Looking beyond the basal ganglia: the spectrum of MRI changes in methylmalonic acidaemia

Validity and reliability of the MetabQoL 1.0 and assessment of neuropsychiatric burden in organic acidemias: Reflections from Turkey

OBJECTIVES

The MetabQoL 1.0 is the first disease-specific health related quality of life (HrQoL) questionnaire for patients with intoxication-type inherited metabolic disorders. Our aim was to assess the validity and reliability of the MetabQoL 1.0, and to investigate neuropsychiatric burden in our patient population.

METHODS

Data from 29 patients followed at a single center, aged between 8 and 18 years with the diagnosis of methylmalonic acidemia (MMA), propionic acidemia (PA) or isovaleric acidemia (IVA), and their parents were included. The Pediatric Quality of Life Inventory (PedsQoL) was used to evaluate the validity and reliability of MetabQoL 1.0.

RESULTS

The MetabQoL 1.0 was shown to be valid and reliable (Cronbach's alpha: 0.64-0.9). Fourteen out of the 22 patients (63.6%) formally evaluated had neurological findings. Of note, 17 out of 20 patients (85%) had a psychiatric disorder when evaluated formally by a child and adolescent psychiatrist. The median mental scores of the MetabQoL 1.0 proxy report were significantly higher than those of the self report (p = 0.023). Patients with neonatal-onset disease had higher MetabQoL 1.0 proxy physical (p = 0.008), mental (p = 0.042), total scores (p = 0.022); and self report social (p = 0.007) and total scores (p = 0.043) than those with later onset disease.

CONCLUSIONS

This study continues to prove that the MetabQoL 1.0 is an effective tool to measure what matters in intoxication-type inherited metabolic disorders. Our results highlight the importance of clinical assessment complemented by patient reported outcomes which further expands the evaluation toolbox of inherited metabolic diseases.

Methylmalonic acidemia: Neurodevelopment and neuroimaging

Methylmalonic acidemia (MMA) is a genetic disease of abnormal organic acid metabolism, which is one of the important factors affecting the survival rate and quality of life of newborns or infants. Early detection and diagnosis are particularly important. The diagnosis of MMA mainly depends on clinical symptoms, newborn screening, biochemical detection, gene sequencing and neuroimaging diagnosis. The accumulation of methylmalonic acid and other metabolites in the body of patients causes brain tissue damage, which can manifest as various degrees of intellectual disability and severe neurological dysfunction. Neuroimaging examination has important clinical significance in the diagnosis and prognosis of MMA. This review mainly reviews the etiology, pathogenesis, and nervous system development, especially the neuroimaging features of MMA.

Cellular and computational models reveal environmental and metabolic interactions in MMUT-type methylmalonic aciduria

Methylmalonyl-coenzyme A (CoA) mutase (MMUT)-type methylmalonic aciduria is a rare inherited metabolic disease caused by the loss of function of the MMUT enzyme. Patients develop symptoms resembling those of primary mitochondrial disorders, but the underlying causes of mitochondrial dysfunction remain unclear. Here, we examined environmental and genetic interactions in MMUT deficiency using a combination of computational modeling and cellular models to decipher pathways interacting with MMUT. Immortalized fibroblast (hTERT BJ5ta) MMUT-KO (MUTKO) clones displayed a mild mitochondrial impairment in standard glucose-based medium, but they did not to show increased reliance on respiratory metabolism nor reduced growth or viability. Consistently, our modeling predicted MUTKO specific growth phenotypes only for lower extracellular glutamine concentrations. Indeed, two of three MMUT-deficient BJ5ta cell lines showed a reduced viability in glutamine-free medium. Further, growth on 183 different carbon and nitrogen substrates identified increased NADH (nicotinamide adenine dinucleotide) metabolism of BJ5ta and HEK293 MUTKO cells compared with controls on purine- and glutamine-based substrates. With this knowledge, our modeling predicted 13 reactions interacting with MMUT that potentiate an effect on growth, primarily those of secondary oxidation of propionyl-CoA, oxidative phosphorylation and oxygen diffusion. Of these, we validated 3-hydroxyisobutytyl-CoA hydrolase (HIBCH) in the secondary propionyl-CoA oxidation pathway. Altogether, these results suggest compensation for the loss of MMUT function by increasing anaplerosis through glutamine or by diverting flux away from MMUT through the secondary propionyl-CoA oxidation pathway, which may have therapeutic relevance.

Mitochondrial disease, mitophagy, and cellular distress in methylmalonic acidemia

Mitochondria—the intracellular powerhouse in which nutrients are converted into energy in the form of ATP or heat—are highly dynamic, double-membraned organelles that harness a plethora of cellular functions that sustain energy metabolism and homeostasis. Exciting new discoveries now indicate that the maintenance of this ever changing and functionally pleiotropic organelle is particularly relevant in terminally differentiated cells that are highly dependent on aerobic metabolism. Given the central role in maintaining metabolic and physiological homeostasis, dysregulation of the mitochondrial network might therefore confer a potentially devastating vulnerability to high-energy requiring cell types, contributing to a broad variety of hereditary and acquired diseases. In this Review, we highlight the biological functions of mitochondria-localized enzymes from the perspective of understanding—and potentially reversing—the pathophysiology of inherited disorders affecting the homeostasis of the mitochondrial network and cellular metabolism. Using methylmalonic acidemia as a paradigm of complex mitochondrial dysfunction, we discuss how mitochondrial directed-signaling circuitries govern the homeostasis and physiology of specialized cell types and how these may be disturbed in disease. This Review also provides a critical analysis of affected tissues, potential molecular mechanisms, and novel cellular and animal models of methylmalonic acidemia which are being used to develop new therapeutic options for this disease. These insights might ultimately lead to new therapeutics, not only for methylmalonic acidemia, but also for other currently intractable mitochondrial diseases, potentially transforming our ability to regulate homeostasis and health.

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.

Novel imaging technologies for genetic diagnoses in the inborn errors of metabolism

Many inborn errors of metabolism and genetic disorders affect the brain. The brain biochemistry may differ from that in the periphery and is not accessible by simple blood and urine sampling. Therefore, neuroimaging has proven to be a valuable tool to not only evaluate the brain structure, but also biochemistry, blood flow and function. Neuroimaging in patients with inborn errors of metabolism can include additional sequences in addition to T1 and T2-weighted imaging because in early stages, there may be no significant findings on the routine sequnces due to the lack of sensitivity or the evolution of abnormalities lags behind the ability of the imaging to detect it. In addition, findings on T1 and T2-weighted imaging of several inborn errors of metabolism may be non-specific and be seen in other non-genetic conditions. Therefore, additional neuroimaging modalities that have been employed including diffusion tensor imaging (DTI), magnetic resonance spectroscopy, functional MRI (fMRI), functional near infrared spectroscopy (fNIRS), or positron emission tomography (PET) imaging may further inform underlying changes in myelination, biochemistry, and functional connectivity. The use of Magnetic Resonance Spectroscopy in certain disorders may add a level of specificity depending upon the metabolite levels that are abnormal, as well as provide information about the process of brain injury (i.e., white matter, gray matter, energy deficiency, toxic buildup or depletion of key metabolites). It is even more challenging to understand how genetic or metabolic disorders contribute to short and/or long term changes in cognition which represent the downstream effects of IEMs. In order to image “cognition” or the downstream effects of a metabolic disorder on domains of brain function, more advanced techniques are required to analyze underlying fiber tracts or alternatively, methods such as fMRI enable generation of brain activation maps after both task based and resting state conditions. DTI can be used to look at changes in white matter tracks. Each imaging modality can explore an important aspect of the anatomy, physiology or biochemisty of the central nervous system. Their properties, pros and cons are discussed in this article. These imaging modalities will be discussed in the context of several inborn errors of metabolism including Galactosemia, Phenylketonruia, Maple syrup urine disease, Methylmalonic acidemia, Niemann-Pick Disease, type C1, Krabbe Disease, Ornithine transcarbamylase deficiency, Sjogren Larsson syndrome, Pelizeaus-Merzbacher disease, Pyruvate dehydrogenase deficiency, Nonketotic Hyperglycinemia and Fabry disease. Space constraints do not allow mention of all the disorders in which one of these modalities has been investigated, or where it would add value to diagnosis or disease progression.

Fibroblast growth factor 21 as a biomarker for long-term complications in organic acidemias

BACKGROUND

There is increasing evidence that long-term complications in organic acidemias are caused by impaired mitochondrial metabolism. Currently, there is no specific biomarker to monitor mitochondrial dysfunction in organic acidemias. Serum fibroblast growth factor 21 (FGF-21) is a biomarker for mitochondrial disease and could be a candidate to monitor mitochondrial function in the deleterious course of disease.

METHODS

Data of 17 patients with classical organic acidemias (11 propionic acidemia (PA), four methylmalonic acidemia (MMA) and two isovaleric acidemia (IVA) patients) were included. The clinical course was evaluated; metabolic decompensations and long-term complications were correlated with plasma FGF-21 levels. Cardiomyopathy, prolonged QT interval, renal failure, and optic neuropathy were defined as long-term complications.

RESULTS

Patients ages ranged from 16 months up to 32 years. Serious long-term complications occurred in eight patients (five PA and three MMA patients). In MMA and PA patients plasma FGF-21 levels during stable metabolic periods were significantly higher in patients with long-term complications (Mdn = 2556.0 pg/ml) compared to patients without (Mdn = 287.0 pg/ml). A median plasma FGF-21 level above 1500 pg/ml during a stable metabolic period, measured before the occurrence of long-term complications, had a positive predictive value of 0.83 and a negative predictive value of 1.00 on long-term complications in MMA and PA patients.

CONCLUSION

This study demonstrates the potential role of FGF-21 as a biomarker for long-term complications in classical organic acidemias, attributed to mitochondrial dysfunction.

Ammonia role in glial dysfunction in methylmalonic acidemia

Hyperammonemia is a common finding in patients with methylmalonic acidemia. However, its contribution to methylmalonate (MMA)-induced neurotoxicity is poorly understood. The aim of this study was evaluate whether an acute metabolic damage to brain during the neonatal period may disrupt cerebral development, leading to neurodevelopmental disorders, as memory deficit. Mice received a single intracerebroventricular dose of MMA and/or NH₄Cl, administered 12 hs after birth. The maze tests showed that MMA and NH₄Cl injected animals (21 and 40 days old) exhibited deficit in the working memory test, but not in the reference memory test. Furthermore, MMA and NH₄Cl increased the levels of 2',7'-dichlorofluorescein-diacetate (DCF), TNF-α, IL-1β in the cortex, hippocampus and striatum of mice. MMA and NH₄Cl also increased glial proliferation in all structures. Since the treatment of MMA and ammonia increased cytokines levels, we suggested that it might be a consequence of the glial activation induced by the acid and ammonia, leading to delay in the developing brain and contributing to behavioral alterations. However, this hypothesis is speculative in nature and more studies are needed to clarify this possibility.

Shunt surgery for early-onset severe hydrocephalus in methylmalonic acidemia: report on two cases and review of the literature

OBJECT

Methylmalonic acidemia (MMA) with early-onset severe hydrocephalus is rare. In this paper, we described two cases of MMA with hydrocephalus and review the literature to elucidate the clinical features of the disease, treatment options, and follow-up results.

METHODS

The PubMed and Embase databases were searched for clinical reports on MMA with severe hydrocephalus, and two unreported cases were presented to illustrate the clinical spectrum.

RESULTS

Six cases of MMA with severe hydrocephalus were observed in the previous literature. Our two patients with severe hydrocephalus but not bulging fontanelle received a ventriculo-peritoneal shunt, and intracranial hypertension was confirmed in both cases during the operation. These patients' clinical symptoms significantly improved after the operation.

CONCLUSIONS

Intracranial hypertension can exist in early-onset severe hydrocephalus in MMA, even if the bulging anterior fontanelle is not apparent. These patients could benefit from a ventriculo-peritoneal shunt.

Metabolic ataxias

The nervous system is vulnerable to intrinsic and extrinsic metabolic perturbations. In particular, the cerebellum, with its large Purkinje cells and its high density of neurons and glial cells, has high metabolic demand and is highly vulnerable to metabolic derangements. As a result, many disorders of intermediary metabolism will preferentially and sometimes selectively target the cerebellum. However, many of these disorders present in a multisystem fashion with ataxia being a part of the neurologic symptom complex. The presentation of these disorders depends on the time of onset and type of metabolic derangement. Early infantile or intrauterine-onset diseases will present in a young child typically with global hypotonia and both nystagmus and ataxia become more apparent later in life, while later-onset diseases usually present primarily with ataxia. It is important to note that the majority of these disorders are progressive if they are untreated. This chapter provides a review of acquired and genetic metabolic disorders that target the cerebellum, and discusses their diagnostic evaluation and therapy.

Ultrasonographic Characteristics of Cortical Sulcus Development in the Human Fetus between 18 and 41 Weeks of Gestation

Background:

Fetal brain development is a complicated process that continues throughout pregnancy. Fetal sulcus development has typical morphological features. Assessment of fetal sulcus development to understand the cortical maturation and development by prenatal ultrasound has become widespread. This study aimed to explore a reliable method to assess cortical sulcus and to describe the normal sonographic features of cortical sulcus development in the human fetus between 18 and 41 weeks of gestation.

Methods:

A cross-sectional study was designed to examine the fetal cortical sulcus development at 18–41 weeks of gestation. Ultrasound was used to examine the insula, sylvian fissure (SF), parieto-occipital fissure (POF), and calcarine fissure (CF). Bland-Altman plots were used for assessing the concordance, and the intraclass correlation coefficient was used for assessing the reliability.

Results:

SF images were successfully obtained in 100% of participants at 22 weeks of gestation, while the POF images and CF images could be obtained in 100% at 23 weeks of gestation and 24 weeks of gestation, respectively. The SF width, temporal lobe depth, POF depth, and the CF depth increased with the developed gestation. The width of uncovered insula and the POF angle decreased with the developed gestation. By 23 weeks of gestation, the insula was beginning to be covered. Moreover, it completed at 35 weeks of gestation. The intra- and inter-observer agreements showed consistent reproducibility.

Conclusions:

This study defined standard views of the fetal sulcus as well as the normal reference ranges of these sulcus measurements between 18 and 41 weeks of gestation. Such ultrasonographic measurements could be used to identify fetuses at risk of fetal neurological structural disorders.

Methylmalonate Induces Inflammatory and Apoptotic Potential: A Link to Glial Activation and Neurological Dysfunction

Methylmalonic acid (MMA) accumulates in tissues in methylmalonic acidemia, a heterogeneous group of inherited childhood diseases characterized by neurological dysfunction, oxidative stress and neuroinflammation; it is associated with degeneration of striatal neurons and cerebral cortical atrophy. It is presently unknown, however, whether transient exposure to MMA in the neonatal period is sufficient to trigger inflammatory and apoptotic processes that lead to brain structural damage. Here, newborn mice were given a single intracerebroventricular dose of MMA at 12 hours after birth. Maze testing of 21- and 40-day-old mice showed that MMA-injected animals exhibited deficit in the working memory test but not in the reference test. MMA-injected mice showed increased levels of the reactive oxygen species marker 2',7'-dichlorofluorescein diacetate, tumor necrosis factor, interleukin-1β, caspases 1, 3, and 8, and increased acetylcholinesterase activity in the cortex, hippocampus and striatum. This was associated with increased astrocyte and microglial immunoreactivity in all brain regions. These findings suggest that transient exposure to MMA may alter the redox state and cause neuroinflammatory/apoptotic processes and glial activation during critical periods of brain development. Similar processes may underlie brain dysfunction and cognitive impairment in patients with methylmalonic acidemia.

The Pediatric Cerebellum in Inherited Neurodegenerative Disorders: A Pattern-recognition Approach

Evaluation of imaging studies of the cerebellum in inherited neurodegenerative disorders is aided by attention to neuroimaging patterns based on anatomic determinants, including biometric analysis, hyperintense signal of structures, including the cerebellar cortex, white matter, dentate nuclei, brainstem tracts, and nuclei, the presence of cysts, brain iron, or calcifications, change over time, the use of diffusion-weighted/diffusion tensor imaging and T2*-weighted sequences, magnetic resonance spectroscopy; and, in rare occurrences, the administration of contrast material.

2-Methylcitric acid impairs glutamate metabolism and induces permeability transition in brain mitochondria

Accumulation of 2-methylcitric acid (2MCA) is observed in methylmalonic and propionic acidemias, which are clinically characterized by severe neurological symptoms. The exact pathogenetic mechanisms of brain abnormalities in these diseases are poorly established and very little has been reported on the role of 2MCA. In the present work we found that 2MCA markedly inhibited ADP-stimulated and uncoupled respiration in mitochondria supported by glutamate, with a less significant inhibition in pyruvate plus malate respiring mitochondria. However, no alterations occurred when α-ketoglutarate or succinate was used as respiratory substrates, suggesting a defect on glutamate oxidative metabolism. It was also observed that 2MCA decreased ATP formation in glutamate plus malate or pyruvate plus malate-supported mitochondria. Furthermore, 2MCA inhibited glutamate dehydrogenase activity at concentrations as low as 0.5 mM. Kinetic studies revealed that this inhibitory effect was competitive in relation to glutamate. In contrast, assays of osmotic swelling in non-respiring mitochondria suggested that 2MCA did not significantly impair mitochondrial glutamate transport. Finally, 2MCA provoked a significant decrease in mitochondrial membrane potential and induced swelling in Ca(2+)-loaded mitochondria supported by different substrates. These effects were totally prevented by cyclosporine A plus ADP or ruthenium red, indicating induction of mitochondrial permeability transition. Taken together, our data strongly indicate that 2MCA behaves as a potent inhibitor of glutamate oxidation by inhibiting glutamate dehydrogenase activity and as a permeability transition inducer, disturbing mitochondrial energy homeostasis. We presume that 2MCA-induced mitochondrial deleterious effects may contribute to the pathogenesis of brain damage in patients affected by methylmalonic and propionic acidemias. We propose that brain glutamate oxidation is disturbed by 2-methylcitric acid (2MCA), which accumulates in tissues from patients with propionic and methylmalonic acidemias because of a competitive inhibition of glutamate dehydrogenase (GDH) activity. 2MCA also induced mitochondrial permeability transition (PT) and decreased ATP generation in brain mitochondria. We believe that these pathomechanisms may be involved in the neurological dysfunction of these diseases.

The effect of WIN 55,212-2 suggests a cannabinoid-sensitive component in the early toxicity induced by organic acids accumulating in glutaric acidemia type I and in related disorders of propionate metabolism in rat brain synaptosomes

Several physiological processes in the CNS are regulated by the endocannabinoid system (ECS). Cannabinoid receptors (CBr) and CBr agonists have been involved in the modulation of the N-methyl-D-aspartate receptor (NMDAr) activation. Glutaric (GA), 3-hydroxyglutaric (3-OHGA), methylmalonic (MMA) and propionic (PA) acids are endogenous metabolites produced and accumulated in the brain of children affected by severe organic acidemias (OAs) with neurodegeneration. Oxidative stress and excitotoxicity have been involved in the toxic pattern exerted by these organic acids. Studying the early pattern of toxicity exerted by these metabolites is crucial to explain the extent of damage that they can produce in the brain. Herein, we investigated the effects of the synthetic CBr agonist WIN 55,212-2 (WIN) on early markers of GA-, 3-OHGA-, MMA- and PA-induced toxicity in brain synaptosomes from adult (90-day-old) and adolescent (30-day-old) rats. As pre-treatment, WIN exerted protective effects on the GA- and MMA-induced mitochondrial dysfunction, and prevented the reactive oxygen species (ROS) formation and lipid peroxidation induced by all metabolites. Our findings support a protective and modulatory role of cannabinoids in the early toxic events elicited by toxic metabolites involved in OAs.

Metabolic disturbances in diseases with neurological involvement

Degeneration of specific neuronal populations and progressive nervous system dysfunction characterize neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. These findings are also reported in inherited diseases such as phenylketonuria and glutaric aciduria type I. The involvement of mitochondrial dysfunction in these diseases was reported, elicited by genetic alterations, exogenous toxins or buildup of toxic metabolites. In this review we shall discuss some metabolic alterations related to the pathophysiology of diseases with neurological involvement and aging process. These findings may help identifying early disease biomarkers and lead to more effective therapies to improve the quality of life of the patients affected by these devastating illnesses.

HIBCH mutations can cause Leigh-like disease with combined deficiency of multiple mitochondrial respiratory chain enzymes and pyruvate dehydrogenase

Background

Deficiency of 3-hydroxy-isobutyryl-CoA hydrolase (HIBCH) caused by HIBCH mutations is a rare cerebral organic aciduria caused by disturbance of valine catabolism. Multiple mitochondrial respiratory chain (RC) enzyme deficiencies can arise from a number of mechanisms, including defective maintenance or expression of mitochondrial DNA. Impaired biosynthesis of iron-sulphur clusters and lipoic acid can lead to pyruvate dehydrogenase complex (PDHc) deficiency in addition to multiple RC deficiencies, known as the multiple mitochondrial dysfunctions syndrome.

Methods

Two brothers born to distantly related Pakistani parents presenting in early infancy with a progressive neurodegenerative disorder, associated with basal ganglia changes on brain magnetic resonance imaging, were investigated for suspected Leigh-like mitochondrial disease. The index case had deficiencies of multiple RC enzymes and PDHc in skeletal muscle and fibroblasts respectively, but these were normal in his younger brother. The observation of persistently elevated hydroxy-C4-carnitine levels in the younger brother led to suspicion of HIBCH deficiency, which was investigated by biochemical assay in cultured skin fibroblasts and molecular genetic analysis.

Results

Specific spectrophotometric enzyme assay revealed HIBCH activity to be below detectable limits in cultured skin fibroblasts from both brothers. Direct Sanger sequence analysis demonstrated a novel homozygous pathogenic missense mutation c.950G <A; p.Gly317Glu in the HIBCH gene, which segregated with infantile-onset neurodegeneration within the family.

Conclusions

HIBCH deficiency, a disorder of valine catabolism, is a novel cause of the multiple mitochondrial dysfunctions syndrome, and should be considered in the differential diagnosis of patients presenting with multiple RC deficiencies and/or pyruvate dehydrogenase deficiency.

Methylmalonic acidemia mimicking diabetic ketoacidosis in an infant

Methylmalonic acidemia (MMA) is an inherited organic acidemia usually present with recurrent episodes of acute illness. A typical episode is ushered in with ketonuria and vomiting, followed by acidosis, dehydration, and lethargy, leading, in the absence of aggressive treatment, to coma and death. We report an infant with MMA presented with diabetes symptoms. A 13-month-old girl complained of polydipsia, diuresis, and loss of weight. She had clinical signs of diabetic ketoacidosis such as dehydration, deep sighing respiration, smell of ketones, lethargy, and vomiting. Laboratory analysis showed hyperglycemia with acidosis and ketonuria. She was treated with parenteral fluid, electrolyte, and insulin infusion. Two days after her discharge, after having a meal rich in protein, she was brought unconscious with hepatomegaly, severe acidosis, ketonuria, and mild hyperammonemia. The absence of hyperglycemia and the presence of neurologic findings suggested organic acidemia. MMA was diagnosed because of methylmalonic aciduria and elevated C3 carnitine esters. Cranial magnetic resonance imaging (MRI) showed increased uptake of radiocontrast material in the basal ganglia bilaterally. A homozygous mutation in exon 4 of the MMAA gene was found in mutation analysis and confirmed the diagnosis of cblA-deficient MMA. Neurologic regression was improved with treatment of low-protein diet, vitamin B12, and l-carnitine. In patients born to consanguineous parents who admit during infancy with severe acidosis refractory to treatment, organic acidemias should be kept in mind, even they have high blood glucose. The definitive diagnosis is important because it may allow a specific treatment and a favorable evolution to prevent the sequelae.

Methylmalonate impairs mitochondrial respiration supported by NADH-linked substrates: involvement of mitochondrial glutamate metabolism

The neurodegeneration that occurs in methylmalonic acidemia is proposed to be associated with impairment of mitochondrial oxidative metabolism resulting from methylmalonate (MMA) accumulation. The present study evaluated the effects of MMA on oxygen consumption by isolated rat brain mitochondria in the presence of NADH-linked substrates (α-ketoglutarate, citrate, isocitrate, glutamate, malate, and pyruvate). Respiration supported either by glutamate or glutamate plus malate was significantly inhibited by MMA (1-10 mM), whereas no inhibition was observed when a cocktail of NADH-linked substrates was used. Measurements of glutamate transport revealed that the inhibitory effect of MMA on respiration maintained by this substrate is not due to inhibition of its mitochondrial uptake. In light of this result, the effect of MMA on the activity of relevant enzymes involved in mitochondrial glutamate metabolism was investigated. MMA had minor inhibitory effects on glutamate dehydrogenase and aspartate aminotransferase, whereas α-ketoglutarate dehydrogenase was significantly inhibited by this metabolite (K(i) = 3.65 mM). Moreover, measurements of α-ketoglutarate transport and mitochondrial MMA accumulation indicated that MMA/α-ketoglutarate exchange depletes mitochondria from this substrate, which may further contribute to the inhibition of glutamate-sustained respiration. To study the effect of chronic in vivo MMA treatment on mitochondrial function, young rats were intraperitoneally injected with MMA. No significant difference was observed in respiration between isolated brain mitochondria from control and MMA-treated rats, indicating that in vivo MMA treatment did not lead to permanent mitochondrial respiratory defects. Taken together, these findings indicate that the inhibitory effect of MMA on mitochondrial oxidative metabolism can be ascribed to concurrent inhibition of specific enzymes and lower availability of respiratory substrates.

Atypical imaging findings in the setting of methylmalonic acidemia in an infant

Classically, methylmalonic acidemia (MMA) is characterized on imaging by abnormalities in the basal ganglia, specifically the globus pallidi, as well as occasional signs of delayed maturation. We report a case of MMA in which abnormal signal and diffusion restriction occurred in the subcortical white matter, sparing the classically involved globus pallidi, a situation that has not been previously reported in the literature. This report demonstrates that diffusion abnormality can be seen in the white matter in MMA, in the absence of basal ganglia involvement, and that MMA may be considered when the diagnosis of metabolic acidemias is raised.

New frontiers in neuroimaging applications to inborn errors of metabolism

Most inborn errors of metabolism (IEMs) are associated with potential for injury to the developing central nervous system resulting in chronic encephalopathy, though the etiopathophysiology of neurological injury have not been fully established in many disorders. Shared mechanisms can be envisioned such as oxidative injury due to over-activation of N-Methyl-d-Aspartate (NMDA) receptors with subsequent glutamatergic damage, but other causes such as energy depletion or inflammation are possible. Neuroimaging has emerged as a powerful clinical and research tool for studying the brain in a noninvasive manner. Several platforms exist to study neural networks underlying cognitive processes, white matter/myelin microstructure, and cerebral metabolism in vivo. The scope and limitations of these methods will be discussed in the context of valuable information they provide in the study and management of selected inborn errors of metabolism. This review is not meant to be an exhaustive coverage of diagnostic findings on MRI in multiple IEMs, but rather to illustrate how neuroimaging modalities beyond T1 and T2 images, can add depth to an understanding of the underlying brain changes evoked by the selected IEMs. Emphasis will be placed on techniques that are available in the clinical setting. Though technically complex, many of these modalities have moved - or soon will - to the clinical arena.

Mitochondrial energy metabolism in neurodegeneration associated with methylmalonic acidemia

Methylmalonic acidemia is one of the most prevalent inherited metabolic disorders involving neurological deficits. In vitro experiments, animal model studies and tissue analyses from human patients suggest extensive impairment of mitochondrial energy metabolism in this disease. This review summarizes changes in mitochondrial energy metabolism occurring in methylmalonic acidemia, focusing mainly on the effects of accumulated methylmalonic acid, and gives an overview of the results found in different experimental models. Overall, experiments to date suggest that mitochondrial impairment in this disease occurs through a combination of the inhibition of specific enzymes and transporters, limitation in the availability of substrates for mitochondrial metabolic pathways and oxidative damage.

Experimental evidence that methylmalonic acid provokes oxidative damage and compromises antioxidant defenses in nerve terminal and striatum of young rats

Methylmalonic acidemia and propionic acidemia are organic acidemias biochemically characterized by predominant tissue accumulation of methylmalonic acid (MMA) and propionic acid (PA), respectively. Affected patients present predominantly neurological symptoms, whose pathogenesis is not yet fully established. In the present study we investigated the in vitro effects of MMA and PA on important parameters of lipid and protein oxidative damage and on the production of reactive species in synaptosomes from cerebrum of developing rats. Synaptosomes correspond to nerve terminals that have been used to investigate toxic properties of compounds on neuronal cells. The in vivo effects of intrastriatal injection of MMA and PA on the same parameters and on enzymatic antioxidant defenses, were also studied. MMA-induced in vitro and in vivo lipid peroxidation and protein oxidative damage. Furthermore, the lipid oxidative damage was attenuated or prevented, pending on the doses utilized, by the free radical scavengers α-tocopherol, melatonin and by the NMDA glutamate receptor antagonist MK-801, implying the involvement of reactive species and glutamate receptor activation in these effects. In addition, 2',7'-dichlorofluorescein diacetate oxidation was significantly increased in synaptosomes by MMA, reinforcing that reactive species generation is elicited by this organic acid. We also verified that glutathione peroxidase activity was inhibited by intrastriatal MMA injection. In contrast, PA did not induce any significant effect on all parameters examined in vitro and in vivo, implying a selective action for MMA. The present data demonstrate that oxidative stress is induced by MMA in vitro in nerve terminals and in vivo in striatum, suggesting the participation of neuronal cells in MMA-elicited oxidative damage.

Evidence for catabolic pathway of propionate metabolism in CNS: expression pattern of methylmalonyl-CoA mutase and propionyl-CoA carboxylase alpha-subunit in developing and adult rat brain

Methylmalonyl-CoA mutase (MCM) and propionyl-CoA carboxylase (PCC) are the key enzymes of the catabolic pathway of propionate metabolism and are mainly expressed in liver, kidney and heart. Deficiency of these enzymes leads to two classical organic acidurias: methylmalonic and propionic aciduria. Patients with these diseases suffer from a whole spectrum of neurological manifestations that are limiting their quality of life. Current treatment does not seem to effectively prevent neurological deterioration and pathophysiological mechanisms are poorly understood. In this article we show evidence for the expression of the catabolic pathway of propionate metabolism in the developing and adult rat CNS. Both, MCM and PCC enzymes are co-expressed in neurons and found in all regions of the CNS. Disease-specific metabolites such as methylmalonate, propionyl-CoA and 2-methylcitrate could thus be formed autonomously in the CNS and contribute to the pathophysiological mechanisms of neurotoxicity. In rat embryos (E15.5 and E18.5), MCM and PCC show a much higher expression level in the entire CNS than in the liver, suggesting a different, but important function of this pathway during brain development.

Peritoneal dialysis for chronic renal failure in a patient with methylmalonic acidaemia

Chronic renal failure is a common complication of methylmalonic acidaemia (MMA). It is usually managed with haemodialysis and renal transplantation. We report the use of continuous cycling peritoneal dialysis (CCPD) for 20 months in a paediatric patient with chronic renal failure due to MMA. This procedure resulted in the elimination of 950 micromol methylmalonate (MM) per day and a fall in the plasma MM concentration from 3.9 to 0.74 mmol/l. As a result of this treatment, the frequency at which this patient was hospitalised was markedly reduced prior to a successful renal transplantation.