Anaplerotic therapy in propionic acidemia

Abstract

Background: Propionic acidemia is a rare metabolic disorder caused by a deficiency of propionyl- CoA carboxylase, the enzyme converting propionyl-CoA to methylmalonyl-CoA that subsequently enters the citric acid cycle as succinyl-CoA. Patients with propionic acidemia cannot metabolize propionic acid, which combines with oxaloacetate to form methylcitric acid. This, with the defective supply of succinyl-CoA, may lead to a deficiency in citric acid cycle intermediates.

Purpose: The objective of this study was to determine whether supplements with glutamine (400mg/kg per day), citrate (7.5mEq/kg per day), or ornithine α-ketoglutarate (400mg/kg per day) (anaplerotic agents that could fill up the citric acid cycle) would affect plasma levels of glutamine and ammonia, the urinary excretion of Krebs cycle intermediates, and the clinical outcome in 3 patients with propionic acidemia.

Methods: Each supplement was administered daily for four weeks with a two week washout period between supplements. The supplement that produced the most favorable changes was supplemented for 30 weeks following the initial study period and then for a 2 year extension.

Results: The urinary excretion of the Krebs cycle intermediates, α-ketoglutarate, succinate, and fumarate increased significantly compared to baseline during citrate supplementation, but not with the other two supplements. For this reason, citrate supplements were continued in the second part of the study. The urinary excretion of methylcitric acid and 3-hydroxypropionic acid did not change with any intervention. No significant changes in ammonia or glutamine levels were observed with any supplement. However, supplementation with any anaplerotic agents normalized the physiological buffering of ammonia by glutamate, with plasma glutamate and alanine levels significantly increasing, rather than decreasing with increasing ammonia levels. No significant side effects were observed with any therapy and safety labs (blood counts, chemistry and thyroid profile) remained unchanged. Motor and cognitive development was severely delayed before the trial and did not change significantly with therapy. Hospitalizations per year did not change during the trial period, but decreased significantly (p<0.05) in the 2years following the study (when citrate was continued) compared to the 2years before and during the study.

Conclusions: These results indicate that citrate entered the Krebs cycle providing successful anaplerotic therapy by increasing levels of the downstream intermediates of the Krebs cycle: α-ketoglutarate, succinate and fumarate. Citrate supplements were safe and might have contributed to reduce hospitalizations in patients with propionic acidemia.

Trial registration: ClinicalTrials.gov NCT00645879.

Keywords: Anaplerosis; Clinical trial; Organic acidemia; Outcome; Propionic acidemia; Sodium citrate.

Conflict of interest statement

Conflict of Interest Notification

The authors have no conflicts of interest.

Copyright © 2017 Elsevier Inc. All rights reserved.

Figures

Figure 1. Anaplerotic therapy in propionic acidemia

The supplements used in this study (ornithine alpha-ketogluratate, glutamine and citrate) are shown underlined in respect to propionic acid metabolism and the tricarboxylic acid cycle. Propionyl CoA is normally carboxylated by propionyl CoA carboxylase to become D-methylmalonyl CoA. With the action of methymalonyl CoA racemase and mutase, this produces succinyl CoA that can enter the Krebs cycle and contribute to energy metabolism. In propionic acidemia, propionyl CoA accumulates and condenses with oxaloacetate to produce methylcitric acid. The decrease in oxaloacetate and succinyl CoA can impair the Krebs cycle, reducing the concentration of alpha-ketoglutarate. This can be repleted to the expense of glutamine and glutamate. Ornithine can be converted to glutamate by the action of two enzymes, ornithine amino transferase and Δ1pyrroline-5-carboxylate dehydrogenase. GDH: glutamate dehydrogenase; MAAT: mitochondrial aspartate amino transferase; OAT: ornithine amino transferase; PDG: phosphate-dependent glutaminase.

Figure 2. Effect of anaplerotic therapy on (A) plasma levels of glutamic acid (Glu), Glutamine (Gln) their sum (Glu+Gln and ammonia and (B) selected urinary organic acids in patients with propionic acidemia

Each value is the average ± SD of at least 12 observations. *p

Figure 3. Effect of anaplerotic therapy on the urinary excretion of methylcitrate (A), 3-hydroxy-propionate (B), propionylglycine (C), and ketone bodies (sum of 30hydroxybutyric acid and acetoacetate) (D) in patients with propionic acidemia

Each value is the average ± SD of at least 12 observations. There were no statistically significant difference between any treatment and baseline.

Figure 4. Correlation of ammonia levels with selected plasma amino acids in patients with propionic acidemia

Correlation of ammonia levels with select plasma amino acids in patients with propionic acidemia during the clinical trial. Data were analyzed by linear regression with results indicated in the figure. The shaded area represents the normal range of variation.

Figure 5. Correlation of ammonia levels with selected plasma amino acids in patients with propionic acidemia

Data were analyzed by linear regression with results indicated in the figure. The shaded area represents the normal range of variation. Note that several lysine levels were above the normal range.