Metabolism of 1-13C-propionate in vivo in patients with disorders of propionate metabolism

Intellectual disability and autism in propionic acidemia: a biomarker-behavioral investigation implicating dysregulated mitochondrial biology

Propionic acidemia (PA) is an autosomal recessive condition (OMIM #606054), wherein pathogenic variants in PCCA and PCCB impair the activity of propionyl-CoA carboxylase. PA is associated with neurodevelopmental disorders, including intellectual disability (ID) and autism spectrum disorder (ASD); however, the correlates and mechanisms of these outcomes remain unknown. Using data from a subset of participants with PA enrolled in a dedicated natural history study (n = 33), we explored associations between neurodevelopmental phenotypes and laboratory parameters. Twenty (61%) participants received an ID diagnosis, and 12 of the 31 (39%) who were fully evaluated received the diagnosis of ASD. A diagnosis of ID, lower full-scale IQ (sample mean = 65 ± 26), and lower adaptive behavior composite scores (sample mean = 67 ± 23) were associated with several biomarkers. Higher concentrations of plasma propionylcarnitine, plasma total 2-methylcitrate, serum erythropoietin, and mitochondrial biomarkers plasma FGF21 and GDF15 were associated with a more severe ID profile. Reduced 1-13C-propionate oxidative capacity and decreased levels of plasma and urinary glutamine were also associated with a more severe ID profile. Only two parameters, increased serum erythropoietin and decreased plasma glutamine, were associated with ASD. Plasma glycine, one of the defining features of PA, was not meaningfully associated with either ID or ASD. Thus, while both ID and ASD were commonly observed in our PA cohort, only ID was robustly associated with metabolic parameters. Our results suggest that disease severity and associated mitochondrial dysfunction may play a role in CNS complications of PA and identify potential biomarkers and candidate surrogate endpoints.

Pathophysiological mechanisms of complications associated with propionic acidemia

Propionic acidemia (PA) is a genetic metabolic disorder caused by mutations in the mitochondrial enzyme, propionyl-CoA carboxylase (PCC), which is responsible for converting propionyl-CoA to methylmalonyl-CoA for further metabolism in the tricarboxylic acid cycle. When this process is disrupted, propionyl-CoA and its metabolites accumulate, leading to a variety of complications including life-threatening cardiac diseases and other metabolic strokes. While the clinical symptoms and diagnosis of PA are well established, the underlying pathophysiological mechanisms of PA-induced diseases are not fully understood. As a result, there are currently few effective therapies for PA beyond dietary restriction. This review focuses on the pathophysiological mechanisms of the various complications associated with PA, drawing on extensive research and clinical reports. Most research suggests that propionyl-CoA and its metabolites can impair mitochondrial energy metabolism and cause cellular damage by inducing oxidative stress. However, direct evidence from in vivo studies is still lacking. Additionally, elevated levels of ammonia can be toxic, although not all PA patients develop hyperammonemia. The discovery of pathophysiological mechanisms underlying various complications associated with PA can aid in the development of more effective therapeutic treatments. The consequences of elevated odd-chain fatty acids in lipid metabolism and potential gene expression changes mediated by histone propionylation also warrant further investigation.

Acetate-encapsulated Linolenic Acid Liposomes Reduce SARS-CoV-2 and RSV Infection

Emergent Coronaviridae viruses, such as SARS-CoV-1 in 2003, MERS-CoV in 2012, and SARS-CoV-2 (CoV-2) in 2019, have caused millions of deaths. These viruses have added to the existing respiratory infection burden along with respiratory syncytial virus (RSV) and influenza. There are limited therapies for respiratory viruses, with broad-spectrum treatment remaining an unmet need. Since gut fermentation of fiber produces short-chain fatty acids (SCFA) with antiviral potential, developing a fatty acid-based broad-spectrum antiviral was investigated. Molecular docking of fatty acids showed α-linolenic acid (ALA) is likely to interact with CoV-2-S, NL63-CoV-S, and RSV-F, and an ALA-containing liposome interacted with CoV-2 directly, degrading the particle. Furthermore, a combination of ALA and a SCFA-acetate synergistically inhibited CoV2-N expression and significantly reduced viral plaque formation and IL-6 and IL-1β transcript expression in Calu-3 cells, while increasing the expression of IFN-β. A similar effect was also observed in RSV-infected A549 cells. Moreover, mice infected with a murine-adapted SARS-CoV-2 (MA10) and treated with an ALA-liposome encapsulating acetate showed significant reductions in plaque-forming units present in lung tissue and in infection-associated lung inflammation and cytokines. Taken together, these results demonstrate that the ALA liposome-encapsulating acetate can be a promising broad antiviral therapy against respiratory infections.

Biomarkers for drug development in propionic and methylmalonic acidemias

There is an unmet need for the development and validation of biomarkers and surrogate endpoints for clinical trials in propionic acidemia (PA) and methylmalonic acidemia (MMA). This review examines the pathophysiology and clinical consequences of PA and MMA that could form the basis for potential biomarkers and surrogate endpoints. Changes in primary metabolites such as methylcitric acid (MCA), MCA:citric acid ratio, oxidation of ¹³ C-propionate (exhaled ¹³ CO₂ ), and propionylcarnitine (C3) have demonstrated clinical relevance in patients with PA or MMA. Methylmalonic acid, another primary metabolite, is a potential biomarker, but only in patients with MMA. Other potential biomarkers in patients with either PA and MMA include secondary metabolites, such as ammonium, or the mitochondrial disease marker, fibroblast growth factor 21. Additional research is needed to validate these biomarkers as surrogate endpoints, and to determine whether other metabolites or markers of organ damage could also be useful biomarkers for clinical trials of investigational drug treatments in patients with PA or MMA. This review examines the evidence supporting a variety of possible biomarkers for drug development in propionic and methylmalonic acidemias.

Severity modeling of propionic acidemia using clinical and laboratory biomarkers

Purpose

To conduct a proof-of-principle study to identify subtypes of propionic acidemia (PA) and associated biomarkers.

Methods

Data from a clinically diverse PA patient population (https://clinicaltrials.gov/ct2/show/NCT02890342) were used to train and test machine learning models, identify PA-relevant biomarkers, and perform validation analysis using data from liver-transplanted participants. k-Means clustering was used to test for the existence of PA subtypes. Expert knowledge was used to define PA subtypes (mild and severe). Given expert classification, supervised machine learning (support vector machine with a polynomial kernel, svmPoly) performed dimensional reduction to define relevant features of each PA subtype.

Results

Forty participants enrolled in the study; five underwent liver transplant. Analysis with k-means clustering indicated that several PA subtypes may exist on the biochemical continuum. The conventional PA biomarkers, plasma total 2-methylctirate and propionylcarnitine, were not statistically significantly different between nontransplanted and transplanted participants motivating us to search for other biomarkers. Unbiased dimensional reduction using svmPoly revealed that plasma transthyretin, alanine:serine ratio, GDF15, FGF21, and in vivo 1-¹³C-propionate oxidation, play roles in defining PA subtypes.

Conclusion

Support vector machine prioritized biomarkers that helped classify propionic acidemia patients according to severity subtypes, with important ramifications for future clinical trials and management of PA.

Graphical Abstract

1-13C-propionate breath testing as a surrogate endpoint to assess efficacy of liver-directed therapies in methylmalonic acidemia (MMA)

PURPOSE

To develop a safe and noninvasive in vivo assay of hepatic propionate oxidative capacity.

METHODS

A modified 1-¹³C-propionate breath test was administered to 57 methylmalonic acidemia (MMA) subjects, including 19 transplant recipients, and 16 healthy volunteers. Isotopomer enrichment (¹³CO₂/¹²CO₂) was measured in exhaled breath after an enteral bolus of sodium-1-¹³C-propionate, and normalized for CO₂ production. 1-¹³C-propionate oxidation was then correlated with clinical, laboratory, and imaging parameters collected via a dedicated natural history protocol.

RESULTS

Lower propionate oxidation was observed in patients with the severe mut⁰ and cblB subtypes of MMA, but was near normal in those with the cblA and mut^- forms of the disorder. Liver transplant recipients demonstrated complete restoration of 1-¹³C-propionate oxidation to control levels. 1-¹³C-propionate oxidation correlated with cognitive test result, growth indices, bone mineral density, renal function, and serum biomarkers. Test repeatability was robust in controls and in MMA subjects (mean coefficient of variation 6.9% and 12.8%, respectively), despite widely variable serum methylmalonic acid concentrations in the patients.

CONCLUSION

Propionate oxidative capacity, as measured with 1-¹³C-propionate breath testing, predicts disease severity and clinical outcomes, and could be used to assess the therapeutic effects of liver-targeted genomic therapies for MMA and related disorders of propionate metabolism.

TRIAL REGISTRATION

This clinical study is registered in www.clinicaltrials.gov with the ID: NCT00078078. Study URL: http://clinicaltrials.gov/ct2/show/NCT00078078.

Structure, interactions and action of Mycobacterium tuberculosis 3-hydroxyisobutyric acid dehydrogenase

Biochemical and crystallographic studies on Mycobacterium tuberculosis 3-hydroxyisobutyric acid dehydrogenase (MtHIBADH), a member of the 3-hydroxyacid dehydrogenase superfamily, have been carried out. Gel filtration and blue native PAGE of MtHIBADH show that the enzyme is a dimer. The enzyme preferentially uses NAD+ as the cofactor and is specific to S-hydroxyisobutyric acid (HIBA). It can also use R-HIBA, l-serine and 3-hydroxypropanoic acid (3-HP) as substrates, but with much less efficiency. The pH optimum for activity is ∼11. Structures of the native enzyme, the holoenzyme, binary complexes with NAD+, S-HIBA, R-HIBA, l-serine and 3-HP and ternary complexes involving the substrates and NAD+ have been determined. None of the already known structures of HIBADH contain a substrate molecule at the binding site. The structures reported here provide for the first time, among other things, a clear indication of the location and interactions of the substrates at the active site. They also define the entrance of the substrates to the active site region. The structures provide information on the role of specific residues at the active site and the entrance. The results obtained from crystal structures are consistent with solution studies including mutational analysis. They lead to the proposal of a plausible mechanism of the action of the enzyme.

Propionyl-CoA carboxylase - A review

Propionyl-CoA carboxylase (PCC) is the enzyme which catalyzes the carboxylation of propionyl-CoA to methylmalonyl-CoA and is encoded by the genes PCCA and PCCB to form a hetero-dodecamer. Dysfunction of PCC leads to the inherited metabolic disorder propionic acidemia, which can result in an affected individual presenting with metabolic acidosis, hyperammonemia, lethargy, vomiting and sometimes coma and death if not treated. Individuals with propionic acidemia also have a number of long term complications resulting from the dysfunction of the PCC enzyme. Here we present an overview of the current knowledge about the structure and function of PCC. We review an updated list of human variants which are published and provide an overview of the disease.

Gene therapy in a murine model of methylmalonic acidemia using rAAV9-mediated gene delivery

Methylmalonic acidemia (MMA), an inherited metabolic disorder caused by deficient activity of methylmalonyl-CoA mutase, carries a poor prognosis for long-term survival. While administration of a recombinant adeno-associated virus serotype 8 vector (rAAV8) can rescue Mut(-/-) mice from neonatal lethality and provide sustained phenotypic correction, translation of gene therapy to human subjects will likely require multiple rounds of systemic administration and, ideally, the use of a vector that transduces the kidney. To examine the effectiveness of alternative rAAVs in the treatment of MMA, a serotype 9 rAAV expressing the Mut cDNA was constructed and delivered to newborn Mut(-/-) mice (n=11). rAAV9 gene therapy directed hepatic transgene expression within 24 h and effectively rescued the Mut(-/-) mice from lethality, conferred long-term survival, markedly improved metabolism and resulted in striking preservation of renal function and histology. Systemic readministration of the vector at a dose similar to that used in human clinical trials (2.5 × 10(9) GC of rAAV9 per gram) to older, treated Mut(-/-) mice (n=5) lowered circulating metabolites, increased in vivo propionate oxidative capacity and produced transgene expression in the kidney and liver. Our data support the use of an rAAV9 vector in the acute and chronic treatment of MMA, and highlight the renal tropism afforded by this novel serotype.

Long-term rescue of a lethal murine model of methylmalonic acidemia using adeno-associated viral gene therapy

Methylmalonic acidemia (MMA) is an organic acidemia caused by deficient activity of the mitochondrial enzyme methylmalonyl-CoA mutase (MUT). This disorder is associated with lethal metabolic instability and carries a poor prognosis for long-term survival. A murine model of MMA that replicates a severe clinical phenotype was used to examine the efficacy of recombinant adeno-associated virus (rAAV) serotype 8 gene therapy as a treatment for MMA. Lifespan extension, body weight, circulating metabolites, transgene expression, and whole animal propionate oxidation were examined as outcome parameters after gene therapy. One-hundred percent of the untreated Mut^−/− mice (n = 58) died by day of life (DOL) 72, whereas >95% of the adeno-associated virus–treated Mut^−/− mice (n = 27) have survived for ≥1 year. Despite a gradual loss of transgene expression and elevated circulating metabolites in the treated Mut^−/− mice, the animals are indistinguishable from unaffected control littermates in size and activity levels. These experiments provide the first definitive evidence that gene therapy will have clinical utility in the treatment of MMA and support the development of gene therapy for other organic acidemias.

Genetic and genomic systems to study methylmalonic acidemia

Methylmalonic acidemia (MMAemia) is the biochemical hallmark of a group of genetic metabolic disorders that share a common defect in the ability to convert methylmalonyl-CoA into succinyl-CoA. This disorder is due to either a mutant methylmalonyl-CoA mutase apoenzyme or impaired synthesis of adenosylcobalamin, the cofactor for this enzyme. In this article, we will provide an overview of the pathways disrupted in these disorders, discuss the known metabolic blocks with a particular focus on molecular genetics, and review the use of selected model organisms to study features of methylmalonic acidemia.

Optimum nutrition: thiamin, biotin and pantothenate

The metabolism of glucose is deranged in thiamin deficiency, but once any deficiency has been corrected there is no further effect of increased thiamin intake on the ability to metabolize glucose through either pyruvate dehydrogenase (EC 1.2.4.1) and the citric acid cycle, or the pentose phosphate pathway, in which transketolase (EC 2.2.1.1) is the thiamin-dependent step. It has been suggested that the Wernicke-Korsakoff syndrome is associated with a genetic variant of transketolase which requires a higher than normal concentration of thiamin diphosphate for activity. This finding would suggest that there may be a group of the population who have a higher than average requirement for thiamin, but the evidence is not convincing. There are no estimates of biotin requirements, but either coenzyme saturation of erythrocyte pyruvate carboxylase, or the excretion of 3-hydroxy-isovalerate (perhaps after a test dose of leucine) could be used to assess requirements in depletion-repletion studies. Biotin deficiency leads to impaired glucose tolerance, but it is unlikely that glucose tolerance could be used to assess optimum biotin status, since other more common factors affect glucose tolerance to a greater extent. Plasma triacylglycerol and nonesterified fatty acids are moderately elevated in pantothenic acid deficiency. However, this is unlikely to be useful in assessing pantothenate status, since again, other more common factors affect plasma lipids. To date there are no biochemical indices of adequate pantothenate nutrition, and no estimates of requirements.

The application of infrared spectroscopy to breath CO2 isotope ratio measurements and the risk of spurious results

Stable CO2 isotope breath tests are established as a valuable tool in diagnostic and investigative medicine with the potential to become more prominent in the future. However, their development and widespread clinical use is limited by the requirement of isotope ratio mass spectroscopic analysis. To overcome this restriction alternative analytical techniques have been developed; the most promising, offering relative simplicity and lower costs, are those instruments using infrared spectroscopy. Clinical investigations using such instruments show them to perform well but very little attention has been given to the possibility of interference from the infrared absorption spectrum of other compounds in the breath. To provide an unambiguous answer to this concern we have analysed literature on over 200 detected breath compounds and their infrared absorption spectra to identify any absorption bands coincident with the nu3 absorption band of CO2. It was found that only five breath trace compounds possess coincident fundamental absorption bands, none of which pose the risk of spurious results. We conclude that the 13C16O2/12C16O2 ratio can confidently be measured for isotopic breath tests using an infrared spectrometer, the position of the nu3 absorption band of CO2 in the infrared spectrum precluding any discernible risk of spurious measurements due to coincidental absorption bands.

Late-onset holocarboxylase synthetase deficiency

We report a 21-month-old female patient whose urine organic acid profile suggested a biotin utilization abnormality consistent with multiple carboxylase deficiency. For most previously reported patients, holocarboxylase synthetase deficiency has correlated with the early-onset variant of multiple carboxylase deficiency; conversely, biotinidase deficiency has been characteristic of the late-onset form. In vitro enzyme studies revealed that our patient suffered from holocarboxylase synthetase deficiency. We suggest that holocarboxylase synthetase deficiency should be considered in the differential diagnosis of older patients in whom there is suspicion of a defect in biotin metabolism.

The management and outcome of propionic and methylmalonic acidaemia

The management of the severe variants of methylmalonic and propionic acidaemia remains difficult. With conventional therapy of diet, carnitine and antibiotics, mortality is high and long-term complications are common. Liver transplantation appears to be an important alternative.

The disposition of acrylic acid in the male Sprague-Dawley rat following oral or topical administration

The disposition of [1-14C] acrylic acid (AA) was characterized in the male Sprague-Dawley rat following oral administration, by gavage in water, at 400 mg/kg and topical application, in acetone, at 501 micrograms/cm2. The oral dose was well absorbed, rapidly and extensively metabolized, and excreted primarily (approx. 80%) as 14CO2 within 24 hr of administration. The rate and extent of 14CO2 evolution from [14C]AA was greater for [1-14CAA] while a significantly lower proportion of the dosed radioactivity remained in the tissue of animals than that reported for [2,3-14C]AA (Winter et al., Drug Metabolism and Disposition 1992, 20, 665). This is consistent with incorporation of AA into a minor beta-oxidation pathway of mitochondrial propionate metabolism by which AA may be metabolized to CO2 or incorporated into cellular constituents. Approximately 5% of the dosed radioactivity was excreted in the urine. The disposition of [1-14C]AA following dermal application was studied using charcoal-containing traps attached to the back of the rats to trap volatilized AA from the dosing sites. Following application of 100 microliters AA [4% (v/v) in acetone] to an area of 8.4 cm2 of the skin of a rat (501 micrograms/cm2), the majority (about 73%) of the dose volatilized and was recovered in the charcoal trap. Percutaneous absorption of AA that did not volatilize was rapid and appeared to have the same metabolic fate as AA administered orally with about 75% of the absorbed dose excreted as 14CO2 within 24 hr.

A new immunochemical assay for biotin

A double antibody technique has been developed to separate free biotin from bound biotin after competitive binding of [3H]biotin and unlabelled biotin to avidin. Antiavidin goat antibody was added followed by the addition of antigoat IgG antibody linked to agarose. Centrifugation separated the free biotin from the biotin bound to the avidin complex. The method was suitable for the detection of the amounts of biotin contained in 100-200 microliters of plasma or 5-10 microliters of urine. Normal values for the concentration of biotin in plasma and urine determined by this assay were 1.27 +/- 0.67 nmol/l and 49.1 +/- 35.7 mumol/mol creatinine, respectively.