Prenatal diagnosis for arginase deficiency by second-trimester fetal erythrocyte arginase assay and first-trimesterARG1 mutation analysis

Arginase-1 deficiency

Arginase-1 (ARG1) deficiency is a rare autosomal recessive disorder that affects the liver-based urea cycle, leading to impaired ureagenesis. This genetic disorder is caused by 40+ mutations found fairly uniformly spread throughout the ARG1 gene, resulting in partial or complete loss of enzyme function, which catalyzes the hydrolysis of arginine to ornithine and urea. ARG1-deficient patients exhibit hyperargininemia with spastic paraparesis, progressive neurological and intellectual impairment, persistent growth retardation, and infrequent episodes of hyperammonemia, a clinical pattern that differs strikingly from other urea cycle disorders. This review briefly highlights the current understanding of the etiology and pathophysiology of ARG1 deficiency derived from clinical case reports and therapeutic strategies stretching over several decades and reports on several exciting new developments regarding the pathophysiology of the disorder using ARG1 global and inducible knockout mouse models. Gene transfer studies in these mice are revealing potential therapeutic options that can be exploited in the future. However, caution is advised in extrapolating results since the lethal disease phenotype in mice is much more severe than in humans indicating that the mouse models may not precisely recapitulate human disease etiology. Finally, some of the functions and implications of ARG1 in non-urea cycle activities are considered. Lingering questions and future areas to be addressed relating to the clinical manifestations of ARG1 deficiency in liver and brain are also presented. Hopefully, this review will spark invigorated research efforts that lead to treatments with better clinical outcomes.

Five novel mutations in ARG1 gene in Chinese patients of argininemia

BACKGROUND

Argininemia is an autosomal recessive genetic disorder caused by hepatocyte arginase deficiency. It could be detected by blood amino acids analysis (high arginine) and confirmed by molecular diagnosis. The clinical manifestations in patients are similar to cerebral palsy so the diagnosis is usually much delayed. Reports of argininemia from mainland China are few, and genetic analyses have not been reported.

PATIENTS AND METHODS

Five Chinese patients with argininemia were investigated. They had progressive spastic tetraplegia, poor physical growth from 1 month to 4 years. When argininemia was found at the ages of 4 to 12 years, four of patients had mental retardation, and three had seizures.

RESULTS

Elevated blood arginine and significantly decreased erythrocyte arginase activity in five patients confirmed the diagnosis of arginase deficiency. Liver dysfunction was found in four patients, two of whom had mildly elevated blood ammonia levels. Cranial magnetic resonance imaging showed progressive cerebral atrophy in three patients. Six mutations in the ARG1 gene were identified, of which only one (c.703 G>A, p.G235R) in exon 7 has been reported before; c.34 G>T (p.G12X) in exon 1, c.67delG (p.G23fsX31) in exon 2, c.539G>C (p.R180 T) in exon 5, c.374C>T (p.A125 V) in exon 4, and c.646-649del CTCA (p.T215fsX219) in exon 6 were novel mutations.

CONCLUSIONS

Argininemia is one of the few treatable causes of pediatric spastic paraparesis. Early metabolic investigation is very important to reach a diagnosis and better outcome. Five Chinese patients with late-diagnosed argininemia were reported. The mutation spectrum of ARG1 gene should be different from other populations.

Hyperargininemia: a family with a novel mutation in an unexpected site

Hyperargininemia is a rare autosomal recessive disorder of the last step of the urea cycle characterized by a deficiency in liver arginase1. Clinically, it differs from other urea cycle defects by a progressive paraparesis of the lower limbs (spasticity and contractures) with hyperreflexia, neurodevelopmental delay and regression in early childhood. Growth is affected as well. Hyperammonemia is episodic, if present at all. The disease is caused by mutations in the ARG1 gene; there are approximately 20 different known ARG1 mutations with considerable genetic heterogeneity. We describe two Arab siblings with a late diagnosis of hyperargininemia and present the genetic findings in their family. As ARG1 sequencing was unrevealing despite suggestive clinical and laboratory findings, molecular cDNA analysis was performed. The ARG1 expression pattern identified a 125-bp out-of-frame insertion between exons 3 and 4, leading to the addition of 41 amino acids and a premature termination codon TGA at the sixth codon downstream. The insertion originated at intron 3 and was attributable to a novel c.305 + 1323 t > c intronic base change that enabled an enhancement phenomenon. This is the first reported exon-splicing-enhancer mutation in patients with hyperargininemia. The clinical course and genetic findings emphasize the possibility that hyperargininemia causes neurological deterioration in children and the importance of analyzing the expression pattern of the candidate gene when sequencing at the DNA level is unrevealing.

Utilization of targeted array comparative genomic hybridization, MitoMet, in prenatal diagnosis of metabolic disorders

Metabolic disorders are inborn errors that often present in the neonatal period with a devastating clinical course. If not treated promptly, these diseases can result in severe, irreversible disease or death. Determining the molecular defects in metabolic diseases is important in providing a definitive diagnosis for patient management. Therefore, prenatal diagnosis for families with known mutations causing metabolic disorders is crucial for timely intervention. Here we present three families in which standard Sanger sequencing failed to provide a definitive diagnosis, but the detection of genomic deletions by array comparative genomic hybridization (CGH) specifically targeted to mitochondrial and metabolic disease genes, MitoMet®, was fundamental in providing accurate prenatal diagnosis. In addition, to our knowledge, two deletions are the smallest detected by oligonucleotide array CGH reported for their respective genes, OTC and ARG1. These data highlight the importance of targeted array CGH in patients with suspected metabolic disorders and incomplete or negative sequencing results, as well as its emerging role in prenatal diagnosis.

Dysregulation of L-arginine metabolism and bioavailability associated to free plasma heme

Severe Plasmodium falciparum malaria is associated with hypoargininemia, which contributes to impaired systemic and pulmonary nitric oxide (NO) production and endothelial dysfunction. Since intravascular hemolysis is an intrinsic feature of severe malaria, we investigated whether and by which mechanisms free heme [Fe(III)-protoporphyrin IX (FP)] might contribute to the dysregulation of l-arginine (l-Arg) metabolism and bioavailability. Carrier systems “y+” [or cationic amino acid transporter (CAT)] and “y+L” transport l-Arg into red blood cells (RBC), where it is hydrolyzed to ornithine and urea by arginase (isoform I) or converted to NO^· and citrulline by endothelial nitric oxide synthase (eNOS). Our results show a significant and dose-dependent impairment of l-Arg transport into RBC pretreated with FP, with a strong inhibition of the system carrier y+L. Despite the impaired l-Arg influx, higher amounts of l-Arg-derived urea are produced by RBC preexposed to FP caused by activation of RBC arginase I. This activation appeared not to be mediated by oxidative modifications of the enzyme. We conclude that l-Arg transport across RBC membrane is impaired and arginase-mediated l-Arg consumption enhanced by free heme. This could contribute to reduced NO production in severe malaria.