Investigations of Methylmalonic Acidemia and Related Disorders

National Human Genome Research Institute

Investigators

Linked publications, trials & patents

Abstract

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 methylmalonic acidemia (MMA) or propionic acidemia (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. Similarly, those with cobalamin C deficiency may have severe and early loss of vision despite medical management. Goal 1. The characterization of patients with MMA, PA 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 continue to provide guidance to the community by writing contemporary reviews on diagnosis and management of MMA and related disorders (reference 1). In the past year, we have contributed to the understanding of the natural history of the transcobalamin receptor deficiency (reference 2) and participated in the identification of the molecular basis for a new form of MMA that features severe brain manifestations (reference 3). Other active and planned efforts include: delineation of the natural history of cobalamin A deficiency; the development of integrated clinical outcomes databases for MMA, PA, and cblC deficiency; the analysis of cardiac phenotypes in cblC deficiency (in collaboration with NHLBI colleagues); defining the natural history of hearing loss in MMA; and the continued identification and validation of biomarkers that correlate with disease severity in MMA, PA and cblC deficiency. Goal 2. Animal modeling continues to be an area of focus. We have used genome editing to construct a series of new knock-in mutations in the Mmut locus to recapitulate the phenotypic and biochemical features of the human disorder, and then used the resulting mice to test three new albumin targeted therapeutic editing approaches. Two target the first intron of albumin as a safe harbor location for the insertion of a therapeutic transgene while the other relies upon a combination of nuclease-enhanced and nuclease independent targeting to replace the 5 end of albumin with methylmalonylcoa mutase. All demonstrated efficacy in the new MMA mouse models. Another report published in the past year (reference 5) has defined the mechanism of MMA using new mouse models and patient biospecimens. Specifically, we have discovered a new post-translational modification termed methylmalonylation and demonstrated that many impaired pathways in MMA, such as the urea cycle, glycine cleavage reaction, and mitochondrial DNA replication, are specifically modified by this aberrant PTM. In addition, we identified the enzyme that removes the PTMs, SIRT5, and further engineered a SIRT5 that resists inactivation. Termed superSIRT5, this rationally designed enzyme also was proven to therapeutically modulate the dysregulated pathways in MMA mice and hence could serve as a universal therapeutic for all forms of MMA, and perhaps other organic acidemias as well. We continue to create and characterize knock-out and transgenic mouse models of disorders that afflict our patients, and have generated new mouse models of Pcca, Pccb, and Mmab deficiency for use in conditional genetic and therapy studies. We have also initiated parallel efforts to model lethal metabolic disorders under study in the clinic using zebrafish, with the anticipation that the zebrafish models will be amendable to the facile testing of small molecules, possibly in a high throughput fashion. In the next year, we will continue to characterize the new animal models using genomic, proteomic and metabolomic analyses, then test new therapeutics, including AAV gene therapy, genome editing, small molecules, and microbiome manipulations. Goal 3. The development, testing and enablement of new genomic therapies for the disorders under study in the NIH clinical center continues as a dominant focus area for our section studies. We have continued to focus on gene therapy as treatment for methylmalonylcoa mutase (MMUT) deficiency, the most common and severe form of isolated MMA. Recent publications include a report demonstrating that the proprietary immunomodulatory agent ImmTOR can be used to block antibody formation against the capsid in MMA mice, and allows redosing of AAV (reference 6). We have also extended our work on to demonstrate that nuclease-free, AAV-mediated genome editing can provide an effective treatment for MMUT deficiency beyond the neonatal period (reference 7) and written a comprehensive review summarizing the state of the field for genomic therapies to treat MMA (reference 8). We have also developed next generation AAV vectors for human translation to treat propionic acidemia, PCCA type, and a patent has been filed to protect this invention (reference 9). We continue to partner with NCATS to through the Platform Vector Gene Therapies Project ( PaVeGT) where we are leading efforts to develop AAV9 gene therapy for PCCA and MMAB deficiencies. In the next year, we will continue to advance AAV gene therapy for MMUT, PCCA, and MMAB deficiencies and move them toward the clinic with industry (MMUT) and government (NCATS) partners.