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

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.

Cubilin, the intrinsic factor-vitamin B12 receptor

Cubilin (CUBN), the intrinsic factor-vitamin B12 receptor is a large endocytic protein involved in various physiological functions: vitamin B12 uptake in the gut; reabsorption of albumin and maturation of vitamin D in the kidney; nutrient delivery during embryonic development. Cubilin is an atypical receptor, peripherally associated to the plasma membrane. The transmembrane proteins amnionless (AMN) and Lrp2/Megalin are the currently known molecular partners contributing to plasma membrane transport and internalization of Cubilin. The role of Cubilin/Amn complex in the handling of vitamin B12 in health and disease has extensively been studied and so is the role of the Cubilin-Lrp2 tandem in renal pathophysiology. Accumulating evidence strongly supports a role of Cubilin in some developmental defects including impaired closure of the neural tube. Are these defects primarily caused by the dysfunction of a specific Cubilin ligand or are they secondary to impaired vitamin B12 or protein uptake? We will present the established Cubilin functions, discuss the developmental data and provide an overview of the emerging implications of Cubilin in the field of cardiovascular disease and cancer pathogenesis.

A 17-Month-old Boy With Pancytopenia Caused by a Rare Genetic Defect of Vitamin B12 Malabsorption

Imerslund-Gräsbeck syndrome is an autosomal recessive disorder of vitamin B12 malabsorption presenting with megaloblastic anemia and mild proteinuria in childhood. The disorder is caused by biallelic pathogenic variants in the CUBN or AMN genes, which encode proteins involved in B12 absorption. We present the case of a 17-month-old boy with failure to thrive, pancytopenia, and fevers. His megaloblastic anemia was overlooked leading to unnecessary invasive testing. Findings on bone marrow biopsy prompted investigation for genetic disorders of B12 metabolism. Exome sequencing uncovered 1 known pathogenic variant and 1 novel likely pathogenic variant in CUBN, confirming the diagnosis of Imerslund-Gräsbeck syndrome.

Novel CUBN Mutation in a Young Child With Megaloblastic Anemia

Inherited disorders of cobalamin (Cbl, vitamin B12) metabolism are rare causes of megaloblastic anemia and neurologic abnormalities. More prevalent in certain ethnic groups, these disorders occur despite adequate Cbl intake and usually result from abnormal vitamin cell transport or processing. Cubilin (CUBN, intrinsic factor-cobalamin receptor) is the intestinal receptor for the endocytosis of intrinsic factor-vitamin B12. Its gene is localized to chromosome 10p13 and mutations involving CUBN have been described in patients with congenital megaloblastic anemia. In this report, we describe a novel CUBN pathogenic variant in a child with megaloblastic anemia.

A child with Imerslund-Gräsbeck syndrome concealed by co-existing α-thalassaemia presenting with subacute combined degeneration of the spinal cord: a case report

BACKGROUND

Imerslund-Gräsbeck syndrome is a rare genetic disease characterised by vitamin B₁₂ deficiency and proteinuria.

CASE PRESENTATION

A 4-year old Sri Lankan boy presented with gradually worsening difficulty in walking for two weeks duration. He was previously diagnosed and managed as having non-transfusion-dependent α-thalassaemia based on the presence of hypochromic microcytic anaemia, haemoglobin H inclusion bodies in the blood film and compound heterozygous α-thalassaemia genotype with a gene deletion. However, his transfusion requirement increased over the past three months and he gradually lost his motor developmental milestones during two weeks before admission. The neurological examination revealed generalised hypotonia, exaggerated knee jerks and extensor plantar response. His complete blood count showed pancytopenia, and bone marrow biopsy revealed megaloblastic changes. Serum vitamin B₁₂ and red blood cell folate levels were low. MRI revealed sub-acute combined degeneration of the spinal cord with characteristic 'inverted V sign'. Urine analysis showed non-nephrotic range proteinuria. The diagnosis of Imerslund-Gräsbeck syndrome was made due to the presence of non-nutritional vitamin B₁₂ deficiency and asymptomatic proteinuria. He showed a rapid haematological and neurological improvement to intramuscular hydroxocobalamin.

CONCLUSIONS

This case report presents a rare occurrence of severe vitamin B₁₂ deficiency due to Imerslund-Gräsbeck syndrome masked by co-existent α-thalassaemia, resulting in serious consequences. It highlights the need for a high index of suspicion in evaluating children with severe anaemia, especially in the presence of mixed pathologies.

Imerslund-Gräsbeck Syndrome presenting with microangiopathic hemolytic anemia in a child

Imerslund-Gräsbeck Syndrome is a rare autosomal recessive disorder characterized by proteinuria and selective malabsorption of cobalamin. Deficiency of cobalamin can lead to megaloblastic anemia, pancytopenia and even "pseudo"-thrombotic microangiopathy (TMA). Signs of mechanical hemolysis on peripheral blood smear, elevated lactate dehydrogenase and thrombocytopenia are common findings of TMA. We report a child presenting with TMA features with cobalamin deficiency. Because of her family history of vitamin B12 deficiency and proteinuria, the performed genetic analysis revealed that an Imerslund-Gräsbeck Syndrome with the detection of a homozygous mutation in AMN gene.

GutSelf: Interindividual Variability in the Processing of Dietary Compounds by the Human Gastrointestinal Tract

Nutritional research is currently entering the field of personalized nutrition, to a large extent driven by major technological breakthroughs in analytical sciences and biocomputing. An efficient launching of the personalized approach depends on the ability of researchers to comprehensively monitor and characterize interindividual variability in the activity of the human gastrointestinal tract. This information is currently not available in such a form. This review therefore aims at identifying and discussing published data, providing evidence on interindividual variability in the processing of the major nutrients, i.e., protein, fat, carbohydrates, vitamins, and minerals, along the gastrointestinal tract, including oral processing, intestinal digestion, and absorption. Although interindividual variability is not a primary endpoint of most studies identified, a significant number of publications provides a wealth of information on this topic for each category of nutrients. This knowledge remains fragmented, however, and understanding the clinical relevance of most of the interindividual responses to food ingestion described in this review remains unclear. In that regard, this review has identified a gap and sets the base for future research addressing the issue of the interindividual variability in the response of the human organism to the ingestion of foods.

Cubilin, the Intrinsic Factor-Vitamin B12 Receptor in Development and Disease

Gp280/Intrinsic factor-vitamin B12 receptor/Cubilin (CUBN) is a large endocytic receptor serving multiple functions in vitamin B12 homeostasis, renal reabsorption of protein or toxic substances including albumin, vitamin D-binding protein or cadmium. Cubilin is a peripheral membrane protein consisting of 8 Epidermal Growth Factor (EGF)-like repeats and 27 CUB (defined as Complement C1r/C1s, Uegf, BMP1) domains. This structurally unique protein interacts with at least two molecular partners, Amnionless (AMN) and Lrp2/Megalin. AMN is involved in appropriate plasma membrane transport of Cubilin whereas Lrp2 is essential for efficient internalization of Cubilin and its ligands. Observations gleaned from animal models with Cubn deficiency or human diseases demonstrate the importance of this protein. In this review addressed to basic research and medical scientists, we summarize currently available data on Cubilin and its implication in renal and intestinal biology. We also discuss the role of Cubilin as a modulator of Fgf8 signaling during embryonic development and propose that the Cubilin-Fgf8 interaction may be relevant in human pathology, including in cancer progression, heart or neural tube defects. We finally provide experimental elements suggesting that some aspects of Cubilin physiology might be relevant in drug design.

Gastrointestinal Handling of Water-Soluble Vitamins

Nine compounds are classified as water-soluble vitamins, eight B vitamins and one vitamin C. The vitamins are mandatory for the function of numerous enzymes and lack of one or more of the vitamins may lead to severe medical conditions. All the vitamins are supplied by food in microgram to milligram quantities and in addition some of the vitamins are synthesized by the intestinal microbiota. In the gastrointestinal tract, the vitamins are liberated from binding proteins and for some of the vitamins modified prior to absorption. Due to their solubility in water, they all require specific carriers to be absorbed. Our current knowledge concerning each of the vitamins differs in depth and focus and is influenced by the prevalence of conditions and diseases related to lack of the individual vitamin. Because of that we have chosen to cover slightly different aspects for the individual vitamins. For each of the vitamins, we summarize the physiological role, the steps involved in the absorption, and the factors influencing the absorption. In addition, for some of the vitamins, the molecular base for absorption is described in details, while for others new aspects of relevance for human deficiency are included. © 2018 American Physiological Society. Compr Physiol 8:1291-1311, 2018.

Amnionless-mediated glycosylation is crucial for cell surface targeting of cubilin in renal and intestinal cells

Mutations in either cubilin (CUBN) or amnionless (AMN) genes cause Imerslund–Gräsbeck syndrome (IGS), a hereditary disease characterised by anaemia attributed to selective intestinal malabsorption of cobalamin and low-molecular weight proteinuria. Although cubilin protein does not have a transmembrane segment, it functions as a multi-ligand receptor by binding to the transmembrane protein, amnionless. We established a system to quantitatively analyse membrane targeting of the protein complex in cultured renal and intestinal cells and analysed the pathogenic mechanisms of mutations found in IGS patients. A novel CUBN mutation, several previously reported CUBN missense mutations and all previously reported AMN missense mutations resulted in endoplasmic reticulum (ER) retention and completely inhibited amnionless-dependent plasma membrane expression of cubilin. The ER retention of cubilin and amnionless was confirmed in renal proximal tubular cells of a patient with IGS. Notably, the interaction between cubilin and amnionless was not sufficient, but amnionless-mediated glycosylation of cubilin was necessary for their surface expression. Quantitative mass spectrometry and mutagenesis demonstrated that N-linked glycosylation of at least 4 residues of cubilin protein was required for its surface targeting. These results delineated the molecular mechanisms of membrane trafficking of cubilin in renal and intestinal cells.

Lmbrd1 expression is essential for the initiation of gastrulation

The rare inborn cblF defect of cobalamin metabolism is caused by mutations in the limb region 1 (LMBR1) domain containing 1 gene (LMBRD1). This defect is characterized by massive accumulation of free cobalamin in lysosomes and loss of mitochondrial succinyl‐CoA synthesis and cytosolic methionine synthesis. Affected children suffer from heart defects, developmental delay and megaloblastic anemia. LMBRD1 encodes for LMBD1, a predicted lysosomal cobalamin transport protein. In this study, we determine the physiological function of LMBRD1 during embryogenesis by generating Lmbrd1 deficient mice using the Cre/LoxP system. Complete loss of Lmbrd1 function is accompanied by early embryonic death in mice. Whole mount in situ hybridization studies against bone morphogenetic protein 4 and Nodal show that initial formation of the proximal–distal axis is unaffected in early embryonic stages whereas the initiation of gastrulation is disturbed shown by the expression pattern of even skipped homeotic gene 1 and fibroblast growth factor 8 in Lmbrd1 deficient mice. We conclude that intact function of LMBD1 is essential for the initiation of gastrulation.

Novel compound heterozygous mutations in AMN cause Imerslund-Gräsbeck syndrome in two half-sisters: a case report

Background

Imerslund-Gräsbeck Syndrome (IGS) is a rare autosomal recessive disease characterized by intestinal vitamin B12 malabsorption. Clinical features include megaloblastic anemia, recurrent infections, failure to thrive, and proteinuria. Recessive mutations in cubilin (CUBN) and in amnionless (AMN) have been shown to cause IGS. To date, there are only about 300 cases described worldwide with only 37 different mutations found in CUBN and 30 different in the AMN gene.

Case presentation

We collected pedigree structure, clinical data, and DNA samples from 2 Caucasian English half-sisters with IGS. Molecular diagnostics was performed by direct Sanger sequencing of all 62 exons of the CUBN gene and 12 exons of the AMN gene. Because of lack of parental DNA, cloning, and sequencing of multiple plasmid clones was performed to assess the allele of identified mutations. Genetic characterization revealed 2 novel compound heterozygous AMN mutations in both half-sisters with IGS. Trans-configuration of the mutations was confirmed.

Conclusion

We have identified novel compound heterozygous mutations in AMN in a family from the United Kingdom with clinical features of Imerslund-Gräsbeck Syndrome.

Detailed investigations of proximal tubular function in Imerslund-Gräsbeck syndrome

BACKGROUND

Imerslund-Gräsbeck Syndrome (IGS) is a rare genetic disorder characterised by juvenile megaloblastic anaemia. IGS is caused by mutations in either of the genes encoding the intestinal intrinsic factor-vitamin B12 receptor complex, cubam. The cubam receptor proteins cubilin and amnionless are both expressed in the small intestine as well as the proximal tubules of the kidney and exhibit an interdependent relationship for post-translational processing and trafficking. In the proximal tubules cubilin is involved in the reabsorption of several filtered plasma proteins including vitamin carriers and lipoproteins. Consistent with this, low-molecular-weight proteinuria has been observed in most patients with IGS. The aim of this study was to characterise novel disease-causing mutations and correlate novel and previously reported mutations with the presence of low-molecular-weight proteinuria.

METHODS

Genetic screening was performed by direct sequencing of the CUBN and AMN genes and novel identified mutations were characterised by in silico and/or in vitro investigations. Urinary protein excretion was analysed by immunoblotting and high-resolution gel electrophoresis of collected urines from patients and healthy controls to determine renal phenotype.

RESULTS

Genetic characterisation of nine IGS patients identified two novel AMN frameshift mutations alongside a frequently reported AMN splice site mutation and two CUBN missense mutations; one novel and one previously reported in Finnish patients. The novel AMN mutations were predicted to result in functionally null AMN alleles with no cell-surface expression of cubilin. Also, the novel CUBN missense mutation was predicted to affect structural integrity of the IF-B12 binding site of cubilin and hereby most likely cubilin cell-surface expression. Analysis of urinary protein excretion in the patients and 20 healthy controls revealed increased urinary excretion of cubilin ligands including apolipoprotein A-I, transferrin, vitamin D-binding protein, and albumin. This was, however, only observed in patients where plasma membrane expression of cubilin was predicted to be perturbed.

CONCLUSIONS

In the present study, mutational characterisation of nine IGS patients coupled with analyses of urinary protein excretion provide additional evidence for a correlation between mutation type and presence of the characteristic low-molecular-weight proteinuria.

Imerslund-Gräsbeck syndrome in a 25-month-old Italian girl caused by a homozygous mutation in AMN

Imerslund-Gräsbeck syndrome is a rare autosomal recessive disorder, characterized by vitamin B12 deficiency due to selective malabsorption of the vitamin and usually results in megaloblastic anemia appearing in childhood. It is responsive to parenteral vitamin B12 therapy.

The estimated prevalence (calculated based on Scandinavian data) is less than 6:1,000,000. However, many cases may be misdiagnosed.

When there is reasonable evidence to suspect that a patient suffers from IGS, a new and straightforward approach to diagnosis is mutational analysis of the appropriate genes. We report for the first time the case of a girl of Italian ancestry with IGS genetically confirmed by the detection of a homozygous missense mutation in the AMN gene (c.208-2 A > G).

An exon 53 frameshift mutation in CUBN abrogates cubam function and causes Imerslund-Gräsbeck syndrome in dogs

Cobalamin malabsorption accompanied by selective proteinuria is an autosomal recessive disorder known as Imerslund-Gräsbeck syndrome in humans and was previously described in dogs due to amnionless (AMN) mutations. The resultant vitamin B12 deficiency causes dyshematopoiesis, lethargy, failure to thrive, and life-threatening metabolic disruption in the juvenile period. We studied 3 kindreds of border collies with cobalamin malabsorption and mapped the disease locus in affected dogs to a 2.9Mb region of homozygosity on canine chromosome 2. The region included CUBN, the locus encoding cubilin, a peripheral membrane protein that in concert with AMN forms the functional intrinsic factor-cobalamin receptor expressed in ileum and a multi-ligand receptor in renal proximal tubules. Cobalamin malabsorption and proteinuria comprising CUBN ligands were demonstrated by radiolabeled cobalamin uptake studies and SDS-PAGE, respectively. CUBN mRNA and protein expression were reduced ~10 fold and ~20 fold, respectively, in both ileum and kidney of affected dogs. DNA sequencing demonstrated a single base deletion in exon 53 predicting a translational frameshift and early termination codon likely triggering nonsense mediated mRNA decay. The mutant allele segregated with the disease in the border collie kindred. The border collie disorder indicates that a CUBN mutation far C-terminal from the intrinsic factor-cobalamin binding site can abrogate receptor expression and cause Imerslund-Gräsbeck syndrome.

How can cobalamin injections be spaced in long-term therapy for inborn errors of vitamin B(12) absorption?

Inborn errors of cobalamin (Cbl, vitamin B(12)) absorption include hereditary intrinsic factor deficiency (HIFD) and Imerslund-Gräsbeck disease (IGD). HIFD is secondary to mutations in the HIF gene while IGD is due to mutations in one of the 2 subunits of the intrinsic factor receptor that is cubilin (CUBN) or amnionless (AMN). These disorders lead to intracellular Cbl depletion which in turn causes megaloblastic bone marrow failure, accumulation of homocysteine and methylmalonic acid (MMA), and methionine depletion. The clinical presentation reflects Cbl deficiency, with gastrointestinal symptoms, pancytopenia, and megaloblastic anemia. Mixed proteinuria, when it is present is strongly suggestive of IGD. Accurate diagnosis is always an emergency because early detection and treatment with life-long parenteral pharmacological doses of hydroxocobalamin are life saving and prevent further deterioration. However, the optimal frequency for cobalamin injections as a maintenance therapy is poorly reported. In order to evaluate the optimal maintenance schedule of cobalamin injections, we retrospectively collected clinical, biological, molecular and treatment data on 7 patients affected with congenital Cbl malabsorption. Unlike previous recommendations, we showed that a maintenance dosage of 1 mg cobalamin twice a year was enough to ensure a normal clinical status and keep the hematological and metabolic parameters in the normal range. These data suggest that patients affected with inborn errors of cobalamin absorption may be safely long-term treated with cobalamin injections every 6 months with careful follow-up of hematological and metabolic parameters. This maintenance regime is beneficial because the patients' quality of life improves.

Inherited cobalamin malabsorption. Mutations in three genes reveal functional and ethnic patterns

BACKGROUND

Inherited malabsorption of cobalamin (Cbl) causes hematological and neurological abnormalities that can be fatal. Three genes have been implicated in Cbl malabsorption; yet, only about 10% of ~400-500 reported cases have been molecularly studied to date. Recessive mutations in CUBN or AMN cause Imerslund-Gräsbeck Syndrome (IGS), while recessive mutations in GIF cause Intrinsic Factor Deficiency (IFD). IGS and IFD differ in that IGS usually presents with proteinuria, which is not observed in IFD. The genetic heterogeneity and numerous differential diagnoses make clinical assessment difficult.

METHODS

We present a large genetic screening study of 154 families or patients with suspected hereditary Cbl malabsorption. Patients and their families have been accrued over a period spanning >12  years. Systematic genetic testing of the three genes CUBN, AMN, and GIF was accomplished using a combination of single strand conformation polymorphism and DNA and RNA sequencing. In addition, six genes that were contenders for a role in inherited Cbl malabsorption were studied in a subset of these patients.

RESULTS

Our results revealed population-specific mutations, mutational hotspots, and functionally distinct regions in the three causal genes. We identified mutations in 126/154 unrelated cases (82%). Fifty-three of 126 cases (42%) were mutated in CUBN, 45/126 (36%) were mutated in AMN, and 28/126 (22%) had mutations in GIF. We found 26 undescribed mutations in CUBN, 19 in AMN, and 7 in GIF for a total of 52 novel defects described herein. We excluded six other candidate genes as culprits and concluded that additional genes might be involved.

CONCLUSIONS

Cbl malabsorption is found worldwide and genetically complex. However, our results indicate that population-specific founder mutations are quite common. Consequently, targeted genetic testing has become feasible if ethnic ancestry is considered. These results will facilitate clinical and molecular genetic testing of Cbl malabsorption. Early diagnosis improves the lifelong care required by these patients and prevents potential neurological long-term complications. This study provides the first comprehensive overview of the genetics that underlies the inherited Cbl malabsorption phenotype.

Ancient founder mutation is responsible for Imerslund-Gräsbeck Syndrome among diverse ethnicities

Background

Imerslund-Gräsbeck syndrome (IGS) was described just over 50 years ago by Olga Imerslund and Ralph Gräsbeck and colleagues. IGS is caused by specific malabsorption of cobalamin (Cbl) due to bi-allelic mutations in either the cubilin gene (CUBN) or the human amnionless homolog (AMN). Mutations in the two genes are commonly seen in founder populations or in societies with a high degree of consanguineous marriages. One particular mutation in AMN, c.208-2A>G, causing an out-of-frame loss of exon 4 in the mRNA, is responsible for some 15% of IGS cases globally. We present evidence that this founder mutation causes a substantial percentage of cases among diverse ethnicities and that the mutation is as old as human civilization.

Methods

Partial genotyping indicated a founder event but its presence in diverse peoples of Arabic, Turkish, Jewish, and Hispanic ancestry suggested that the mutation might be recurrent. We therefore studied the flanking sequence spanning 3.5 Mb to elucidate the origin of the haplotype and estimate the age of the mutation using a Bayesian inference method based on observed linkage disequilibrium.

Results

The mutation's distribution, the size of the shared haplotype, and estimates of growth rate and carrier frequency indicated that the mutation was a single prehistoric event. Dating back to the ancient Middle East around 11,600 BC, the mutation predates the advent of writing, farming, and the monotheistic religions of the region.

Conclusions

This mutation causes over 50% of the IGS cases among Arabic, Turkish, and Sephardic Jewish families, making it a primary target for genetic screening among diverse IGS cases originating from the Middle East. Thus, rare founder mutations may cause a substantial number of cases, even among diverse ethnicities not usually thought to be related.

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.

Juvenile selective vitamin B₁₂ malabsorption: 50 years after its description-10 years of genetic testing

Fifty years have passed since the description of juvenile selective malabsorption of cobalamin (Cbl). Quality of life improvements have dramatically reduced the incidence of parasite-induced or nutritional Cbl deficiency. Consequently, inherited defects have become a leading cause of Cbl deficiency in children, which is not always expressed as anemia. Unfortunately, the gold standard for clinical diagnosis, the Schilling test, has increasingly become unavailable, and replacement tests are only in their infancy. Genetic testing is complicated by genetic heterogeneity and differential diagnosis. This review documents the history, research, and advances in genetics that have elucidated the causes of juvenile Cbl malabsorption. Genetic research has unearthed many cases in the past decade, mostly in Europe and North America, often among immigrants from the Middle East or North Africa. Lack of suitable clinical testing potentially leaves many patients inadequately diagnosed. The consequences of suboptimal Cbl levels for neurological development are well documented. By raising awareness, we wish to push for fast track development of better clinical tools and suitable genetic testing. Clinical awareness must include attention to ethnicity, a sensitive topic but effective for fast diagnosis. The treatment with monthly parenteral Cbl for life offers a simple and cost-effective solution once proper diagnosis is made.

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.

Inborn errors of cobalamin absorption and metabolism

Derivatives of cobalamin (vitamin B(12)) are required for activity of two enzymes in humans. Adenosylcobalamin is required for activity of mitochondrial methylmalonylCoA mutase and methylcobalamin is required for activity of cytoplasmic methionine synthase. Deficiency in cobalamin, or inability to absorb cobalamin normally, can result in accumulation of methylmalonic acid and homocysteine in blood and urine. Methylmalonic acidemia can result in metabolic acidosis which in severe cases may be fatal. Hyperhomocysteinemia along with hypomethioninemia can result in hematologic (megaloblastic anemia, neutropenia, thrombocytopenia) and neurologic (subacute combined degeneration of the cord, dementia, psychosis) defects. Inborn errors affecting cobalamin absorption (inherited intrinsic factor deficiency, Imerslund–Gra¨ sbeck syndrome) and transport (transcobalamin deficiency) have been described. A series of inborn errors of intracellular cobalamin metabolism, designated cblA-cblG, have been differentiated by complementation analysis. These can give rise to isolated methylmalonic acidemia (cblA, cblB, cblD variant 2), isolated hyperhomocysteinemia (cblD variant 1, cblE, cblG) or combined methylmalonic acidemia and hyperhomocysteinemia (cblC, classic cblD, cblF). All these disorders are inherited as autosomal recessive traits. The genes underlying each of these disorders have been identified. Two other disorders, haptocorrin deficiency and transcobalamin receptor deficiency, have been described, but it is not clear that they have any consistent clinical phenotype.

[Imerslund-Gräsbeck syndrome]

Anemia is a very common symptom encountered in numerous clinical situations in pediatrics. Etiologies range from classic iron-deficiency anemia to the more particular etiologies. We report on a clinical history where usual symptoms such as asthenia, drowsiness and proteinuria provided a rare diagnosis: Imerslund-Gräsbeck syndrome. We discuss the exams to be done with aregenerative macrocytic anemia so as not to underestimate these diagnoses, which each require adapted treatments.

Receptor-mediated endocytosis in renal proximal tubule

Proteins filtered in renal glomeruli are removed from the tubular fluid by endocytosis in the proximal tubule mediated by the two receptors megalin and cubilin. After endocytic uptake, the proteins are transferred to lysosomes for degradation, while the receptors are returned to the apical cell membrane by receptor recycling in dense apical tubules. In the renal proximal tubule, there is no significant transcellular transport of protein. The reabsorptive process is extremely efficient as evidenced by the virtual protein free urine in humans. The two receptors bind a variety of ligands. The process serves not only to remove the proteins from the ultrafiltrate but also to conserve a variety of essential substances such as vitamins and trace elements carried by plasma proteins. The endocytic apparatus is highly developed in the proximal tubule demonstrating the high capacity of the cells; however, under certain circumstances like diseases affecting the glomeruli, the system is overloaded resulting in proteinuria.

Genome-wide association study of vitamin B6, vitamin B12, folate, and homocysteine blood concentrations

The B vitamins are components of one-carbon metabolism (OCM) that contribute to DNA synthesis and methylation. Homocysteine, a by-product of OCM, has been associated with coronary heart disease, stroke and neurological disease. To investigate genetic factors that affect circulating vitamin B6, vitamin B12, folate and homocysteine, a genome-wide association analysis was conducted in the InCHIANTI (N = 1175), SardiNIA (N = 1115), and BLSA (N = 640) studies. The top loci were replicated in an independent sample of 687 participants in the Progetto Nutrizione study. Polymorphisms in the ALPL gene (rs4654748, p = 8.30 x 10(-18)) were associated with vitamin B6 and FUT2 (rs602662, [corrected] p = 2.83 x 10(-20)) with vitamin B12 serum levels. The association of MTHFR, a gene consistently associated with homocysteine, was confirmed in this meta-analysis. The ALPL gene likely influences the catabolism of vitamin B6 while FUT2 interferes with absorption of vitamin B12. These findings highlight mechanisms that affect vitamin B6, vitamin B12 and homocysteine serum levels.

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.

[Hereditary juvenile cobalamin deficiency due to mutations in GIF gene]

Inborn errors of cobalamin (Cbl) metabolism affect its absorption, transport, as well as its intracellular metabolism. Hereditary juvenile megaloblastic anaemia due to cobalamin deficiency, results from defects in Cbl absorption. There is a lack of vitamin B12 in congenital pernicious anaemia due to intrinsic factor deficiency and megaloblastic anaemia 1 due to selective intestinal malabsorption of vitamin B12 or Imerslund-Gräsbeck syndrome. Differential diagnosis can't be accomplished only by clinical and biochemical findings. We present a patient from Spain with a megaloblastic anaemia due to intrinsic factor deficiency (IFD). The patient is a compound heterozygous in GIF gene for a splice site mutation inherited from his mother and a missense change inherited from his father. The identification of disease-causing mutations in specific genes has improved our ability to diagnose many of these conditions.

Common variants of FUT2 are associated with plasma vitamin B12 levels

We identified a strong association (P = 5.36 x 10(-17)) between rs492602 in FUT2 and plasma vitamin B(12) levels in a genome-wide scan (n = 1,658) and an independent replication sample (n = 1,059) from the Nurses' Health Study. Women homozygous for the rs492602[G] allele had higher B(12) levels. This allele is in strong linkage disequilibrium with the FUT2 nonsecretor variant encoding W143X, suggesting a plausible mechanism for altered B(12) absorption and plasma levels.

Imerslund-Gräsbeck syndrome in a 15-year-old German girl caused by compound heterozygous mutations in CUBN

Imerslund-Gräsbeck syndrome (IGS) is a recessive disorder of intestinal cobalamin (Cbl) absorption and renal tubular protein reabsorption sometimes accompanied by urinary tract malformation. Mutations in the cubilin (CUBN) and amnionless (AMN) genes have been described as causal defects. CUBN and AMN proteins form the cubam complex that functions as the receptor for the intrinsic factor-Cbl (IF-Cbl) complex in the ileum and for proteins found in the primary urine in the kidney. We report the case of a 15-year-old German girl who presented with megaloblastic anaemia and funicular myelosis due to Cbl-deficiency and selective proteinuria. We clinically diagnosed- and for the first time in a patient of German ancestry-genetically confirmed IGS by detecting a compound heterozygous gene deletion and missense mutation in the CUBN gene. In conclusion IGS should be considered in paediatric patients presenting with symptoms like megaloblastic anaemia, funicular myelosis and benign proteinuria. Diagnosis should be confirmed genetically to avoid further invasive diagnostics, administer proper lifelong treatment and offer genetic counselling.

Cobalamin-mediated regulation of transcobalamin receptor levels in rat organs

Total gastrectomy (TG) causes cobalamin (Cbl) deficiency followed by increases in tumor necrosis factor (TNF)-alpha levels in the spinal cord (SC) of the rat. In order to understand how Cbl deficiency may influence cell Cbl transport, we have measured by immunoblotting protein levels of the receptor for the Cbl-transcobalamin (TC) complex (TC-R) in both animal and cell models. TC-R protein levels were elevated in the total membranes of duodenal mucosa, kidneys, liver, and SC of rats made Cbl-deficient (Cbl-D) by means of TG or feeding with a Cbl-D diet. Postoperative Cbl-replacement treatment normalized the TC-R protein levels in each of the tested organs, regardless of whether this treatment was given during the first two post-TG or during the third and fourth post-TG mo. In Caco-2 cells, progressively increasing TNF-alpha concentrations supplemented to culture medium induced an up-regulation of TC-R protein levels. We provide the first evidence of the regulation of a Cbl-specific receptor by the vitamin itself in some rat organs.

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.

Imerslund-Grasbeck syndrome associated with recurrent aphthous stomatitis and defective neutrophil function

Vitamin B(12) deficiency is a well-known cause of recurrent aphthous stomatitis (RAS). However, the mechanism by which this deficiency causes the stomatitis is not well understood. Imerslund-Grasbeck syndrome (IGS) causes vitamin B(12) deficiency and proteinuria due to a defect in the vitamin B(12) receptor. We sought to determine whether the RAS observed in IGS patients is associated with neutrophil dysfunction. We report 3 infants with vitamin B(12) deficiency due to IGS, who presented with borderline or normal hemoglobin concentrations, RAS, and a neutrophil function defect. All 3 patients were homozygous for a splice site mutation affecting exon 4 of the AMN gene. A direct correlation was observed between low serum vitamin B12 levels and defective neutrophil function (low chemotaxis and elevated superoxide production) in the patients. Vitamin B(12) therapy led to an immediate resolution of aphthous stomatitis and full correction of neutrophil function. We demonstrated that serum vitamin B(12) deficiency is associated with a neutrophil chemotactic defect and RAS in IGS patients. We suggest that the RAS observed in these patients is due to this defect.

Diagnosis of megaloblastic anaemias

There are a large number of causes of megaloblastic anaemia. The most frequent are disorders resulting in vitamin B(12) or folate deficiency. The diagnostic process often consists first of establishing the presence of B(12) or folate deficiency and then of determining the cause of deficiency. The blood count, blood film, serum B(12) assay, and red cell and serum folate assays are the primary investigations. Other useful investigations include serum/plasma methylmalonic acid (MMA), plasma total homocysteine (tHCYS) and serum holo-transcobalamin II assays. All currently used tests have limitations regarding specificity or sensitivity or both and the metabolite assays are not widely available. An understanding of these limitations is essential in formulating any diagnostic strategy. The wide use of serum B(12) and metabolite assays has resulted in the increasingly early diagnosis of B(12) deficiency, often in patients without B(12)-related symptoms (subclinical deficiency). Food cobalamin malabsorption is the most frequent cause of a low serum B(12). At least 25% of low serum B(12) levels are not associated with elevated metabolite levels and may not indicate B(12) deficiency. Some of these are caused by partial deficiency of transcobalamine I.

The kidney in vitamin B12and folate homeostasis: characterization of receptors for tubular uptake of vitamins and carrier proteins

Over the past 10 years, animal studies have uncovered the molecular mechanisms for the renal tubular recovery of filtered vitamin and vitamin carrier proteins. Relatively few endocytic receptors are responsible for the proximal tubule uptake of a number of different vitamins, preventing urinary losses. In addition to vitamin conservation, tubular uptake by endocytosis is important to vitamin metabolism and homeostasis. The present review focuses on the receptors involved in renal tubular recovery of folate, vitamin B12, and their carrier proteins. The multiligand receptor megalin is important for the uptake and tubular accumulation of vitamin B12. During vitamin load, the kidney accumulates large amounts of free vitamin B12, suggesting a possible storage function. In addition, vitamin B12is metabolized in the kidney, suggesting a role in vitamin homeostasis. The folate receptor is important for the conservation of folate, mediating endocytosis of the vitamin. Interaction between the structurally closely related, soluble folate-binding protein and megalin suggests that megalin plays an additional role in the uptake of folate bound to filtered folate-binding protein. A third endocytic receptor, the intrinsic factor-B12receptor cubilin-amnionless complex, is essential to the renal tubular uptake of albumin, a carrier of folate. In conclusion, uptake is mediated by interaction with specific endocytic receptors also involved in the renal uptake of other vitamins and vitamin carriers. Little is known about the mechanisms regulating intracellular transport and release of vitamins, and whereas tubular uptake is a constitutive process, this may be regulated, e.g., by vitamin status.

Acquired and inherited disorders of cobalamin and folate in children

Cobalamin deficiency in the newborn usually results from cobalamin deficiency in the mother. Megaloblastic anaemia, pancytopenia and failure to thrive can be present, accompanied by neurological deficits if the diagnosis is delayed. Most cases of spina bifida and other neural tube defects result from maternal folate and/or cobalamin insufficiency in the periconceptual period. Polymorphisms in a number of genes involved in folate and cobalamin metabolism exacerbate the risk. Inborn errors of cobalamin metabolism affect its absorption, (intrinsic factor deficiency, Imerslund-Gräsbeck syndrome) and transport (transcobalamin deficiency) as well as its intracellular metabolism affecting adenosylcobalamin synthesis (cblA and cblB), methionine synthase function (cblE and cblG) or both (cblC, cblD and cblF). Inborn errors of folate metabolism include congenital folate malabsorption, severe methylenetetrahydrofolate reductase deficiency and formiminotransferase deficiency. The identification of disease-causing mutations in specific genes has improved our ability to diagnose many of these conditions, both before and after birth.

Imerslund-Gräsbeck syndrome (selective vitamin B(12) malabsorption with proteinuria)

Imerslund-Gräsbeck syndrome (IGS) or selective vitamin B(12) (cobalamin) malabsorption with proteinuria is a rare autosomal recessive disorder characterized by vitamin B(12) deficiency commonly resulting in megaloblastic anemia, which is responsive to parenteral vitamin B(12) therapy and appears in childhood. Other manifestations include failure to thrive and grow, infections and neurological damage. Mild proteinuria (with no signs of kidney disease) is present in about half of the patients. Anatomical anomalies in the urinary tract were observed in some Norwegian patients. Vitamin B(12) absorption tests show low absorption, not corrected by administration of intrinsic factor. The symptoms appear from 4 months (not immediately after birth as in transcobalamin deficiency) up to several years after birth. The syndrome was first described in Finland and Norway where the prevalence is about 1:200,000. The cause is a defect in the receptor of the vitamin B(12)-intrinsic factor complex of the ileal enterocyte. In most cases, the molecular basis of the selective malabsorption and proteinuria involves a mutation in one of two genes, cubilin (CUBN) on chromosome 10 or amnionless (AMN) on chromosome 14. Both proteins are components of the intestinal receptor for the vitamin B(12)-intrinsic factor complex and the receptor mediating the tubular reabsorption of protein from the primary urine. Management includes life-long vitamin B(12) injections, and with this regimen, the patients stay healthy for decades. However, the proteinuria persists. In diagnosing this disease, it is important to be aware that cobalamin deficiency affects enterocyte function; therefore, all tests suggesting general and cobalamin malabsorption should be repeated after abolishment of the deficiency.

New insights into carrier binding and epithelial uptake of the erythropoietic nutrients cobalamin and folate

PURPOSE OF REVIEW

In addition to malnutrition several genetic and acquired conditions may affect the homeostasis of cobalamin (vitamin B12) and folate, leading to megaloblastic anemia and other diseases. The present review describes new insight into protein handling of cobalamin and folate.

RECENT FINDINGS

The recent solution of the three-dimensional structure of the cobalamin binder transcobalamin shows two separate domains enclosing the vitamin. This structure apparently also applies for the other homologous cobalamin binders, intrinsic factor and haptocorrin. Genetic studies of inherited cobalamin malabsorption and biochemical studies have now revealed that the functional receptor for uptake of intrinsic factor-vitamin cobalamin complexes also is a complex itself consisting of two different gene products, cubilin and amnionless. A role in folate uptake of megalin, an endocytic receptor for epithelial uptake of various proteins including transcobalamin, is now also indicated by the observation that megalin can mediate uptake of soluble folate receptor.

SUMMARY

New data show the structure of cobalamin carriers and reveal novel proteins involved in the epithelial uptake of cobalamin and folate. Genetic abnormalities in three different genes encoding proteins in the epithelial uptake of cobalamin are now known to cause malabsorption of cobalamin and megaloblastic anemia.

Role of megalin and cubilin in renal physiology and pathophysiology

Megalin and cubilin are endocytic receptors highly expressed in the endocytic apparatus of the renal proximal tubule. These receptors appear to be responsible for the tubular clearance of most proteins filtered in the glomeruli. Cubilin is a peripheral membrane protein, and therefore it does not have an endocytosis signaling sequence. It appears that megalin is responsible for internalization of cubilin and its ligands in addition to internalizing its own ligands. The proteinuria observed in megalin-deficient mice, in dogs lacking functional cubilin, and in patients with distinct mutations of the cubilin gene illustrates the importance of the receptors.