Ileal mucosa in familial selective vitamin B 12 malabsorption

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.

Exome sequencing reveals cubilin mutation as a single-gene cause of proteinuria

In two siblings of consanguineous parents with intermittent nephrotic-range proteinuria, we identified a homozygous deleterious frameshift mutation in the gene CUBN, which encodes cubulin, using exome capture and massively parallel re-sequencing. The mutation segregated with affected members of this family and was absent from 92 healthy individuals, thereby identifying a recessive mutation in CUBN as the single-gene cause of proteinuria in this sibship. Cubulin mutations cause a hereditary form of megaloblastic anemia secondary to vitamin B(12) deficiency, and proteinuria occurs in 50% of cases since cubilin is coreceptor for both the intestinal vitamin B(12)-intrinsic factor complex and the tubular reabsorption of protein in the proximal tubule. In summary, we report successful use of exome capture and massively parallel re-sequencing to identify a rare, single-gene cause of nephropathy.

Genetic heterogeneity of megaloblastic anaemia type 1 in Tunisian patients

Megaloblastic anaemia 1 (MGA1) is a rare autosomal recessive condition characterized by selective intestinal vitamin B12 malabsorption and proteinuria. More than 200 MGA1 patients have been identified worldwide, but the disease is relatively prevalent in Finland, Norway and several Eastern Mediterranean regions. MGA1 is genetically heterogeneous and can be caused by mutations in either the cubilin (CUBN) or the amnionless (AMN) gene. In the present study we investigated the molecular defect underlying MGA1 in nine Tunisian patients belonging to six unrelated consanguineous families. Haplotype and linkage analyses, using microsatellite markers surrounding both CUBN and AMN genes, indicated that four out of the six families were likely to be linked to the CUBN gene. Patients from these families were screened for the Finnish, Mediterranean and Arabian mutations already published. None of the screened mutations could be detected in our population. One family showed a linkage to AMN gene. Direct screening of the AMN gene allowed the identification of the c.208-2A>G mutation, previously described in a Jewish Israeli patient of Tunisian origin and in Turkish patients. This suggests that the c.208-2A>G mutation may derive from a single Mediterranean founder ancestor. For the last family, haplotype analysis excluded both CUBN and AMN genes, suggesting the existence of a third locus that may cause MGA1.

Canine Imerslund-Gräsbeck syndrome maps to a region orthologous to HSA14q

Selective malabsorption of cobalamin (vitamin B(12)) accompanied by proteinuria, known as Imerslund-Gräsbeck syndrome or megaloblastic anemia 1 (I-GS, MGA1; OMIM 261100), is a rare autosomal recessive disorder. In Finnish kindreds, I-GS is caused by mutations in the cubilin gene ( CUBN), located on human Chromosome (Chr) 10. However, not all patients have CUBN mutations, and three distinct mutations in the amnionless gene, AMN, were very recently identified in patients from Norwegian and Israeli families. The present study demonstrates that in a large canine I-GS pedigree, the disease is genetically linked (peak multipoint LOD score 11.74) to a region on dog Chr 8 that exhibits conserved synteny with human Chr 14q. Multipoint analysis indicates that the canine disease gene lies in an interval between the echinoderm microtubule-associated, protein-like 1 ( EML1) gene and the telomere. A single critical recombinant further suggests that the disease gene is between markers in EML1 and the G protein-coupled receptor ( G2A) gene, defining an I-GS interval in the human genome that contains the AMN gene. Thus, these comparative-mapping data provide evidence that canine I-GS is a homologue of one form of the human disease and will provide a useful system for understanding the molecular mechanisms underlying the disease in humans.

Genetically heterogeneous selective intestinal malabsorption of vitamin B12: founder effects, consanguinity, and high clinical awareness explain aggregations in Scandinavia and the Middle East

Selective intestinal malabsorption of vitamin B(12) causing juvenile megaloblastic anemia (MGA; MIM# 261100) is a recessively inherited disorder that is believed to be rare except for notable clusters of cases in Finland, Norway, and the Eastern Mediterranean region. The disease can be caused by mutations in either the cubilin (CUBN; MGA1; MIM# 602997) or the amnionless (AMN; MIM# 605799) gene. To explain the peculiar geographical distribution, we hypothesized that mutations in one of the genes would mainly be responsible for the disease in Scandinavia, and mutations in the other gene in the Mediterranean region. We studied 42 sibships and found all cases in Finland to be due to CUBN (three different mutations) and all cases in Norway to be due to AMN (two different mutations), while in Turkey, Israel, and Saudi Arabia, there were two different AMN mutations and three different CUBN mutations. Haplotype evidence excluded both CUBN and AMN conclusively in five families and tentatively in three families, suggesting the presence of at least one more gene locus that can cause MGA. We conclude that the Scandinavian cases are typical examples of enrichment by founder effects, while in the Mediterranean region high degrees of consanguinity expose rare mutations in both genes. We suggest that in both regions, physician awareness of this disease causes it to be more readily diagnosed than elsewhere; thus, it may well be more common worldwide than previously thought.

Identification of a 4-base deletion in the gene in inherited intrinsic factor deficiency

A 4-base deletion has been identified in the coding region of the gene for gastric intrinsic factor (IF) in an 11-year-old girl with severe anemia and cobalamin (Cbl) deficiency. The bone marrow showed frank megaloblastic morphology, and the Schilling test indicated a failure to absorb Cbl that was corrected by coadministration of IF. Pentagastrin administration induced acid secretion, but the gastric juice lacked IF as determined by CbI binding, by fractionation of protein-bound CbI, and by immunoprecipitation with anti-IF antiserum. Individual exons were amplified by the polymerase chain reaction by using primers to the flanking intronic regions, and the nucleotide sequence analysis identified a 4-base deletion (c183_186delGAAT) spanning positions 104 to 107 in exon 2, resulting in premature termination of translation. This mutation also eliminates a site for Bst XI endonuclease and introduces a site for BsaBI for identifying this deletion in hereditary IF deficiency.

Mutations in CUBN, encoding the intrinsic factor-vitamin B12 receptor, cubilin, cause hereditary megaloblastic anaemia 1

Megaloblastic anaemia 1 (MGA1, OMIM 261100) is a rare, autosomal recessive disorder characterized by juvenile megaloblastic anaemia, as well as neurological symptoms that may be the only manifestations. At the cellular level, MGA1 is characterized by selective intestinal vitamin B12 (B12, cobalamin) malabsorption. MGA1 occurs worldwide, but its prevalence is higher in several Middle Eastern countries and Norway, and highest in Finland (0.8/100,000). We previously mapped the MGA1 locus by linkage analysis in Finnish and Norwegian families to a 6-cM region on chromosome 10p12.1 (ref. 8). A functional candidate gene encoding the intrinsic factor (IF)-B12 receptor, cubilin, was recently cloned; the human homologue, CUBN, was mapped to the same region. We have now refined the MGA1 region by linkage disequilibrium (LD) mapping, fine-mapped CUBN and identified two independent disease-specific CUBN mutations in 17 Finnish MGA1 families. Our genetic and molecular data indicate that mutations in CUBN cause MGA1.

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.

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.

A case report of 46,XX,del(21)(q22) de novo deletion associated with Imerslund-Grasbeck syndrome

We describe a 2-year-old female patient who had megaloblastic anaemia caused by selective vitamin B12 malabsorption (Imerslund-Grasbeck syndrome) and del(21)(q22). To our knowledge, this is the first observation of Imerslund-Grasbeck syndrome associated with del(21)(q22) in the literature.

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

BACKGROUND/AIMS

The pathogenesis of inherited intestinal cobalamin malabsorption (Gräsbeck-Imerslund disease) remains unknown. The authors studied whether the disease corresponds to a defective expression and/or function of the intrinsic factor-cobalamin receptor in the ileum.

METHODS

Intrinsic factor-cobalamin receptor activity was measured using radioisotope assay and gel-filtration exclusion chromatography in ileal biopsy specimens and urine concentrates from 4 patients with Gräsbeck-Imerslund disease and 5 controls.

RESULTS

Receptor activity was 164 +/- 13 fmol/mg of protein in control biopsy specimens and < 2.6 fmol/mg protein in specimens from patients. The association constant was estimated to be 3.8 +/- 0.4 (nmol/L)-1 in controls. A dramatic decrease in receptor activity was also observed in urine concentrate from patients with an association constant of 1.9 and 3.3 (nmol/L)-1. Isoelectrofocusing of the cross-linked intrinsic factor-cobalamin receptor complex showed an isoelectric point at 4.8 in a patient as well as in control samples.

CONCLUSIONS

It is concluded that Gräsbeck-Imerslund disease is related to decreased intrinsic factor-receptor activity in intestinal mucosa; the receptor assay in urine can be helpful for diagnosis.

Familial selective vitamin B12 malabsorption (Imerslund-Gräsbeck syndrome) in a pool of Turkish patients

Thirty-six patients with Imerslund-Gräsbeck syndrome are presented. The mean ages at presentation and diagnosis were 4.7 +/- 3.7 years and 7.2 +/- 4.2 years, respectively. The mean hemoglobin level was 5.8 +/- 2.2 g/dL, the mean cell volume was 104.9 +/- 11.6 fL, the white blood cell count was 4479 +/- 2022/mm3, and the serum vitamin B12 level was 96.9 +/- 73 pg/mL. At diagnosis, 5 of the 36 patients, aged 5 to 16 years, had neurologic symptoms. All the patients had severe megaloblastic changes in bone marrow precursor cells. Proteinuria was detected in 78% of them. Patients with proteinuria had a younger age of onset (P < 0.0001) and diagnosis (P < 0.001) compared with those without proteinuria. In all patients, vitamin B12 excretion unbound to intrinsic factor after a flushing dose of vitamin B12 was lower than normal, and there was no appreciable correction in urinary vitamin B12 excretion after binding of intrinsic factor. The impairment of vitamin B12 absorption studies in Schilling tests; however, showed great variation among patients. Serum haptoglobin values were close to zero in all patients, indicating the presence of that intravascular hemolysis in Imerslund-Gräsbeck syndrome. Variations among patients in the age of presentation, degree of impairment of vitamin B12 absorption, and presence or absence of proteinuria suggest a heterogeneity in etiology of Imerslund-Gräsbeck syndrome at the molecular level.

Imerslund-Grasbeck syndrome in a Chinese family with distinct skin lesions refractory to vitamin B12

Two brothers in a Chinese family with selective malabsorption of vitamin B12 associated with proteinuria (Imerslund-Grasbeck syndrome) presented with widespread mottled skin pigmentation, termed poikiloderma. In contrast to anaemia, this pigmentary disturbance remained unresponsive to vitamin B12 replacement. This is different from the reported hyperpigmentation sometimes seen in vitamin B12 deficiency which is reversible following treatment. As far as is known, an irreversible and persistent skin disorder has not been reported in this syndrome before.

Imerslund-Gräsbeck syndrome in a Saudi family

Imerslund-Gräsbeck syndrome, an autosomal recessive trait of defective uptake of intrinsic factor-vitamin B12 complex by terminal ileum, is described in a Saudi family (two siblings and their first cousin). This rare disease has previously been reported only in Northern Europe and North African Jewish ethnic groups.

Reversal of severe neurological abnormalities after vitamin B12 replacement in the Imerslund-Grasbeck syndrome

Vitamin B12 malabsorption in the ileum has been postulated as the underlying cause of the Imerslund-Grasbeck syndrome comprising megaloblastic anemia, proteinuria, and multiple neurological abnormalities. A young Saudi child with spasticity, truncal ataxia, cerebral atrophy, megaloblastic anaemia and proteinuria is described. Replacement therapy with parenteral vitamin B12 resulted in the complete resolution of his neurological findings and brain atrophy.

Intrinsic factor secretion and cobalamin absorption. Physiology and pathophysiology in the gastrointestinal tract

Intrinsic factor is produced by the gastric parietal cell. Its secretion is stimulated via all pathways known to stimulate gastric acid secretion: histamine, gastrin, and acetylcholine. There is, however, a different mode of secretion for both substances: atropine, vagotomy, and H2 receptor antagonists inhibit both intrinsic factor and acid secretion, but secretin and the hydrogen-potassium ATPase antagonist omeprazole have no effect on intrinsic factor while substantially reducing acid secretion. Cobalamin in food is bound to animal protein. Cobalamin deficiency due to inadequate dietary intake is rarely seen in extreme vegetarians (vegans). In the stomach cobalamin is liberated from its protein binding by peptic digestion and bound to R-proteins. Hypochlorhydria or achlorhydria, whether medically induced or not, may impair cobalamin uptake. The cobalamin-R-protein complex is split by pancreatic enzymes in the duodenum, where cobalamin is bound to intrinsic factor. Pancreatic insufficiency may lead to cobalamin deficiency. Lack of intrinsic factor is the commonest cause of cobalamin deficiency; very rarely, aberrant forms of intrinsic factor are produced, but the clinical syndrome is similar. Gram-negative anaerobe bacteria bind the cobalamin-intrinsic factor complex, and bacterial overgrowth of the small intestine diminishes cobalamin resorption. Parasitic infections with fish tape-worm and Giardia lamblia are also associated with cobalamin malabsorption. The cobalamin-intrinsic factor complex binds to the ileal receptors in the terminal ileum. Cobalamin absorption may be impaired after resection or by diseases affecting more than 50 cm of the terminal ileum, such as Crohn's disease, coeliac disease, tuberculosis, lymphoma or radiation. There is clearly a wide diversity in the aetiology of cobalamin deficiency, which requires a versatile diagnostic approach.

Imerslund syndrome with dolichocephaly

Imerslund syndrome is a rare autosomal recessive disorder of megaloblastic anemia as a result of selective vitamin B12 malabsorption associated with proteinuria. An Arabic Muslim family is described, with three children who had inherited selective vitamin B12 malabsorption with proteinuria. Dolichocephaly was noted in all the male children of this family in association with congenital megaloblastic anemia and proteinuria. The findings of this anemia are compatible with Imerslund-Gräsbeck syndrome, and coexistence of this syndrome with dolichocephaly in a single family has not been previously reported.

Imerslund-Grasbeck syndrome in a Libyan boy

Immerslund-Grasbeck syndrome is an uncommon disease, characterized by megaloblastic anaemia and persistent proteinuria. A Libyan boy with the characteristic findings is presented. He received intramuscular vitamin B12 injections and there followed a remarkable clinical and haematological improvement.

Intrinsic factor within parietal cells of patients with juvenile pernicious anemia. A retrospective immunohistochemical study

One of the diverse group of disorders that cause pernicious anemia in childhood, juvenile pernicious anemia, has been characterized by normal acid secretion, normal gastric mucosal histology, adequate intestinal absorption of cobalamin in the presence of exogenous gastric intrinsic factor (IF), but the inadequate "production" of IF from birth. Inadequate production has been inferred from the absence of measured IF in stimulated gastric secretions. To assess whether immunogenic IF was commonly present within the parietal cells of subjects with juvenile pernicious anemia, we studied paraffin-embedded biopsy material from the largest reported series of childhood pernicious anemia, using a well-characterized indirect immunoperoxidase method. Preliminary studies were able to identify IF in fundic mucosal biopsy specimens that had been stored for as long as 27 yr. In a blinded evaluation, six of the nine fundic biopsy specimens from children with juvenile pernicious anemia demonstrated immunogenic IF. Two sets of siblings were concordant for the presence or absence of intracellular IF, and six gastric biopsy specimens from patients with Imerslund's syndrome were all positive for IF. These findings indicate that juvenile pernicious anemia is a heterogeneous group of disorders whose similar clinical expression might be caused by (a) inadequate synthesis of IF, (b) a block in IF secretion, (c) the secretion of an abnormal IF that does not bind to cobalamin, or (d) the secretion of other abnormal IFs that could contain a number of other functional defects.

Absent ileal uptake of IF-bound vitamin B12 in vivo in the Imerslund-Grasbeck syndrome (familial vitamin B12 malabsorption with proteinuria)

A Syrian family is described with three children who had inherited selective vitamin B12 malabsorption associated with proteinuria. (Imerslund-Grasbeck syndrome). Although inherited the defect was apparently not present at birth. A third child had less severe vitamin B12 malabsorption, was not vitamin B12 deficient and had no proteinuria. Studies on two of the affected children with subcellular fractionation of the uptake of radioactive vitamin B12 by ileal tissue in vivo indicate a defect in the ileal receptors for IF-bound vitamin B12. These findings are different from the single in vitro experiment on a patient with this condition that has been previously reported.

Biochemistry and clinical chemistry of vitamin B12 transport and the related diseases

This review focusses on research performed by the author and coworkers. The absorption, turnover and excretion of cobalamin and the pathogenesis of cobalamin deficiency states are described and the laboratory tests used to diagnose these states are discussed. Topics dealt with in detail include: overall turnover, daily need, enterohepatic circulation and excretion of cobalamin and other corrins . The soluble proteins mediating cobalamin transport and their cellular receptors are described and their nomenclature, isolation, structure and mode of action, the role of calcium in the membrane transport, the evolution of these systems and the analogies with transport systems for other substrates are discussed together with deficiency states, especially fish tapeworm anemia and familial selective vitamin B12 malabsorption with proteinuria. Folate deficiency is a relatively rare cause of megaloblastic anemia in Scandinavia but common in North America and explanations for this difference are suggested. The methods of assaying cobalamin in serum and plasma and the performance of radiovitamin B12 absorption tests are critically evaluated. The measurement of intrinsic factor in gastric juice, serum, amniotic fluid and urine is described.

Imerslund-Gräsbeck anemia. A long-term follow-up study

A follow-up study has been performed on 14 patients, now aged 6-46 years, with Imerslund-Gräsbeck anemia (congenital, hereditary selective malasorption of vitamin B12). On intramuscular vitamin B12 therapy, the patients are clinically and hematologically normal. Those who had constant proteinuria in childhood continue to excrete protein in the urine. Our patients excrete an average of 750 mg of protein per 24 hours (range 13-1460 mg). The proteinuria is predominantly of glomerular origin, but some is also of tubular origin. Renal biopsies of the two oldest patients were normal on light microscopy. Electron microscopy revealed moderate signs of chronic glomerulopathy of mesangioproliferative type in both patients. The renal lesions do not seem to be progressive.

Failure to thrive associated with the Imerslund-Grasbeck syndrome

Two siblings are described who presented with failure to thrive and in whom a diagnosis of Imerslund-Grasbeck syndrome was made. Following treatment with i.m. hydroxocobalamin there was a good and rapid increase in weight and their haematology returned to normal.

Selective vitamin B12 malabsorption (Imerslund-Gräsbeck syndrome). Studies on gastroenterological and nephrological problems

In a girl 10 years of age with selective vitamin B12 malabsorption associated with proteinuria and residual symptoms of funicular myelosis an extensive study of the intestinal and nephrologic functions was done. Repeated Schilling tests pointed to a malabsorption pattern of vitamin B12. Gastric acid and intrinsic factor secretion as well as gastric morphology were normal. There were no antibodies against intrinsic factor and parietal cells in serum. Ileal mucosa showed on light- and electron-microscopy no pathologic changes. Pancreatic exocrine function as well as pH and calcium concentrations in the lumen of the gut were within the normal range. A general malabsorption syndrome could be excluded. A high selective glomerular proteinuria was found through different methods. Inulin clearance was slightly reduced, PAH clearance, however, markedly so. There was no further evidence for renal tubular dysfunction. Renal biopsy showed a minimal proliferative intercapillary glomerulonephritis (minimal changes). In electron-microscopic studies a fusion of a part of the foot processes of the podocytes was found. No familialhistory of the syndrome could be demonstrated in our patient.