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Molecular mechanisms leading to three different phenotypes in the cblD defect of intracellular cobalamin metabolism


Stucki, Martin; Coelho, David; Suormala, Terttu; Burda, Patricie; Fowler, Brian; Baumgartner, Matthias R (2012). Molecular mechanisms leading to three different phenotypes in the cblD defect of intracellular cobalamin metabolism. Human Molecular Genetics, 21(6):1410-1418.

Abstract

The cblD defect of intracellular vitamin B(12) metabolism can lead to isolated methylmalonic aciduria (cblD-MMA) or homocystinuria (cblD-HC), or combined methylmalonic aciduria and homocystinuria (cblD-MMA/HC). We studied the mechanism whereby MMADHC mutations can lead to three phenotypes. The effect of various expression vectors containing MMADHC modified to contain an enhanced mitochondrial leader sequence or mutations changing possible downstream sites of reinitiation of translation or mutations introducing stop codons on rescue of adenosyl- and methylcobalamin (MeCbl) formation was studied. The constructs were transfected into cell lines derived from various cblD patient's fibroblasts. Expression of 10 mutant alleles from 15 cblD patients confirmed that the nature and location of the mutations correlate with the biochemical phenotype. In cblD-MMA/HC cells, improving mitochondrial targeting of MMADHC clearly increased the formation of adenosylcobalamin (AdoCbl) with a concomitant decrease in MeCbl formation. In cblD-MMA cells, this effect was dependent on the mutation and showed a negative correlation with endogenous MMADHC mRNA levels. These findings support the hypothesis that a single protein exists with two different functional domains that interact with either cytosolic or mitochondrial targets. Also a delicate balance exists between cytosolic MeCbl and mitochondrial AdoCbl synthesis, supporting the role of cblD protein as a branch point in intracellular cobalamin trafficking. Furthermore, our data indicate that the sequence after Met116 is sufficient for MeCbl synthesis, whereas the additional sequence between Met62 and Met116 is required for AdoCbl synthesis. Accordingly, western blot studies reveal proteins of the size expected from the stop codon position with subsequent reinitiation of translation.

The cblD defect of intracellular vitamin B(12) metabolism can lead to isolated methylmalonic aciduria (cblD-MMA) or homocystinuria (cblD-HC), or combined methylmalonic aciduria and homocystinuria (cblD-MMA/HC). We studied the mechanism whereby MMADHC mutations can lead to three phenotypes. The effect of various expression vectors containing MMADHC modified to contain an enhanced mitochondrial leader sequence or mutations changing possible downstream sites of reinitiation of translation or mutations introducing stop codons on rescue of adenosyl- and methylcobalamin (MeCbl) formation was studied. The constructs were transfected into cell lines derived from various cblD patient's fibroblasts. Expression of 10 mutant alleles from 15 cblD patients confirmed that the nature and location of the mutations correlate with the biochemical phenotype. In cblD-MMA/HC cells, improving mitochondrial targeting of MMADHC clearly increased the formation of adenosylcobalamin (AdoCbl) with a concomitant decrease in MeCbl formation. In cblD-MMA cells, this effect was dependent on the mutation and showed a negative correlation with endogenous MMADHC mRNA levels. These findings support the hypothesis that a single protein exists with two different functional domains that interact with either cytosolic or mitochondrial targets. Also a delicate balance exists between cytosolic MeCbl and mitochondrial AdoCbl synthesis, supporting the role of cblD protein as a branch point in intracellular cobalamin trafficking. Furthermore, our data indicate that the sequence after Met116 is sufficient for MeCbl synthesis, whereas the additional sequence between Met62 and Met116 is required for AdoCbl synthesis. Accordingly, western blot studies reveal proteins of the size expected from the stop codon position with subsequent reinitiation of translation.

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Item Type:Journal Article, refereed, original work
Communities & Collections:04 Faculty of Medicine > University Children's Hospital Zurich > Medical Clinic
04 Faculty of Medicine > Center for Integrative Human Physiology
Dewey Decimal Classification:570 Life sciences; biology
610 Medicine & health
Language:English
Date:2012
Deposited On:06 Jul 2012 08:26
Last Modified:05 Apr 2016 15:52
Publisher:Oxford University Press
ISSN:0964-6906
Additional Information:This is a pre-copy-editing, author-produced PDF of an article accepted for publication in Human Molecular Genetics following peer review. The definitive publisher-authenticated version Stucki, Martin; Coelho, David; Suormala, Terttu; Burda, Patricie; Fowler, Brian; Baumgartner, Matthias R (2012). Molecular mechanisms leading to three different phenotypes in the cblD defect of intracellular cobalamin metabolism. Human Molecular Genetics, 21(6):1410-1418. is available online at: dx.doi.org/0.1093/hmg/ddr579
Publisher DOI:10.1093/hmg/ddr579
PubMed ID:22156578
Permanent URL: http://doi.org/10.5167/uzh-63106

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