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 2.9 Mb 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 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.

Prospective long‐term evaluation of parenteral hydroxocobalamin supplementation in juvenile beagles with selective intestinal cobalamin malabsorption (Imerslund‐Gräsbeck syndrome)

“…17 It is an autosomal recessive trait caused by 2 distinct AMN mutations in Giant Schnauzers 17 and Australian Shepherds. 5 Our group and others recently identified 2 independent CUBN mutations in Border Collies 18,19 and Beagles. 10,20 Although the disease is rare, substantial carrier frequency has been noted in affected breeds 20 and thus disease occurrence is more common regionally.…”

Selective Intestinal Cobalamin Malabsorption with Proteinuria (Imerslund‐Gräsbeck Syndrome) in Juvenile Beagles

“…Cubam is a heteromeric, multi ligand, endocytic receptor composed of 2 distinct protein subunits, cubilin (CUBN) and amnionless (AMN) . While several AMN and CUBN mutations occur in humans with I‐GS, only 2 AMN mutations, and most recently, a single CUBN mutation, have been found in 3 different affected dog breeds.…”

Daily oral cyanocobalamin supplementation in Beagles with hereditary cobalamin malabsorption (Imerslund‐Gräsbeck syndrome) maintains normal clinical and cellular cobalamin status

“…The results of our study using IGS‐affected dogs with a known functional defect of cubam but otherwise healthy intestinal tract indicate for the first time that passive intestinal Cbl absorption alone leads to a normal clinical and cellular Cbl status in dogs. This is especially true for the Beagle breed because a recent study showed that CUBN expression was undetectable in tissue samples from an affected dog, whereas in Border Collies with IGS, expression of small amounts of residual CUBN retaining IF‐Cbl binding recently was reported . However, this laboratory finding does not appear to be associated with effective Cbl absorption because all available publications concerning Border Collies with IGS have reported the presence of severe Cbl deficiency …”

“…In dogs, selective intestinal Cbl malabsorption, comparable to IGS in humans, has been described in Australian Shepherds, Beagles, Border Collies, and Giant Schnauzers . It is an autosomal recessive trait caused by 2 distinct AMN mutations in Giant Schnauzers and Australian Shepherds, and our group and others recently identified 2 independent CUBN mutations in Border Collies and Beagles . The condition is lethal in affected dogs unless detected and treated appropriately .…”