 0.4 to 0.5 times the concentrations seen in phenotypically normal animals. In summary, the AI-deficient mouse duplicates several pathobiological aspects of the human condition and should prove to be a useful model for further study of the disease mechanism(s) and to explore treatment options, such as pharmaceutical administration of sodium phenylbutyrate and/or ornithine and development of gene therapy protocols.Arginase (EC 3.5.3.1) is the fifth and final enzyme of the urea cycle, the major pathway for the detoxification of ammonia in mammals. There are at least two forms of arginase in mammals, AI and AII, located in the cytoplasm and mitochondrion, respectively. The principal ureagenic enzyme activity (AI) is most abundant in normal mammalian liver and acts in coordination with the other enzymes of the urea cycle to sequester and eliminate excess nitrogen from the body (7). The second form (AII) is found in many organs, with the highest levels found in kidney and prostate and lower levels in macrophages, lactating mammary glands, and brain, often in the absence of the other urea cycle enzymes (7,18). In humans, deficiency of the liver isoform (AI) causes hyperargininemia (OMIM 207800; Online Mendelian Inheritance in Man [http:// www.ncbi.nlm.nih.gov/entrez/query.fcgi?dbϭOMIM]), a metabolic disorder characterized by neurological impairment, with deterioration of the cortex and pyramidal tracts and progressive dementia, spasticity, and growth retardation and punctuated by infrequently fatal episodes of hyperammonemia (7

Ablation of Arg1 in hematopoietic cells improves respiratory function of lung parenchyma, but not that of larger airways or inflammation in asthmatic mice

“…We flanked exon 4 with loxP sites to create a conditionally Arg1-deficient mouse. When these Arg1 fl/fl mice were crossed with deleterCre females (48) to delete exon 4 in the germ line, the resulting Arg1 Ϫ/Ϫ mice died between postnatal days 10 and 14, as reported for constitutive Arg1-deficient mice (20). At postnatal day 8, plasma levels of arginine and glutamine were approximately nine-and approximately twofold higher, respectively, whereas lysine levels were decreased to ϳ50% in Arg1…”

Auxotrophy-Based Detection of Hyperornithinemia in Mouse Blood and Urine

“…To refine diagnoses, perhaps testing a small panel of single-gene auxotrophs (such as argF, argG, or argH loss-of-function variants; see Figure 2) for growth rescue on minimal media by physiological fluids in conjunction with argE mutants (or as a follow-up for growth-positive samples) might be informative. Animal models for these urea cycle disorders [31][32][33][34][35][36] might be helpful in testing and refining such a screening strategy.…”

Clinical, biochemical, and molecular spectrum of hyperargininemia due to arginase I deficiency

“…Plasma arginine in the homozygous mutant animals was significantly higher than in the wild type animals. A knockout mouse for the ARG1 gene has also been generated [Iyer et al, 2002]. The level of ARG1 mRNA was virtually zero with undetectable protein levels.…”