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Organic acidemias (OA) are a group of rare autosomal recessive disorders of intermediary metabolism that result in a systemic elevation of organic acid. Despite optimal dietary and cofactor therapy, OA patients still suffer from potentially lethal metabolic instability and experience long‐term multisystemic complications. Severely affected patients can benefit from elective liver transplantation, which restores hepatic enzymatic activity, improves metabolic stability, and provides the theoretical basis for the pursuit of gene therapy as a new treatment for patients. Because of the poor outcomes reported in those with OA, especially methylmalonic and propionic acidemia, multiple gene therapy approaches have been explored in relevant animal models. Here, we review the results of gene therapy experiments performed using MMA and PA mouse models to illustrate experimental paradigms that could be applicable for all forms of OA. [ABSTRACT FROM AUTHOR]

Methylmalonic Acidemia (MMA) is a heterogenous group of inborn errors of metabolism caused by a defect in the methylmalonyl‐CoA mutase (MMUT) enzyme or the synthesis and transport of its cofactor, 5′‐deoxy‐adenosylcobalamin. It is characterized by life‐threatening episodes of ketoacidosis, chronic kidney disease, and other multiorgan complications. Liver transplantation can improve patient stability and survival and thus provides clinical and biochemical benchmarks for the development of hepatocyte‐targeted genomic therapies. Data are presented from a US natural history protocol that evaluated subjects with different types of MMA including mut‐type (N = 91), cblB‐type (15), and cblA‐type MMA (17), as well as from an Italian cohort of mut‐type (N = 19) and cblB‐type MMA (N = 2) subjects, including data before and after organ transplantation in both cohorts. Canonical metabolic markers, such as serum methylmalonic acid and propionylcarnitine, are variable and affected by dietary intake and renal function. We have therefore explored the use of the 1‐13C‐propionate oxidation breath test (POBT) to measure metabolic capacity and the changes in circulating proteins to assess mitochondrial dysfunction (fibroblast growth factor 21 [FGF21] and growth differentiation factor 15 [GDF15]) and kidney injury (lipocalin‐2 [LCN2]). Biomarker concentrations are higher in patients with the severe mut0‐type and cblB‐type MMA, correlate with a decreased POBT, and show a significant response postliver transplant. Additional circulating and imaging markers to assess disease burden are necessary to monitor disease progression. A combination of biomarkers reflecting disease severity and multisystem involvement will be needed to help stratify patients for clinical trials and assess the efficacy of new therapies for MMA. [ABSTRACT FROM AUTHOR]

Hereditary methylmalonic acidemia (MMA) caused by deficiency of the enzyme methylmalonyl‐CoA mutase (MMUT) is a relatively common and severe organic acidemia. The recalcitrant nature of the condition to conventional dietary and medical management has led to the use of elective liver and combined liver‐kidney transplantation in some patients. However, liver transplantation is intrinsically limited by organ availability, the risks of surgery, procedural and life‐long management costs, transplant comorbidities, and a remaining underlying risk of complications related to MMA despite transplantation. Here, we review pre‐clinical studies that present alternative approaches to solid organ transplantation as a treatment for MMUT MMA, including adeno‐associated viral gene addition therapy, mRNA therapy, and genome editing, with and without nuclease enhancement. [ABSTRACT FROM AUTHOR]

Methylmalonic acidemia (MMA) is a severe, and sometimes lethal, monogenic metabolic disorder in need of improved treatments. A number of new genomic therapies, which include canonical adeno-associated virus gene addition, genome editing, and systemic mRNA therapy, have shown great promise in murine models of MMA. Each approach has unique advantages and disadvantages for treating genetic disorders like MMA. This article reviews traditional viral gene therapy experiments that have provided enabling proof of concept studies in animal models, and newer approaches that may emerge as effective treatments for MMA and related disorders of organic acid metabolism. [ABSTRACT FROM AUTHOR]

Methylmalonic acidemia patients have complex rehabilitation needs that can be targeted to optimize societal independence and quality of life. Thirty-seven individuals with isolated MMA (28 mut, 5 cblA, 4 cblB), aged 2-33 years, were enrolled in a natural history study, and underwent age-appropriate clinical assessments to characterize impairments and disabilities. Neurological examination and brain imaging studies were used to document movement disorders and the presence of basal ganglia injury. A range of impairments and disabilities were identified by a team of physical medicine experts. Movement disorders, such as chorea and tremor, were common (n = 31, 83%), even among patients without evidence of basal ganglia injury. Joint hypermobility (n = 24, 69%) and pes planus (n = 22, 60%) were frequent and, in many cases, under-recognized. 23 (62%) patients required gastrostomy feedings. 18/31 patients >4 years old (58%) had difficulties with bathing and dressing. 16 of 23 school-aged patients received various forms of educational support. Five of the 10 adult patients were employed or in college; three lived independently. Unmet needs were identified in access to rehabilitation services, such as physical therapy (unavailable to 14/31), and orthotics (unavailable to 15/22). We conclude that patients with MMA are challenged by a number of functional limitations in essential activities of mobility, self-care, and learning, in great part caused by movement disorders and ligamentous laxity. Early assessment, referral, and implementation of age-appropriate rehabilitation services should significantly improve independence and quality of life. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate that human methylmalonyl-CoA mutase (MUT), delivered using an AAV serotype 8 vector, rescues the lethal phenotype displayed by mice with MMA and provides long-term phenotypic correction. In addition to defining a lower limit of effective dosing, our studies establish that neither a species barrier to mitochondrial processing nor an apparent immune response to MUT limits the murine model as an experimental platform to test the efficacy of human gene therapy vectors for MMA. [Copyright &y& Elsevier]

Methylmalonic acidemia (MMA) is a rare and severe inherited metabolic disease typically caused by mutations of the methylmalonyl-CoA mutase (MMUT) gene. Despite medical management, patients with MMA experience frequent episodes of metabolic instability, severe morbidity, and early mortality. In several preclinical studies, systemic gene therapy has demonstrated impressive improvement in biochemical and clinical phenotypes of MMA murine models. One approach uses a promoterless adeno-associated viral (AAV) vector that relies upon homologous recombination to achieve site-specific in vivo gene addition of MMUT into the last coding exon of albumin (Alb) , generating a fused Alb-MMUT transcript after successful editing. We have previously demonstrated that nuclease-free AAV mediated Alb editing could effectively treat MMA mice in the neonatal period and noted that hepatocytes had a growth advantage after correction. Here, we use a transgenic knock-out mouse model of MMA that recapitulates severe clinical and biochemical symptoms to assess the benefits of Alb editing in juvenile animals. As was first noted in the neonatal gene therapy studies, we observe that gene edited hepatocytes in the MMA mice treated as juveniles exhibit a growth advantage, which allows them to repopulate the liver slowly but dramatically by 8–10 months post treatment, and subsequently manifest a biochemical and enzymatic response. In conclusion, our results suggest that the benefit of AAV mediated nuclease-free gene editing of the Alb locus to treat MMA could potentially be therapeutic for older patients. [ABSTRACT FROM AUTHOR]

Abstract: Methylmalonic acidemia is a lethal inborn error of metabolism that causes mitochondrial impairment, multi-organ dysfunction and a shortened lifespan. Previous transmission electron microscope studies of thin sections from normal (Mut +/+) and diseased (Mut −/−) tissue found that the mitochondria appear to occupy a progressively larger volume of mutant cells with age, becoming megamitochondria. To assess changes in shape and volume of mitochondria resulting from the mutation, we carried out ion-abrasion scanning electron microscopy (IA–SEM), a method for 3D imaging that involves the iterative use of a focused gallium ion beam to abrade the surface of the specimen, and a scanning electron beam to image the newly exposed surface. Using IA–SEM, we show that mitochondria are more convoluted and have a broader distribution of sizes in the mutant tissue. Compared to normal cells, mitochondria from mutant cells have a larger surface-area-to-volume ratio, which can be attributed to their convoluted shape and not to their elongation or reduced volume. The 3D imaging approach and image analysis described here could therefore be useful as a diagnostic tool for the evaluation of disease progression in aberrant cells at resolutions higher than that currently achieved using confocal light microscopy. [Copyright &y& Elsevier]

Abstract: We have utilized Caenorhabditis elegans to study human methylmalonic acidemia. Using bioinformatics, a full complement of mammalian homologues for the conversion of propionyl-CoA to succinyl-CoA in the genome of C. elegans, including propionyl-CoA carboxylase subunits A and B (pcca-1, pccb-1), methylmalonic acidemia cobalamin A complementation group (mmaa-1), co(I)balamin adenosyltransferase (mmab-1), MMACHC (cblc-1), methylmalonyl-CoA epimerase (mce-1) and methylmalonyl-CoA mutase (mmcm-1) were identified. To verify predictions that the entire intracellular adenosylcobalamin metabolic pathway existed and was functional, the kinetic properties of the C. elegans mmcm-1 were examined. RNA interference against mmcm-1, mmab-1, mmaa-1 in the presence of propionic acid revealed a chemical phenotype of increased methylmalonic acid; deletion mutants of mmcm-1, mmab-1 and mce-1 displayed reduced 1-[14C]-propionate incorporation into macromolecules. The mutants produced increased amounts of methylmalonic acid in the culture medium, proving that a functional block in the pathway caused metabolite accumulation. Lentiviral delivery of the C. elegans mmcm-1 into fibroblasts derived from a patient with mut o class methylmalonic acidemia could partially restore propionate flux. The C. elegans mce-1 deletion mutant demonstrates for the first time that a lesion at the epimerase step of methylmalonyl-CoA metabolism can functionally impair flux through the methylmalonyl-CoA mutase pathway and suggests that malfunction of MCEE may cause methylmalonic acidemia in humans. The C. elegans system we describe represents the first lower metazoan model organism of mammalian propionate spectrum disorders and demonstrates that mass spectrometry can be employed to study a small molecule chemical phenotype in C. elegans RNAi and deletion mutants. [Copyright &y& Elsevier]