Physiological Perspectives on the Use of Triheptanoin as Anaplerotic Therapy for Long Chain Fatty Acid Oxidation Disorders

Abstract

Inborn errors of mitochondrial fatty acid oxidation (FAO) comprise the most common group of disorders identified through expanded newborn screening mandated in all 50 states in the United States, affecting 1:10,000 newborns. While some of the morbidity in FAO disorders (FAODs) can be reduced if identified through screening, a significant gap remains between the ability to diagnose these disorders and the ability to treat them. At least 25 enzymes and specific transport proteins are responsible for carrying out the steps of mitochondrial fatty acid metabolism, with at least 22 associated genetic disorders. Common symptoms in long chain FAODs (LC-FAODs) in the first week of life include cardiac arrhythmias, hypoglycemia, and sudden death. Symptoms later in infancy and early childhood may relate to the liver or cardiac or skeletal muscle dysfunction, and include fasting or stress-related hypoketotic hypoglycemia or Reye-like syndrome, conduction abnormalities, arrhythmias, dilated or hypertrophic cardiomyopathy, and muscle weakness or fasting- and exercise-induced rhabdomyolysis. In adolescent or adult-onset disease, muscular symptoms, including rhabdomyolysis, and cardiomyopathy predominate. Unfortunately, progress in developing better therapeutic strategies has been slow and incremental. Supplementation with medium chain triglyceride (MCT; most often a mixture of C8-12 fatty acids containing triglycerides) oil provides a fat source that can be utilized by patients with long chain defects, but does not eliminate symptoms. Three mitochondrial metabolic pathways are required for efficient energy production in eukaryotic cells: oxidative phosphorylation (OXPHOS), FAO, and the tricarboxylic (TCA) cycle, also called the Krebs cycle. Cell and mouse studies have identified a deficiency in TCA cycle intermediates in LC-FAODs, thought to be due to a depletion of odd chain carbon compounds in patients treated with a predominantly MCT fat source. Triheptanoin (triheptanoyl glycerol; UX007, Ultragenyx Pharmaceuticals) is chemically composed of three heptanoate (seven carbon fatty acid) molecules linked to glycerol through ester bonds that has the potential to replete TCA cycle intermediates through production of both acetyl-CoA and propionyl-CoA through medium chain FAO. Compassionate use, retrospective, and recently completed prospective studies demonstrate significant reduction of hypoglycemic events and improved cardiac function in LC-FAOD patients, but a less dramatic effect on muscle symptoms.

Keywords: anaplerosis; energy metabolism; fatty acid oxidation; fatty acid oxidation disorders; inborn errors of metabolism; metabolomics; tricarboxylic acid cycle; very long chain acyl-CoA dehydrogenase.

FIGURE 1

The carnitine cycle involves generation of a long chain acylcarnitine from activated fatty acids by CPT I, transport across the mitochondrial membranes by CACT, and release of the activated acyl-CoA substrate for FAO into the mitochondrial matrix by CPT II. Carnitine itself is transported into cells by a high affinity transporter (OCTN2). All abbreviations are as defined in the text. Interactions of mitochondrial energy metabolism pathways. The carnitine cycle transport long-chain CoAs into the mitochondrial matrix with carnitine ultimately being restored to the cytoplasm in exchange for release of an acyl-CoA in the mitochondrial matrix. Medium chain fats directly diffuse into mitochondria. ETC and the enzymes of LC-FAO functionally and physically interact. Mitochondrial TFP interacts with and passes its reducing equivalents to the matrix arm of ETC complex I. VLCAD interacts with TFP, but its reducing equivalents are transferred to ETC complex III via electron transfer flavoprotein (ETF, not shown) and ETF dehydrogenase (not shown). The acetyl-CoA product of one cycle of FAO enters the TCA cycle to produce citrate. While standard medium chain trioctanoylglycerol (C8) provides double the amount of acetyl-CoA compared to triheptanoin (C7), the latter provides a propionyl-CoA that enters the TCA cycle through succinyl-CoA. Anaplerotic amino acids are listed in red. Pyruvate and citrate enter the mitochondria through specific carriers (the latter with counter transport of malate to the cytoplasm). All abbreviations are as defined in the text.

FIGURE 2

The matrix reactions of FAO consist of four steps that result in one molecule of acetyl-CoA and an acyl-CoA that is shortened by two-carbon units. For long-chain substrates, VLCAD, which catalyzes the first step is long-chain fatty acid β-oxidation, is the most important acyl-CoA dehydrogenase for energy generation. The next three steps are performed by a single multifunctional enzyme with three active sites, the mitochondrial TFP. All abbreviations are as defined in the text.

FIGURE 3

Concentrations of TCA cycle intermediates in the blood from VLCAD deficiency patients treated with standard MCT oil (MCT1–5) or triheptanoin (C1–6). The values are given as Z-score, defined as the number of standard deviations from age and sex matched control means.

FIGURE 4

Concentrations of odd chain long-chain complex lipids in VLCAD patients treated with MCG oil (MCT 1–5) or triheptanoin (C7 1–6). The values are given as Z-score, defined as the number of standard deviations from age and sex matched control means. Compound abbreviation are as defined in Table 1.