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Identification of a putative lysosomal cobalamin exporter altered in the cblF defect of vitamin B12 metabolism


Vitamin B12 (cobalamin) is essential in animals for metabolism of branched chain amino acids and odd chain fatty acids, and for remethylation of homocysteine to methionine1. In the cblF inborn error of vitamin B12 metabolism, free vitamin accumulates in lysosomes, thus hindering its conversion to cofactors2,3. Using homozygosity mapping in 12 unrelated cblF individuals and microcell-mediated chromosome transfer, we identified a candidate gene on chromosome 6q13, LMBRD1, encoding LMBD1, a lysosomal membrane protein with homology to lipocalin membrane receptor LIMR. We identified five different frameshift mutations in LMBRD1 resulting in loss of LMBD1 function, with 18 of the 24 disease chromosomes carrying the same mutation embedded in a common 1.34-Mb haplotype. Transfection of fibroblasts of individuals with cblF with wild-type LMBD1 rescued cobalamin coenzyme synthesis and function. This work identifies LMBRD1 as the gene underlying the cblF defect of cobalamin metabolism and suggests that LMBD1 is a lysosomal membrane exporter for cobalamin.

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Figure 1: Mapping of a locus for cblF on chromosome 6q13.
Figure 2: Colocalization of LMBD1 with the lysosomal membrane marker LAMP1.
Figure 3: Multiple sequence alignment and structure prediction of LMBD1.
Figure 4: Complementation of the cblF defect by transfection with the wild-type human LMBD1 construct.

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We thank the following individuals who provided subject samples and/or clinical information: D. Applegarth, J. Bever, S. Cederbaum, G. Davidson, B.S. Dowton, G. Hoganson, R. Laframboise, U. Lichter, M. Lindner, M. Seashore, V. Shih, G.P.A. Smit, J. Vockley and H.E. Wiltse. We thank E. Kirst, C. Kluck and U. Botschen for expert technical assistance. F.R. and S.G. were supported by the fund “Innovative Medical Research” of University of Münster Medical School. B.G. and C.S. were supported by the Centre National de la Recherche Scientifique, the Institut national de la santé et de la recherche médicale and the Agence Nationale de la Recherche - Programme Pluriannuel de Recherche sur les Maladies Rares. I.R.M. was supported by a studentship from the Fonds de la Recherche en Santé du Québec. D.S.R. and I.R.M. were supported by funds from the Canadian Institutes of Health Research and the Hess B. and Diane Finestone Laboratory in memory of Jacob and Jenny Finestone. Financial support was further given by the Swiss National Foundation and the German Federal Ministry of Education and Research through the National Genome Research Network.

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Authors and Affiliations



F.R. and P.N. designed the study, analyzed the data and wrote the manuscript. S.G. designed primers, collected data, performed sequencing analysis, LMBD1 expression analysis and biochemical rescue experiments. I.R.M. collected subject information and performed microcell-mediated chromosome transfer. T.S. performed biochemical rescue experiments with the wild-type construct. C.S., I.B., A.S., H.R. and B.G. analyzed the subcellular localization of the tagged construct. M.R.T. performed pyrosequencing of the control chromosomes. G.N. performed the bioinformatic analysis and analyzed the haplotype data. T.W. performed sequencing analysis and edited the manuscript. M.S. and M.B. cloned the construct. C.B. performed the genome scan. T.M., B.G., D.S.R. and B.F. were involved in study design. W.H. and B.G. analyzed the structure and homology of LMBD1. All authors discussed the results and commented on the manuscript.

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Correspondence to Frank Rutsch.

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Rutsch, F., Gailus, S., Miousse, I. et al. Identification of a putative lysosomal cobalamin exporter altered in the cblF defect of vitamin B12 metabolism. Nat Genet 41, 234–239 (2009).

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