ARG was discovered in mammalian liver tissue in 1904. Two isoenzymes, ARG1 and ARG2, were encoded by different genes in mammals[13, 14]; ARG1 was expressed highest in the liver cell cytosol, while ARG2 was mainly expressed in the prostate and kidneys. ARG1 was the final enzyme of the urea cycle, converting arginine to urea and ornithine. And the deficiency of ARG1 could lead to impaired ureagenesis, which characterized by hyperargininemia. The ARG1 gene was located on chromosome 6 (6q23.2) and comprised eight exons, and mutations in the gene caused changes in the enzyme structure that prevent it from functioning correctly. Although the functions of the two enzymes are not the same, functional overlap between them may explain why severe hyperammonemia was absent in patients with argininemia[5, 16].
The pathological mechanism of argininemia in neurological damage is still unknown, which may be related to the direct effect of arginine or its metabolites, such as guanidino compounds and nitric oxide(NO). Previous studies have suggested that guanidino compounds could cause damage to the brain by oxidative stress. Plasma metabolomic data from 13 argininemia patients showed elevated guanidino compounds, guanidinoacetate, argininate, 2-oxoarginine, and N-acetylarginine contributions to the phenotype. The conventional treatment can lower plasma arginine levels and prevent symptoms, but the arginine’s catabolites can remain elevated[4, 8]. LT can normalize the arginine level and its metabolites to average ranges, which is not achieved by traditional treatment. In this research, the neurological status and the quality of life improved in most patients, supporting the idea that LT can prevent progressive aggravation of neurological impairment; not all neurological impairments were reversed. According to this study, it appeared that the ankle spasms were challenging to disappear.
In addition, the neurophysiological evaluation data of one patient (not shown) showed that the central motor conduction time was prolonged, while the peripheral motor conduction time, somatosensory evoked potential, electromyography, and nerve conduction velocity were within the normal range, suggesting that corticospinal tract impairment was involved in the pathophysiologic mechanism. The latest research shows that argininemia can cause dysmyelination in brain white matter, indicating that the pathogenesis of argininemia is related to a reduction of oligodendrocytes. Mitochondrial homeostasis was disrupted for the accumlation of arginine in the arginase deficiency model of Caenorhabditis elegans. At the same time, the genetic inactivation of the mitochondrial basic amino acid transporter SLC-25A29 could rescue the mitochondrial defects, meaning energy metabolism impairment may damage the nervous system. At the same time, NO induces demyelination by impairing the energy metabolism of oligodendrocytes. Lesion of corticospinal injury caused by oligodendrocyte energy metabolism disorder may be responsible for dyskinesia. Additional studies will be needed to elucidate the pathophysiological mechanism.
A study has found that patients with two severe mutant alleles may indicate of LT because they are less effective at rigorous dietary control. This group of patients had poor responses to dietary therapy. The genetic analysis revealed that they were compound heterozygotes, and the mutations in six patients(Cases 3–8) were considered severe. Compared with homozygotes, animal models showed heterozygotes with mild symptoms; however, many studies have suggested that there is no significant correlation between genotype and phenotype. The data of this study implicated that heterozygotes without clinical symptoms can act as donors for argininemia. Age of onset, duration of arginine effect, peak plasma arginine, the sensitivity of the nervous system to arginine and its metabolites, and genotype may be related to recovery, but the underlying mechanism and correlation need to be further studied.
In addition to nervous system damage, the effect on the liver was often manifested as mildly elevated ammonia and transaminase. Although the mechanism of liver injury in UCDs was still unclear, hyperammonemia was considered to be a significant cause of liver damage[6, 7]. Coagulation disorder with hyperammonemia episodes has been described previously, while there is no consensus on their relationship[3, 6]. The histopathological results of this group are consistent with those previously reported and are similar to the trend of hepatocellular edema and reversible changes caused by hyperammonemia. Combined with imaging results and laboratory data, we speculate that liver dysfunction might be caused by hyperammonemia in patients with argininemia.
It has been claimed that hyperammonemia can restrict liver protein synthesis, especially for liver-derived proteins with short plasma half-life. Decreased clotting factor Ⅶ levels and increased INR have been observed in patients with ornithine transcarbamylase deficiency (OTCD) during hyperammonemia episodes. In patients affected by acute liver failure, the increased INR was positively correlated with plasma ammonia levels, suggesting it was a sensitive index to reflect liver dysfunction associated with plasma ammonia. However, one case reported no correlation between coagulation dysfunction and plasma ammonia or serum albumin levels, suggesting heterogeneity of etiology of liver damage in patients with OTCD. Clotting factor Ⅴ is considered a sensitive indicator of synthetic liver function, but it was within the normal range in patients with argininemia. According to the report, the deficiency in clotting factors did not result from the hepatic involvement of arginase deficiency. In our study, APTT and INR were positively correlated, indicating a common pathway for coagulopathy. No correlation was found between INR and plasma ammonia; we speculated that coagulopathy was independent of the patient’s metabolic status and unrelated to hyperammonemia.
PT was prolonged and INR was increased in nine patients. APTT was prolonged > 10 s in five patients with hyperammonemia, which is different from a previous report. The phenomenon may be related to the disease severity of patients in another study. Despite the severe clotting disorder, none of the patients had a life-threatening hemorrhage, suggesting some underlying metabolic abnormalities may be involved in those patients, such as the decrease in clotting and anticlotting factors[7, 26]. Unfortunately, we have not verified the hypothesis.
In conclusion, we suppose that hyperammonemia may cause hepatocyte damage in patients with argininemia. Still, it may be irrelevant to coagulation dysfunction, and other potential mechanisms may play an important role. The coagulation dysfunction in hyperargininemia may be caused by NO, for the cycle of arginine metabolism is disrupted by arginase deficiency, leading to greater production of NO[3–5, 16], which has an anticoagulant effect. The active form of coagulation factor Ⅷ stabilizes blood clotting; however, NO has an inhibitory effect on it by S-nitrosylation of a cysteine residue, resulting in clot solubilization or suppression of clot formation, which may be responsible for the prolonged PT and INR. Also, as the principal substrate for NO synthase, L-arginine suppresses the expression of tissue factors in human microvascular endothelial cells, which is a critical determinant of thrombin generation. We hypothesized that clotting dysfunction is related to inhibition of tissue factors by high levels of arginine and clot suppression by overproduction of NO in patients with ARG1 deficiency.
In this study, the coagulation abnormalities returned to normal after LT because the arginine cycle can be restored by LT through arginase supplementation. On the other hand, the persistence of clotting dysfunction in other groups supporting the function of NO because metabolites of arginine were not normalized by conventional treatment[5, 6, 8]. In the future, mechanism research on NO will help to explain the phenomenon of patients with argininemia.
Although it was the largest study on LT for argininemia, selection bias may be inevitable as a single-center retrospective study. Due to the rarity of the disease, it is difficult to conduct large cohort studies. Advanced and detailed laboratory analyses, longer follow-up, research on the arginine cycle, and the animal model establishment will help elucidate the exact mechanism of argininemia. In the future, developmental delay, protein tolerance, medication, and neurological status need to be observed in the long run to explore the efficacy of LT in patients with argininemia, and we are doing so.