Metabolic toxicity and neurological dysfunction in methylmalonic acidemia: from mechanisms to therapeutics

Methylmalonic acidemia (MMAemia) is an inborn error of organic acid metabolism characterized by the accumulation of toxic metabolites-including methylmalonic acid (MMA), 2-methylcitric acid (2-MCA), propionic acid (PA), homocysteine (Hcy), ammonia, and lactate-due to defects in methylmalonyl-CoA mutase or impaired cobalamin metabolism. These metabolites exert profound effects on the central nervous system, contributing to neurological injury through tightly interconnected mechanisms, including mitochondrial dysfunction, neuroinflammation, and excitotoxicity. This review synthesizes current evidence on how these metabolites trigger neurological dysfunction, integrating findings from clinical studies, animal models, and cellular systems. We also highlight the increasingly recognized role of aberrant post-translational modifications (e.g., methylmalonylation, propionylation, lactylation) in disrupting metabolic network architecture and reprogramming cellular metabolism. Despite advances in supportive therapies, intracerebral metabolite accumulation remains a therapeutic challenge. We discuss emerging strategies targeting mitochondrial protection, redox homeostasis, and inflammation-including enzyme replacement, gene therapy, antioxidant regimens, and exosome-based delivery. A deeper mechanistic understanding of metabolite-driven neurotoxicity is critical to the development of targeted interventions that can improve neurological outcomes in MMAemia.