Propionic acidaemia: clinical, biochemical and therapeutic aspects. Experience in 30 patients

Challenges and strategies for clinical trials in propionic and methylmalonic acidemias

Clinical trial development in rare diseases poses significant study design and methodology challenges, such as disease heterogeneity and appropriate patient selection, identification and selection of key endpoints, decisions on study duration, choice of control groups, selection of appropriate statistical analyses, and patient recruitment. Therapeutic development in organic acidemias (OAs) shares many challenges with other inborn errors of metabolism, such as incomplete understanding of natural history, heterogenous disease presentations, requirement for sensitive outcome measures and difficulties recruiting a small sample of participants. Here, we review strategies for the successful development of a clinical trial to evaluate treatment response in propionic and methylmalonic acidemias. Specifically, we discuss crucial decisions that may significantly impact success of the study, including patient selection, identification and selection of endpoints, determination of the study duration, consideration of control groups including natural history controls, and selection of appropriate statistical analyses. The significant challenges associated with designing a clinical trial in rare disease can sometimes be successfully met through strategic engagement with experts in the rare disease, seeking regulatory and biostatistical guidance, and early involvement of patients and families.

Yenidoğan hipoglisemisine güncel yaklaşım

Neonatal hipoglisemi yenidoğan döneminde sık görülen, yaşamın ilerleyen dönemlerini de etkileyen bir sağlık sorunudur. Hipoglisemi akut ve kronik komplikasyonlara neden olarak yenidoğan bebeğin büyümesi ve gelişmesinde kalıcı ve geri dönüşümsüz nörolojik sorunlara yol açabilmektedir. Hipoglisemiyi saptamak için en duyarlı test halen plazma glukozu ile ölçümü olsa da, yıllardır kullanılan geleneksel glukoz ölçüm yöntemleri ile birlikte son yıllarda noninvaziv glukoz ölçüm yöntemlerindeki gelişmeler ile riskli bebeklerde hipogliseminin tanı ve tedavisinde bu teknolojik ölçüm yöntemleri yaygın kullanılmaya başlamıştır. Glukometre, kan gazı analizatörleri, laboratuvarda enzimatik ölçüm geleneksel ölçüm yöntemleri iken, sürekli glukoz ölçüm sistemleri yeni nesil glukoz ölçümünde yer almaya başlayan yeni bir tekniktir. Hipoglisemi gelişimini engellemek için koruyucu önlemler alırken erken dönemde anne sütü ile emzirme desteği sağlanmalı ve teşvik edilmelidir. Hangi durumların hipoglisemi için risk oluşturduğu, hangi glukoz eşik değerinde tedavi vermek gerektiği ve son kullanılan yeni nesil glukoz ölçüm yöntemlerinin etkileri ve güncel tedaviler bu derlemede tartışılmıştır. Riskli bebeklerde kan şekeri ölçümleri ve klinik bulguların gözlenmesi yanında hipoglisemide ayırıcı tanı yapılarak tedavinin yönetilmesi de prognozda önemli bir yer tutar.

Immunological Memory and Affinity Maturation After Vaccination in Patients With Propionic Acidemia

Earlier studies have recommended routine childhood immunization in patients with propionic acidemia (PA); however, the literature presents insufficient data on the response to vaccines, notably specific IgG concentrations and avidity maturation, after measles, mumps, rubella (MMR), and diphtheria/tetanus (DiphtTe) vaccinations in this population. In patients with PA, cellular and humoral changes of the immune system (e.g. a decreased CD4+ T cell count, with a reversal of CD4/CD8 T cell ratio, a deficient gamma-globulin fraction, and in one case a decreased lymphocyte blastogenesis) have been reported. Former reports also detected pancytopenias accompanying febrile infections in PA patients. In the current study, we analyzed vaccine-specific IgG concentrations and avidity maturation after MMR and DiphtTe vaccinations in 10 patients with PA. Compared to gender and age matched controls, all 10 had protective IgG concentrations for at least one tested antigen, and in 6 out of 10 patients high relative avidity indices for measles and rubella were detected. In summary, the present study revealed a sufficient immune response and outcome, indicating an acceptable humoral memory in patients with PA after booster vaccinations.

Analysis of the relationship between phenotypes and genotypes in 60 Chinese patients with propionic acidemia: a fourteen-year experience at a tertiary hospital

Background

Propionic acidemia is a severe inherited metabolic disorder, caused by the deficiency of propionyl-CoA carboxylase which encoded by the PCCA and PCCB genes. The aim of the study was to investigate the clinical features and outcomes, molecular epidemiology and phenotype-genotype relationship in Chinese population.

Methods

We conducted a retrospective study of 60 Chinese patients diagnosed at Peking University First Hospital from 2007 to 2020. Their clinical and laboratory data were reviewed. The next-generation sequencing was conducted on blood samples from 58 patients.

Results

Only 5 (8.3%) patients were identified by newborn screening. In the rest 55 patients, 25 had early-onset (≤ 3 months) disease and 30 had late-onset (> 3 months) disease. Neurological abnormalities were the most frequent complications. Five cases detected by newborn screening had basically normal development. Nine (15%) cases died in our cohort. 24 patients (41.4%) harbored PCCA variants, and 34 (58.6%) harbored PCCB variants. 30 (11 reported and 19 novel) variants in PCCA and 28 (18 reported and 10 novel) variants in PCCB mere identified. c.2002G>A and c.937C>T in PCCA, and c.838dupC in PCCB were the most common variants in this cohort, with the frequency of 13.9% (6/44 alleles), 13.9% (6/44 alleles) and 12.5% (8/64 alleles), respectively. There was no difference in clinical features and outcomes between patients with PCCA and PCCB variants. Certain variants with high frequencies and homozygotes may be associated with early-onset or late-onset propionic acidemia.

Conclusions

Although the genotype–phenotype correlation is still unclear, certain variants seemed to be related to early-onset or late-onset propionic acidemia. Our study further delineated the complex clinical manifestations of propionic acidemia and expanded the spectrum of gene variants associated with propionic acidemia.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02271-3.

Case Report: Novel Mutations in the PCCB Gene Causing Late-Onset Propionic Acidemia

Introduction: Propionic acidemia is an autosomal recessive metabolic disorder and the patients with adult onset are very rare.

Methods: Two PCCB mutations were identified. Clinical data were collected from a patient, and metabolic screening and clinical exome sequencing analysis were performed.

Results: Two novel mutations were identified in the PCCB gene: M1:c.404_406del:p.G135del and M2:c.632C>T:p.T211I.

Conclusion: Late-onset propionic acidemia should be taken into account, and metabolic screening as well as gene analysis should be performed to make a definite diagnosis timely.

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.

Analysis of 2-methylcitric acid, methylmalonic acid, and total homocysteine in dried blood spots by LC-MS/MS for application in the newborn screening laboratory: A dual derivatization approach

Inborn errors of propionate, cobalamin and methionine metabolism are targets for Newborn Screening (NBS) in most programs world-wide, and are primarily screened by analyzing for propionyl carnitine (C3) and methionine in dried blood spot (DBS) cards using tandem mass spectrometry (MS/MS). Single-tier NBS approaches using C3 and methionine alone lack specificity, which can lead to an increased false-positive rate if conservative cut-offs are applied to minimize the risk of missing cases. Implementation of liquid chromatography tandem mass spectrometry (LC-MS/MS) second-tier testing for 2-methylcitric acid (MCA), methylmalonic acid (MMA), and homocysteine (HCY) from the same DBS card can improve disease screening performance by reducing the false-positive rate and eliminating the need for repeat specimen collection. However, DBS analysis of MCA, MMA, and HCY by LC-MS/MS is challenging due to limited specimen size and analyte characteristics leading to a combination of low MS/MS sensitivity and poor reverse-phase chromatographic retention. Sufficient MS response and analytical performance can be achieved for MCA by amidation using DAABD-AE and by butylation for MMA and HCY. Herein we describe the validation of a second-tier dual derivatization LC-MS/MS approach to detect elevated MCA, MMA, and HCY in DBS cards for NBS. Clinical utility was demonstrated by retrospective analysis of specimens, an interlaboratory method comparison, and assessment of external proficiency samples. Imprecision was <10.8% CV, with analyte recoveries between 90.2 and 109.4%. Workflows and analytical performance characteristics of this second-tier LC-MS/MS approach are amenable to implementation in the NBS laboratory.

Orthopaedic Problems in 35 Patients With Organic Acid Disorders

INTRODUCTION

Organic acid disorders (OADs) are a subset of inborn errors of metabolism that result in a toxic accumulation of organic acids in the body, which can lead to metabolic derangements and encephalopathy. Patients with these disorders are managed by a team of biochemical geneticists and metabolic nutritionists. However, subspecialists such as neurologists and orthopaedic surgeons are often needed to help manage the sequelae of the metabolic derangements. The breadth of orthopaedic sequelae of these disease states is poorly understood. Herein, we describe orthopaedic problems associated with 5 types of OAD most commonly seen at our institution: maple syrup urine disease, methylmalonic aciduria, propionic aciduria, pyruvate dehydrogenase deficiency, and glutaric aciduria type 1.

METHODS

We retrospectively reviewed medical records of 35 patients with an OAD who were seen at our academic tertiary care center from May 1999 to May 2020. Patients were grouped into cohorts according to OAD type and analyzed for orthopaedic presentations of hip, knee, or foot disorders, presence and severity of scoliosis, history of fracture, movement disorders, and osteopenia/osteoporosis.

RESULTS

Of the 35 patients, 13 had maple syrup urine disease, 12 had methylmalonic aciduria, 4 had propionic aciduria, 4 had pyruvate dehydrogenase deficiency, and 2 had glutaric aciduria type 1. Associated orthopaedic problems included spasticity causing neuromuscular scoliosis and/or hip subluxation or dislocation (10 patients), fractures (7 patients), and osteopenia/osteoporosis (7 patients). Overall, 22 of 35 patients had some orthopaedic condition.

CONCLUSIONS

Most in this cohort of patients with OAD also had an orthopaedic abnormality. It is important for physicians treating these patients to understand their propensity for musculoskeletal problems. When treating patients with OAD, it is important to initiate and maintain communication with specialists in several disciplines and to develop collaborative treatments for this unique population.

LEVEL OF EVIDENCE

Level IV-prognostic study.

Guidelines for the diagnosis and management of methylmalonic acidaemia and propionic acidaemia: First revision

Isolated methylmalonic acidaemia (MMA) and propionic acidaemia (PA) are rare inherited metabolic diseases. Six years ago, a detailed evaluation of the available evidence on diagnosis and management of these disorders has been published for the first time. The article received considerable attention, illustrating the importance of an expert panel to evaluate and compile recommendations to guide rare disease patient care. Since that time, a growing body of evidence on transplant outcomes in MMA and PA patients and use of precursor free amino acid mixtures allows for updates of the guidelines. In this article, we aim to incorporate this newly published knowledge and provide a revised version of the guidelines. The analysis was performed by a panel of multidisciplinary health care experts, who followed an updated guideline development methodology (GRADE). Hence, the full body of evidence up until autumn 2019 was re-evaluated, analysed and graded. As a result, 21 updated recommendations were compiled in a more concise paper with a focus on the existing evidence to enable well-informed decisions in the context of MMA and PA patient care.

Long-term follow-up with filter paper samples in patients with propionic acidemia

BACKGROUND

Propionic acidemia (PA) is an inherited disorder caused by deficiency of propionyl CoA carboxylase. Most patients with this disorder are diagnosed during the neonatal period because of severe metabolic acidosis and hyperammonemia. Patients are required to undergo blood and urine analysis at least 3 to 4 times per year, depending on age and metabolic control.

METHODS

We designed a prospective study in which we investigated the results from blood and urinary samples collected monthly in filter paper from 10 PA patients followed in a single metabolic reference center from January 2015 to September 2017. The aim of this study was to evaluate the usefulness of filter paper samples in the follow-up of the PA patients.

RESULTS

During the follow-up period, 163 dried blood spot (DBS) and 119 urine dried spot samples were analyzed and compared with 160 plasma and 103 liquid urine specimens; 64 specimens of plasma were analyzed for odd-numbered long-chain fatty acids (OLCFAs). A total of 40 metabolic crises, 18 of them with hyperammonemia were documented. We observed a strong correlation between the filter paper and the urine/plasma samples for the main PA parameters both in stable metabolic conditions as well as in acute decompensations. Also, there was a strong correlation between OLCFAs measured in plasma and quantification of odd number acylcarnitines in DBS.

CONCLUSIONS

We conclude that filter paper blood and urinary samples can be used for the follow-up of the patients with PA, correctly reflecting their metabolic situation.

Disruption of mitochondrial functions and oxidative stress contribute to neurologic dysfunction in organic acidurias

Organic acidurias (OADs) are inherited disorders of amino acid metabolism biochemically characterized by accumulation of short-chain carboxylic acids in tissues and biological fluids of the affected patients and clinically by predominant neurological manifestations. Some of these disorders are amenable to treatment, which significantly decreases mortality and morbidity, but it is still ineffective to prevent long-term neurologic and systemic complications. Although pathogenesis of OADs is still poorly established, recent human and animal data, such as lactic acidosis, mitochondrial morphological alterations, decreased activities of respiratory chain complexes and altered parameters of oxidative stress, found in tissues from patients and from genetic mice models with these diseases indicate that disruption of critical mitochondrial functions and oxidative stress play an important role in their pathophysiology. Furthermore, organic acids that accumulate in the most prevalent OADs were shown to compromise bioenergetics, by decreasing ATP synthesis, mitochondrial membrane potential, reducing equivalent content and calcium retention capacity, besides inducing mitochondrial swelling, reactive oxygen and nitrogen species generation and apoptosis. It is therefore presumed that secondary mitochondrial dysfunction and oxidative stress caused by major metabolites accumulating in OADs contribute to tissue damage in these pathologies.

Propionic and Methylmalonic Acidemias: Initial Clinical and Biochemical Presentation

PA and MAA have numerous nonspecific presentations, potentially leading to delayed diagnosis or misdiagnosis. In this paper, we present the clinical and biochemical characteristics of MMA and PA patients at initial presentation. Results. This is a retrospective review of 20 patients with PA (n = 10) and MMA (n = 10). The most observed symptoms were vomiting (85%) and refusing feeding (70%). Ammonia was 108.75 ± 9.3 μmol/l, showing a negative correlation with pH and bicarbonate and positive correlation with lactate and anion gap. Peak ammonia did not correlate with age of onset (r = 0.11 and p = 0.64) or age at diagnosis (r = 0.39 and p = 0.089), nor did pH (r = 0.01, p = 0.96; r = −0.25, p = 0.28) or bicarbonate (r = 0.07, p = 0.76; r = −0.22, p = 0.34). There was no correlation between ammonia and C3 : C2 (r = 0.1 and p = 0.96) or C3 (r = 0.23 and p = 0.32). The glycine was 386 ± 167.1 μmol/l, and it was higher in PA (p = 0.003). There was a positive correlation between glycine and both pH (r = 0.56 and p = 0.01) and HCO₃ (r = 0.49 and p = 0.026). There was no correlation between glycine and ammonia (r = −0.435 and p = 0.055) or lactate (r = 0.32 and p = 0.160). Conclusion. Clinical presentation of PA and MMA is nonspecific, though vomiting and refusing feeding are potential markers of decompensation. Blood gas, lactate, and ammonia levels are also good predictors of decompensation, though increasing levels of glycine may not indicate metabolic instability.

A novel delins (c.773_819+47delinsAA) mutation of the PCCA gene associated with neonatal-onset propionic acidemia: a case report

Background

Propionic acidemia (PA)(OMIM#606054) is an inborn error of branched-chain amino acid metabolism, caused by defects in the propionyl-CoA carboxylase (PCC) enzyme which encoded by the PCCA and PCCB genes.

Case presentation

Here we report a Chinese neonate diagnosed with suspected PA based on the clinical symptoms, gas chromatography-mass spectrometry (GC/MS), and brain imaging tests. Targeted next-generation sequencing (NGS) was performed on the proband. We detected only one heterozygous recurrent nonsense variant (c.937C > T, p.Arg313Ter) in the PCCA gene. When we manually checked the binary alignment map (BAM) diagram of PCCA gene, we found a heterozygous deletion chr13:100915039-100915132delinsAA (c.773_819 + 47delinsAA) (GRCh37.p13) inside the exon 10 in the PCCA gene. The results were validated by Sanger sequencing and qPCR method in the family: the variant (c.937C > T, p.Arg313Ter) was in the maternal allele, and the delins was in the paternal allele. When the mother was pregnant again, prenatal diagnosis was carried out through amniocentesis at 18 weeks gestation, the fetus carried neither of the two mutations. After birth, newborn screening was undertaken, the result was negative.

Conclusions

We identified a recurrent c.937C > T and a novel c.773_819 + 47delinsAA mutations in the PCCA gene, which may be the genetic cause of the phenotype of this patient. Our findings expanded the spectrum of causative genotype-phenotype of the PCCA gene. For the cases, the NGS results revealed only a heterozygous mutation in autosomal recessive disease when the gene is associated with phenotypes, it is necessary to manually check the BAM diagram to improve the detection rate. Targeted NGS is an effective technique to detect the various genetic lesions responsible for the PA in one step. Genetic testing is essential for genetic counselling and prenatal diagnosis in the family to avoid birth defects.

Disturbance of bioenergetics and calcium homeostasis provoked by metabolites accumulating in propionic acidemia in heart mitochondria of developing rats

Propionic acidemia is caused by lack of propionyl-CoA carboxylase activity. It is biochemically characterized by accumulation of propionic (PA) and 3-hydroxypropionic (3OHPA) acids and clinically by severe encephalopathy and cardiomyopathy. High urinary excretion of maleic acid (MA) and 2-methylcitric acid (2MCA) is also found in the affected patients. Considering that the underlying mechanisms of cardiac disease in propionic acidemia are practically unknown, we investigated the effects of PA, 3OHPA, MA and 2MCA (0.05-5 mM) on important mitochondrial functions in isolated rat heart mitochondria, as well as in crude heart homogenates and cultured cardiomyocytes. MA markedly inhibited state 3 (ADP-stimulated), state 4 (non-phosphorylating) and uncoupled (CCCP-stimulated) respiration in mitochondria supported by pyruvate plus malate or α-ketoglutarate associated with reduced ATP production, whereas PA and 3OHPA provoked less intense inhibitory effects and 2MCA no alterations at all. MA-induced impaired respiration was attenuated by coenzyme A supplementation. In addition, MA significantly inhibited α-ketoglutarate dehydrogenase activity. Similar data were obtained in heart crude homogenates and permeabilized cardiomyocytes. MA, and PA to a lesser degree, also decreased mitochondrial membrane potential (ΔΨm), NAD(P)H content and Ca²⁺ retention capacity, and caused swelling in Ca²⁺-loaded mitochondria. Noteworthy, ΔΨm collapse and mitochondrial swelling were fully prevented or attenuated by cyclosporin A and ADP, indicating the involvement of mitochondrial permeability transition. It is therefore proposed that disturbance of mitochondrial energy and calcium homeostasis caused by MA, as well as by PA and 3OHPA to a lesser extent, may be involved in the cardiomyopathy commonly affecting propionic acidemic patients.

Propionic Acidemia with Novel Mutation Presenting as Recurrent Pancreatitis in a Child

Propionic acidemia (PA) is a rare organic acidemia resulting from a deficiency of the mitochondrial enzyme propionyl-coenzyme A carboxylase. Most cases are diagnosed after the detection of metabolic abnormalities-such as hyperammonemia, metabolic acidosis, and ketosis-associated with complaints of vomiting, feeding difficulties, and hypotonia during the neonatal period. However, in rare late-onset cases, mild or vague symptoms make the diagnosis more challenging. Even though acute pancreatitis is relatively uncommon in children, it can occur in association with PA. We present the case of a 4-year-old child who was admitted owing to the complaint of recurrent pancreatitis and had not previously been diagnosed with having metabolic disease. During inpatient treatment for acute pancreatitis, convulsions occurred with concomitant hyperammonemia, metabolic acidosis, coagulopathy, and shock 1 week after the administration of total parenteral nutrition. He was diagnosed to have PA after a metabolic work-up and confirmed to have novel mutation by molecular genetic analysis. Because children with PA may have acute pancreatitis, although rare, vomiting and abdominal pain should raise a suspicion of acute pancreatitis. On the contrary, even among children who have never been diagnosed with a metabolic disease, if a child has recurrent pancreatitis, metabolic pancreatitis caused by organic acidemia should be considered.

Chronic kidney disease in propionic acidemia

Purpose

Propionic acidemia (PA) is a severe metabolic disorder characterized by multiorgan pathology, including renal disease. The prevalence of chronic kidney disease (CKD) in PA patients and factors associated with CKD in PA are not known.

Methods

Thirty-one subjects diagnosed with PA underwent laboratory and clinical evaluations through a dedicated natural history study at the NIH (ClinicalTrials.gov identifier: NCT02890342).

Results

Cross-sectional analysis of the creatinine-based estimated glomerular filtration rate (eGFR) in subjects with native kidneys revealed an age-dependent decline in renal function (P <0.002). Among adults with PA, 4/8 (50%) had eGFR <60 mL/min/1.73 m². There was a significant discrepancy between eGFRs calculated using estimating equations based on serum creatinine compared to serum cystatin C (P <0.0001). The tubular injury marker, plasma lipocalin-2, and plasma uric acid were strongly associated with CKD (P <0.0001). The measured 24-hour creatinine excretion was below normal, even after adjusting for age, height, and sex.

Conclusions

CKD is common in adults with PA and is associated with age. The poor predictive performance of standard eGFR estimating equations, likely due to reduced creatine synthesis in kidney and liver, could delay the recognition of CKD and management of ensuing complications in this population.

Epilepsy in Propionic Acidemia: Case Series of 14 Saudi Patients

Propionic acidemia is an inborn error of metabolism that is inherited in an autosomal recessive manner. It is characterized by a deficient propionyl-CoA carboxylase due to mutations in either of its beta or alpha subunits. In the literature, there is a clear association between propionic acidemia and epilepsy. In this cohort, we retrospectively reviewed the data of 14 propionic acidemia patients in Saudi Arabia and compared the findings to those of former studies. Six of the 14 (43%) patients developed epileptic seizure, mainly focal seizures. All patients were responsive to conventional antiepileptic drugs as their seizures are controlled. The predominant electroencephalographic (EEG) findings were diffuse slowing in 43% and multifocal epileptiform discharges in 14% of the patients. In 1 patient, burst suppression pattern was detected, a pattern never before reported in patients with propionic acidemia. Brain magnetic resonance imaging (MRI) findings mainly consisted of signal changes of the basal ganglia (36%), generalized brain atrophy (43%), and delayed myelination (43%).The most common genotype in our series is the homozygous missense mutation in the PCCA gene (c.425G>A; p. Gly142Asp). However, there is no clear genotype-seizure correlation. We conclude that seizure is not an uncommon finding in patients with propionic acidemia and not difficult to control. Additional studies are needed to further elaborate on genotype-seizure correlation.

Propionyl-CoA carboxylase pcca-1 and pccb-1 gene deletions in Caenorhabditis elegans globally impair mitochondrial energy metabolism

Propionic acidemia (PA) is a classical inborn error of metabolism with high morbidity that results from the inability of the propionyl-CoA carboxylase (PCC) enzyme to convert propionyl-CoA to methylmalonyl-CoA. PA is inherited in an autosomal recessive fashion due to functional loss of both alleles of either PCCA or PCCB. These genes are highly conserved across evolutionarily diverse species and share extensive similarity with pcca-1 and pccb-1 in the nematode, Caenorhabditis elegans. Here, we report the global metabolic effects of deletion in a single PCC gene, either pcca-1 or pccb-1, in C. elegans. Animal lifespan was significantly reduced relative to wild-type worms in both mutant strains, although to a greater degree in pcca-1. Mitochondrial oxidative phosphorylation (OXPHOS) capacity and efficiency as determined by direct polarography of isolated mitochondria were also significantly reduced in both mutant strains. While in vivo quantitation of mitochondrial physiology was normal in pccb-1 mutants, pcca-1 deletion mutants had significantly increased mitochondrial matrix oxidant burden as well as significantly decreased mitochondrial membrane potential and mitochondrial content. Whole worm steady-state free amino acid profiling by UPLC revealed reduced levels in both mutant strains of the glutathione precursor cysteine, possibly suggestive of increased oxidative stress. Intermediary metabolic flux analysis by GC/MS with 1,6-¹³C₂-glucose further showed both PCC deletion strains had decreased accumulation of a distal tricarboxylic acid (TCA) cycle metabolic intermediate (+1 malate), isotopic enrichment in a proximal TCA cycle intermediate (+1 citrate), and increased +1 lactate accumulation. GC/MS analysis further revealed accumulation in the PCC mutants of a small amount of 3-hydroxypropionate, which appeared to be metabolized in C. elegans to oxalate through a unique metabolic pathway. Collectively, these detailed metabolic investigations in translational PA model animals with genetic-based PCC deficiency reveal their significantly dysregulated energy metabolism at multiple levels, including reduced mitochondrial OXPHOS capacity, increased oxidative stress, and inhibition of distal TCA cycle flux, culminating in reduced animal lifespan. These findings demonstrate that the pathophysiology of PA extends well beyond what has classically been understood as a single PCC enzyme deficiency with toxic precursor accumulation, and suggest that therapeutically targeting the globally disrupted energy metabolism may offer novel treatment opportunities for PA.

SUMMARY

Two C. elegans model animals of propionic acidemia with single-gene pcca-1 or pccb-1 deletions have reduced lifespan with significantly reduced mitochondrial energy metabolism and increased oxidative stress, reflecting the disease's broader pathophysiology beyond a single enzyme deficiency with toxic precursor accumulation.

Seventeen Novel Mutations in PCCA and PCCB Genes in Indian Propionic Acidemia Patients, and Their Outcomes

AIMS

The goal of this study was to identify mutations in the propionyl-CoA carboxylase alpha subunit (PCCA) and propionyl-CoA carboxylase beta subunit (PCCB) genes, and to assess their effects on propionic academia (PA) patients.

METHODOLOGY

Twenty-five Indian children with PA were enrolled in this study. Bidirectional Sanger sequencing was performed on both the coding and flanking regions of the PCCA and PCCB genes and the chromatograms were analyzed. Bioinformatic tools were used to classify novel variations into pathogenic or benign.

RESULTS

The majority of the cases (19/25, 76%) were of the early-onset (<90 days of age) type and 5 were of the late-onset type. The majority of patients had mutations in the PCCA gene (18/25). A total of 26 mutations were noted: 20 in the PCCA gene and 6 in PCCB gene. Seventeen mutations were novel (14 in PCCA and 3 in PCCB). The SNP c.937C>T (p.Arg313Ter), was noted in 9/36 (25%) alleles in the PCCA gene. All of the children were symptomatic and only three survived who are doing well with no major disabilities.

CONCLUSION

The spectrum of mutations in the PCCA and PCCB genes among Indians is distinct from other populations. The absence of a common mutation signifies the heterogeneity and admixture of various subpopulations. These findings also suggest that individuals of Indian origin may not benefit from the mutation-based "carrier screening panels" offered by many genetic laboratories.

Death of a Neonate With a Negative Autopsy and Ketoacidosis: A Case Report of Propionic Acidemia

Facing a sudden neonatal death in the forensic setting brings to mind enormous differential diagnostic possibilities. This case report demonstrates that at times, when no anatomical cause of death is apparent after a postmortem examination, ancillary testing can lead to diagnosis. In this case, ancillary testing showed ketosis and further workup showed presence of propionic acidemia in a 3-day-old neonate.

Emerging trends in management of propionic acidemia

OBJETIVO

To evaluate the therapeutic agents used during metabolic crises and in long-term management of patients with propionic acidemia (PA).

MATERIALS AND METHODS

The records of PA patients were retrospectively evaluated.

RESULTS

The study group consisted of 30 patients with 141 admissions. During metabolic crises, hyperammonemia was found in 130 (92%) admissions and almost all patients were managed with normal saline, ≥ 10% dextrose, and restriction of protein intake. In 56 (40%) admissions, management was done in intensive care unit, 31 (22%) with mechanical ventilation, 10 (7%) with haemodialysis, 16 (11%) with vasopressor agents, and 12 (9%) with insulin. In the rescue procedure, L-carnitine was used in 135 (96%) patients, sodium bicarbonate in 116 (82%), sodium benzoate in 76 (54%), and metronidazole in 10 (7%), biotin in about one-quarter, L-arginine in one third, and antibiotics in three-quarter of the admissions. Blood/packed RBCs were used in 28 (20%) patients, platelets in 26 (18%), fresh frozen plasma in 8 (6%), and granulocyte-colony stimulating factors in 10 (7%) admissions. All patients were managed completely/partially with medical nutrition formula plus amino acid mixture, vitamins and minerals. For long-term management 24 (80%) patients were on L-carnitine, 22 (73%) on sodium benzoate, 6 (20%) on biotin, one half on alkaline therapy and 4 (13%) on regular metronidazole use. Almost all patients were on medical formula and regular follow-up.

CONCLUSION

Aggressive and adequate management of acute metabolic crises with restriction of protein intake, stabilization of patient, reversal of catabolism, and removal of toxic metabolites are essential steps. Concerted efforts to ensure adequate nutrition, to minimize the risk of acute decompensation and additional therapeutic advances are imperative to improve the outcome of PA patients.

Propionic acidemia in the Arab World

The autosomal recessive disease propionic acidemia (PA) is an inborn error of metabolism with highly variable clinical manifestations, caused by a deficiency of propionyl-CoA carboxylase (PCC) enzyme, due to mutations in either PCCA or PCCB genes, which encode the alpha and beta subunits of the PCC enzyme, respectively. The classical clinical presentation consists of poor feeding, vomiting, metabolic acidosis, hyperammonemia, lethargy, neurological problems, and developmental delay. PA seems to be a prevalent disease in the Arab World. Arab patients with PA seem to have the same classical clinical picture for PA with distinctive associated complications and other diseases. Most of the mutations found in Arab patients seem to be specific to the Arab population, and not observed in other ethnic groups. In this review, I will discuss in details the clinical and molecular profile of Arab patients with PA.

Proposed guidelines for the diagnosis and management of methylmalonic and propionic acidemia

Methylmalonic and propionic acidemia (MMA/PA) are inborn errors of metabolism characterized by accumulation of propionic acid and/or methylmalonic acid due to deficiency of methylmalonyl-CoA mutase (MUT) or propionyl-CoA carboxylase (PCC). MMA has an estimated incidence of ~ 1: 50,000 and PA of ~ 1:100’000 -150,000. Patients present either shortly after birth with acute deterioration, metabolic acidosis and hyperammonemia or later at any age with a more heterogeneous clinical picture, leading to early death or to severe neurological handicap in many survivors. Mental outcome tends to be worse in PA and late complications include chronic kidney disease almost exclusively in MMA and cardiomyopathy mainly in PA. Except for vitamin B₁₂ responsive forms of MMA the outcome remains poor despite the existence of apparently effective therapy with a low protein diet and carnitine. This may be related to under recognition and delayed diagnosis due to nonspecific clinical presentation and insufficient awareness of health care professionals because of disease rarity.

These guidelines aim to provide a trans-European consensus to guide practitioners, set standards of care and to help to raise awareness. To achieve these goals, the guidelines were developed using the SIGN methodology by having professionals on MMA/PA across twelve European countries and the U.S. gather all the existing evidence, score it according to the SIGN evidence level system and make a series of conclusive statements supported by an associated level of evidence. Although the degree of evidence rarely exceeds level C (evidence from non-analytical studies like case reports and series), the guideline should provide a firm and critical basis to guide practice on both acute and chronic presentations, and to address diagnosis, management, monitoring, outcomes, and psychosocial and ethical issues. Furthermore, these guidelines highlight gaps in knowledge that must be filled by future research. We consider that these guidelines will help to harmonize practice, set common standards and spread good practices, with a positive impact on the outcomes of MMA/PA patients.

Effects of adeno-associated virus serotype and tissue-specific expression on circulating biomarkers of propionic acidemia

Propionic acidemia (PA) is an autosomal recessive inborn error of metabolism caused by deficiency of propionyl-CoA carboxylase (PCC). This enzyme is composed of six PCCA and six PCCB subunits and mediates a critical step in catabolism of odd chain fatty acids and certain amino acids. Current treatment options for PA are limited to stringent dietary restriction of protein consumption and some patients undergo elective liver transplantation. We previously generated a hypomorphic model of PA, designated Pcca(-/-)(A138T), with 2% of wild-type enzyme activity that mimics many aspects of the human disease. In this study, we used the differing tissue tropisms of adeno-associated virus (AAV) to probe the ability of liver or muscle-directed gene therapy to treat systemic aspects of this disease that affects many cell types. Systemic therapy with muscle-biased AAV1, liver-biased AAV8, and broadly tropic AAVrh10 mediated significant biochemical corrections in circulating propionylcarnitine (C3) and methyl citrate by all vectors. The innate tissue bias of AAV1 and AAV8 gene expression was made more specific by the use of muscle-specific muscle creatine kinase (specifically MCK6) and hepatocyte-specific transthyretin (TTR) promoters, respectively. Under these targeted conditions, both vectors mediated significant long-term correction of circulating metabolites, demonstrating that correction of muscle and likely other tissue types in addition to liver is necessary to fully correct pathology caused by PA. Liver-specific AAV8-TTR-PCCA mediated better correction than AAV1-MCK-PCCA. These data suggest that targeted gene therapy may be a viable alternative to liver transplantation for PA. They also demonstrate the effects of tissue-specific and broad gene therapy on a cell autonomous systemic genetic disease.

Liver transplantation for pediatric metabolic disease

Liver transplantation (LTx) was initially developed as a therapy for liver diseases known to be associated with a high risk of near-term mortality but is based upon a different set of paradigms for inborn metabolic diseases. As overall outcomes for the procedure have improved, LTx has evolved into an attractive approach for a growing number of metabolic diseases in a variety of clinical situations. No longer simply life-saving, the procedure can lead to a better quality of life even if not all symptoms of the primary disorder are eliminated. Juggling the risk-benefit ratio thus has become more complicated as the list of potential disorders amenable to treatment with LTx has increased. This review summarizes presentations from a recent conference on metabolic liver transplantation held at the Children's Hospital of Pittsburgh of UPMC on the role of liver or hepatocyte transplantation in the treatment of metabolic liver disease.

Clinical spectrum of propionic acidaemia

Objectives. To evaluate the clinical features, physical findings, diagnosis, and laboratory parameters of the patients with propionic acidaemia (PA). Methods. The records of diagnosed cases of propionic acidaemia were reviewed, retrospectively. Results. Twenty-six patients with PA had 133 admissions. The majority (85%) of the patients exhibited clinical manifestations in the 1st week of life. Regarding clinical features, lethargy, fever, poor feeding, vomiting, dehydration, muscular hypotonia, respiratory symptoms, encephalopathy, disturbance of tone and reflexes, and malnutrition were observed in 51–92% admissions. Metabolic crises, respiratory diseases, hyperammonaemia, metabolic acidosis, hypoalbuminaemia, and hypocalcaemia were observed in 30–96% admissions. Pancytopenia, ketonuria, hypoproteinemia, hypoglycaemia, and mildly disturbed liver enzymes were found in 12–41% admissions. Generalised brain oedema was detected in 17% and cerebral atrophy in 25% admissions. Gender-wise odd ratio analysis showed value of 1.9 for lethargy, 1.99 for respiratory diseases, 0.55 for anaemia, and 1.82 for hypocalcaemia. Conclusion. Propionic acidaemia usually presents with wide spectrum of clinical features and disturbances of laboratory parameters in early neonatal age. It is associated with significant complications which deteriorate the patients' quality of life. Perhaps with early diagnosis of the disease and in time intervention, these may be preventable.

Intractable metabolic acidosis in a child with propionic acidemia undergoing liver transplantation -a case report-

Propionic acidemia (PA) is a rare autosomal recessive disorder of metabolism caused by deficient activity of the mitochondrial enzyme propionyl-CoA carboxylase. The clinical manifestations are metabolic acidosis, poor feeding, lethargy, vomiting, osteoporosis, neurological dysfunction, pancytopenia, developmental retardation and cardiomyopathy. Liver transplantation has recently been considered as one of the treatment options for patients with PA. This case report describes several anesthetic considerations for patients with PA undergoing liver transplantation. Understanding the patient's status and avoiding events that may precipitate metabolic acidosis are important for anesthetic management of patients with PA. In conclusion, anesthesia should be focused on minimizing the severity of metabolic acidosis with following considerations: (1) maintaining optimal tissue perfusion by avoiding hypotension, (2) preventing hypoglycemia, and (3) providing bicarbonate to compensate for the acidosis.

Generation of a hypomorphic model of propionic acidemia amenable to gene therapy testing

Propionic acidemia (PA) is a recessive genetic disease that results in an inability to metabolize certain amino acids and odd-chain fatty acids. Current treatment involves restricting consumption of these substrates or liver transplantation. Deletion of the Pcca gene in mice mimics the most severe forms of the human disease. Pcca(-) mice die within 36 hours of birth, making it difficult to test intravenous systemic therapies in them. We generated an adult hypomorphic model of PA in Pcca(-) mice using a transgene bearing an A138T mutant of the human PCCA protein. Pcca(-/-)(A138T) mice have 2% of wild-type PCC activity, survive to adulthood, and have elevations in propionyl-carnitine, methylcitrate, glycine, alanine, lysine, ammonia, and markers associated with cardiomyopathy similar to those in patients with PA. This adult model allowed gene therapy testing by intravenous injection with adenovirus serotype 5 (Ad5) and adeno-associated virus 2/8 (AAV8) vectors. Ad5-mediated more rapid increases in PCCA protein and propionyl-CoA carboxylase (PCC) activity in the liver than AAV8 and both vectors reduced propionylcarnitine and methylcitrate levels. Phenotypic correction was transient with first generation Ad whereas AAV8-mediated long-lasting effects. These data suggest that this PA model may be a useful platform for optimizing systemic intravenous therapies for PA.

Propionic acidemia: clinical course and outcome in 55 pediatric and adolescent patients

Background

Propionic acidemia is an inherited disorder caused by deficiency of propionyl-CoA carboxylase. Although it is one of the most frequent organic acidurias, information on the outcome of affected individuals is still limited.

Study design/methods

Clinical and outcome data of 55 patients with propionic acidemia from 16 European metabolic centers were evaluated retrospectively. 35 patients were diagnosed by selective metabolic screening while 20 patients were identified by newborn screening. Endocrine parameters and bone age were evaluated. In addition, IQ testing was performed and the patients’ and their families’ quality of life was assessed.

Results

The vast majority of patients (>85%) presented with metabolic decompensation in the neonatal period. Asymptomatic individuals were the exception. About three quarters of the study population was mentally retarded, median IQ was 55. Apart from neurologic symptoms, complications comprised hematologic abnormalities, cardiac diseases, feeding problems and impaired growth. Most patients considered their quality of life high. However, according to the parents’ point of view psychic problems were four times more common in propionic acidemia patients than in healthy controls.

Conclusion

Our data show that the outcome of propionic acidemia is still unfavourable, in spite of improved clinical management. Many patients develop long-term complications affecting different organ systems. Impairment of neurocognitive development is of special concern. Nevertheless, self-assessment of quality of life of the patients and their parents yielded rather positive results.

Propionic acidemia: neonatal versus selective metabolic screening

BACKGROUND

Whereas propionic acidemia (PA) is a target disease of newborn screening (NBS) in many countries, it is not in others. Data on the benefit of NBS for PA are sparse.

STUDY DESIGN

Twenty PA patients diagnosed through NBS were compared to 35 patients diagnosed by selective metabolic screening (SMS) prompted by clinical findings, family history, or routine laboratory test results. Clinical and biochemical data of patients from 16 metabolic centers in Germany, Austria, and Switzerland were evaluated retrospectively. Additionally, assessment of the intelligent quotient (IQ) was performed. In a second step, the number of PA patients who have died within the past 20 years was estimated based on information provided by the participating metabolic centers.

RESULTS

Patients diagnosed through NBS had neither a milder clinical course regarding the number of metabolic crises nor a better neurological outcome. Among NBS patients, 63% were already symptomatic at the time of diagnosis, and <10% of all patients remained asymptomatic. Among all PA patients, 76% were found to be at least mildly mentally retarded, with an IQ <69. IQ was negatively correlated with the number of metabolic decompensations, but not simply with the patients' age. Physical development was also impaired in the majority of patients. Mortality rates tended to be lower in NBS patients compared with patients diagnosed by SMS.

CONCLUSION

Early diagnosis of PA through NBS seems to be associated with a lower mortality rate. However, no significant benefit could be shown for surviving patients with regard to their clinical course, including the number of metabolic crises, physical and neurocognitive development, and long-term complications.

Amino acid metabolism in patients with propionic acidaemia

Propionic acidaemia (PA) is an inborn error of intermediary metabolism caused by deficiency of propionyl-CoA carboxylase. The metabolic block leads to a profound failure of central metabolic pathways, including the urea and the citric acid cycles. This review will focus on changes in amino acid metabolism in this inborn disorder of metabolism. The first noted disturbance of amino acid metabolism was hyperglycinaemia, which is detectable in nearly all PA patients. Additionally, hyperlysinaemia is a common observation. In contrast, concentrations of branched chain amino acids, especially of isoleucine, are frequently reported as decreased. These non-proportional changes of branched-chain amino acids (BCAAs) compared with aromatic amino acids are also reflected by the Fischer's ratio (concentration ratio of BCAAs to aromatic amino acids), which is decreased in PA patients. As restricted dietary intake of valine and isoleucine as precursors of propionyl-CoA is part of the standard treatment in PA, decreased plasma concentrations of BCAAs may be a side effect of treatment. The concentration changes of the nitrogen scavenger glutamine have to be interpreted in the light of ammonia levels. In contrast to other hyperammonaemic syndromes, in PA plasma glutamine concentrations do not increase in hyperammonaemia, whereas CSF glutamine concentrations are elevated. Despite lactic acidaemia in PA patients, hyperalaninaemia is only rarely reported. The mechanisms underlying the observed changes in amino acid metabolism have not yet been elucidated, but most of the changes can be at least partly interpreted as consequence of disturbance of anaplerosis.

Natural history of propionic acidemia

Propionic acidemia is an organic acidemia that can lead to metabolic acidosis, coma and death, if not treated appropriately in the acute setting. Recent advancements in treatment have allowed patients with propionic acidemia to live beyond the neonatal period and acute presentation. The natural history of the disease is just beginning to be elucidated as individuals reach older ages. Recent studies have identified the genomic mutations in the genes PCCA and PCCB. However, as of yet no clear genotype-phenotype correlations are known. As patients age, the natural progression of propionic acidemia illuminates intellectual difficulties, increased risk for neurological complications, including stroke-like episodes, cardiac complications, and gastrointestinal difficulties, as well as a number of other complications. This article reviews the available literature for the natural history of propionic acidemia.

Cardiomyopathies in propionic aciduria are reversible after liver transplantation

OBJECTIVE

To evauluate the relationship between propionic acidemia (PA) and cardiomyopathy.

STUDY DESIGN

We retrospectively compared clinical and metabolic results of patients with PA with and without cardiomyopathy.

RESULTS

Of 26 patients with PA who survived the first year of age, a dilated cardiomyopathy developed in 6 (group 1) at a median age of 7 years (range, 5-11 years). They were compared with 14 patients without cardiomyopathy for whom data were available (group 2). Their median age at the time of the study was 11 years (range, 3-21 years). PA was diagnosed in the neonatal period in 5 of 6 patients in group 1 and 11 of 14 patients in group 2. All patients received similar medical treatment. Two patients in group 1 died of cardiac arrest. In 2 patients, the cardiomyopathy was reversed during the year after orthotopic liver transplantation (OLT). In 2 other patients, OLT was contraindicated because of severe heart disease. The number of metabolic distress episodes was similar in both groups. Excretion of propionate metabolites in urine did not correlate with the occurrence of cardiomyopathy.

CONCLUSION

Dilated cardiomyopathy, a frequent complication of PA, develops independent of any specific metabolic profile and is reversible after OLT.

Changes in plasma amino acid concentrations with increasing age in patients with propionic acidemia

The objective of the study is to analyze plasma amino acid concentrations in propionic acidemia (PA) for the purpose of elucidating possible correlations between propionyl-CoA carboxylase deficiency and distinct amino acid behavior. Plasma concentrations of 19 amino acids were measured in 240 random samples from 11 patients (6 families) with enzymatically and/or genetically proven propionic acidemia (sampling period, January 2001-December 2007). They were compared with reference values from the literature and correlated with age using the Pearson correlation coefficient test. Decreased plasma concentrations were observed for glutamine, histidine, threonine, valine, isoleucine, leucine, phenylalanine and arginine. Levels of glycine, alanine and aspartate were elevated, while values of serine, asparagine, ornithine and glutamate were normal. For lysine, proline and methionine a clear association was not possible. Significant correlations with age were observed for 13 amino acids (positive correlation: asparagine, glutamine, proline, alanine, histidine, threonine, methionine, arginine; negative correlation: leucine, phenylalanine, ornithine, glutamate and aspartate). This study gives new insight over long-term changes in plasma amino acid concentrations and may provide options for future therapies (e.g., substitution of anaplerotic substances) in PA patients.

Long-term needs of adult patients with organic acidaemias: outcome and prognostic factors

BACKGROUND

With improvements in the treatment of children with organic acidaemias (OA), the number surviving to adulthood is increasing. To plan appropriate services for their care it is important to know what their needs are.

OBJECTIVE

To describe the clinical and social problems affecting adult patients with OA.

PATIENTS AND METHODS

We reviewed the medical records of 15 adult patients diagnosed with OA. Social attainment (housing, schooling and occupation) was analysed. Nutritional status was evaluated by body mass index (BMI) and laboratory studies. Neurological and visceral complications were noted. Cognitive outcome was evaluated by psychometric testing and/or educational attainment.

RESULTS

Seven had methylmalonic acidaemia (MMA), 4 isovaleric acidaemia (IVA) and 4 propionic acidaemia (PA). Ten were female, and median age was 23.5 years (range 18-48). All but three had late-onset disease. Two patients became pregnant during follow up. Four patients had obtained university degrees and were working. Three-quarters of the patients required some kind of social support. All had a good nutritional status. Height was normal in IVA and 3 PA patients. Osteoporosis was present in 2 out of 8 patients assessed. A variety of neurocognitive or visceral complications were seen in two-thirds of the patients. Metabolic decompensations were unusual.

CONCLUSIONS

The approach to adult patients with OA has to be multidisciplinary, with the clinician and dietician as the core of the team, but with the collaboration of clinical nurses specialists, social workers and other specialist services and the support of a biochemical and molecular laboratory.

[Propionic acidemia and sensorineural hearing loss: is there a connection at the molecular genetics level?]

CURRENT KNOWLEDGE

Propionic acidemia is caused by a gene defect leading to malfunction of the enzyme propionyl-CoA carboxylase (PCC) and in turn to a pathologic accumulation of propionic acid. Many mutations have been found at the molecular genetic level over the past 20 years, and their implications for the limitation of enzyme activity of PCC in propionic acidemia are discussed.

SCIENTIFIC QUESTION AND AIMS OF THE STUDY

As an elevated incidence of deafness has been observed in patients with propionic acidemia, the question arises of whether mutations primarily responsible for this disease could also be the underlying cause for a genetic form of deafness.

METHODS AND RESULTS

As well as a standard pure tone audiogram, a pedigree was elaborated and DNA isolated for each family concerned. In one family several subjects displayed mutations of both the PCCA and the PCCB -subunits; these included only one girl whose phenotype was affected, however.

CONCLUSIONS

Mutation of the PCCB subunit p.R113X has not previously been mentioned in the literature. According to our present knowledge no connection can be assumed between either of the two mutations and the severe sensorineural hearing loss.

Neurodegeneration and chronic renal failure in methylmalonic aciduria--a pathophysiological approach

In the last decades the survival of patients with methylmalonic aciduria has been improved. However, the overall outcome of affected patients remains disappointing. The disease course is often complicated by acute life-threatening metabolic crises, which can result in multiple organ failure or even death, resembling primary defects of mitochondrial energy metabolism. Biochemical abnormalities during metabolic derangement, such as metabolic acidosis, ketonaemia/ketonuria, lactic acidosis, hypoglycaemia and hyperammonaemia, suggest mitochondrial dysfunction. In addition, long-term complications such as chronic renal failure and neurological disease are frequently found. Neuropathophysiological studies have focused on various effects caused by accumulation of putatively toxic organic acids, the so-called 'toxic metabolite' hypothesis. In previous studies, methylmalonate (MMA) has been considered as the major neurotoxin in methylmalonic aciduria, whereas more recent studies have highlighted a synergistic inhibition of mitochondrial energy metabolism (pyruvate dehydrogenase complex, tricarboxylic acid cycle, respiratory chain, mitochondrial salvage pathway of deoxyribonucleoside triphosphate (dNTP)) induced by propionyl-CoA, 2-methylcitrate and MMA as the key pathomechanism of inherited disorders of propionate metabolism. Intracerebral accumulation of toxic metabolites ('trapping' hypothesis') is considered a biochemical risk factor for neurodegeneration. Secondary effects of mitochondrial dysfunction, such as oxidative stress and impaired mtDNA homeostasis, contribute to pathogenesis of these disorders. The underlying pathomechanisms of chronic renal insufficiency in methylmalonic acidurias are not yet understood. We hypothesize that renal and cerebral pathomechanisms share some similarities, such as an involvement of dicarboxylic acid transport. This review aims to give a comprehensive overview on recent pathomechanistic concepts for methylmalonic acidurias.

Auxiliary liver transplantation for propionic acidemia: a 10-year follow-up

Orthotopic liver transplantation (OLT) is an established treatment for patients with liver-based metabolic disorders that produce structural and functional impairment. Auxiliary liver transplantation (ALT) has been proposed as an alternative approach due to the potential advantage of preserving the native liver that could be used for future gene therapy and also serves as a back-up should the graft fail. The aim of our study was to determine if ALT has the long-term potential to correct the underlying abnormality in propionic acidemia (PA). A retrospective analysis was performed on graft function, metabolic parameters and effects on development in a child who underwent ALT for PA at our institute. The clinical and biochemical parameters are near normal with no diet restrictions and with good graft survival. A normal growth and an acceptable neurological and psychomotor development were achieved in the child. ALT is feasible and provides adequate liver mass to prevent metabolic decompensation in PA.

Prolonged QTc intervals and decreased left ventricular contractility in patients with propionic acidemia

OBJECTIVE

To investigate electrophysiological and functional signs of myocardial damage in patients with propionic acidemia (PA), an inborn error of metabolism caused by deficiency of propionyl CoA carboxylase (PCC).

STUDY DESIGN

In an observational longitudinal study 10 patients with PA (6 boys and 4 girls) ranging between 2.5 and 20.2 (median 9.0) years of age at last follow-up were investigated over a period of up to 20 (mean 7.4) years using 12-lead electrocardiograms (ECGs), 24-hour continuous ECG recordings, bicycle exercise testings, and echocardiography with special focus on repolarization abnormalities such as corrected QT interval (QTc) prolongation, ventricular dysrhythmias, and left ventricular systolic function.

RESULTS

QTc interval was prolonged (>440 ms) in 70% of patients beyond infanthood. Continuous ECG recordings revealed rhythm disturbances in 20% of patients. M-mode echocardiographic left ventricular function was reduced (fractional shortening [FS] <30%) in 40%. One patient showed signs of dilated cardiomyopathy.

CONCLUSIONS

The majority of patients with PA (even in clinically stable situations) have disturbances in cardiac electrophysiology that can contribute to cardiac complications. Possible mechanisms include effects of toxic metabolites or deprivation of essential substrates. To avoid life-threatening complications, we recommend regular cardiological evaluations in this group of patients.

Evaluation and management of patients with propionic acidemia undergoing liver transplantation: a comprehensive review

Propionic acidemia is a rare metabolic disorder that often results in episodic hyperammonemia, basal ganglia infarction, mental retardation, and cardiomyopathy. OLT has been used as a treatment for propionic acidemia, but its benefit in patients with this disease is unclear. The current study was undertaken to clarify the role of OLT in the management of this disease. The medical literature, a national registry of US OLT recipients, and a single institution liver transplant experience were reviewed for cases of OLT for propionic acidemia. Accumulated cases demonstrate that OLT has resulted in clear evidence of clinical improvement in several patients, often obviating the need for dietary restriction or other forms of medical management. OLT appears to halt the decline in neurocognitive function often associated with propionic acidemia. In total, 12 patients with propionic acidemia have undergone a total of 14 OLTs. A quantitative analysis of outcomes shows an overall patient survival rate of 72.2% at one year after OLT. In conclusion, OLT should be considered a treatment option for patients with propionic acidemia who continue to experience episodes of hyperammonemia in spite of maximal medical therapy. Early OLT may limit the development of mental retardation and/or cardiomyopathy.

Methylmalonic and propionic aciduria

Methylmalonic and propionic aciduria (PA) are the most frequent forms of branched-chain organic acidurias. These autosomal recessive disorders result from deficient activity of methylmalonyl-CoA mutase and propionyl-CoA carboxylase, respectively. Clinically, acute or chronic neurologic signs are caused by the accumulation of toxic compounds proximal to the metabolic block. Phenotype varies from severe neonatal-onset forms with high mortality and poor outcome to milder forms with a later onset. In both cases the clinical course is dominated by the risk of relapses of life-threatening episodes of metabolic decompensation and of severe organ failure. Despite improvement of treatment, the overall outcome remains disappointing with no major differences between the two diseases. The diagnosis is based on the presence of characteristic compounds in body fluids as detected by organic acid analysis in urine and acylcarnitine profile in blood. Therapy is based on low-protein high-energy diet, carnitine supplementation, and metronidazole. Some patients with methylmalonic aciduria (MMA) respond to pharmacological doses of vitamin B12. Given the poor long-term prognosis, liver transplantation has been recently attempted as an alternative therapy to conventional medical treatment to cure the underlying metabolic defect. Nevertheless, the overall experience to date does not clearly demonstrate its effectiveness in preventing further deterioration or improving survival and quality of life. The recent implementation of neonatal screening by electrospray tandem mass spectrometry has decreased early mortality and improved the short-term outcome, without changing the detection rate of both diseases in the screening population compared to clinically detected cases. However, the limited number of patients and the short duration of their follow-up do not yet permit drawing final conclusions on its effect on the long-term outcome of methylmalonic and propionic acidemia.

Liver transplantation for non-hepatotoxic inborn errors of metabolism

Hepatic-based inborn errors of metabolism are targets for treatment with liver transplantation in children, in whom the metabolic defect causes irreversible damage to the liver. However, certain metabolic defects originate with enzyme deficiencies localized in the liver but then give rise to toxic intermediates that damage extrahepatic organs without any significant compromise of general liver function. Here, the rationale of using liver transplantation to replace an organ that is functioning normally except for a specific metabolic pathway raises difficult questions about indications for transplantation, timing, amount of replacement tissue needed to correct the defect, and whether heterozygote parents are suitable living donors for liver transplantation in their affected children. This review explores these questions and others, including the role of hepatocyte transplantation, in this select group of disorders. Until the promise of specific gene or enzyme replacement therapy is realized, liver and hepatocyte transplantation offers the best chance of achieving metabolic control in these challenging patients.