Methylmalonic aciduria presenting in an adult

Hyperammonemia and lactic acidosis in adults: Differential diagnoses with a focus on inborn errors of metabolism

The adult endocrinologist may be asked to consult on a patient for unexplained biochemical disturbances that could be caused by an underlying inborn error of metabolism. A genetic disorder is generally less likely to be the cause as these disorders are individually rare, however inborn errors of metabolism are collectively not infrequent and important to consider as they may be treatable and tragic outcomes avoided. Hyperammonemia or lactic acidosis are most often secondary markers of an acquired primary disease process, but they may be a clue to the presence of a genetic disorder. Herein is presented an approach to the differential diagnosis of elevated ammonia and lactate, and a brief discussion of management for when an inborn error is diagnosed.

Lethal late onset cblB methylmalonic aciduria

OBJECTIVE

To alert the physicians to the possibility of a late-onset inborn error of metabolism in an apparently previously healthy patient with acute clinical presentation.

DESIGN

Case report.

SETTING

Pediatric unit and general intensive care unit.

PATIENT

An apparently previously healthy 12-yr-old female presented acutely with vomiting, fever, bronchopneumonia, and progressive loss of consciousness associated with ketoacidosis, hyperglycemia, and hyperammonemia. She died 3 days later with a diagnosis of insulin-dependent diabetes mellitus.

INTERVENTIONS

Intravenous hydration, glucose and insulin, mechanical ventilation.

MEASUREMENTS AND MAIN RESULTS

Organic acid analysis on a postmortem sample of aqueous humor revealed high levels of methylmalonic acid. Enzymatic studies on cultured fibroblasts were consistent with the diagnosis of cblB methylmalonic aciduria.

CONCLUSIONS

The diagnosis of cblB methylmalonic aciduria was made in a postmortem patient who died with a misdiagnosis of insulin-dependent diabetes mellitus. Unclear biochemical findings and positive family history should strongly lead to suspicion of an inborn error of metabolism in an apparently previously healthy critically ill patient.

Methylmalonyl-CoA mutase induction by cerebral ischemia and neurotoxicity of the mitochondrial toxin methylmalonic acid

Differential screening of gerbil brain hippocampal cDNA libraries was used to search for genes expressed in ischemic, but not normal, brain. The methylmalonyl-CoA mutase (MCM) cDNA was highly expressed after ischemia and showed a 95% similarity to mouse and 91% similarity to the human MCM cDNAs. Transient global ischemia induced a fourfold increase in MCM mRNA on Northern blots from both hippocampus and whole forebrain. MCM protein exhibited a similar induction on Western blots of gerbil cerebral cortex 8 and 24 hr after ischemia. Treatment of primary brain astrocytes with either the branched-chain amino acid (BCAA) isoleucine or the BCAA metabolite, propionate, induced MCM mRNA fourfold. Increased concentrations of BCAAs and odd-chain fatty acids, both of which are metabolized to propionate, may contribute to inducing the MCM gene during ischemia. Methylmalonic acid, which is formed from the MCM substrate methylmalonyl-CoA and which inhibits succinate dehydrogenase (SDH), produced dose-related cell death when injected into the basal ganglia of adult rat brain. This neurotoxicity is similar to that of structurally related mitochondrial SDH inhibitors, malonate and 3-nitropropionic acid. Methylmalonic acid may contribute to neuronal injury in human conditions in which it accumulates, including MCM mutations and B12 deficiency. This study shows that methylmalonyl-CoA mutase is induced by several stresses, including ischemia, and would serve to decrease the accumulation of an endogenous cellular mitochondrial inhibitor and neurotoxin, methylmalonic acid.

Methylmalonic acidemia with a severe chemical but benign clinical phenotype

A 5-year-old boy of West African origin had methylmalonic acidemia with a mut- enzyme phenotype, no clinical response to hydroxycobalamin, and metabolite measurements indicative of the severe form of mutase deficiency. His development, both mental and physical, was satisfactory and he had no episodes of metabolic decompensation. The explanation for the neurotoxic effects and metabolic decompensation in typical methylmalonic acidemia and the (allelic) genotype that explains this patient's phenotype are uncertain.