Advances and challenges in the treatment of branched-chain amino/keto acid metabolic defects

Abstract

Disorders of branched-chain amino/keto acid metabolism encompass diverse entities, including maple syrup urine disease (MSUD), the 'classical' organic acidurias isovaleric acidemia (IVA), propionic acidemia (PA), methylmalonic acidemia (MMA) and, among others, rarely described disorders such as 2-methylbutyryl-CoA dehydrogenase deficiency (MBDD) or isobutyryl-CoA dehydrogenase deficiency (IBDD). Our focus in this review is to highlight the biochemical basis underlying recent advances and ongoing challenges of long-term conservative therapy including precursor/protein restriction, replenishment of deficient substrates, and the use of antioxidants and anaplerotic agents which refill the Krebs cycle. Ongoing clinical assessments of affected individuals in conjunction with monitoring of disease-specific biochemical parameters remain essential. It is likely that mass spectrometry-based 'metabolomics' may be a helpful tool in the future for studying complete biochemical profiles and diverse metabolic phenotypes. Prospective studies are needed to test the effectiveness of adjunct therapies such as antioxidants, ornithine-alpha-ketoglutarate (OKG) or creatine in addition to specialized diets and to optimize current therapeutic strategies in affected individuals. With the individual life-time risk and degree of severity being unknown in asymptomatic individuals with MBDD or IBDD, instructions regarding risks for metabolic stress and fasting avoidance along with clinical monitoring are reasonable interventions at the current time. Overall, it is apparent that carefully designed prospective clinical investigations and multicenter cohort-controlled trials are needed in order to leverage that knowledge into significant breakthroughs in treatment strategies and appropriate approaches.

Fig. 1

Pathways of branched-chain amino acid catabolism. Not all steps and intermediates are outlined. Abbreviations employed: OH, hydroxyl; BCKDH, branched-chain ketoacid dehydrogenase; IBD, isobutyryl-CoA dehydrogenase; DH, dehydrogenase; PCC, propionyl-CoA carboxylase; MHBD, 2-methyl-3-hydroxybutyryl-CoA dehydro-genase; MBD, 2-methylbutyryl-CoA dehydrogenase; IVD, isovaleryl-CoA dehydrogenase; MCC, 3-methylcrotonyl-CoA carboxylase. Metabolites that are identified in increased amounts in physiological fluids derived from patients are depicted in italics; enzyme defects discussed here are given in bold, required cofactors are presented in ellipses. Image modified from Zschocke & Hoffmann, Vademecum Metabolicum, Schattauer, Stuttgart, with permission