Decreased activity of intestinal and urinary intrinsic factor receptor in Gräsbeck-Imerslund disease [corrected]

Imerslund-Gräsbeck syndrome: a comprehensive review of reported cases

Imerslund-Gräsbeck syndrome (IGS) is a rare autosomal recessive disorder characterized by vitamin B12 malabsorption. Most patients present with non-specific symptoms attributed to vitamin B12 deficiency, and proteinuria. Patients may if untreated, develop severe neurocognitive manifestations. If recognized and treated with sufficient doses of vitamin B12, patients recover completely. We provide, for the first time, an overview of all previously reported cases of IGS. In addition, we provide a complete review of IGS and describe two new patients.

Clinical, phenotypic and genetic landscape of case reports with genetically proven inherited disorders of vitamin B12 metabolism: A meta-analysis

Inherited disorders of B₁₂ metabolism produce a broad spectrum of manifestations, with limited knowledge of the influence of age and the function of related genes. We report a meta-analysis on 824 patients with a genetically proven diagnosis of an inherited disorder of vitamin B₁₂ metabolism. Gene clusters and age categories are associated with patients' manifestations. The "cytoplasmic transport" cluster is associated with neurological and ophthalmological manifestations, the "mitochondrion" cluster with hypotonia, acute metabolic decompensation, and death, and the "B₁₂ availability" and "remethylation" clusters with anemia and cytopenia. Hypotonia, EEG abnormalities, nystagmus, and strabismus are predominant in the younger patients, while neurological manifestations, such as walking difficulties, peripheral neuropathy, pyramidal syndrome, cerebral atrophy, psychiatric disorders, and thromboembolic manifestations, are predominant in the older patients. These results should prompt systematic checking of markers of vitamin B₁₂ status, including homocysteine and methylmalonic acid, when usual causes of these manifestations are discarded in adult patients.

Vitamin B12 absorption and malabsorption

Vitamin B12 is assimilated and transported by complex mechanisms that involve three transport proteins, intrinsic factor (IF), haptocorrin (HC) and transcobalamin (TC) and their respective membrane receptors. Vitamin deficiency is mainly due to inadequate dietary intake in vegans, and B12 malabsorption is related to digestive diseases. This review explores the physiology of vitamin B12 absorption and the mechanisms and diseases that produce malabsorption. In the stomach, B12 is released from food carrier proteins and binds to HC. The degradation of HC by pancreatic proteases and the pH change trigger the transfer of B12 to IF in the duodenum. Cubilin and amnionless are the two components of the receptor that mediates the uptake of B12 in the distal ileum. Part of liver B12 is excreted in bile, and undergoes an enterohepatic circulation. The main causes of B12 malabsorption include inherited disorders (Intrinsic factor deficiency, Imerslund-Gräsbeck disease, Addison's pernicious anemia, obesity, bariatric surgery and gastrectomies. Other causes include pancreatic insufficiency, obstructive Jaundice, tropical sprue and celiac disease, bacterial overgrowth, parasitic infestations, Zollinger-Ellison syndrome, inflammatory bowel diseases, chronic radiation enteritis of the distal ileum and short bowel. The assessment of B12 deficit is recommended in the follow-up of subjects with bariatric surgery. The genetic causes of B12 malabsorption are probably underestimated in adult cases with B12 deficit. Despite its high prevalence in the general population and in the elderly, B12 malabsorption cannot be anymore assessed by the Schilling test, pointing out the urgent need for an equivalent reliable test.

Vitamin B12 deficiency

Vitamin B₁₂ (B12; also known as cobalamin) is a B vitamin that has an important role in cellular metabolism, especially in DNA synthesis, methylation and mitochondrial metabolism. Clinical B12 deficiency with classic haematological and neurological manifestations is relatively uncommon. However, subclinical deficiency affects between 2.5% and 26% of the general population depending on the definition used, although the clinical relevance is unclear. B12 deficiency can affect individuals at all ages, but most particularly elderly individuals. Infants, children, adolescents and women of reproductive age are also at high risk of deficiency in populations where dietary intake of B12-containing animal-derived foods is restricted. Deficiency is caused by either inadequate intake, inadequate bioavailability or malabsorption. Disruption of B12 transport in the blood, or impaired cellular uptake or metabolism causes an intracellular deficiency. Diagnostic biomarkers for B12 status include decreased levels of circulating total B12 and transcobalamin-bound B12, and abnormally increased levels of homocysteine and methylmalonic acid. However, the exact cut-offs to classify clinical and subclinical deficiency remain debated. Management depends on B12 supplementation, either via high-dose oral routes or via parenteral administration. This Primer describes the current knowledge surrounding B12 deficiency, and highlights improvements in diagnostic methods as well as shifting concepts about the prevalence, causes and manifestations of B12 deficiency.

Luminal expression of cubilin is impaired in Imerslund-Grasbeck syndrome with compound AMN mutations in intron 3 and exon 7

Juvenile megaloblastic anaemia 1 (OMIM # 261100) is a rare autosomic disorder characterized by selective cobalamin mal-absorption and inconstant proteinuria produced by mutations in either CUBN or AMN genes. Amnionless, the gene product of AMN, is a transmembrane protein that binds tightly to the N-terminal end of cubilin, the gene product of CUBN. Cubilin binds to intrinsic factor-cobalamin complex and is expressed in the distal intestine and the proximal renal tubule. We report a compound AMN heterozygosity with c.742C>T, p.Gln248X and c.208-2A>G mutations in 2 siblings that led to premature termination codon in exon 7 and exon 6, respectively. It produced a dramatic decrease in receptor activity in urine, despite absence of CUBN mutation and normal affinity of the receptor for intrinsic factor binding. Heterozygous carriers for c.742T and c.208-2G had no pathological signs. These results indicate that amnionless is essential for the correct luminal expression of cubilin in humans.

Amnionless (AMN) mutations in Imerslund-Gräsbeck syndrome may be associated with disturbed vitamin B12 transport into the CNS

Familial selective vitamin B12 (cobalamin, Cbl) malabsorption (Imerslund-Gräsbeck syndrome, IGS, OMIM 261100) is a group of autosomal recessive disorders characterized by selective malabsorption of Cbl from the terminal ileum in the presence of normal histology. Mutations in the amnionless (AMN) and cubilin (CUBN) genes are known to be causes of IGS. Their gene products combine to form a receptor complex (cubam), which is instrumental in the binding and transport of Cbl in the gut. As opposed to Cbl transport in the terminal ileum, normal transport of Cbl into the CNS is poorly understood and little is known regarding its molecular basis. Studies in adults with neuropsychiatric disease have suggested the presence of an active transport mechanism into the central nervous system constituting a blood-brain barrier (BBB) for Cbl. A child with IGS, compound heterozygous for a missense and a nonsense mutation in the amnionless (AMN) protein gene, was noted to have a high daily cobalamin (Cbl) requirement for neuropsychiatric, but not for systemic metabolic and haematological, remission. Measurements of CSF Cbl revealed evidence that the transport of Cbl into the central nervous system was impaired, and a standard Schilling test was consistent with a dose response of cobalamin transport across the terminal ileum. Amnionless protein is known to be expressed in the fetal and postnatal central nervous system, and is known to be involved in Cbl transport in other tissues such as kidney as well as the gut. It is possible that an active Cbl transport mechanism at the BBB exists, and that the amnionless (AMN) protein may be part of this mechanism, as it is in cobalamin transport in the terminal ileum.

The Finnish Disease Heritage III: the individual diseases

This article is the third and last in a series entitled The Finnish Disease Heritage I-III. All the 36 rare hereditary diseases belonging to this entity are described for clinical and molecular genetic purposes, based on the Finnish experience gathered over a period of half a century. In addition, five other diseases are mentioned. They may be included in the list of the "Finnish diseases" after adequate complementary studies.

Hyperhomocysteinemia is related to a decreased blood level of vitamin B12 in the second and third trimester of normal pregnancy

Hyperhomocysteinemia has been associated with several pregnancy complications. We have investigated the variation of plasma total homocysteine (tHcys) during the 2 last trimesters of normal pregnancy and related it to blood vitamin B12 and folate and to the excretion of the degraded intrinsic factor receptor (IFCR) in urine, in a follow-up study of 15 cases. A significant rise in tHcys was observed between the beginning of the second trimester and the third trimester with respective values (median) 6.1, 5.8 and 6.7 micromol/l (p = 0.038). The tHcys/albumin ratio also increased significantly, while no correlation was found between albumin and folate blood concentration. In contrast, a significant decrease in vitamin B12 was observed (279, 225 and 199 pmol/l, between the 4th and 6th, and the 6th and 9th month, respectively (p = 0.017-0.002)). A significant negative correlation was found between tHcys between the 4th and 9th month of pregnancy and the ratio of vitamin B12 between the 4th and 9th month of pregnancy (r = 0.55, p = 0.037). The urine excretion of IFCR was increased and was not related to vitamin B12 and tHcys. In conclusion, we have observed a rise in tHcys between the beginning of the second trimester and the third trimester of pregnancy which was related to the decreased blood level of vitamin B12. Subclinical deficiency of vitamin B12 should be further investigated in pregnant women who remain on inadequate diet.

Production of gastric intrinsic factor, transcobalamin, and haptocorrin in opossum kidney cells

Opossum kidney epithelial cells were shown previously to synthesize and secrete two cobalamin (Cbl)-binding proteins, presumed to be haptocorrin (Hc) and transcobalamin II (TCII). The present study examines the hypothesis that renal tubular cells also produce intrinsic factor (IF), and this production provides an explanation for the presence of IF in urine. By using antisera raised against human IF and against TCII, the presence of TCII was confirmed, and that of IF discovered in the media of opossum kidney (OK) cells in culture. The apparent molecular weight of IF and TCII was 68 and 43 kDa, respectively. Immunoreactivity on Western blot of the putative IF protein was blocked by recombinant human IF. When proteins secreted into the media were separated electrophoretically under nondenaturing conditions after binding with [(57)Co]Cbl, a broad major band migrated at a relative front independently of recombinant IF or TCII, and probably represents Hc, as the Cbl binding is blocked by cobinamide. Small amounts of bound [(57)Co]Cbl migrated in the position of both IF and TCII, when cobinamide was present. The presence of IF and TCII in OK cells was confirmed by immunohistology. Specific reactivity for IF (blocked by recombinant IF) was found in proximal tubules of opossum kidney, but not in other portions of the nephron, confirming the ability of anti-human IF antiserum to detect opossum IF. A 732-bp fragment of IF, nearly identical in sequence to rat IF, was isolated by RT-PCR from opossum kidney mRNA, and Western blot confirmed the presence of IF protein. The presence of IF was also documented in rat kidney by isolation of an RT-PCR fragment, immunocytochemistry, and Western blot. IF should be added to the list of renal (proximal) tubular antigens that are shared by other epithelia.

Receptor-mediated endocytosis of cobalamin (vitamin B12)

Dietary cobalamin (Cbl) (vitamin B12) is utilized as methyl-Cbl and the coenzyme 5'-deoxyadenosyl Cbl by cells of the body that have the enzymes methionine synthase and methyl malonyl CoA mutase, which convert homocysteine to methionine and methyl malonyl CoA to succinyl CoA, respectively. Prior to conversions and utilizations as the active alkyl forms of Cbl, dietary Cbl is absorbed and transported across cellular plasma membranes by two receptor-mediated events. First, dietary and biliary Cbl bound to gastric intrinsic factor (IF) presented apically to the ileal absorptive enterocytes is transported to the circulation by receptor-mediated endocytosis via apically expressed IF-Cbl receptor. Second, Cbl bound to plasma transcobalamin (TC) II is taken up from the circulation by all cells via a TC II receptor expressed in the plasma membrane of these cells, and in polarized cells via a TC II receptor expressed in the basolateral membranes. This review updates recent work and focuses on (a) the molecular and cellular aspects of Cbl binding protein ligands, IF and TC II, and their cell-surface receptors, IF-Cbl receptor and TC II receptor; (b) the cellular sorting pathways of internalized Cbl bound to IF and TC II in polarized epithelial cells; and (c) the absorption and transport disorders that cause Cbl deficiency.

Molecular dissection of the intrinsic factor-vitamin B12 receptor, cubilin, discloses regions important for membrane association and ligand binding

Cubilin, the receptor for intrinsic factor-vitamin B12, is a novel type of high molecular weight receptor consisting of a 27 CUB (complement components C1r/C1s, Uegf, and bone morphogenic protein-1) domain cluster preceded by 8 epidermal growth factor repeats and a short N-terminal sequence. In addition to binding the vitamin B12-carrier complex, cubilin also binds receptor-associated protein. To delineate the structures for membrane association and ligand binding we established a panel of stable transfected Chinese hamster ovary cells expressing overlapping segments of rat cubilin. Analysis of conditioned media and cell extracts of transfected cells revealed that the N-terminal cubilin region conveys membrane association. Helical plotting of this region demonstrated a conserved amphipathic helix pattern (Lys74-Glu109) as a candidate site for hydrophobic interactions. Ligand affinity chromatography and surface plasmon resonance analysis of the secreted cubilin fragments showed ligand binding in the CUB domain region. Further dissection of binding-active fragments localized the binding site for intrinsic factor-vitamin B12 to CUB domains 5-8 and a receptor-associated protein-binding site to CUB domains 13-14. In conclusion, the N-terminal cubilin region seems crucial for membrane association, whereas the CUB domain cluster harbors distinct sites for ligand binding.

Assimilation of [57Co]-labeled cobalamin in human fetal gastrointestinal xenografts into nude mice

Cobalamin (Cbl) and its Cbl-binding proteins are present in amniotic fluid. Because amniotic fluid is swallowed by the embryo-fetus, we studied the ability of Cbl to be transported and metabolized across the embryo-fetal digestive tract. Human embryonic stomachs and intestines were transplanted into nude mice. The basal secretion of Cbl-binding proteins was studied by gel filtration of the graft juices. Intrinsic factor (IF) was looked for in gastric mucosa by immunohistochemistry. The uptake of [57Co]-labeled Cbl by the intestinal graft was studied by Schilling tests and HPLC. IF, haptocorrin, and a transcobalamin-like protein were detected in gastric juice, with concentration ranges of 5.0-26.4, 1.9-27.1, and 5.2-12.6 pmol/mL, respectively. The IF [57Co]Cbl complex had a single isoprotein with a pI at 5.6, which was maintained after incubation with neuraminidase. Urine excretion percentages (Schilling tests) ranged from 5.5 to 21.2% and from 0.3 to 1.6% when cyano-[57Co]Cbl-IF or cyano-[57Co]Cbl, respectively, was instilled in intestinal grafts. Chloroquine reduced significantly the percentage of excreted [57Co]Cbl. The [57Co]Cbl was mainly excreted as cyano-[57Co]Cbl in urines, showing a low coenzyme conversion. In conclusion, IF is secreted by the nonstimulated embryonic stomach and lacks sialic acid. Cbl binds to it and is subsequently transported across the xenografted embryo-fetal intestine. This suggests that amniotic fluid may contribute to Cbl delivery to the embryo-fetus.

Vitamin B12 absorption test and oral treatment in 14 children with selective vitamin B12 malabsorption

Oral vitamin B12 (VB12) absorption was studied in 12 patients with selective VB12 malabsorption and in 6 age-matched healthy controls. Serum VB12 level was measured before and 3 h after oral administration of VB12 100 or 1000 micrograms. After administration of 1000 micrograms of VB12 an appreciable increase in the serum VB12 level was observed in patients as well as in controls. The mean of the increase in the serum VB12 level did not differ between patients and the controls (273 +/- 203 pg/mL, 180 +/- 71 pg/mL, respectively P > .05). Twelve patients previously treated by parenteral VB12 were switched to, and 2 newly diagnosed patients were started on, oral VB12 treatment of 1000 micrograms given every 2 weeks. Hematological parameters and serum VB12 levels remained stable after switching to oral therapy in the 12 patients. In the two newly diagnosed patients anemia was cured by orally administrated VB12. This study lends further support to the use of megadoses of VB12 as an alternative treatment for selective VB12 malabsorption.

Overexpression of an unstable intrinsic factor-cobalamin receptor in Imerslund-Gräsbeck syndrome

Two sisters with Imerslund-Gräsbeck syndrome who presented with clinical features of cobalamin deficiency are described. Intrinsic factor-cobalamin receptor (IFCR) activity and protein levels were determined in ileal biopsy specimens by using radioisotope assay and immunoblotting, respectively. IFCR activities in ileal homogenates expressed as femtomoles of ligand binding per milligram of protein were 38 +/- 4 in control tissue, 494 +/- 24 in patient 1, and 94 +/- 7 in patient 2. However, when assayed in the presence of IFCR antiserum, the ligand binding was inhibited by > 90% in both normal control and the patients with Imerslund-Gräsbeck syndrome. Immunoblotting of total membranes from the biopsy specimen of these 2 patients failed to detect an immunoreactive band of molecular mass of 185 kilodaltons. These findings are at variance with reports of decreased IFCR activity and indicate a new phenotype in which an active but an unstable receptor is overexpressed in Imerslund-Gräsbeck syndrome.

The Human Intrinsic Factor-Vitamin B12 Receptor,Cubilin: Molecular Characterization and Chromosomal Mapping of the Gene to 10p Within the Autosomal Recessive Megaloblastic Anemia (MGA1) Region

Uptake of vitamin B12 (cyanocobalamin) is facilitated by the cobalamin-binder gastric intrinsic factor (IF), which recognizes a 460-kD receptor, cubilin, present in the epithelium of intestine and kidney. Surface plasmon resonance analysis of ligand-affinity-purified human cubilin demonstrated a high-affinity calcium- and cobalamin-dependent binding of IF-cobalamin. Complete cDNA cloning of the human receptor showed a 3597 amino acid peripheral membrane protein with 69% identity to rat cubilin. Amino-terminal sequencing of the receptor indicates that the cDNA sequence encodes a precursor protein undergoing proteolytic processing due to cleavage at a recognition site (Arg7-Glu8-Lys9-Arg10) for the trans-Golgi proteinase furin. Using fluorescence in situ hybridization, radiation hybrid mapping, and screening of YAC clones, the human cubilin gene was mapped between the markers D10S1661 and WI-5445 on the short arm of chromosome 10. This is within the autosomal recessive megaloblastic anemia (MGA1) 6-cM region harboring the unknown recessive-gene locus of juvenile megaloblastic anemia caused by intestinal malabsorption of cobalamin (Imerslund-Gräsbeck's disease). In conclusion, the present molecular and genetic information on human cubilin now provides circumstantial evidence that an impaired synthesis, processing, or ligand binding of cubilin is the molecular background of this hereditary form of megaloblastic anemia.

The Human Intrinsic Factor-Vitamin B12 Receptor,Cubilin: Molecular Characterization and Chromosomal Mapping of the Gene to 10p Within the Autosomal Recessive Megaloblastic Anemia (MGA1) Region

Uptake of vitamin B12 (cyanocobalamin) is facilitated by the cobalamin-binder gastric intrinsic factor (IF), which recognizes a 460-kD receptor, cubilin, present in the epithelium of intestine and kidney. Surface plasmon resonance analysis of ligand-affinity-purified human cubilin demonstrated a high-affinity calcium- and cobalamin-dependent binding of IF-cobalamin. Complete cDNA cloning of the human receptor showed a 3597 amino acid peripheral membrane protein with 69% identity to rat cubilin. Amino-terminal sequencing of the receptor indicates that the cDNA sequence encodes a precursor protein undergoing proteolytic processing due to cleavage at a recognition site (Arg7-Glu8-Lys9-Arg10) for the trans-Golgi proteinase furin. Using fluorescence in situ hybridization, radiation hybrid mapping, and screening of YAC clones, the human cubilin gene was mapped between the markers D10S1661 and WI-5445 on the short arm of chromosome 10. This is within the autosomal recessive megaloblastic anemia (MGA1) 6-cM region harboring the unknown recessive-gene locus of juvenile megaloblastic anemia caused by intestinal malabsorption of cobalamin (Imerslund-Gräsbeck's disease). In conclusion, the present molecular and genetic information on human cubilin now provides circumstantial evidence that an impaired synthesis, processing, or ligand binding of cubilin is the molecular background of this hereditary form of megaloblastic anemia.

Membrane receptors for endocytosis in the renal proximal tubule

The renal proximal tubule exhibits a very extensive apical endocytic apparatus consisting of an elaborate network of coated pits and small coated and noncoated endosomes. In addition, the cells contain a large number of late endosomes/prelysosomes, lysosomes, and so-called dense apical tubules involved in receptor recycling from the endosomes to the apical plasma membrane. This endocytic apparatus is involved in the reabsorption of molecules filtered in the glomeruli. The process is very effective as demonstrated by the fact that although several grams of protein are filtered daily in the human glomeruli, human urine is virtually devoid of proteins under physiological conditions. Several key receptors appear to be involved in this function, which serves not only to conserve protein as such for the organism but also to reabsorb vital substances such as different vitamins in complex with their binding proteins. Recent research has established megalin, a 600-kDa protein belonging to the LDL receptor family, as probably the most important receptor in this process in the proximal tubule mediating endocytosis of a large variety of ligands and therefore classifying it as a scavenger receptor. More specific receptors like the folate receptor, IGF-II/Man-6-P receptor, and gp280/IFR, identical to the intrinsic factor receptor, are also functioning in the apical endocytic pathway of renal proximal tubules. A better understanding of these receptors will give us new insight into these very important processes for the organism.

The intrinsic factor-vitamin B12 receptor and target of teratogenic antibodies is a megalin-binding peripheral membrane protein with homology to developmental proteins

The present report shows the molecular characterization of the rat 460-kDa epithelial glycoprotein that functions as the receptor facilitating uptake of intrinsic factor-vitamin B12 complexes in the intestine and kidney. The same receptor represents also the yolk sac target for teratogenic antibodies causing fetal malformations in rats. Determination of its primary structure by cDNA cloning identified a novel type of peripheral membrane receptor characterized by a cluster of eight epidermal growth factor type domains followed by a cluster of 27 CUB domains. In accordance with the absence of a hydrophobic segment, the receptor could be released from renal cortex membranes by nonenzymatic and nonsolubilizing procedures. The primary structure has no similarity to known endocytic receptors but displays homology to epidermal growth factor and CUB domain proteins involved in fetal development, e.g. the bone morphogenic proteins. Electron microscopic immunogold double labeling of rat yolk sac and renal proximal tubules demonstrated subcellular colocalization with the endocytic receptor megalin, which is expressed in the same epithelia as the 460-kDa receptor. Furthermore, megalin affinity chromatography and surface plasmon resonance analysis revealed a calcium-dependent high affinity binding of the 460-kDa receptor to megalin, which thereby may mediate its vesicular trafficking. Due to the high number of CUB domains, accounting for 88% of the protein mass, we propose the name cubilin for the novel receptor.

A urinary radioisotope-binding assay to diagnose Gräsbeck-Imerslund disease

BACKGROUND

Gräsbeck-Imerslund disease (congenital familial selective vitamin B12-malabsorption with proteinuria, MGA1, MIM No. 261100) is a rare disorder displaying autosomal recessive inheritance. This study was designed to investigate the usefulness of measuring the activity of the urinary receptor for the intrinsic factor-cobalamin complex as a tool to diagnose this disease.

METHODS

The receptor activity was measured by a radioisotope-binding assay, using phenyl-Sepharose gel as the adsorbant solid phase of the receptor.

RESULTS

In 10 Finnish patients, urinary receptor activity was on the average 640 times (15-1400 times) lower than that in 13 healthy control subjects: mean values of 0.1 nmol/mol (range, 0.01-0.32 nmol/mol) and 6.4 nmol/mol (range, 3.8-12.4 nmol/mol) creatinine, respectively. The mean value of urinary receptor activity in 11 first-degree, healthy relatives of the patients was 4.6 nmol/mol (range, 1.1-10.4 nmol/mol) creatinine, a difference from levels in control subjects that is not statistically significant. When the first-degree relatives were divided into heterozygotes (parents and siblings heterozygous for the haplotype of genetic markers associated with the disease gene) and wild-type homozygotes (siblings not displaying the disease haplotype), no difference was seen.

CONCLUSION

Determination of receptor activity in the urine is a highly accurate method for diagnosis of Gräsbeck-Imerslund disease at an early stage, but it does not detect carriers of the disorder.

[Imerslund's disease. Clinical and biological aspects. Apropos of 6 cases]

BACKGROUND

Imerslund syndrome, a recessive autosomal disease, initially described by Imerslund and Grasbeck in 1960, associates megaloblastic anemia and proteinuria.

CASE REPORT

We report on six cases, studied in five different families. All patients (mean age: 3.5 years) had clinical symptoms of anemia, three had malabsorption, proteinuria was present in five, at the time of diagnosis. Hemogram and decreased serum vitamin B12 levels were consistent with the diagnosis in all cases. Intra-muscular injections of cyanocobalamine was instituted on a life-time basis and the long term prognosis is good.

CONCLUSION

The diagnosis should be evoked when the three typical features are present: macrocytic anemia, decreased serum B12 level and proteinuria. It will be confirmed by the bone marrow megaloblastic aspects and the Schilling test findings.

Oral treatment in selective vitamin B12 malabsorption

PURPOSE

The efficacy of oral treatment with megadose vitamin B12 in a patient with selective vitamin B12 malabsorption is studied.

PATIENTS AND METHODS

An 8-year-old boy with megaloblastic anemia due to selective vitamin B12 malabsorption is presented. His history was significant for anemia of 4 years duration, requiring transfusion on two occasions. On admission, the Hb was 7.9 g/dL, WBC 6 x 10(9)/L, mean corpuscular volume 124 fl, red cell distribution width 16.8%, platelets 156 x 10(9)/L, reticulocyte 0.04%, and the serum vitamin B12 level 87 pmol/L. There was proteinuria. Replacement treatment with oral B12 1,000 micrograms/daily was instituted.

RESULTS

Reticulocytosis was observed on the third day of treatment, which was followed by a gradual increase in Hb level to 12 g/dL in 3 weeks. A Schilling test performed after a 5-day interruption of therapy was compatible with malabsorption.

CONCLUSIONS

Our study suggests that the oral route is as effective as the parenteral route when vitamin B12 is given at a dose larger than that of parenteral therapy.

Purification by cobalamin-Sepharose affinity chromatography and intrinsic factor-binding activity of an extramembrane proteolytic product from pig ileal mucosa

We have purified a cobalamin-binding protein obtained by papain digestion of pig intestine by cobalamin-AH-Sepharose affinity chromatography, with a purification factor of 17,300, a yield of 63% and a cobalamin-binding activity of 11,260 pmol/mg of protein. The protein contained 3.8% carbohydrate and was O- and N-glycosylated. Its molecular mass was 69 kDa on SDS/PAGE and its isoelectric point was 5.1. It had a binding activity for both [57Co]cobalamin and [57Co]cobalamin-intrinsic factor in native PAGE autoradiography and it inhibited the binding of intrinsic factor to the intact intestinal receptor with an IC50 of 49.31 nmol/l in a radioisotope assay. In conclusion, the purified protein shared a binding activity for both cobalamin and intrinsic factor-cobalamin complexes and could correspond to the extracellular domain of the ileal intrinsic factor receptor.