Research Notes: Methylmalonic Acidemia

From U.S. National Institute of Health - Home Genetic Reference:

Methylmalonic acidemias are a group of heterogeneous disorders characterized by the accumulation of methylmalonic acid (MMA) and its byproducts because the body is unable to process certain proteins and fats (lipids) properly. The effects of methylmalonic acidemia usually appear in early infancy and vary from mild to life-threatening. Affected infants experience vomiting, dehydration, weak muscle tone (hypotonia), excessive tiredness (lethargy), and failure to gain weight and grow at the expected rate (failure to thrive). Long-term complications can include feeding problems, mental retardation, chronic kidney disease, and inflammation of the pancreas (pancreatitis). Without treatment, methylmalonic acidemia can lead to coma and death in some cases.

Methylmalonic acidemias occur in an estimated 1 in 50,000 to 100,000 people and are caused by mutations in the MMAA (chromosome 4), MMAB (chromosome 12), and MUT genes (chromosome 6). About half of methylmalonic acidemia cases are caused by mutations in the MUT gene, which provides instructions for making an enzyme called methylmalonyl CoA mutase which works with vitamin B12 to break down the amino acids isoleucine, methionine, threonine, and valine, as well as certain lipids and cholesterol. First, several chemical reactions convert the amino acids, lipids, or cholesterol to a molecule called methylmalonyl CoA. Then, working with another compound (a derivative of vitamin B12 called adenosylcobalamin or AdoCbl), methylmalonyl CoA mutase converts methylmalonyl CoA to a compound called succinyl CoA. Other enzymes break down succinyl CoA into molecules that are later used for energy. As a result, a substance called methylmalonyl CoA and other potentially toxic compounds can accumulate in the body's organs and tissues, causing the signs and symptoms of methylmalonic acidemia.

More than 150 mutations in the MUT gene have been identified in people with methylmalonic acidemia. Mutations in the MUT gene that prevent the production of any functional enzyme result in a form of the condition designated mut⁰. Mut⁰ is the most severe form of methylmalonic acidemia and has the poorest outcome. Mutations that change the structure of methylmalonyl CoA mutase but do not eliminate its activity cause a form of the condition designated mut^-. The mut^- form is typically less severe, with more variable symptoms than the mut⁰ form.

More than 20 mutations that cause methylmalonic acidemia have been identified in the MMAA gene that provides instructions for making a protein involved in the formation of adenosylcobalamin (AdoCbl) which as noted above, is necessary for normal function of methylmalonyl CoA mutase. Research indicates that the MMAA protein may play a role in one of the last steps in AdoCbl formation, the transport of vitamin B12 into mitochondria (specialized structures inside cells that serve as energy-producing centers). Additional chemical reactions then convert vitamin B12 into AdoCbl. Other studies suggest that the MMAA protein may help stabilize methylmalonyl CoA mutase and protect the enzyme from being inactivated.

More than 20 mutations that cause methylmalonic acidemia have been identified in the MMAB gene that like the MMAA, provides instructions for making a protein involved in the formation of adenosylcobalamin (AdoCbl) which as noted above, is necessary for normal function of methylmalonyl CoA mutase.

It is likely that other unidentified genes also cause methylmalonic acidemia. Methylmalonic acidemia is inherited in an autosomal recessive pattern, which means two copies of the MUT, MMAA, or MMAB gene are altered in each cell. Most often, the parents of an individual with an autosomal recessive disorder are carriers of one copy of the altered gene but do not show signs and symptoms of the disorder.

From Pediatrix:

Clinical: Because of the dependence of methylmalonyl-CoA mutase activity upon cobalamin metabolism and function, the different defects producing MMA have a similar clinical presentation. The picture of methymalonic acidemia as recurrent vomiting, dehydration, respiratory distress, muscle hypotonia, and lethargy that can lead to coma and death is often seen in the first week of life. Metabolic acidosis is pronounced. Ketoacidosis, hyperglycinemia, hypoglycemia, and hyperammonemia are often found, along with leukopenia, thrombocytopenia, and anemia. This same scenario can present later in the first month of life, manifesting as failure-to-thrive and mental retardation. All patients are reportedly susceptible to infection. A long-term complication of MMA is renal failure.

Testing: Newborns can be screened for MMA using tandem mass spectrometry analysis of a heel-stick dried blood spot specimen. The finding of elevated three-carbon acylcarnitine (C3) indicates a possible metabolic defect, either MMA or propionic acidemia. With MMA, C4-dicarboxylic acylcarnitine may be found as well. To make a diagnosis, further testing is required. Urine organic acid analysis of a patient with MMA will reveal massive elevation of Methylmalonic acid, together with precursor metabolites beta-hydroxypropionate and methylcitrate. These metabolites and others inhibit mitochondrial function. Methylmalonyl-CoA Mutase activity and cobalamin metabolism can be studied in several tissues. A trial of vitamin B12 therapy has diagnostic importance in identifying those patients who have defects in cobalamin metabolism. Prenatal diagnosis is possible by measuring methylmalonic acid in amniotic fluid or maternal urine, and by enzyme activity studies in cultured amniocytes.

Treatment: Treatment of patients with MMA involves reducing protein intake, particularly the branched-chain amino acids valine and isoleucine, along with methionine and threonine. Special formulas are commercially available for this purpose. All patients should be given a trial of cobalamin supplementation to evaluate a response, since the management of B12-responsive patients is considerably easier and the prognosis is better. Carnitine supplementation has proven beneficial. Oral antibiotics help control infections and hypothetically reduce intestinal bacteria, which produce Propionic acid that can be absorbed through the gut and contribute to methylmalonic acid production. Strict control is most crucial throughout childhood. Several older patients with mild metabolic defects are reported to function untreated.

Because the diagnosis and therapy of MMA is complex, the pediatrician is advised to manage the patient in close collaboration with a consulting pediatric metabolic disease specialist and dietician. It is recommended that parents travel with a letter of treatment guidelines from the patient's physician.

Brain. 2007 Feb 14.
SUCLA2 mutations are associated with mild methylmalonic aciduria, Leigh-like encephalomyopathy, dystonia and deafness.
Carrozzo R, Dionisi-Vici C, Steuerwald U, Lucioli S, Deodato F, Di Giandomenico S, Bertini E, Franke B, Kluijtmans LA, Meschini MC, Rizzo C, Piemonte F, Rodenburg R, Santer R, Santorelli FM, van Rooij A, Vermunt-de Koning D, Morava E, Wevers RA.
Unit of Molecular Medicine, Division of Metabolism, Bambino Gesu Children's Hospital, Rome, Italy, Department of Occupational and Public Health, Torshavn, Faroe Islands, Department of Pediatrics and Genetics, Laboratory of Pediatrics and Neurology, Radboud University, Nijmegen Medical Centre, Nijmegen, The Netherlands and Molecular Genetics Laboratory, University Children's Hospital, Hamburg, Germany.

One pedigree with four patients has been recently described with mitochondrial DNA depletion and mutation in SUCLA2 gene leading to succinyl-CoA synthase deficiency. Patients had a Leigh-like encephalomyopathy and deafness but besides the presence of lactic acidosis, the profile of urine organic acid was not reported. We have studied 14 patients with mild 'unlabelled' methylmalonic aciduria (MMA) from 11 families. Eight of the families are from the Faroe Islands, having a common ancestor, and three are from southern Italy. Since the reaction catalysed by succinyl-CoA synthase in the tricarboxylic acid (TCA) cycle represents a distal step of the methylmalonic acid pathway, we investigated the SUCLA2 gene as a candidate gene in our patients. Genetic analysis of the gene in the 14 patients confirmed the defect in all patients and led to the identification of three novel mutations (p.Gly118Arg; p.Arg284Cys; c.534 + 1G --> A). The defect could be convincingly shown at the protein level and our data also confirm the previously described mitochondrial DNA depletion. Defects in SUCLA2 can be found at the metabolite level and are defined by mildly elevated methylmalonic acid and C4-dicarboxylic carnitine concentrations in body fluids in association with variable lactic acidosis. Clinically the diagnosis should be considered in patients with early/neonatal onset encephalomyopathy, dystonia, deafness and Leigh-like MRI abnormalities mainly affecting the putamen and the caudate nuclei. The frequency of the mutated allele in the Faroese population amounted to 2%, corresponding with an estimated homozygote frequency of 1 : 2500. Our data extend knowledge on the genetic defects causing MMA. Our patients present with an early infantile Leigh-like encephalomyopathy with deafness, and later on a progressive dystonia. Mild MMA, lactic acidosis and specific abnormalities in the carnitine ester profile are the biochemical hallmarks of the disease. In view of the frequency of the mutated allele on the Faroe Islands, measures become feasible to prevent the occurrence of the disease on the islands. We confirm and extend the findings on this inborn error of metabolism in the TCA cycle that must be carefully investigated by accurate metabolite analyses.

Brain. 2007 Feb 7.
Mitochondrial encephalomyopathy with elevated methylmalonic acid is caused by SUCLA2 mutations.
Ostergaard E, Hansen FJ, Sorensen N, Duno M, Vissing J, Larsen PL, Faeroe O, Thorgrimsson S, Wibrand F, Christensen E, Schwartz M.
Department of Clinical Genetics, Department of Pediatrics, Department of Pediatrics, Hillerod Hospital, Department of Neurology, Copenhagen University Hospital Rigshospitalet, Copenhagen, Denmark, John F. Kennedy Institute, Glostrup, Denmark, Landssjukrahusid, Department of Paediatrics, Torshavn, Faroe Islands and The University Hospital of Iceland, Reykjavik, Iceland.

We have identified 12 patients with autosomal recessive mitochondrial encephalomyopathy with elevated methylmalonic acid. The disorder has a high incidence of 1 in 1700 in the Faroe Islands due to a founder effect, and a carrier frequency of 1 in 33. The symptoms comprise hypotonia, muscle atrophy, hyperkinesia, severe hearing impairment and postnatal growth retardation. Neuroimaging showed demyelination and central and cortical atrophy, including atrophy of the basal ganglia, and some of the patients fulfilled the criteria for Leigh syndrome. Urine and plasma methylmalonic acid were elevated. Homozygosity mapping with the Affymetrix 10 K array revealed a homozygous region on chromosome 13q14 harbouring the SUCLA2 gene. Mutations in SUCLA2 were recently shown to cause a similar disorder in a small Israeli family. Mutation analysis identified a novel splice site mutation in SUCLA2, IVS4 + 1G --> A, leading to skipping of exon 4. The SUCLA2 gene encodes the ATP-forming beta subunit of the Krebs cycle enzyme succinyl-CoA ligase. The hallmark of the condition, elevated methylmalonic acid, can be explained by an accumulation of the substrate of the enzyme, succinyl-CoA, which in turn leads to elevated methylmalonic acid, because the conversion of methylmalonyl-CoA to succinyl-CoA is inhibited.

J Inherit Metab Dis. 2003.
Infantile mitochondrial DNA depletion syndrome associated with methylmalonic aciduria and 3-methylcrotonyl-CoA and propionyl-CoA carboxylase deficiencies in two unrelated patients: a new phenotype of mtDNA depletion syndrome.
Yano S, Li L, Le TP, Moseley K, Guedalia A, Lee J, Gonzalez I, Boles RG.
Medical Genetics, Department of Pediatrics, Children's Hospital Los Angeles, University of Southern California, Keck School of Medicine, Los Angeles, California, USA.

Mitochondrial DNA (mtDNA) depletion refers to a quantitative defect in mtDNA and is heterogeneous with regard to causal genotypes and the associated clinical phenotypes. We report two unrelated infants with mtDNA depletion. A diagnosis of methylmalonic aciduria was initially raised in both on the basis of high urine methylmalonic acid and related organic acids and elevated propionylcarnitine and methylmalonylcarnitine. Carboxylase assay with skin fibroblasts revealed low propionyl-CoA and 3-methylcrotonyl-CoA carboxylase and normal pyruvate carboxylase activities. Quantitative Southern blot analysis of mitochondrial and nuclear DNA with muscle tissues revealed the patients' mtDNA to be depleted to 24% and 39% of normal controls. Our two patients showed multiple mitochondrial dysfunction including respiratory chain defects and deficiencies in the two nuclear DNA encoded carboxylases resulting in abnormal urine organic acids. To our knowledge, there is no obvious connection between the defective pathways other than their mitochondrial locations. These two cases may represent a new entity of mitochondrial disease that might be due to a defective common mechanism, such as assembly, maintenance and transport, affecting various mitochondrial enzymes and functions. Mitochondrial depletion should be considered in infants with atypical organic aciduria that may resemble methylmalonicaciduria, propionic acidaemia, or 3-methylcrotonyl-CoA carboxylase deficiency.

Pediatr Res. 1984 Dec.
Urinary excretion of l-carnitine and acylcarnitines by patients with disorders of organic acid metabolism: evidence for secondary insufficiency of l-carnitine.
Chalmers RA, Roe CR, Stacey TE, Hoppel CL.

Concentrations of l-carnitine and acylcarnitines have been determined in urine from patients with disorders of organic acid metabolism associated with an intramitochondrial accumulation of acyl-CoA intermediates. These included propionic acidemia, methylmalonic aciduria, isovaleric acidemia, multicarboxylase deficiency, 3-hydroxy-3-methylglutaric aciduria, methylacetoacetyl-CoA thiolase deficiency, and various dicarboxylic acidurias including glutaric aciduria, medium-chain acyl-CoA dehydrogenase deficiency, and multiple acyl-CoA dehydrogenase deficiency. In all cases, concentrations of acylcarnitines were greatly increased above normal with free carnitine concentrations ranging from undetectable to supranormal values. The ratios of acylcarnitine/carnitine were elevated above the normal value of 2.0 +/- 1.1. l-Carnitine was given to three of these patients; in each case, concentrations of plasma and urine carnitines increased accompanied by a marked increase in concentrations of short-chain acylcarnitines. These acylcarnitines have been examined using fast atom bombardment mass spectrometry in some of these diseases and have been shown to be propionylcarnitine in methylmalonic aciduria and propionic acidemia, isovalerylcarnitine in isovaleric acidemia, and hexanoylcarnitine and octanoylcarnitine in medium-chain acyl-CoA dehydrogenase deficiency. The excretion of these acylcarnitines is compatible with the known accumulation of the corresponding acyl-CoA esters in these diseases. In this group of disorders, the increased acylcarnitine/carnitine ratio in urine and plasma indicates an imbalance of mitochondrial mass action homeostasis and, hence, of acyl-CoA/CoA ratios. Despite naturally occurring attempts to increase endogeneous l-carnitine biosynthesis, there is insufficient carnitine available to restore the mass action ratio as demonstrated by the further increase in acylcarnitine excretion when patients were given oral l-carnitine.

Arch Dis Child. 1983 Nov.
Metabolic response to carnitine in methylmalonic aciduria. An effective strategy for elimination of propionyl groups.
Roe CR, Hoppel CL, Stacey TE, Chalmers RA, Tracey BM, Millington DS.

Patients with methylmalonic aciduria have an excessive intramitochondrial accumulation of acyl-coenzyme A compounds that may reduce the availability of free coenzyme A (CoA) for normal metabolic requirements, producing profound metabolic disturbances. Giving carnitine to a patient with methylmalonic aciduria produced an increase in hippurate excretion (an index of intramitochondrial adenosine triphosphate (ATP) and CoA availability), a large increase in short chain urinary acylcarnitines, and a reduction in excretion of methylmalonate and methylcitrate. These acylcarnitines were shown by fast atom bombardment and B/E linked scan mass spectrometry to be propionylcarnitine and acetylcarnitine. Carnitine acts by removing (detoxifying) propionyl groups, thereby releasing CoA and restoring ATP biosynthesis and concentrations towards normal. L-carnitine may play a central role in maintenance of mitochondrial and cellular homoeostasis in methylmalonic aciduria and propionic acidaemia. These principles may provide an approach to the treatment of this and other disorders, inherited and acquired, in which accumulation of acyl CoA metabolites results in sequestration of free CoA, thereby perturbing metabolic homoeostasis.