Project

Discipline

vitamin B12 / cobalamin ; intracellular cobalamin metabolism; methylmalonic acid; inborn error of metabolism ; neurological disease; protein complex formation and interaction; induced pluripotent stem cells / iPSC; vascular disease; homocysteine

Lead
Vitamin B12 (Cobalamin) ist ein wasserlösliches Vitamin, das über die Nahrung aufgenommen wird und als Kofaktor für die Funktion von 2 Enzymen im Stoffwechsel von Aminosäuren unerlässlich ist. Dazu wird Vitamin B12 in der Zelle mithilfe von mehreren Proteinen in Adenosylcobalamin und Methylcobalamin umgewandelt, seine biologisch aktiven Kofaktor-Formen. Fehlen diese aufgrund einer zu geringen Aufnahme von Vitamin B12 über die Nahrung oder aufgrund eines genetischen Defekts in der Umwandlung, führt dies zu schwerwiegenden Krankheiten, die bereits beim Neugeborenen oder während der Kindheit zum Tod führen können. Je nach Art des Defekts häufen sich Methylmalonsäure und/oder Homocystein in Blut und Urin an, weshalb man die Krankheiten auch mit Methylmalonazidurie (MMA) und Homocystinurie bezeichnet.
Lay summary
Unser übergeordnetes Ziel ist, das Verständnis über die Funktionen und Interaktionen der Proteine, die zur Umwandlung von Vitamin B12 in der Zelle notwendig sind, zu vertiefen. Zu diesem Zweck werden wir i) die molekularen Mechanismen und Interaktionen des Cobalamin-Stoffwechsels determinieren und  ii) die Krankheitsmechanismen der Hirnschädigung bei MMA in einem neuronalen Zellmodell untersuchen. Schliesslich werden wir  iii) neue therapeutische Interventionen bei MMA wie z.B. Therapie mit Antioxidantien evaluieren.  Unsere wissenschaftlichen Erkenntnisse werden einen Einblick in die bisher nicht gänzlich verstandene zelluläre und molekulare Biologie des intrazellulären Cobalamin-Stoffwechsels geben, zu einem besseren Verständnis der Hirnschädigung bei der MMA beitragen und die Grundlagen für Entwicklung von besseren Therapien führen.

Direct link to Lay Summary

Last update: 23.11.2017

Natural persons

Active participation

Self-organised

Title	Year

Number	Title	Start	Funding scheme

Background: In man, vitamin B12 (cobalamin) is required as coenzyme for only 2 reactions, both of which are essential for life. Adenosylcobalamin is the cofactor for mitochondrial methylmalonyl-CoA mutase (MUT), which converts methylmalonyl-CoA to succinyl-CoA, and methylcobalamin is required by cytosolic methionine synthase (MS) for the remethylation of homocysteine to methionine. Deficiencies of either protein, due to mutation of the enzymes themselves or incomplete processing and delivery of their cofactor forms, results in methylmalonic aciduria (MMAuria, MUT-deficiency) and/or hyperhomocystinemia (HCemia, MS-deficiency). MMAuria and HCemia related defects are rare but devastating disease-causing disorders that in their most severe forms result in death in infancy or the newborn period, while milder forms may lead to a wide range of symptoms with varying severity. Those individuals who survive the first crises often suffer serious long-term consequences, including chronic renal failure and/or neurological deterioration in MMAuria. In the past years, we have discovered several genes in intracellular cobalamin processing and have made instrumental contributions to increasing knowledge of mutational, structural, biochemical and whole organism aspects of disease mechanisms. Given the potentially severe phenotypic and pathological outcomes, there is an imminent need to search for novel therapeutic approaches. However, the ability to treat patients effectively requires a complete understanding of the processes involved - which itself is dependent upon knowledge of the basic metabolic steps. For example, recent findings of physical interaction among cobalamin pathway proteins, which is blocked by patient mutations, illustrate the need for continuous sequestration of this rare and highly reactive vitamin within protein complexes. Further, the tissue specificity of long-term complications, despite ubiquitous enzyme expression, points to a variable dependence of different tissues on the disturbed metabolic pathways and/or a higher sensitivity to the accumulating metabolites brought about by their dysfunction. In order to understand these dysfunctions, the cell types specifically affected by disease must be investigated. We have developed innovative tools which for the first time bring these affected cell types within reach, including novel mouse models of MMAuria and induced pluripotent stem cells (iPSCs) from patient fibroblasts. Finally, in order to improve treatment, the molecular basis of the currently used and any newly identified therapy must be thoroughly investigated, so that a complete understanding of their efficacy may be determined.Hypotheses: (i) Cobalamin is processed within protein interaction networks that form multi-protein complexes which are disrupted in mutation and disease. (ii) Accumulation of neurotoxic metabolites and defective mitochondria are key pathogenic mechanisms of neurological dysfunction and cell death in MMAuria.Specific Aims and Experimental Design:1. Identification and reconstruction of the MUT-associated mitochondrial complex of cobalamin and related proteins2. Determination of pathomechanisms in neuronal models of MMAuria derived from patient iPSCs and isogenic controls3. Evaluation of classical and novel therapeutic interventions in MMAuria including cobalamin and corrinoid therapy, pharmacological chaperones and antioxidant treatmentExpected value of the proposed project: Knowledge gained in the proposed studies will provide insight into the so far incompletely understood cellular and molecular biology of intracellular cobalamin processing. This has implications for not only severe disease, but also milder deficiencies, which are common in certain populations and the elderly, as well as implications for other inborn errors and wider biological processes. Our studies will increase insight into the poorly understood pathomechanisms of neurological damage in MMAuria profiting from the use of the first human iPSC and neuronal models of this disease. Further, these cell lines may provide the basis for genetic correction and future use as individualized cell therapy. Knowledge gained in MMAuria will be extendable throughout the cobalamin pathway (e.g. HCuria) as well as to related organic acid disorders (e.g. glutaric and propionic acidemia). Any treatments identified in this project can be tested in our mouse models of disease and eventually in patient cohorts to which we have access.