Investigations of Methylmalonic Acidemia and Related Disorders

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The disorders of propionyl-CoA and vitamin B12 (cobalamin) metabolism comprise a group of collectively common inherited enzymopathies in the organic acidemia category. Affected patients can display a wide range of symptoms and rate of disease progression, making delineating the natural history and designing clinical trials very challenging. In aggregate, the frequency of complications, their precipitants, long-term sequelae, optimal treatment regimens and effects of early intervention remain ill-defined for this large group of patients, who still face substantial disease associated morbidity and a poor prognosis for long-term survival. Because newborn screening for this group of disorders has become routine in the US, Australia, and many developed nations, there are growing cohorts of affected infants and children, who face a grim prognosis in the absence of disease-specific therapies. Liver, kidney, or combined organ transplantation, the only currently available treatment option for the more severely affected patients, ameliorates metabolic instability but does not provide a cure for MMA or PA patients, who remain at risk for extrahepatic disease manifestations including neurological progression, vision and hearing loss, and sequelae of transplantation such as chronic immunosuppression, organ rejection and other transplant complications. Goal 1. The characterization of patients with methylmalonic acidemia (MMA) and related disorders is accomplished via dedicated natural history studies, Clinical and Basic Investigations of Methylmalonic Acidemia and Related Disorders (ClinicalTrials.gov Identifier: NCT00078078) and Natural History, Physiology, Microbiome and Biochemistry Studies of Propionic Acidemia (ClinicalTrials.gov Identifier:NCT02890342). Through these clinical protocols, we have assembled the largest single center cohort of MMA and PA patients in the world, and continue to accumulate natural history data that will improve clinical care and help define outcomes for planned interventional trials. We have continued to to important reference textbooks (reference 1), and this past year we described an extension of a high dose hydroxocobalamin treatment study for patients with cobalamin C deficiency (reference 2). Our study demonstrates that high-dose hydroxocobalamin achieves rapid and sustainable metabolic control and an improvement in neuropsychiatric outcomes in adults with late-onset cblC disease. In an extension of this work and previous efforts, a treatment trial with high dose hydroxocobalamin for cobalamin C deficiency is planned. An EIR to enable these studies based on mouse model data has been filed (E-147-2020-0, High Concentration Methylcobalamin (Me-Cbl) Or Combination Of Methyl- And Hydroxocobalamin (Me/OH-Cbl) For The Treatment Of Cobalamin C Deficiency And Related Disorders, Sloan JL, Manoli I, Venditti CP). Other active efforts include defining the metabolic phenotype of MMA and PA patients with stable isotopes; the construction of an integrated clinical outcomes database for MMA and PA; the analysis of cardiac phenotypes in cblC (in collaboration with NHLBI colleagues); the delineation of the natural history of PA; and the identification and validation of protein biomarkers that correlate with disease severity in MMA, PA and cblC. Goal 2. Animal modeling continues to be an area of focus. We reported on a new zebrafish model of cobalamin C deficiency (cblC), the most common inborn error of intracellular cobalamin metabolism (reference 3). This recessive disorder is characterized by a failure to metabolize cobalamin into adenosyl- and methylcobalamin, which results in the biochemical perturbations of methylmalonic acidemia, hyperhomocysteinemia and hypomethioninemia, can be accompanied by a wide spectrum of clinical manifestations, including progressive blindness, and, in mice, manifests with very early embryonic lethality. We used genome editing to study the loss of mmachc function and to develop the first viable animal model of cblC deficiency. This new zebrafish model recapitulates several of the phenotypic and biochemical features of the human disorder, including ocular pathology, and shows a response to established treatments. We have also initiated a parallel effort to model lethal metabolic disorders such as MMUT MMA and propionic acidemia in zebrafish with the anticipation that the zebrafish models will be amendable to the facile testing of small molecules, possibly in a high throughput fashion. We continue to create and characterize knock-out and transgenic mouse models of disorders that afflict our patients, and have generated new knock-in mouse models of Mmut deficiency, filed EIRs (E-160-2020-0, Mmut P.G715V/ P.G715V Knock-In Methylmalonyl-CoA Mutase (Mmut) Allele For The Study Of Methylmalonic Acidemia (MMA), Schneller J, Chandler RJ, and Venditti CP; E-161-2020, Mmut P.Pro207_Lys210del/ P.Pro207_Lys210del Knock-In Methylmalonyl-CoA Mutase (Mmut) Allele For The Study Of Methylmalonic Acidemia (MMA); Schneller J, Chandler RJ, and Venditti CP; E-162-2020, Mmut P.R106C/ P.R106C Knock-In Methylmalonyl-CoA Mutase (Mmut) Allele For The Study Of Methylmalonic Acidemia (MMA), Schneller J, Chandler RJ, and Venditti CP), and the used these mice for genome editing studies. We have also begun an exploration of a new pathophysiological axis in MMA defined by aberrant acylation (E-054-2020-0, Discovery Of Aberrant Post-translational Modifications (PTMs) In Methyl- And Propionic Acidemia And The Construction Of A Novel Sirtuin SIRT Enzyme To Metabolize PTMs; Head PS, Venditti CP). These studies have involved generating new antibodies (E-132-2020, Novel Antibodies And Supporting Reagents To Detect Methylmalonyl-lysine, A New Post-transitional Modification (PTM) Related To Hereditary Methylmalonic Acidemia And Disordered Vitamin B12 Metabolism; Head P, Venditti CP), as well as a patent that protects a mutated SIRT that could represent a universal therapy for all organic acidemias, including MMA and PA (reference 7). A publication describing our results is in preparation. In the next year, we will continue to characterize the Mmut, Pcca, Pccb, and Mmachc mutant mice using genomic, proteomic and metabolomic analyses, then test new therapeutics, such as mRNA therapy, AAV gene therapy, genome editing and microbiome manipulations. Goal 3. The development, testing and enablement of new genomic therapies for the disorders under study by the section continues as another, and complementary, focus area for our animal model studies. We have continued to focus on gene therapy as treatment for methylmalonyl-CoA mutase (MMUT) deficiency, the most common and severe form of isolated MMA, and have developed next generation AAV vectors for human translation (E-167-2020-0, Novel adeno-associated viral (AAV) vectors to treat hereditary methylmalonic acidemia (MMA), Chandler RJ, Venditti CP). An AAV mediated, nuclease free genome editing approach to target albumin has been refined, a patent has been filed (reference 4), and a publication describing our results is under review. We have also developed new AAV vectors and synthetic genes to treat propionic acidemia due to PCCB mutations, and filed a patent to protect and enable this important new therapy (reference 5). We also remain leaders in the field of AAV integration, and have assisted collaborators assess hepatic tumors in GSD1A mice (reference 6). In the next year, we will refine AAV gene therapy for MMUT, PCCA, PCCB and MMACHC deficiencies and move them toward the clinic with government (NCATS) and industry partners.