| *602997 | |||||||||||||||||||||||||||||||||||||||||||||
| CUBILIN; CUBN | |||||||||||||||||||||||||||||||||||||||||||||
| Alternative titles; symbols | |||||||||||||||||||||||||||||||||||||||||||||
| INTRINSIC FACTOR-COBALAMIN RECEPTOR; IFCR | |||||||||||||||||||||||||||||||||||||||||||||
| HGNC Approved Gene Symbol: CUBN | |||||||||||||||||||||||||||||||||||||||||||||
| Cytogenetic location: 10p13 Genomic coordinates (GRCh37): 10:16,865,964 - 17,171,815 (from NCBI) | |||||||||||||||||||||||||||||||||||||||||||||
| Gene Phenotype Relationships | |||||||||||||||||||||||||||||||||||||||||||||
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| Description | |||||||||||||||||||||||||||||||||||||||||||||
| Cubilin is the intestinal receptor for the endocytosis of intrinsic factor (IF; 609342)-vitamin B12 and a receptor in epithelial apoA-I/HDL (see 107680) metabolism (summarized by Kozyraki et al., 1998). It is mutant in megaloblastic anemia-1 (261100). | |||||||||||||||||||||||||||||||||||||||||||||
| Cloning | |||||||||||||||||||||||||||||||||||||||||||||
| By surface plasmon resonance analysis of ligand-affinity-purified human cubilin, Kozyraki et al. (1998) demonstrated a high affinity calcium- and cobalamin (vitamin B12)-dependent binding of gastric intrinsic factor (IF)-cobalamin. Complete cDNA cloning of the human receptor showed a 3,597-amino acid peripheral membrane protein with 69% identity to rat cubilin. Amino-terminal sequencing of the receptor indicated that the cDNA sequence encodes a precursor protein undergoing proteolytic processing due to cleavage at a recognition site (arg-7/glu-8/lys-9/arg-10) for the trans-Golgi proteinase furin (136950). | |||||||||||||||||||||||||||||||||||||||||||||
| Mapping | |||||||||||||||||||||||||||||||||||||||||||||
| Using fluorescence in situ hybridization, radiation hybrid mapping, and screening of YAC clones, Kozyraki et al. (1998) mapped the human cubilin gene between markers D10S1661 and WI-5445 on the short arm of chromosome 10. This was within the 6-cM region harboring the gene responsible for megaloblastic anemia-1 (MGA1; 261100). All of this was considered circumstantial evidence that an impaired synthesis, processing, or ligand binding of cubilin is the molecular basis of Imerslund-Grasbeck disease (MGA1). | |||||||||||||||||||||||||||||||||||||||||||||
| Gene Function | |||||||||||||||||||||||||||||||||||||||||||||
| Studies in rodents showed that uptake of cobalamin in complex with IF is facilitated by an intestinal 460-kD protein (Birn et al., 1997; Seetharam et al., 1997), designated cubilin (Moestrup et al., 1998). Cubilin is suggested to traffic by means of megalin (LRP2; 600073), a 600-kD endocytic receptor expressed in the same tissues and mediating uptake of a number of ligands, including transcobalamin-cobalamin complexes. Like megalin, cubilin has a significantly higher expression in the renal proximal tubules compared with the intestine, and, because IF is only present in minute amounts in nongastrointestinal tissues, cubilin might also have multiligand properties. Cubilin binds receptor-associated protein (RAP; 104225), a 40-kD endoplasmic reticulum protein also binding with high affinity to the multiligand giant receptors (e.g., megalin) belonging to the low density lipoprotein receptor (LDLR; 606945) protein family. RAP may function as a chaperone during folding of the receptors. Moestrup et al. (1998) determined the primary structure of rat cubilin and showed that almost the entire sequence is accounted for by a cluster of 8 epidermal growth factor (EGF) repeats, followed by a large cluster of 27 CUB domains which led to the designation of the receptor. (The name CUB, introduced by Bork and Beckmann (1993), is an abbreviation for complement subcomponents C1r/C1s (613785; 120580), Uegf, and bone morphogenetic protein-1 (BMP1; 112264).) Although cubilin is the intestinal receptor for the endocytosis of intrinsic factor-vitamin B12, several lines of evidence, including a high expression in kidney and yolk sac, indicated that it may have additional functions. Using cubilin affinity chromatography, Kozyraki et al. (1999) isolated apolipoprotein A-I (APOA1; 107680), the main protein of high density lipoprotein (HDL). They demonstrated a high-affinity binding of APOA1 and HDL to cubilin, and cubilin-expressing yolk sac cells showed efficient endocytosis of iodine-labeled HDL that could be inhibited by IgG antibodies against APOA1 and cubilin. The physiologic relevance of the cubilin-APOA1 interaction was further emphasized by urinary APOA1 loss in some known cases of functional cubilin deficiency (Imerslund-Grasbeck syndrome). Therefore, cubilin is a receptor in epithelial APOA1/HDL metabolism. Megalin binds a large number of structurally unrelated ligands, and coreceptors may confer ligand specificity by sequestering and presenting their cargo to megalin (summarized by Nykjaer et al., 2001). For example, IF-B12 complex is taken up in the intestine by a tandem receptor-mediated mechanism; the complex is first bound to a receptor, cubilin, anchored to the outer leaflet of the plasma membrane possibly by an amphipathic helix, followed by endocytosis of cubilin and its cargo mediated by megalin. The pivotal role of intestinal cubilin is underscored by the vitamin B12 deficiency observed in patients with Imerslund-Grasbeck disease characterized by defective cubilin incapable of binding IF-B12. These patients have low molecular weight proteinuria in addition to megaloblastic anemia, indicating dysfunction of cubilin coexpressed with megalin in kidney proximal tubules. Nykjaer et al. (2001) identified cubilin as an important coreceptor in the endocytic pathway for retrieval of 25(OH)D3-DBP complexes by megalin-mediated endocytosis in the kidney. They showed that absence of cubilin or inhibition of its function markedly reduces cellular uptake of the steroid-carrier complex, and animals or patients lacking functional cubilin are characterized by abnormal vitamin D metabolism. They identified patients with mutations in an endocytic pathway that regulates steroid hormone metabolism. Cubilin recognizes intrinsic factor (IF)-cobalamin and various other proteins to be endocytosed in the intestine and kidney, respectively. Fyfe et al. (2004) showed that cubilin and amnionless (AMN; 605799) colocalize in the endocytic apparatus of polarized epithelial cells and copurify as a tight complex during IF-cobalamin affinity and nondenaturing gel filtration chromatography. In transfected cells expressing either AMN or a truncated IF-cobalamin-binding cubilin construct, neither protein alone conferred ligand endocytosis. Other studies indicated that cubilin and AMN are subunits of a novel cubilin/AMN (cubam) complex, where AMN binds to the N-terminal third of cubilin and directs subcellular localization and endocytosis of cubilin with its ligand. Fyfe et al. (2004) concluded that mutations affecting either of the 2 proteins may abrogate function of the cubam complex and cause Imerslund-Grasbeck syndrome. | |||||||||||||||||||||||||||||||||||||||||||||
| Biochemical Features | |||||||||||||||||||||||||||||||||||||||||||||
| Crystal Structure Andersen et al. (2010) presented the crystal structure of the complex between gastric intrinsic factor (IF; 609342)-cobalamin and the CUBN-IF-cobalamin-binding region determined at 3.3-angstrom resolution. The structure provided insight into how several CUB (complement C1r/C1s, Uegf, Bmp1) domains collectively function as modular ligand-binding regions, and how 2 distant CUB domains embrace the cobalamin molecule by binding the 2 IF domains in a calcium-dependent manner. This dual-point model provided a probable explanation of how cobalamin indirectly induces ligand-receptor coupling. Finally, the comparison of calcium-binding CUB domains and the LDLR-type A modules suggested that the electrostatic pairing of a basic ligand arginine/lysine residue with calcium-coordinating acidic aspartates/glutamates is a common theme of calcium-dependent ligand-receptor interactions. | |||||||||||||||||||||||||||||||||||||||||||||
| Molecular Genetics | |||||||||||||||||||||||||||||||||||||||||||||
| Megaloblastic anemia-1 (MGA1; 261100) is a rare, autosomal recessive disorder characterized by juvenile megaloblastic anemia, as well as neurologic symptoms that may be the only manifestations. At the cellular level, MGA1 is characterized by selective intestinal B12 malabsorption. MGA1 occurs worldwide, but its prevalence is higher in several Middle Eastern countries and in Norway, and highest in Finland (0.8 in 100,000). Aminoff et al. (1995) mapped the MGA1 locus by linkage analysis in Finnish and Norwegian families to a 6-cM region on 10p12.1. As the receptor for intrinsic factor-B12 complex (IF-B12), the CUBN gene was a logical candidate for the site of the mutation and was also a positional candidate because it maps to the same region. Aminoff et al. (1999) refined the MGA1 region by linkage disequilibrium (LD) mapping, fine-mapped the CUBN gene in 17 Finnish MGA1 families, and identified 2 independent disease-specific CUBN mutations. Nykjaer et al. (2001) found that a patient with megaloblastic anemia-1 who was homozygous for the intronic single-nucleotide substitution causing the activation of a cryptic splice site and leading to an insertion into domain 6 of the CUBN (602997.0002) gene showed urinary loss of DBP and 25(OH)D3. On the other hand, patients with the missense mutation P1297L (602997.0001) reabsorbed DBP normally, suggesting that the binding site for 25(OH)D3-DBP is distinct from the binding site for IF-B12. In a patient with megalobastic anemia-1, Finnish type, Storm et al. (2011) identified homozygosity for a splice site mutation in the CUBN gene (602997.0003). This patient showed no immunogenic reaction to cubilin and was found to have an abnormal cytoplasmic, vesicular distribution of amnionless, its receptor partner, in renal biopsy specimen, indicating that amnionless depends on cubilin for correct localization in the human proximal tubule. | |||||||||||||||||||||||||||||||||||||||||||||
| Animal Model | |||||||||||||||||||||||||||||||||||||||||||||
| Mixed breed dogs exhibiting autosomal recessive inheritance of cubilin malexpression have been reported (Fyfe et al., 1991; Xu et al., 1999). In these dogs, Nykjaer et al. (2001) showed that there is a disturbance of vitamin D metabolism as well as severe vitamin B12 deficiency similar to that of patients with Imerslund-Grasbeck disease. | |||||||||||||||||||||||||||||||||||||||||||||
| History | |||||||||||||||||||||||||||||||||||||||||||||
| Megaloblastic anemia and neurologic disturbances are common symptoms of deficiency of the coenzyme vitamin B12 (cyanocobalamin). The cellular uptake of the vitamin and its modified forms depends on the binding to the carrier proteins, intrinsic factor (IF; 609342) produced in the stomach, and transcobalamin, present in the circulation and various tissue fluids. Hereditary forms of cobalamin deficiency are known to relate to qualitatively abnormal IF (see 261000), to decreased synthesis of transcobalamin (275350), and to a defect of the intestinal epithelium leading to decreased uptake of IF-cobalamin and failure to absorb cobalamin (Imerslund-Grasbeck disease (IGS), or megaloblastic anemia-1 (MGA1); 261100). Imerslund-Grasbeck disease has been shown by linkage studies to be caused by mutation in a region designated MGA1 (megaloblastic anemia-1), located on 10p between markers D10S548 and D10S466. The defect has been thought to be related to abnormal epithelial translocation of cobalamin, perhaps due to decreased receptor function/expression. | |||||||||||||||||||||||||||||||||||||||||||||
| ALLELIC VARIANTS (Selected Examples): | |||||||||||||||||||||||||||||||||||||||||||||
| Table View | |||||||||||||||||||||||||||||||||||||||||||||
| .0001 MEGALOBLASTIC ANEMIA 1, FINNISH TYPE | |||||||||||||||||||||||||||||||||||||||||||||
| CUBN, PRO1297LEU [dbSNP:rs121434430] | |||||||||||||||||||||||||||||||||||||||||||||
| In 16 of 17 Finnish families segregating megaloblastic anemia-1 (261100), Aminoff et al. (1999) found that 31 of 34 disease chromosomes carried a 3916C-T transition in the CUBN gene, resulting in a pro1297-to-leu amino acid substitution in cubilin. The mutation was found in only 1 of 316 control Finnish chromosomes. All parents were heterozygous. By site-directed mutagenesis, mammalian expression, and functional comparison of the purified wildtype and Finnish mutant forms of the IF-cobalamin-binding region of cubilin (amino acids 928-1386), Kristiansen et al. (2000) investigated the functional implications of the P1297L mutation. They found that the mutation impairs recognition of intrinsic factor-vitamin B12 complex by cubilin. | |||||||||||||||||||||||||||||||||||||||||||||
| .0002 MEGALOBLASTIC ANEMIA 1, FINNISH TYPE | |||||||||||||||||||||||||||||||||||||||||||||
| CUBN, IVS6, C-G | |||||||||||||||||||||||||||||||||||||||||||||
| Aminoff et al. (1999) found that in 1 Finnish family with MGA1 (261100), the proband was homozygous for an intronic mutation which was found in none of the 302 control Finnish chromosomes. An apparent in-frame insertion had occurred due to a point mutation in the intron interrupting domain 6 of the CUBN gene. The change was a C-to-G transversion. | |||||||||||||||||||||||||||||||||||||||||||||
| .0003 MEGALOBLASTIC ANEMIA 1, FINNISH TYPE | |||||||||||||||||||||||||||||||||||||||||||||
| CUBN, IVS23, G-T, +1 | |||||||||||||||||||||||||||||||||||||||||||||
| Storm et al. (2011) identified a patient with Imerslund-Grasbeck syndrome (261100) with a novel homozygous G-to-T transversion at the conserved donor splice site of exon 23 of the CUBN gene. A renal biopsy specimen from the patient showed no immunologic reaction for cubilin and an abnormal cytoplasmic, vesicular distribution of amnionless (605799), indicating that amnionless depends on cubilin for correct localization in the human proximal tubule. | |||||||||||||||||||||||||||||||||||||||||||||
| REFERENCES | |||||||||||||||||||||||||||||||||||||||||||||
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