A Prospective Study of Pediatric Participants up to 16 Years of Age With Methylmalonic Acidemia (MMA) Due to Mutations in the MMUT Gene

The term "isolated methylmalonic acidemia" refers to a group of inborn errors of metabolism associated with elevated methylmalonic acid (MMA) concentration in the blood and urine that result from the failure to metabolize methylmalonyl-coenzyme A (CoA) into succinyl-CoA during propionyl-CoA metabolism in the mitochondrial matrix, without hyperhomocysteinemia or homocystinuria, hypomethioninemia, or variations in other metabolites, such as malonic acid (Manoli 2016).

Although, the diagnosis of isolated MMA can result from identification of biallelic pathogenic variants in either MCEE, MMAA, MMAB, MMADHC, or MMUT, this study will focus on better understanding clinical outcomes of participants with the MMUT biallelic mutation only.

Isolated MMA, associated with MMUT gene is the most common cause of Isolated MMA, exists as a continuous spectrum of disease depending on the amount of residual enzyme activity and clinical symptoms. Historically, the disease has been categorized as either complete (mut0) or partial (mut-) deficiency of the enzyme methylmalonyl-CoA mutase. (Kruszka 2013, Manoli 2016) In modern practice, severe and non-severe forms of MMA are described rather than describing patient subtype by genotype. This is because the phenomenon of interallelic complementation makes prediction of genotype/phenotype/enzyme activity difficult because some individuals who have two pathogenic variants can have a mut- enzymatic subtype in the compound state but a mut0 enzymatic subtype in the homozygous state (Acquaviva 2005). Severity of disease is therefore distinguished primarily by the type of genetic mutation but also the severity of symptoms. Severe MMA is often associated with mutations that completely eliminate the activity of the enzyme methylmalonyl-CoA mutase (typically mut0 form) which presents early in infancy and has more pronounced clinical features such as severe metabolic acidosis, developmental delays, and frequent metabolic crises. Non-severe MMA more frequently involves mutations that reduce but do not eliminate enzyme activity (typically mut- form) and may present later with milder features (Forny 2021).

The routine management of MMA involves strict dietary restrictions, specifically limiting certain amino acids (isoleucine, valine, methionine, threonine), carnitine supplementation, and treatment of hyperammonemia with ammonia scavengers and limiting the endogenous production of odd-chain fatty acids. Early diagnosis and prompt initiation of treatment are crucial to minimize metabolic crises, improve growth and development, and reduce neurological complications. However, despite these interventions, MMA remains a progressive and life-limiting condition. Liver and kidney transplants are the only effective treatments available for severe cases of MMA. These procedures, while offering hope come with their own set of risks, including the need for compatible donors, the risk of rejection, the requirement for long-term immunosuppression, and the possibility of other clinical complications.

Severe MMA is managed with dietary management, specific therapies and patients may receive liver transplant while patients with non-severe MMA are unlikely to receive liver transplant. Long-term without liver transplantation patients with severe MMA will develop higher rates of irreversible morbidity and mortality (Forny 2021).