Plasma or serum methylmalonic acid (MMA) is a clinical biomarker used to assess functional vitamin B12 deficiency or to diagnose inborn errors of metabolism (IEM) associated with accumulation of this metabolite. MMA is synthesized from propionyl-CoA, which is generated from the breakdown of propiogenic amino acids (valine, isoleucine, methionine, threonine), nucleic acids (thymidine, uracil), and in the terminal reaction of odd-chain fatty acids oxidation (1). MMA is further metabolized to succinyl-CoA by methylmalonyl-CoA mutase (MMUT), which requires cobalamin (vitamin B12) as a cofactor. Therefore, either impaired activity of methylmalonyl-CoA mutase, for example due to biallelic pathogenic variants in the MMUT gene, or vitamin B12 deficiency can cause elevated concentrations of MMA. In fact, plasma MMA level is a more reliable test for vitamin B12 insufficiency (subclinical cobalamin deficiency) than plasma Vitamin B12 (2). The most widely used methods for measuring MMA are based on liquid chromatography-tandem mass spectrometry (LC-MS/MS), though originally gas chromatography-mass spectrometry (GC-MS) was used for a long time. The reference interval for MMA reported by most laboratories is ≤ 0.40 nmol/mL or µmol/L.
A recent clinical report of a 72-year-old male showed a case of acquired inborn error of metabolism for methyl malonyl CoA mutase deficiency (3). The patient came for a routine follow-up visit, and several laboratory tests were ordered. He had normal metabolic panel results, vitamin B12 within reference range and markedly elevated MMA of >150 µmol/L, much higher than that typically seen in a vitamin B12 deficiency. The patient’s hematocrit was in the reference interval, hemoglobin was only slightly low, and was normocytic. Given the highly abnormal level of MMA, methylmalonic acidemia, an inborn error of metabolism, was suspected; however, patient’s medical history and old age was not consistent with this possible diagnosis. Reviewing the patient’s previous medical history revealed the patient had a domino liver transplant from a donor with known methylmalonic acidemia due to methylmalonyl-CoA mutase deficiency. “Domino” liver transplant is a situation where the healthy liver from a normal donor is transplanted to a younger patient who typically has a metabolic disease and the liver from this younger patient is transplanted to older individuals.
It is interesting that the patient’s clinical findings were unremarkable despite such high MMA level. It has been suggested that the accumulation of toxic organic acid metabolites alone cannot account for the wide range of clinical features seen in organic acidurias. A recent study in Science Translational Medicine journal (4) showed that accumulation of MMA in patients with methylmalonic acidemia resulted in widespread methylmalonylation as a posttranslational modification of metabolic regulatory proteins; with mitochondrial and key cellular pathway proteins shown to be dysregulated with MMA modification. In hepatic extracts, the aberrant methylmalonylation of enzymes of the urea cycle and glycine cleavage pathway resulted in inhibition of their activity (3), which might explain hyperammonemia and hyperglycinemia often seen in these patients. The authors also demonstrated that sirtuin 5 (SIRT5), a member of deacetylase enzyme family, can remove the methylmalonyl group from enzymes and ameliorate hyperammonemia in mutant mice. Overall, this study identified methylmalonylation as a novel disease mechanism in methylmalonic acidemia and proposed SIRT5 as a potential therapy for these patients.
The negative effect of high MMA is not limited to its toxicity and to induction of aberrant posttranslational modification. Another recent study in Nature journal (5) highlighted MMA as a factor in tumor progression and metastasis. Specifically, when cultured cancer cells were exposed to sera of older individuals (>60 years of age), they acquired pro-metastatic properties and became resistant to two widely used chemotherapeutic drugs. Sera from younger donors (<30-year-old) did not have these effects on cell cultures. Metabolomic studies revealed consistently increased MMA in the sera from the aged donors. Furthermore, a global transcriptome analysis showed that MMA mediates tumor progression by modulating the transcriptional reprogramming through the transcription factor SOX4, a known metastatic marker that is abundantly expressed in aggressive cancers. Finally, injections of cancerous cells into mice with elevated circulatory MMA levels resulted in faster tumor growth, metastatic spread, and lower survival than in mice with lower MMA. Taken together, the results implicated MMA as a novel therapeutic target for advanced carcinomas.
In summary, recent studies increased our awareness of acquired inborn error of metabolism in a domino liver transplant recipient (3), expanded on the pathophysiology of methylmalonic acidemia (4) and uncovered the role of MMA in tumorigenesis (5). The link between ageing, increase in circulatory MMA and tumor progression also raises a question about MMA as a novel biomarker for ageing associated cancers. Future clinical and research studies focused on evaluating the association of MMA with various cancers should explore the utility of clinical laboratory MMA testing for cancer surveillance and prognosis.
Kundrapu S, Noguez J. Laboratory Assessment of Anemia. Adv Clin Chem. 2018; 83:197-225.
Carmel R. Biomarkers of cobalamin (vitamin B-12) status in the epidemiologic setting: a critical overview of context, applications, and performance characteristics of cobalamin, methylmalonic acid, and holotranscobalamin II. The American Journal of Clinical Nutrition, Volume 94, Issue 1, July 2011, Pages 348S–358S
Zabel KM, Fitzgerald RL. Elevated methylmalonic acid as an acquired inborn error of metabolism in a domino liver transplant recipient. Clin Chim Acta. 2022 Dec 1;5 37:74-76.
Head PE, Myung S, Chen Y, Schneller JL, Wang C, Duncan N, Hoffman P, Chang D, Gebremariam A, Gucek M, Manoli I, Venditti CP. Aberrant methylmalonylation underlies methylmalonic acidemia and is attenuated by an engineered sirtuin. Science Transl Med. 2022 May 25;14(646)
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