Characterization of a monoclonal antibody with specificity for holo-transcobalamin

Pregnancy homocysteine and cobalamin status predict childhood metabolic health in the offspring

BACKGROUND

Inadequate pregnancy cobalamin status has been associated with adverse offspring metabolic health in Indian and Nepalese studies. Studies of pregnancy cobalamin status and mid-childhood health outside of Asia are scarce.

METHODS

Associations between pregnancy fasting plasma total homocysteine (tHcy), cobalamin status (plasma cobalamin, holotranscobalamin (holoTC), methylmalonic acid (MMA)) and mid-childhood metabolic score (MetSco) ((including fat mass index (zFMI), homeostatic model assessment of insulin resistance (zHOMA-IR) and dyslipidemia (zTG - zHDLc)/2) z-scores)) were investigated in a prospective study of 293 mother-child dyads.

RESULTS

Highest versus low-mid pregnancy tHcy tertile was associated with higher mid-childhood MetSco, specifically with higher child zFMI. Stratifying by sex, the maternal tHcy-child MetSco association was limited to boys and confirmed for zFMI and zHOMA-IR. The maternal tHcy-child zFMI association was not mediated by birth weight z-score. First trimester plasma cobalamin was not associated with child outcomes, but other indicators of cobalamin status were. Lowest versus mid-high plasma holoTC tertile was associated with MetSco (specifically zFMI and zHOMA-IR) and highest versus low-mid plasma MMA tertile with higher MetSco and dyslipidemia in boys.

CONCLUSIONS

Moderately elevated pregnancy tHcy and low cobalamin status were associated with mid-childhood metabolic score in boys. The pregnancy tHcy-child zFMI association was not mediated by birth weight.

IMPACT

Fasting plasma total homocysteine (tHcy) during pregnancy and low cobalamin status during early pregnancy are associated with mid-childhood metabolic score and its components in the offspring. These findings were only significant in male offspring. The study provides new evidence that impaired one carbon metabolism during pregnancy is associated with negative health outcomes in the offspring, in a population with low prevalence of cobalamin deficiency. The maternal-offspring associations were observed in the functional markers of cobalamin status (holotranscobalamin and methylmalonic acid) and tHcy, not with plasma cobalamin concentration. Screening for low pregnancy cobalamin status should be considered.

False low holotranscobalamin levels in a patient with a novel TCN2 mutation

BACKGROUND

Measurement of holotranscobalamin (holoTC) is increasingly used as a screening test for cobalamin (Cbl) deficiency. A level well below the reference interval strongly supports a deficient state. We examined a 21-year-old woman diagnosed as Cbl deficient because of an extremely low holoTC level as measured by the Abbott Architect Assay.

METHODS

The patient was evaluated for Cbl deficiency employing an in-house holoTC method as well as other routine markers of Cbl status. Further analyses included exploration of the Cbl binding proteins employing gel filtration of a serum sample saturated with 57 Co-labeled Cbl and Sanger sequencing of exons 1-9 and the intron-exon boundaries of the TCN2 gene, the gene coding for transcobalamin (TC).

RESULTS

The patient had normal hematological variables throughout. Despite initial treatment with Cbl, holoTC as measured by the Abbott assay remained low, while holoTC measured with the in-house assay was normal, and behaved as TC upon gel-filtration. By Sanger sequencing, we detected a homozygous single point mutation c.855T>A in exon 6 of TCN2, corresponding to a asparagine (Asn) to lysine (Lys) substitution in position 267 of the mature protein.

CONCLUSIONS

We describe a novel point mutation of the TCN2 gene. The mutation does not seem to interfere with the function of TC, but the mutation may well explain the low level of holoTC detected by the Abbott assay. Our results underscores that mutations of TCN2 have to be considered when implausible holoTC results are obtained.

Structural basis of transcobalamin recognition by human CD320 receptor

Cellular uptake of vitamin B12 (cobalamin) requires capture of transcobalamin (TC) from the plasma by CD320, a ubiquitous cell surface receptor of the LDLR family. Here we present the crystal structure of human holo-TC in complex with the extracellular domain of CD320, visualizing the structural basis of the TC-CD320 interaction. The observed interaction chemistry can rationalize the high affinity of CD320 for TC and lack of haptocorrin binding. The in vitro affinity and complex stability of TC-CD320 were quantitated using a solid-phase binding assay and thermostability analysis. Stable complexes with TC were also observed for the disease-causing CD320ΔE88 mutant and for the isolated LDLR-A2 domain. We also determined the structure of the TC-CD320ΔE88 complex, which revealed only minor changes compared with the wild-type complex. Finally, we demonstrate significantly reduced in vitro affinity of TC for CD320 at low pH, recapitulating the proposed ligand release during the endocytic pathway.

Cellular uptake of vitamin B12 (cobalamin) requires the binding of holo-transcobalamin (TC) from plasma by CD320. Here, the authors report the structure of a complex between CD320 and TC loaded with cyanocobalamin, alongside additional functional analysis.

Association of a transcobalamin II genetic variant with falsely low results for the holotranscobalamin immunoassay

BACKGROUND

The clinical use of holotranscobalamin (holoTC) testing to evaluate vitamin B12 status has increased in recent years. We present two patients (African Caribbean and Indian heritage), in which the holoTC assay indicated severe B12 deficiency (< 5 pmol/L). Additional clinical tests revealed that these patients had normal levels of total vitamin B12 in blood and unremarkable levels of two other markers of vitamin B12 status, homocysteine and methylmalonic acid. We hypothesized that these patients carry a variant in the transcobalamin gene (TCN2) that influences the most widely commercially available holoTC test - Active-B12 (Axis-Shield Diagnostics Ltd).

DESIGN

Exon sequencing of the TCN2 gene was carried out. Protein characterization included total transcobalamin (TCN2) detection by Western blot, and holoTC by (57) Co-labelled B12 binding followed by size fractionation.

RESULTS

Exon sequencing of TCN2 revealed both patients were homozygous for the minor allele of rs35838082 (p.R215W). Western blot and chromatographic analyses revealed that the serum of these patients contains intact transcobalamin and that this variant-containing protein binds vitamin B12 . The variant is rare in Caucasians (minor allele frequency (MAF) < 0·01) but more common in South Asians (MAF ~ 0·02) and those of African origin (MAF ~ 0·25).

CONCLUSIONS

The impeded ability to detect normal levels of holoTC in these two patients may be due to this variant interfering with the detection of holoTC by one or both of the monoclonal antibodies currently employed in the Active-B12 test. Laboratories should be aware of this variant and use confirmatory tests when applicable.

Physiological and molecular aspects of cobalamin transport

Minute doses of a complex cofactor cobalamin (Cbl, vitamin B12) are essential for metabolism. The nutritional chain for humans includes: (1) production of Cbl by bacteria in the intestinal tract of herbivores; (2) accumulation of the absorbed Cbl in animal tissues; (3) consumption of food of animal origin. Most biological sources contain both Cbl and its analogues, i.e. Cbl-resembling compounds physiologically inactive in animal cells. Selective assimilation of the true vitamin requires an interplay between three transporting proteins - haptocorrin (HC), intrinsic factor (IF), transcobalamin (TC) - and several receptors. HC is present in many biological fluids, including gastric juice, where it assists in disposal of analogues. Gastric IF selectively binds dietary Cbl and enters the intestinal cells via receptor-mediated endocytosis. Absorbed Cbl is transmitted to TC and delivered to the tissues with blood flow. The complex transport system guarantees a very efficient uptake of the vitamin, but failure at any link causes Cbl-deficiency. Early detection of a negative B12 balance is highly desirable to prevent irreversible neurological damages, anaemia and death in aggravated cases. The review focuses on the molecular mechanisms of cobalamin transport with emphasis on interaction of corrinoids with the specific proteins and protein-receptor recognition. The last section briefly describes practical aspects of recent basic research concerning early detection of B12-related disorders, medical application of Cbl-conjugates, and purification of corrinoids from biological samples.

Low holotranscobalamin and cobalamins predict incident fractures in elderly men: the MrOS Sweden

In a population-based study on cobalamin status and incident fractures in elderly men (n = 790) with an average follow-up of 5.9 years, we found that low levels of metabolically active and total cobalamins predict incident fractures, independently of body mass index (BMI), bone mineral density (BMD), plasma total homocysteine (tHcy), and cystatin C.

INTRODUCTION

Cobalamin deficiency in elderlies may affect bone metabolism. This study aims to determine whether serum cobalamins or holotranscobalamin (holoTC; the metabolic active cobalamin) predict incident fractures in old men.

METHODS

Men participating in the Gothenburg part of the population-based Osteoporotic Fractures in Men (MrOS) Sweden cohort and without ongoing vitamin B medication were included in the present study (n = 790; age range, 70-81 years).

RESULTS

During an average follow-up of 5.9 years, 110 men sustained X-ray-verified fractures including 45 men with clinical vertebral fractures. The risk of fracture (adjusted for age, smoking, BMI, BMD, falls, prevalent fracture, tHcy, cystatin C, 25-OH-vitamin D, intake of calcium, and physical activity (fully adjusted)), increased per each standard deviation decrease in cobalamins (hazard ratio (HR), 1.38; 95% confidence intervals (CI), 1.11-1.72) and holoTC (HR, 1.26; 95% CI, 1.03-1.54), respectively. Men in the lowest quartile of cobalamins and holoTC (fully adjusted) had an increased risk of all fracture (cobalamins, HR = 1.67 (95% CI, 1.06-2.62); holoTC, HR = 1.74 (95% CI, 1.12-2.69)) compared with quartiles 2-4. No associations between folate or tHcy and incident fractures were seen.

CONCLUSIONS

We present novel data showing that low levels of holoTC and cobalamins predicting incident fracture in elderly men. This association remained after adjustment for BMI, BMD, tHcy, and cystatin C. However, any causal relationship between low cobalamin status and fractures should be explored in a prospective treatment study.

Holotranscobalamin is not influenced by decreased renal function in elderly men: the MrOS Sweden study

BACKGROUND

Subclinical cobalamin deficiency is common in the elderly, but the sensitivity and specificity of serum total cobalamin for this diagnosis is poor. Serum holotranscobalamin (holoTC), a measure of biologically available cobalamin, is considered a better marker for early cobalamin depletion than total cobalamin. However, in elderly populations, health-related reference intervals for holoTC and correlations to renal function are not entirely clear.

METHODS

HoloTC was determined with an automated microparticle enzyme immunoassay (AxSYM®) in 790 elderly non-vitamin-supplemented Swedish men, median age 75.3 years. Renal function was assessed with creatinine, cystatin C and estimated glomerular filtration rate (eGFR calculated from creatinine).

RESULTS

Median holoTC was 51.8 pmol/L, the health-related reference interval 19.6-132.3 pmol/L. There was no significant difference in mean holoTC in probands with normal compared to high creatinine (P = 0.80) and cystatin C (P = 0.82). No significant differences between the quartiles of creatinine or cystatin C in mean of log holoTC were seen. HoloTC correlated strongly with total cobalamin (r = 0.69, P < 0.001), weaker with eGFRcreatinine (r = -0.09, P < 0.05) and creatinine (r = 0.09, P < 0.05), the latter correlation was only seen in subjects with creatinine <100 µmol/L. HoloTC correlated negatively with plasma total homocysteine (r = -0.24, P < 0.001), but not with cystatin C and age.

CONCLUSIONS

Serum holoTC in healthy elderly men shows the same distribution as earlier described for a younger reference population. In this group of elderly subjects, holoTC did not correlate to reduced renal function. Thus, holoTC appears to be a promising tool for evaluating cobalamin status also in elderly populations.

Cellular uptake of cobalamin: transcobalamin and the TCblR/CD320 receptor

Cellular uptake of cobalamin is facilitated by a receptor-mediated endocytosis process involving transcobalamin, a plasma protein that binds cobalamin and a cell surface receptor that specifically binds transcobalamin saturated with cobalamin. Intracellular Cbl concentration is maintained by modulating the expression of the receptor, which is cell cycle associated with highest expression in actively proliferating cells and an efflux system that shunts the excess cobalamin out of the cells for mobilization to other tissues where it is most needed. This review describes the process, proteins involved and genes encoding these proteins.

Peptide discovery using bacterial display and flow cytometry

Peptides are increasingly used as therapeutic and diagnostic agents. The combination of bacterial cell-surface display peptide libraries with magnetic- and fluorescence-activated cell sorting technologies provides an efficient and highly effective methodology to identify and engineer peptides for a growing number of molecular recognition applications. Here, detailed protocols for both the generation and screening of bacterial display peptide libraries are presented. The methods described enable the discovery and evolutionary optimization of protein-binding peptides, cell-specific peptides, and enzyme substrates for diverse biotechnology applications.

Holotranscobalamin, a marker of vitamin B-12 status: analytical aspects and clinical utility

Approximately one-quarter of circulating cobalamin (vitamin B-12) binds to transcobalamin (holoTC) and is thereby available for the cells of the body. For this reason, holoTC is also referred to as active vitamin B-12. HoloTC was suggested as an optimal marker of early vitamin B-12 deficiency >20 y ago. This suggestion led to the development of suitable assays for measurement of the compound and clinical studies that aimed to show the benefit of measurement of holoTC rather than of vitamin B-12. Today holoTC can be analyzed by 3 methods: direct measurement of the complex between transcobalamin and vitamin B-12, measurement of vitamin B-12 attached to transcobalamin, or measurement of the amount of transcobalamin saturated with vitamin B-12. These 3 methods give similar results, but direct measurement of holoTC complex is preferable in the clinical setting from a practical point of view. HoloTC measurement has proven useful for the identification of the few patients who suffer from transcobalamin deficiency. In addition, holoTC is part of the CobaSorb test and therefore useful for assessment of vitamin B-12 absorption. Clinical studies that compare the ability of holoTC and vitamin B-12 to identify individuals with vitamin B-12 deficiency (elevated concentration of methylmalonic acid) suggest that holoTC performs better than total vitamin B-12. To date, holoTC has not been used for population-based assessments of vitamin B-12 status, but we suggest that holoTC is a better marker than total vitamin B-12 for such studies.

Use of Saccharomyces cerevisiae and Caenorhabditis elegans as model organisms to study the effect of cocoa polyphenols in the resistance to oxidative stress

Developing functional foods to improve the quality of life for elderly people has great economic and social impact. Searching for and validating ingredients with in vivo antioxidant effects is one of the key steps in developing this kind of food. Here we describe the combined use of simple biological models and transcriptomics to define the functional intracellular molecular targets of a polyphenol-enriched cocoa powder. Cocoa powder supplemented culture medium led to increased resistance to oxidative stress, in both the budding yeast Saccharomyces cerevisiae and the nematode Caenorhabditis elegans, and, in the latter, lifespan was also increased. These effects are fully dependent on the polyphenols present in the cocoa powder and on the sirtuins Hst3 (yeast) and SIR-2.1 (worm). The transcription factor DAF-16 also plays an important role in the case of the nematode, indicating that the insulin/IGF-1 (insulin-like growth factor) signaling pathway is related with the antioxidative effect of cocoa polyphenols. All in all, these results confirm that this polyphenol-enriched cocoa powder, with antioxidant activity, has great potential use as a functional food ingredient for elderly people. Furthermore, this work reveals the value of using simple biological models to screen for compounds that are of interest for the food and pharmacological industry.

Associations between holotranscobalamin, vitamin B12, homocysteine and depressive symptoms in community-dwelling elders

BACKGROUND

Vitamin B12 and homocysteine have been shown to be associated with depression or depressive symptoms, but the relationship has not been universal. Both vitamin B12 and homocysteine may exert an effect via vascular mechanisms; it is possible that other mechanisms apply. Holotranscobalamin is a novel, more accurate measure of tissue vitamin B12.

OBJECTIVES

To examine associations between vitamin B12, serum folate, holotranscobalamin, homocysteine and depressive symptoms in a sample of healthy elderly.

METHODS

Cross-sectional, observational community based study.

RESULTS

Lower levels of holotranscobalamin and vitamin B12 were associated with higher levels of depressive symptoms when controlled for Mini-mental state examination scores and psychosocial and cardiovascular risk factors. Homocysteine was not associated with depressive symptoms when biological and psychosocial covariates were included.

CONCLUSIONS

It is possible that low levels of vitamin B12 or holotranscobalamin are associated with depressive symptoms via mechanisms other than vascular pathology.

Preferential selection of Cys-constrained peptides from a random phage-displayed library by anti-glucitollysine antibodies

Phage-displayed peptides recognized by two monoclonal antibodies against glucitollysine were selected. The most prominent feature of the peptide panel was the presence of paired Cys in most of them (21/24 peptides). The availability of a wide variety of peptides having differently spaced paired Cys, as well as truly linear Cys-free peptides, gave the opportunity to explore the role of disulfide bridges in phage selection. Some Cys-containing peptides came from a Cys-flanked cyclic 9-mer library, but most of them (18/21) were derived from a totally random 12-mer library, and hence the presence of Cys was dictated by the selector antibodies. Motifs shared by several peptides (potentially involved in binding) often contained or were flanked by Cys residues. Binding of all Cys-containing phage-displayed peptides was abolished/decreased after a reducing treatment. Screening a random peptide library (without invariant Cys residues) is powerful enough to clearly reveal the need, preferences, and diversity of Cys-mediated structural constraints for recognition.

Indicators for assessing folate and vitamin B12 status and for monitoring the efficacy of intervention strategies

Deficiencies of folate or of vitamin B12 are widespread and constitute a major global burden of morbidity affecting all age groups. Detecting or confirming the presence of folate or vitamin B12 deficiency and distinguishing one from the other depends, ultimately, on laboratory testing. Tests to determine the presence of folate or vitamin B12 deficiency are used singly or in combination to establish the nutritional status and prevalence of deficiencies of the vitamins in various populations. The efficacy of interventions through the use of fortification or supplements is monitored using the same laboratory tests. Tests currently in use have limitations that can be either technical or have a biological basis. Consequently, each single test cannot attain perfect sensitivity, specificity, or predictive value. Laboratory indicators of vitamin B12 or folate status involve measurement of either the total or a physiologically relevant fraction of the vitamin in a compartment such as the blood. Thus, assays to measure vitamin B12 or folate in plasma or serum as well as folate in red blood cells are in widespread use, and more recently, methods to measure vitamin B12 associated with the plasma binding protein transcobalamin (holotranscobalamin) have been developed. Alternatively, levels of surrogate biochemical markers that reflect the metabolic function of the vitamin can be used. Surrogates most commonly used are plasma homocysteine, for detection of either vitamin B12 or folate deficiency and methylmalonic acid for detection of vitamin B12 deficiency. The general methods as well as their uses, indications, and limitations are presented.

Active B12: a rapid, automated assay for holotranscobalamin on the Abbott AxSYM analyzer

BACKGROUND

Conventional tests for vitamin B(12) deficiency measure total serum vitamin B12, whereas only that portion of vitamin B12 carried by transcobalamin (holotranscobalamin) is metabolically active. Measurement of holotranscobalamin (holoTC) may be more diagnostically accurate for detecting B(12) deficiency that requires therapy. We developed an automated assay for holoTC that can be used on the Abbott AxSYM immunoassay analyzer.

METHODS

AxSYM Active B12 is a 2-step sandwich microparticle enzyme immunoassay. In step 1, a holoTC-specific antibody immobilized onto latex microparticles captures holoTC in samples of serum or plasma. In step 2, the captured holoTC is detected with a conjugate of alkaline phosphatase and antiTC antibody.

RESULTS

Neither apoTC nor haptocorrin exhibited detectable cross-reactivity. The detection limit was < or = 0.1 pmol/L. Within-run and total imprecision (CV ranges) were 3.4%-5.1% and 6.3%-8.5%, respectively. Assay CVs were < 20% from at least 3 pmol/L to 107 pmol/L. With diluted serum samples, measured concentrations were 104%-114% of the expected values in the working range of the assay. No interference from bilirubin, hemoglobin, triglycerides, erythrocytes, rheumatoid factor, or total protein was detected at expected (abnormal) concentrations. A comparison of the AxSYM Active B12 assay with a commercial RIA for holoTC yielded the regression equation: AxSYM = 0.98RIA + 4.7 pmol/L (S(y x), 11.4 pmol/L; n = 204). Assay throughput was 45 tests/h. A 95% reference interval of 19-134 pmol/L holoTC was established with samples from 292 healthy individuals.

CONCLUSIONS

The AxSYM Active B12 assay allows rapid, precise, sensitive, specific, and automated measurement of human holoTC in serum and plasma.

Longitudinal study of the effect of pregnancy on maternal and fetal cobalamin status in healthy women and their offspring

Compromised cobalamin status during pregnancy may put both mother and child at risk of deficiency during lactation and subsequent development. We investigated whether changes in cobalamin status during pregnancy are associated with impaired status in the mother and the cord. Plasma cobalamin, methylmalonic acid (MMA), and holotranscobalamin (holoTC) concentrations were determined in 92 women at preconception, 8, 20, and 32 wk of pregnancy, at labor, and in the cord. All variables [geometric mean percentiles 10, 90 (P(10), P(90))] were significantly reduced from preconception [cobalamin: 293 (155, 535) pmol/L; holoTC: 63 (38,98) pmol/L; MMA: 0.12 (0.09, 0.17) micromol/L] by 20 wk of pregnancy [cobalamin: 230 (123, 432) pmol/L; holoTC: 48 (34,78) pmol/L; MMA: 0.11 (0.08, 0.15) micromol/L P < 0.001]. Plasma cobalamin and holoTC remained lower throughout the remainder of pregnancy [32 wk: 198 (107, 339); labor: 224 (117, 444); P < 0.001] and [32 wk: 45 (26,82); labor: 40 (23,79); P < 0.05], respectively. By 32 wk, MMA was greater than preconception [0.14 (0.09, 0.20) micromol/L; P < 0.01]. Plasma holoTC at 32 wk and at labor was negatively correlated with cord MMA (r = -0.51, P < 0.001 and r = -0.40, P < 0.01, respectively). Women with lower holoTC at preconception had greater increases in MMA at 32 wk and at labor. Maternal MMA at 32 wk and at labor was significantly and independently associated with cord MMA only in women with lower holoTC at preconception (regression models: R(2) = 0.707, 0.682, respectively; P < 0.01). The moderate increases observed in the cobalamin biomarker, MMA, during pregnancy may indicate a functional depletion in intracellular cobalamin status.