Decreased affinity of urinary intrinsic factor-cobalamin receptor in a case of Gräsbeck-Imerslund syndrome

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.

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.

An analysis of published trials of interferon monotherapy in children with chronic hepatitis C

BACKGROUND

Although no therapeutic regimen has received Food and Drug Administration approval for treating children with chronic hepatitis C viral infection (CHC), there have been a number of pediatric interferon-alpha (IFN-alpha) trials. The purpose of this study was to perform a critical review of these trials to determine 1) end-of-treatment (ETR) and sustained-response (SR) rates, 2) predictors of response to therapy, and 3) safety of and tolerance to IFN-alpha in children with CHC.

METHODS

Relevant studies in the English-language medical literature and abstracts (January 1990 through November 2000) were identified by searching for manuscripts that contained the key words "children," "hepatitis C," and "interferon." Trials were considered eligible for inclusion in this analysis if criteria for treatment included positive serum polymerase chain reaction for hepatitis C virus RNA (HCV PCR).

RESULTS

Twenty published manuscripts of the use of IFN-alpha in children with CHC were found, of which 12 met our inclusion criteria. Twenty-two abstracts, of which seven met our inclusion criteria, were identified. In the 19 included trials, 366 treated and 105 untreated children were observed; five countries were represented. Average ETR was 54% (0%-91%) and average SR was 36% (0%-73%). The SR in children with genotype 1 was 27% versus 70% for nongenotype 1 ( P = 0.001). Five of 105 (5%) untreated controls exhibited spontaneous viral clearance.

CONCLUSIONS

To date, there is no published large-scale, multicenter, prospective, placebo-controlled randomized trial of the use of IFN-alpha in children with CHC. The data in this review suggest that IFN-alpha in children with CHC does have reasonable efficacy and safety. This review highlights the need for a more systematic design of future pediatric CHC trials. Ideally, such trials would be large scale, prospective, and controlled, and would include HCV genotype and viral load, histology, quality of life measures, and systematic recording of adverse events and of effects of therapy on growth and development.

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.

Cubilin expression and posttranslational modification in the canine gastrointestinal tract

Cubilin is an endocytic receptor of the apical brush border membrane that is essential for intrinsic factor-mediated cobalamin absorption in small intestine. However, cubilin is more highly expressed in kidney and yolk sac, and recent molecular characterization of the receptor has focused on these tissues. The aim of this investigation was to examine tissue-specific cubilin expression and posttranslational modifications with an emphasis on the gastrointestinal tract. Intrinsic factor-cobalamin binding activity, cubilin immunoreactivity, and cubilin mRNA levels were determined in multiple segments of canine gastrointestinal mucosa and other tissues. These aspects of cubilin expression varied in parallel, suggesting that the major determinant of regional cubilin expression in the gastrointestinal tract is modulation of cubilin mRNA. Cell fractionation indicated that ileal cubilin is not strongly membrane associated. An approximately 185-kDa brush border specific and two >400-kDa precursor forms of cubilin were identified. Asparagine-linked oligosaccharide modifications characterized by differential glycosidase digestion of affinity-purified cubilin from ileal mucosa and renal cortex differed, but ileal and renal intracellular cubilin comigrated on SDS-PAGE at approximately 400 kDa after oligosaccharide removal, thus reconciling previous conflicting size estimates of the cubilin polypeptide.