title: Amnionless function is required for cubilin brush-border expression and intrinsic factor-cobalamin (vitamin B12) absorption in vivo.
abstract: Amnionless (AMN) and cubilin gene products appear to be essential functional subunits of an endocytic receptor called cubam. Mutation of either gene causes autosomal recessive Imerslund-Gräsbeck syndrome (I-GS, OMIM no. 261100) in humans, a disorder characterized by selective intestinal malabsorption of cobalamin (vitamin B12) and urinary loss of several specific low-molecular-weight proteins. Vital insight into the molecular pathology of I-GS has been obtained from studies of dogs with a similar syndrome. In this work, we show that I-GS segregates in a large canine kindred due to an in-frame deletion of 33 nucleotides in exon 10 of AMN. In a second, unrelated I-GS kindred, affected dogs exhibit a homozygous substitution in the AMN translation initiation codon. Studies in vivo demonstrated that both mutations abrogate AMN expression and block cubilin processing and targeting to the apical membrane. The essential features of AMN dysfunction observed in vivo are recapitulated in a heterologous cell-transfection system, thus validating the system for analysis of AMN-cubilin interactions. Characterization of canine AMN mutations that cause I-GS establishes the canine model as an ortholog of the human disorder well suited to studies of AMN function and coevolution with cubilin.
authors.affiliation: Department of Microbiology and Molecular Genetics, College of Veterinary Medicine, Michigan State University, East Lansing, MI, USA.
authors.forename: Qianchuan
authors.initials: Q
authors.lastname: He
authors.forename: Mette
authors.initials: M
authors.lastname: Madsen
authors.forename: Adam
authors.initials: A
authors.lastname: Kilkenney
authors.forename: Brittany
authors.initials: B
authors.lastname: Gregory
authors.forename: Erik I
authors.initials: EI
authors.lastname: Christensen
authors.forename: Henrik
authors.initials: H
authors.lastname: Vorum
authors.forename: Peter
authors.initials: P
authors.lastname: Højrup
authors.forename: Alejandro A
authors.initials: AA
authors.lastname: Schäffer
authors.forename: Ewen F
authors.initials: EF
authors.lastname: Kirkness
authors.forename: Stephan M
authors.initials: SM
authors.lastname: Tanner
authors.forename: Albert
authors.initials: A
authors.lastname: de la Chapelle
authors.forename: Urs
authors.initials: U
authors.lastname: Giger
authors.forename: Søren K
authors.initials: SK
authors.lastname: Moestrup
authors.forename: John C
authors.initials: JC
authors.lastname: Fyfe
mesh_terms: D000818:Animals; D018387:Codon, Initiator; D004252:DNA Mutational Analysis; D004285:Dogs; D005091:Exons; D007408:Intestinal Absorption / Q000235:genetics*; D008565:Membrane Proteins; D008871:Microvilli / Q000378:metabolism*; D023421:Models, Animal; D009154:Mutation; D010375:Pedigree; D011506:Proteins; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017384:Sequence Deletion; D013577:Syndrome; D014806:Vitamin B 12 Deficiency / Q000209:etiology* / Q000235:genetics
chemical_list: C484714:AMN protein, human; D018387:Codon, Initiator; D008565:Membrane Proteins; D011506:Proteins; D011956:Receptors, Cell Surface; C029486:intrinsic factor-cobalamin receptor