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Mol Genet Metab. 2013 Aug;109(4):390-6. doi: 10.1016/j.ymgme.2013.05.006. Epub 2013 May 22.

An exon 53 frameshift mutation in CUBN abrogates cubam function and causes Imerslund-Gräsbeck syndrome in dogs.

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Laboratory of Comparative Medical Genetics, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824, USA.


Cobalamin malabsorption accompanied by selective proteinuria is an autosomal recessive disorder known as Imerslund-Gräsbeck syndrome in humans and was previously described in dogs due to amnionless (AMN) mutations. The resultant vitamin B12 deficiency causes dyshematopoiesis, lethargy, failure to thrive, and life-threatening metabolic disruption in the juvenile period. We studied 3 kindreds of border collies with cobalamin malabsorption and mapped the disease locus in affected dogs to a 2.9Mb region of homozygosity on canine chromosome 2. The region included CUBN, the locus encoding cubilin, a peripheral membrane protein that in concert with AMN forms the functional intrinsic factor-cobalamin receptor expressed in ileum and a multi-ligand receptor in renal proximal tubules. Cobalamin malabsorption and proteinuria comprising CUBN ligands were demonstrated by radiolabeled cobalamin uptake studies and SDS-PAGE, respectively. CUBN mRNA and protein expression were reduced ~10 fold and ~20 fold, respectively, in both ileum and kidney of affected dogs. DNA sequencing demonstrated a single base deletion in exon 53 predicting a translational frameshift and early termination codon likely triggering nonsense mediated mRNA decay. The mutant allele segregated with the disease in the border collie kindred. The border collie disorder indicates that a CUBN mutation far C-terminal from the intrinsic factor-cobalamin binding site can abrogate receptor expression and cause Imerslund-Gräsbeck syndrome.


AMN; Amnionless; Animal model; BC; CUB; CUBN; Cubam; Cubilin; GIF; I-GS; Imerslund-Gräsbeck syndrome; Inborn error; MMA; Methylmalonic aciduria; RT; SNP; UCBC; Vitamin B(12); amnionless; an ~110 residue protein domain structure first defined in Complement proteins C1r/C1s, sea Urchin EGF-domain-containing protein (Uegf), Bone morphogenic protein 1 (Bmp1); border collie; cubilin; gastric intrinsic factor; methylmalonic acid; qPCR; quantitative polymerase chain reaction; reverse transcriptase; single nucleotide polymorphism; the functional receptor complex composed of AMN and CUBN subunits; unsaturated cobalamin binding capacity

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