Summary:

Inborn errors of metabolism (IEMs) can be relatively straight forward to screen for and diagnose; these disorders however, have currently very limited options for treatment. Methylmalonic acidemia (MMA), is an IEM caused by complete or partial deficiency of a mitochondrial enzyme named methylmalonyl-CoA mutase (MUT). MMA intervention is currently limited to dietary restriction and continuous medical monitoring of the patient. When possible, liver-kidney transplantation can be considered which has been observed to ameliorate biochemical perturbations and reduce risks of death and disability. However, limitations such as lack of compatible donors and surgical complications dictate that new and efficient intervention methods are greatly needed for MMA. mRNA gene therapy is a potential therapeutic candidate for disorders similar to MMA where lack of a functional protein is the underlying cause of symptoms. For this purpose, researchers from Moderna Therapeutics, in collaboration with a team from National Human Genome Research Institute (NIH), have developed an mRNA lipid nanoparticle. This nanoparticle, manufactured using the Precision NanoSystems NanoAssemblr platform, allows for intracellular expression of MUT and thus leads to dose-dependant elimination of symptoms in two MMA murine models. Lipid nanoparticles are efficacious drug delivery systems that are commonly used for delivery nucleic acids, and specifically siRNA. mRNA delivery however, is more challenging than siRNA due to its instability and larger size. Unlike viral gene therapy methods, mRNA does not lead to host genome integration, or trigger massive immune responses and neutralizing antibodies. Lipid nanoparticles developed by An et al. can efficaciously encapsulate and shield mRNA from degradation and lead to cellular uptake and thus efficient protein expression.

In their article published in 2017 in the journal Cell, Martini et al. first electroporated fibroblasts isolated from MMA patients to transfect them with human MUT mRNA. The results indicated significantly increased expression of hMUT and its colocalization with mitochondria, indicating the proper cellular localization of expressed protein. In the mitochondrion, MUT catalyzes the isomerization of L-methylmalonyl-CoA into succinyl-CoA, and plasma concentrations of methylmalonic acid can be used as a measure of MUT activity. The hMUT mRNA was then encapsulated in lipid nanoparticles and injected intravenously for pharmacokinetics characterization. The results indicated that the LNP was highly biodegradable. Moreover, hMUT measured was 3-fold higher than endogenous murine MUT in wild type mice. hMUT mRNA lipid nanoparticle was then injected to a null MUT mouse model which recapitulates characteristics observed in severe MMA patients. This injection led to near complete elimination of plasma methylmalonic acid concentration up to 2 days. Dose titration studies indicated that at mRNA injection doses as low as 0.2 mg/kg, plasma methylmalonic acid levels remained negligible for 1 week post injection and the treated mice gained weight and were healthier overall than untreated mice. Furthermore, weekly injections of the mRNA lipid nanoparticles for 5 weeks led to survival of all mice in the hMUT mRNA treated group, whereas all the mice in the control group perished with the exception of one subject. Overall, it was found that treatment with hMUT mRNA Lipid nanoparticles reduced or eliminated biochemical and growth abnormalities in a null MUT MMA mouse model.

The hMUT mRNA lipid nanoparticle was then tested a hypomorphic Mut mutant murine model where the mice don’t show complete loss of MUT activity, similar to patients who harbour the MUT p.G717V mutation. Upon treatment, these mice indicated acute metabolic correction, similar to the results observed in the MUT null MMA model. However, the hypomorphic model was even more responsive to the treatment and sustained low plasma methylmalonic acid levels for 2 weeks after a single injection. This is hypothesized to be due to slower turn over rate of introduced MUT in the pool of mutant MUT, or that the introduced MUT stabilizes the mutant molecules and acts as a chaperone leading to the being partially functional. Toxicity studies in the hypomorphic MMA model indicated no change in liver toxicity markers upon mRNA lipid nanoparticle treatment. Also, no significant change in anti-hMUT antibody levels was observed suggesting that mRNA treatment did not lead to immune response in mice.

In brief, the mRNA lipid nanoparticle system developed by Martini et al. can be considered as an emergency treatment for MMA as they can successfully restore plasma MUT levels and help sustain it for a reasonable period of time. This study is indicative of effectiveness of mRNA therapy in disorders caused by the lack of a functional protein. mRNA is superior to viral gene therapy as in does not induce genotoxicity or severe immune responses. Lipid nanoparticles developed here provide a robust mRNA delivery system without significant liver toxicity and immunogenicity and can potentially be applied to related IEMs.

Abstract:

Isolated methylmalonic acidemia/aciduria (MMA) is a devastating metabolic disorder with poor outcomes despite current medical treatments. Like other mitochondrial enzymopathies, enzyme replacement therapy (ERT) is not available, and although promising, AAV gene therapy can be limited by pre-existing immunity and has been associated with genotoxicity in mice. To develop a new class of therapy for MMA, we generated a pseudoU-modified codon-optimized mRNA encoding human methylmalonyl-CoA mutase (hMUT), the enzyme most frequently mutated in MMA, and encapsulated it into biodegradable lipid nanoparticles (LNPs). Intravenous (i.v.) administration of hMUT mRNA in two different mouse models of MMA resulted in a 75%–85% reduction in plasma methylmalonic acid and was associated with increased hMUT protein expression and activity in liver. Repeat dosing of hMUT mRNA reduced circulating metabolites and dramatically improved survival and weight gain. Additionally, repeat i.v. dosing did not increase markers of liver toxicity or inflammation in heterozygote MMA mice.