Genetic evidence of an accessory activity required specifically for cubilin brush-border expression and intrinsic factor-cobalamin absorption

Cubilin is a high molecular weight multiligand receptor that mediates intestinal absorption of intrinsic factor-cobalamin and selective protein reabsorption in renal tubules. The genetic basis of selective intestinal cobalamin malabsorption with proteinuria was investigated in a canine model closely resembling human Imerslund-Grasbeck syndrome caused by cubilin mutations. Canine CUBN cDNA was cloned and sequenced, showing high identity with human and rat CUBN cDNAs, An intragenic CUBN marker was identified in the canine family and used to test the hypothesis of genetic linkage of the disease and CUBN loci. Linkage was rejected, indicating that the canine disorder resembling Imerslund-Grasbeck syndrome is caused by defect of a gene product other than cubilin. These results imply that there may be locus heterogeneity among human kindreds with selective intestinal cobalamin malabsorption and proteinuria and that normal brush-border expression of cubilin requires the activity of an accessory protein. (C) 1999 by The American Society of Hematology.