Residency · Residency · Medical Genetics Genomics

Organic Acidemias: Methylmalonic and Propionic Acidemia

Overview

Organic acidemias (organic acidurias) are a group of inborn errors of metabolism characterized by accumulation of organic acids in blood and urine. Methylmalonic acidemia (MMA) and propionic acidemia (PA) are the two most clinically significant organic acidemias, both involving the propionate catabolic pathway (branched-chain amino acid catabolism). Both follow autosomal recessive inheritance, present with acute metabolic crises (neonatal or later onset), and carry risk of chronic multisystem complications. They are detected by newborn screening via elevated propionylcarnitine (C3) on acylcarnitine profile.

Biochemical Pathway

Propionate Metabolism

Isoleucine, valine, methionine, threonine, odd-chain fatty acids, and cholesterol side chains are catabolized to propionyl-CoA. Propionyl-CoA carboxylase (PCC) converts propionyl-CoA to D-methylmalonyl-CoA -- deficiency of PCC causes propionic acidemia. D-methylmalonyl-CoA is racemized to L-methylmalonyl-CoA, which is then converted to succinyl-CoA by methylmalonyl-CoA mutase (MCM), requiring adenosylcobalamin (vitamin B12) as a cofactor -- deficiency of MCM causes methylmalonic acidemia. Succinyl-CoA enters the TCA cycle.

Toxic Metabolites

In PA, propionic acid, methylcitrate, 3-hydroxypropionate, and propionylglycine accumulate. In MMA, methylmalonic acid accumulates along with the same upstream metabolites. These organic acids inhibit gluconeogenesis, ureagenesis, and mitochondrial energy metabolism. Secondary hyperammonemia occurs through inhibition of N-acetylglutamate synthase, reducing carbamyl phosphate synthetase activity. Mitochondrial toxicity contributes to cardiomyopathy, renal disease, and neurological complications.

<image>Diagram of the propionate catabolic pathway showing the enzymatic steps from branched-chain amino acid catabolism through propionyl-CoA, methylmalonyl-CoA, to succinyl-CoA, with the sites of enzyme deficiency in PA and MMA highlighted</image>

Propionic Acidemia (PA)

Genetics

PA results from deficiency of propionyl-CoA carboxylase, a biotin-dependent mitochondrial enzyme composed of alpha (PCCA, 13q32) and beta (PCCB, 3q22) subunits arranged as a heterododecamer. Inheritance is autosomal recessive with an incidence of approximately 1 in 100,000-150,000. PCCB mutations are more common overall, with specific mutations varying by population.

Clinical Presentation

Neonatal onset (most common) presents with poor feeding, vomiting, lethargy, and hypotonia progressing to coma within the first week of life. Late-onset forms present with recurrent episodes of metabolic decompensation triggered by illness, fasting, or high-protein intake. Metabolic findings include severe metabolic acidosis with elevated anion gap, hyperammonemia, ketonuria, hypoglycemia, and pancytopenia.

Chronic Complications

Neurological complications encompass developmental delay, intellectual disability, movement disorders (dystonia, chorea), metabolic stroke affecting the basal ganglia, and epilepsy. Cardiac complications include dilated cardiomyopathy (25-30% of patients), prolonged QTc, and arrhythmias with sudden death risk. Hematological manifestations include pancytopenia from bone marrow suppression. Additional complications include failure to thrive, short stature, osteoporosis, and recurrent pancreatitis.

Methylmalonic Acidemia (MMA)

Genetic Subtypes

The mut0 subtype involves complete absence of MCM activity (MMUT gene, 6p12.3) and is most severe. The mut- subtype retains residual MCM activity. The cblA subtype (MMAA gene deficiency) involves adenosylcobalamin synthesis and is vitamin B12 responsive. The cblB subtype (MMAB gene deficiency) is partially B12 responsive. The cblC subtype (MMACHC gene) is the most common cobalamin defect, causing combined MMA plus homocysteinemia due to impaired cytoplasmic processing of cobalamin affecting both adenosylcobalamin and methylcobalamin. Rare subtypes include cblD, cblF, and cblJ.

Clinical Presentation

The mut0, mut-, cblA, and cblB forms present similarly to PA with neonatal or late-onset metabolic crises. cblC disease (combined MMA plus homocysteinemia) has a distinct phenotype: the early-onset form presents with severe multisystem disease in infancy including developmental delay, megaloblastic anemia, thrombocytopenia, retinal degeneration, and thromboembolic events. The late-onset form presents with cognitive decline, psychiatric symptoms, myelopathy, and thromboembolism.

Chronic Complications

In addition to PA-shared complications, MMA (especially the mut0 form) causes progressive tubulointerstitial nephritis leading to chronic kidney disease and end-stage renal disease -- this is the most important long-term complication. Optic nerve atrophy is more common in MMA than PA. Metabolic stroke with bilateral basal ganglia necrosis occurs during acute decompensation.

<image>Comparison table of propionic acidemia and methylmalonic acidemia subtypes showing genetic cause, enzyme deficiency, B12 responsiveness, key biochemical markers, and distinguishing chronic complications</image>

Diagnosis

Newborn Screening

Elevated propionylcarnitine (C3) and elevated C3/C2 ratio on acylcarnitine profile are detected, though C3 elevation is shared between PA and MMA, requiring further testing to distinguish them. Some NBS programs perform methylmalonic acid and/or methylcitric acid as second-tier markers.

Confirmatory Testing

Urine organic acids show diagnostic patterns: PA produces elevated 3-hydroxypropionate, methylcitrate, propionylglycine, and tiglylglycine; MMA produces elevated methylmalonic acid plus the PA metabolites (since propionate is upstream). Plasma amino acids show elevated glycine (secondary) and may show elevated homocysteine in cblC. Plasma total homocysteine is elevated in cblC/D/F/J forms. Serum vitamin B12 and methylmalonic acid should be checked to rule out nutritional B12 deficiency. Molecular testing (PCCA, PCCB, MMUT, MMAA, MMAB, MMACHC) has largely replaced fibroblast complementation analysis.

Acute Management of Metabolic Crisis

Emergency Protocol

Management begins with stopping protein intake immediately (though prolonged protein-free states promote catabolism and must be limited). High-calorie IV fluids (10% dextrose at 1.5 times maintenance, with intralipid if needed) suppress catabolism. Metabolic acidosis is corrected with sodium bicarbonate IV if pH is below 7.2 or bicarbonate below 12 mmol/L. Hyperammonemia is treated with sodium benzoate plus sodium phenylacetate (Ammonul) IV if ammonia exceeds 150-200 micromol/L, with hemodialysis for severe hyperammonemia (above 500 micromol/L or rapidly rising). N-carbamylglutamate (carglumic acid/Carbaglu) directly activates CPS-I and is FDA-approved for PA/MMA-associated hyperammonemia. L-carnitine IV promotes excretion of propionyl-carnitine and replenishes depleted stores. A hydroxocobalamin trial (IM) should be initiated for MMA patients until B12-responsive forms are excluded. Protein intake is gradually resumed within 24-48 hours.

Long-Term Management

Dietary Therapy

Management involves restriction of propiogenic precursors (isoleucine, valine, methionine, threonine), propiogenic amino acid-free medical formula to meet protein requirements, individualized natural protein allowance, avoidance of prolonged fasting, and emergency "sick day" protocols for intercurrent illness.

Medications and Supplements

L-carnitine (100-300 mg/kg/day) prevents secondary carnitine deficiency. Hydroxocobalamin (IM, 1 mg daily to weekly) treats B12-responsive MMA forms. Metronidazole reduces propionic acid production by gut bacteria through intermittent courses. N-carbamylglutamate may reduce baseline ammonia levels.

Organ Transplantation

Liver transplantation corrects hepatic enzyme deficiency and reduces metabolic crises. In PA, it can reduce cardiac complications and metabolic instability. In MMA mut0, liver transplant does not prevent renal deterioration because MCM is also expressed in the kidney. Combined liver-kidney transplantation is the most effective approach for MMA mut0 patients with progressive CKD. Isolated kidney transplantation for ESRD provides some metabolic benefit. Gene therapy using AAV-mediated MMUT delivery to the liver is in clinical trials.

<image>Long-term management flowchart for organic acidemias showing dietary management, medication regimen, monitoring schedule (biochemical markers, cardiac, renal, neurological), and criteria for considering organ transplantation</image>

Monitoring and Surveillance

Regular assessments include metabolic labs (plasma amino acids, acylcarnitine profile, urine organic acids, ammonia, blood gases), renal function (creatinine, cystatin C, GFR estimation especially for MMA), cardiac monitoring (echocardiogram and ECG with QTc monitoring at least annually for PA), neurodevelopmental assessments, nutritional monitoring, ophthalmologic examination (optic nerve atrophy in MMA), and neuroimaging as indicated.

Clinical Pearls

Hyperammonemia in a neonate with metabolic acidosis should raise immediate suspicion for organic acidemia, in contrast to urea cycle defects which typically present with respiratory alkalosis. N-carbamylglutamate (carglumic acid) is a critical acute therapy for hyperammonemia in organic acidemias -- it directly activates CPS-I and bypasses the NAG synthase inhibition caused by organic acid accumulation. A B12 trial should always be performed in new MMA diagnoses, as B12-responsive forms (cblA, cblB) have dramatically better prognosis with hydroxocobalamin therapy. Combined MMA plus homocysteinemia (cblC) has a distinct phenotype and treatment (hydroxocobalamin plus betaine plus methionine restriction). Liver transplant in MMA mut0 does not prevent renal deterioration, and combined liver-kidney transplant should be considered when CKD is present or progressing. Dilated cardiomyopathy and prolonged QTc are leading causes of mortality in PA, making annual cardiac surveillance mandatory. Gut flora contribute significantly to propionate production, and intermittent metronidazole courses can reduce baseline metabolite levels.

References

• Manoli I, Sloan JL, Venditti CP. "Isolated methylmalonic acidemia." In: Adam MP, et al., editors. GeneReviews. University of Washington, Seattle. Updated 2022.
• Wongkittichote P, Ah Mew N, Chapman KA. "Propionyl-CoA carboxylase -- a review." Molecular Genetics and Metabolism. 2017;122(4):145-152.
• Fraser JL, Venditti CP. "Methylmalonic and propionic acidemias: clinical management update." Current Opinion in Pediatrics. 2016;28(6):682-693.
• Haijes HA, Jans JJM, Tas SY, et al. "Pathophysiology of propionic and methylmalonic acidemias. Part 1: complications." Journal of Inherited Metabolic Disease. 2019;42(5):730-744.
• Forny P, Hörster F, Ballhausen D, et al. "Guidelines for the diagnosis and management of methylmalonic and propionic acidemia." Journal of Inherited Metabolic Disease. 2021;44(3):566-592.

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