Methylmalonic acidemia (MMA), one of the most common inborn errors of organic acid metabolism, is heterogeneous in etiology and clinical manifestations. Affected patients with mut, cblA, and cblB classes of MMA are medically fragile and can suffer from complications such as metabolic stroke or infarction of the basal ganglia, pancreatitis, end stage renal failure, growth impairment, osteoporosis, and developmental delay. The frequency of these complications and their precipitants remain undefined. Furthermore, current outcomes have continued to demonstrate substantial morbidity and mortality in the patient population. Increasingly, solid organ transplantation (liver, and/or kidney) has been used to treat patients. Disordered transport and intracellular metabolism of vitamin B12 produces a distinct group of disorders that feature methylmalonic acidemia as well as (hyper)homocysteinemia. These conditions are named after the corresponding cellular complementation class (cblC, cblD, cblF, cblJ and cblX) and are also heterogeneous, clinically and biochemically. The genetic disorders underlying cblE and cblG feature an isolated impairment of the activity of methionine synthase, a critical enzyme involved in the conversion of homocysteine to methionine and these disorders feature (hyper)homocysteinemia. Lastly, a group of patients can have increased methylmalonic acid and/or homocysteine and/or malonic acid in the blood or urine caused by variant(s) in recently identified (ACSF3) and unknown genes.

In this protocol, we will clinically evaluate patients with methylmalonic acidemia and cobalamin metabolic defects. Routine inpatient admissions and outpatient evaluations will last up to 4-5 days and involve urine collection, blood drawing, ophthalmological examination, radiological procedures, MRI/MRS, skin biopsies in some, and developmental testing.

The study objectives will be to further delineate the spectrum of phenotypes and characterize the natural history of these enzymopathies, query for genotype/enzymatic/phenotype correlations, search for new genetic causes of methylmalonic acidemia and/or homocysteinemia, identify new disease biomarkers and define clinical outcome parameters for future clinical trials. The population will consist of participants previously evaluated at NIH, physician referrals, and families directed to the study from clinicaltrials.gov as well as the Organic Acidemia Association, Homocystinuria Network America and other national and international support groups. Most participants will be evaluated only at the NIH Clinical Center. However, if the NIH team decides that a patient under the age of 2 years is a candidate subject for this research protocol, that patient may enroll at the Children s National Medical Center (CNMC) site, pending approval by Dr. Chapman, the Principal Investigator of the CNMC location. Individuals may also enroll in the tissue collection only part of the study at the UPMC Children s Hospital of Pittsburgh or share medical history and clinical data via telemedicine visits remotely. Outcome measures will largely be descriptive and encompass correlations between clinical, biochemical and molecular parameters.