title: An exon 53 frameshift mutation in CUBN abrogates cubam function and causes Imerslund-Gräsbeck syndrome in dogs.
abstract: 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.
authors.affiliation: Laboratory of Comparative Medical Genetics, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824, USA. fyfe@msu.edu
authors.forename: John C
authors.initials: JC
authors.lastname: Fyfe
authors.forename: Shelby L
authors.initials: SL
authors.lastname: Hemker
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Venta
authors.forename: Caitlin A
authors.initials: CA
authors.lastname: Fitzgerald
authors.forename: Catherine A
authors.initials: CA
authors.lastname: Outerbridge
authors.forename: Sherry L
authors.initials: SL
authors.lastname: Myers
authors.forename: Urs
authors.initials: U
authors.lastname: Giger
mesh_terms: D000749:Anemia, Megaloblastic; D000818:Animals; D004285:Dogs; D005091:Exons; D005260:Female; D016368:Frameshift Mutation; D005786:Gene Expression Regulation; D006801:Humans; D007082:Ileum / Q000378:metabolism; D007668:Kidney / Q000378:metabolism; D008286:Malabsorption Syndromes / Q000209:etiology / Q000235:genetics* / Q000378:metabolism; D008297:Male; D011485:Protein Binding; D011507:Proteinuria / Q000209:etiology / Q000235:genetics* / Q000378:metabolism; D020871:RNA Stability / Q000235:genetics; D011956:Receptors, Cell Surface / Q000235:genetics* / Q000378:metabolism; D014805:Vitamin B 12 / Q000235:genetics / Q000378:metabolism*; D014806:Vitamin B 12 Deficiency / Q000209:etiology / Q000235:genetics* / Q000378:metabolism
chemical_list: D011956:Receptors, Cell Surface; C029486:intrinsic factor-cobalamin receptor; D014805:Vitamin B 12
keywords: 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