Lessons in biology from patients with inborn errors of vitamin B12 metabolism

Acute Lymphoblastic Leukemia in Combined Methylmalonic Acidemia and Homocysteinemia (cblC Type): A Case Report and Literature Review

Background: Methylmalonic acidemia (MMA) can display many clinical manifestations, among which acute lymphoblastic leukemia (ALL) has not been reported, and congenital heart disease (CHD) is also rare.

Case presentation: We report an MMA case with ALL and CHD in a 5.5-year-old girl. With developmental delay and local brain atrophy in MRI, she was diagnosed with cerebral palsy at 9 months old. Rehabilitation was performed since then. This time she was admitted to hospital because of weakness and widespread bleeding spots. ALL-L2 (pre-B-cell) was confirmed by bone marrow morphology and immunophenotyping. Echocardiography showed patent foramen ovale. The girl was treated with VDLD and CAML chemotherapy, during which she developed seizures, edema and renal insufficiency. Decrease of muscle strength was also found in physical examination. Screening for inherited metabolic disorders showed significantly elevated levels of methylmalonate-2, acetylcarnitine (C2), propionylcarnitine (C3), C3/C2 and homocysteine. Gene analysis revealed a compound heterozygous mutaion in MMACHC (NM_015,560): c.80A > G (p.Gln27Arg) and c.609G > A (p.Trp203*). CblC type MMA was diagnosed. Intramuscular injection of cyanocobalamin and intravenous L-carnitine treatment were applied. The edema vanished gradually, and chemotherapy of small dosage of vindesine was given intermittently when condition permitted. 2 months later, muscle strength of both lower limbs were significantly improved to nearly grade 5. The levels of methylmalonic acid and homocysteine were improved.

Conclusion: Metabolic disease screening and gene analysis are very necessary for diseases with complex clinical symptoms. ALL can be a rare manifestation for MMA.

Synopsis: We report a case of methylmalonic acidemia with acute lymphoblastic leukemia and congenital heart disease, which uncovered the importance of genetic testing and metabolic diseases screening in patients with multiple systemic organ involvement.

Structural Study of the Complex of cblC Methylmalonic Aciduria and Homocystinuria-Related Protein MMACHC with Cyanocobalamin

MMACHC is an essential protein for the body to metabolise vitamin B12, and its deficiency will cause cblC-type methylmalonic aciduria and homocystinuria. MMACHC can interact with cyanocobalamin (a type of vitamin B12) cofactor and plays an important role in targeting cyanocobalamin to the enzyme of interest. In this paper, the GST-tag fusion-tagged MMACHC protein was successfully expressed by Escherichia coli (E. coli) low-temperature induction, and the high-purity MMACHC protein was successfully purified by affinity chromatography and gel filtration. Further, the crystal structure of MMACHC and cyanocobalamin complex was obtained with a resolution of 1.93 Å using X-ray diffraction. By analysing the complex structure of MMACHC and cyanocobalamin, we revealed the reasons for the diversity of MMACHC substrates and explained the reasons for the differences in disease conditions caused by different MMACHC site mutations. The acquisition of the complex structure of MMACHC and cyanocobalamin will play a significant role in promoting research on the metabolic pathway of vitamin B12.

Mutations in Hcfc1 and Ronin result in an inborn error of cobalamin metabolism and ribosomopathy

Combined methylmalonic acidemia and homocystinuria (cblC) is the most common inborn error of intracellular cobalamin metabolism and due to mutations in Methylmalonic Aciduria type C and Homocystinuria (MMACHC). Recently, mutations in the transcriptional regulators HCFC1 and RONIN (THAP11) were shown to result in cellular phenocopies of cblC. Since HCFC1/RONIN jointly regulate MMACHC, patients with mutations in these factors suffer from reduced MMACHC expression and exhibit a cblC-like disease. However, additional de-regulated genes and the resulting pathophysiology is unknown. Therefore, we have generated mouse models of this disease. In addition to exhibiting loss of Mmachc, metabolic perturbations, and developmental defects previously observed in cblC, we uncovered reduced expression of target genes that encode ribosome protein subunits. We also identified specific phenotypes that we ascribe to deregulation of ribosome biogenesis impacting normal translation during development. These findings identify HCFC1/RONIN as transcriptional regulators of ribosome biogenesis during development and their mutation results in complex syndromes exhibiting aspects of both cblC and ribosomopathies.

Combined methylmalonic acidemia (MMA) and hyperhomocysteinemias are inborn errors of vitamin B12 metabolism, and mutations in the transcriptional regulators HCFC1 and RONIN (THAP11) underlie some forms of these disorders. Here the authors generated mouse models of a human syndrome due to mutations in RONIN (THAP11) and HCFC1, and show that this syndrome is both an inborn error of vitamin B12 metabolism and displays some features of ribosomopathy.

Ocular manifestations in patients with inborn errors of intracellular cobalamin metabolism: a systematic review

Inherited disorders of cobalamin (cbl) metabolism (cblA-J) result in accumulation of methylmalonic acid (MMA) and/or homocystinuria (HCU). Clinical presentation includes ophthalmological manifestations related to retina, optic nerve and posterior visual alterations, mainly reported in cblC and sporadically in other cbl inborn errors.We searched MEDLINE EMBASE and Cochrane Library, and analyzed articles reporting ocular manifestations in cbl inborn errors. Out of 166 studies a total of 52 studies reporting 163 cbl and 24 mut cases were included. Ocular manifestations were found in all cbl defects except for cblB and cblD-MMA; cblC was the most frequent disorder affecting 137 (84.0%) patients. The c.271dupA was the most common pathogenic variant, accounting for 70/105 (66.7%) cases. One hundred and thirty-seven out of 154 (88.9%) patients presented with early-onset disease (0-12 months). Nystagmus and strabismus were observed in all groups with the exception of MMA patients while maculopathy and peripheral retinal degeneration were almost exclusively found in MMA-HCU patients. Optic nerve damage ranging from mild temporal disc pallor to complete atrophy was prevalent in MMA-HCU.and MMA groups. Nystagmus was frequent in early-onset patients. Retinal and macular degeneration worsened despite early treatment and stabilized systemic function in these patients. The functional prognosis remains poor with final visual acuity < 20/200 in 55.6% (25/45) of cases. In conclusion, the spectrum of eye disease in Cbl patients depends on metabolic severity and age of onset. The development of visual manifestations over time despite early metabolic treatment point out the need for specific innovative therapies.

Causes and consequences of impaired methionine synthase activity in acquired and inherited disorders of vitamin B12 metabolism

Methyl-Cobalamin (Cbl) derives from dietary vitamin B₁₂ and acts as a cofactor of methionine synthase (MS) in mammals. MS encoded by MTR catalyzes the remethylation of homocysteine to generate methionine and tetrahydrofolate, which fuel methionine and cytoplasmic folate cycles, respectively. Methionine is the precursor of S-adenosyl methionine (SAM), the universal methyl donor of transmethylation reactions. Impaired MS activity results from inadequate dietary intake or malabsorption of B₁₂ and inborn errors of Cbl metabolism (IECM). The mechanisms at the origin of the high variability of clinical presentation of impaired MS activity are classically considered as the consequence of the disruption of the folate cycle and related synthesis of purines and pyrimidines and the decreased synthesis of endogenous methionine and SAM. For one decade, data on cellular and animal models of B₁₂ deficiency and IECM have highlighted other key pathomechanisms, including altered interactome of MS with methionine synthase reductase, MMACHC, and MMADHC, endoplasmic reticulum stress, altered cell signaling, and genomic/epigenomic dysregulations. Decreased MS activity increases catalytic protein phosphatase 2A (PP2A) and produces imbalanced phosphorylation/methylation of nucleocytoplasmic RNA binding proteins, including ELAVL1/HuR protein, with subsequent nuclear sequestration of mRNAs and dramatic alteration of gene expression, including SIRT1. Decreased SAM and SIRT1 activity induce ER stress through impaired SIRT1-deacetylation of HSF1 and hypomethylation/hyperacetylation of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α), which deactivate nuclear receptors and lead to impaired energy metabolism and neuroplasticity. The reversibility of these pathomechanisms by SIRT1 agonists opens promising perspectives in the treatment of IECM outcomes resistant to conventional supplementation therapies.

Antivitamins B12 -Some Inaugural Milestones

The recently delineated structure- and reactivity-based concept of antivitamins B12 has begun to bear fruit by the generation, and study, of a range of such B12 -dummies, either vitamin B12 -derived, or transition metal analogues that also represent potential antivitamins B12 or specific B12 -antimetabolites. As reviewed here, this has opened up new research avenues in organometallic B12 -chemistry and bioinorganic coordination chemistry. Exploratory studies with antivitamins B12 have, furthermore, revealed some of their potential, as pharmacologically interesting compounds, for inducing B12 -deficiency in a range of organisms, from hospital resistant bacteria to laboratory mice. The derived capacity of antivitamins B12 to induce functional B12 -deficiency in mammalian cells and organs also suggest their valuable potential as growth inhibitors of cancerous human and animal cells.

Biological function of cobalamin: causes and effects of hypocobalaminemia at the molecular, cellular, tissue and organism level

Cobalamin (vitamin B12) is a complex compound, which is classified as a water-soluble vitamin. Absorption of cobalamin in the gut and its transport to cells is a unique process, in which many proteins are involved. The loss of function of these proteins causes serious cell homeostasis disturbance, which may result in the dysfunction of many tissues and organs. Vitamin B12, a cofactor of methionine synthase, provides methylation process and nucleic acid synthesis. Cobalamin is also necessary for methylmalonyl-CoA mutase activity. The enzyme synthesizes succinyl-CoA, an intermediate in tricarboxylic acid cycle. Vitamin B12 deficiency is an important and current health problem. It may be caused by insufficient dietary intake, age, or disease-related malabsorption and genetic defects of mechanisms involved in the absorption, transport and metabolism of cobalamin. Hypocobalaminemia can also result from long-term pharmacotherapy with medicines: metformin, proton pump inhibitors (e.g. omeprazole) and H2-receptor antagonists (e.g. ranitidine). Significant clinical symptoms of cobalamin deficiency include hematological abnormalities, mainly megaloblastic anemia, as well as neurological disorders resulting from degeneration within the nervous system. Early diagnosis and starting treatment with vitamin B12 increase chances for a complete cure. Therefore, the diagnostically important symptom of hypocobalaminemia may be skin manifestations, mainly hyperpigmentations, but also premature graying of hair. The aim of this review article was to summarize the current state of knowledge on the biological function of cobalamin, as well as the causes and consequences of its deficiency at the molecular, cellular, tissue and organism level.

Mouse models to study the pathophysiology of combined methylmalonic acidemia and homocystinuria, cblC type

Combined methylmalonic acidemia and homocystinuria, cblC type, is the most common inherited disorder of cobalamin metabolism and is characterized by severe fetal developmental defects primarily impacting the central nervous system, hematopoietic system, and heart. CblC was previously shown to be due to mutations in the MMACHC gene, which encodes a protein thought to function in intracellular cobalamin trafficking and biosynthesis of adenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl). These coenzymes are required for the production of succinyl-CoA and methionine, respectively. However, it is currently unclear whether additional roles for MMACHC exist outside of cobalamin metabolism. Furthermore, due to a lack of sufficient animal models, the exact pathophysiology of cblC remains unknown. Here, we report the generation and characterization of two new mouse models to study the role of MMACHC in vivo. CRISPR/Cas9 genome editing was used to develop a Mmachc floxed allele (Mmachc^flox/flox), which we validated as a conditional null. For a gain-of-function approach, we generated a transgenic mouse line that over-expresses functional Mmachc (Mmachc-OE^+/tg) capable of rescuing Mmachc homozygous mutant lethality. Surprisingly, our data also suggest that these mice may exhibit a partially penetrant maternal-effect rescue, which might have implications for in utero therapeutic interventions to treat cblC. Both the Mmachc^flox/flox and Mmachc-OE^+/tg mouse models will be valuable resources for understanding the biological roles of MMACHC in a variety of tissue contexts and allow for deeper understanding of the pathophysiology of cblC.

SIRT1 activation rescues the mislocalization of RNA-binding proteins and cognitive defects induced by inherited cobalamin disorders

BACKGROUND

The molecular consequences of inborn errors of vitamin B12 or cobalamin metabolism are far from being understood. Moreover, innovative therapeutic strategies are needed for the treatment of neurological outcomes that are usually resistant to conventional treatments. Our previous findings suggest a link between SIRT1, cellular stress and RNA binding proteins (RBP) mislocalization in the pathological mechanisms triggered by impaired vitamin B12 metabolism.

OBJECTIVES AND METHODS

The goal of this study was to investigate the effects of the pharmacological activation of SIRT1 using SRT1720 on the molecular mechanisms triggered by impaired methionine synthase activity. Experiments were performed in vitro with fibroblasts from patients with the cblG and cblC inherited defects of vitamin B12 metabolism and in vivo with an original transgenic mouse model of methionine synthase deficiency specific to neuronal cells. Subcellular localization of the RBPs HuR, HnRNPA1, RBM10, SRSF1 and Y14 was investigated by immunostaining and confocal microscopy in patient fibroblasts. RBPs methylation and phosphorylation were studied by co-immunoprecipitation and proximity ligation assay. Cognitive performance of the transgenic mice treated with SRT1720 was measured with an aquatic maze.

RESULTS

Patient fibroblasts with cblC and cblG defects of vitamin B12 metabolism presented with endoplasmic reticulum stress, altered methylation, phosphorylation and subcellular localization of HuR, HnRNPA1 and RBM10, global mRNA mislocalization and increased HnRNPA1-dependent skipping of IRF3 exons. Incubation of fibroblasts with cobalamin, S-adenosyl methionine and okadaic acid rescued the localization of the RBPs and mRNA. The SIRT1 activating compound SRT1720 inhibited ER stress and rescued RBP and mRNA mislocalization and IRF3 splicing. Treatment with this SIRT1 agonist prevented all these hallmarks in patient fibroblasts but it also improved the deficient hippocampo-dependent learning ability of methionine synthase conditional knock-out mice.

CONCLUSIONS

By unraveling the molecular mechanisms triggered by inborn errors of cbl metabolism associating ER stress, RBP mislocalization and mRNA trafficking, our study opens novel therapeutic perspectives for the treatment of inborn errors of vitamin B12 metabolism.

Megaloblastic Anemia Progressing to Severe Thrombotic Microangiopathy in Patients with Disordered Vitamin B12 Metabolism: Case Reports and Literature Review

We describe 2 children with cobalamin G disease, a disorder of vitamin B₁₂ metabolism with normal serum B₁₂ levels. They presented with megaloblastic anemia progressing rapidly to severe thrombotic microangiopathy. In infants presenting with acute thrombotic microangiopathy, cobalamin disorders should be considered early as diagnosis and targeted treatment can be lifesaving.

Inherited disorders of cobalamin metabolism disrupt nucleocytoplasmic transport of mRNA through impaired methylation/phosphorylation of ELAVL1/HuR

The molecular mechanisms that underlie the neurological manifestations of patients with inherited diseases of vitamin B12 (cobalamin) metabolism remain to date obscure. We observed transcriptomic changes of genes involved in RNA metabolism and endoplasmic reticulum stress in a neuronal cell model with impaired cobalamin metabolism. These changes were related to the subcellular mislocalization of several RNA binding proteins, including the ELAVL1/HuR protein implicated in neuronal stress, in this cell model and in patient fibroblasts with inborn errors of cobalamin metabolism and Cd320 knockout mice. The decreased interaction of ELAVL1/HuR with the CRM1/exportin protein of the nuclear pore complex and its subsequent mislocalization resulted from hypomethylation at R-217 produced by decreased S-adenosylmethionine and protein methyl transferase CARM1 and dephosphorylation at S221 by increased protein phosphatase PP2A. The mislocalization of ELAVL1/HuR triggered the decreased expression of SIRT1 deacetylase and genes involved in brain development, neuroplasticity, myelin formation, and brain aging. The mislocalization was reversible upon treatment with siPpp2ca, cobalamin, S-adenosylmethionine, or PP2A inhibitor okadaic acid. In conclusion, our data highlight the key role of the disruption of ELAVL1/HuR nuclear export, with genomic changes consistent with the effects of inborn errors of Cbl metabolisms on brain development, neuroplasticity and myelin formation.

Construction of Fluorescent Analogs to Follow the Uptake and Distribution of Cobalamin (Vitamin B12) in Bacteria, Worms, and Plants

Vitamin B₁₂ is made by only certain prokaryotes yet is required by a number of eukaryotes such as mammals, fish, birds, worms, and Protista, including algae. There is still much to learn about how this nutrient is trafficked across the domains of life. Herein, we describe ways to make a number of different corrin analogs with fluorescent groups attached to the main tetrapyrrole-derived ring. A further range of analogs were also constructed by attaching similar fluorescent groups to the ribose ring of cobalamin, thereby generating a range of complete and incomplete corrinoids to follow uptake in bacteria, worms, and plants. By using these fluorescent derivatives we were able to demonstrate that Mycobacterium tuberculosis is able to acquire both cobyric acid and cobalamin analogs, that Caenorhabditis elegans takes up only the complete corrinoid, and that seedlings of higher plants such as Lepidium sativum are also able to transport B₁₂.

Transcellular transport of cobalamin in aortic endothelial cells

Cobalamin [Cbl (or B₁₂)] deficiency causes megaloblastic anemia and a variety of neuropathies. However, homeostatic mechanisms of cyanocobalamin (CNCbl) and other Cbls by vascular endothelial cells are poorly understood. Herein, we describe our investigation into whether cultured bovine aortic endothelial cells (BAECs) perform transcytosis of B₁₂, namely, the complex formed between serum transcobalamin and B₁₂, designated as holo-transcobalamin (holo-TC). We show that cultured BAECs endocytose [⁵⁷Co]-CNCbl-TC (source material) via the CD320 receptor. The bound Cbl is transported across the cell both via exocytosis in its free form, [⁵⁷Co]-CNCbl, and via transcytosis as [⁵⁷Co]-CNCbl-TC. Transcellular mobilization of Cbl occurred in a bidirectional manner. A portion of the endocytosed [⁵⁷Co]-CNCbl was enzymatically processed by methylmalonic aciduria combined with homocystinuria type C (cblC) with subsequent formation of hydroxocobalamin, methylcobalamin, and adenosylcobalamin, which were also transported across the cell in a bidirectional manner. This demonstrates that transport mechanisms for Cbl in vascular endothelial cells do not discriminate between various β-axial ligands of the vitamin. Competition studies with apoprotein- and holo-TC and holo-intrinsic factor showed that only holo-TC was effective at inhibiting transcellular transport of Cbl. Incubation of BAECs with a blocking antibody against the extracellular domain of the CD320 receptor inhibited uptake and transcytosis by ∼40%. This study reveals that endothelial cells recycle uncommitted intracellular Cbl for downstream usage by other cell types and suggests that the endothelium is self-sufficient for the specific acquisition and subsequent distribution of circulating B₁₂ via the CD320 receptor. We posit that the endothelial lining of the vasculature is an essential component for the maintenance of serum-tissue homeostasis of B₁₂.-Hannibal, L., Bolisetty, K., Axhemi, A., DiBello, P. M., Quadros, E. V., Fedosov, S., Jacobsen, D. W. Transcellular transport of cobalamin in aortic endothelial cells.

Vitamin B12

The biosynthesis of B₁₂, involving up to 30 different enzyme-mediated steps, only occurs in bacteria. Thus, most eukaryotes require an external source of B₁₂, and yet the vitamin appears to have only two functions in eukaryotes: as a cofactor for the enzymes methionine synthase and methylmalonylCoA mutase. These two functions are crucial for normal health in humans, and in particular, the formation of methionine is essential for providing methyl groups for over 100 methylation processes. Interference with the methionine synthase reaction not only depletes the body of methyl groups but also leads to the accumulation of homocysteine, a risk factor for many diseases. The syndrome pernicious anemia, characterized by lack of intrinsic factor, leads to a severe, sometimes fatal form of B₁₂ deficiency. However, there is no sharp cutoff for B₁₂ deficiency; rather, there is a continuous inverse relationship between serum B₁₂ and a variety of undesirable outcomes, including neural tube defects, stroke, and dementia. The brain is particularly vulnerable; in children, inadequate B₁₂ stunts brain and intellectual development. Suboptimal B₁₂ status (serum B₁₂<300pmol/L) is very common, occurring in 30%-60% of the population, in particular in pregnant women and in less-developed countries. Thus, many tens of millions of people in the world may suffer harm from having a poor B₁₂ status. Public health steps are urgently needed to correct this inadequacy.

Vitamin B12 deficiency

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

Cell Type-Specific Modulation of Cobalamin Uptake by Bovine Serum

Tracking cellular ⁵⁷Co-labelled cobalamin (⁵⁷Co-Cbl) uptake is a well-established method for studying Cbl homeostasis. Previous studies established that bovine serum is not generally permissive for cellular Cbl uptake when used as a supplement in cell culture medium, whereas supplementation with human serum promotes cellular Cbl uptake. The underlying reasons for these differences are not fully defined. In the current study we address this question. We extend earlier observations by showing that fetal calf serum inhibits cellular ⁵⁷Co-Cbl uptake by HT1080 cells (a fibrosarcoma-derived fibroblast cell line). Furthermore, we discovered that a simple heat-treatment protocol (95°C for 10 min) ameliorates this inhibitory activity for HT1080 cell ⁵⁷Co-Cbl uptake. We provide evidence that the very high level of haptocorrin in bovine serum (as compared to human serum) is responsible for this inhibitory activity. We suggest that bovine haptocorrin competes with cell-derived transcobalamin for Cbl binding, and that cellular Cbl uptake may be minimised in the presence of large amounts of bovine haptocorrin that are present under routine in vitro cell culture conditions. In experiments conducted with AG01518 cells (a neonatal foreskin-derived fibroblast cell line), overall cellular ⁵⁷Co-Cbl uptake was 86% lower than for HT1080 cells, cellular TC production was below levels detectable by western blotting, and heat treatment of fetal calf serum resulted in only a modest increase in cellular ⁵⁷Co-Cbl uptake. We recommend a careful assessment of cell culture protocols should be conducted in order to determine the potential benefits that heat-treated bovine serum may provide for in vitro studies of mammalian cell lines.

Biomarkers and Algorithms for the Diagnosis of Vitamin B12 Deficiency

Vitamin B12 (cobalamin, Cbl, B12) is an indispensable water-soluble micronutrient that serves as a coenzyme for cytosolic methionine synthase (MS) and mitochondrial methylmalonyl-CoA mutase (MCM). Deficiency of Cbl, whether nutritional or due to inborn errors of Cbl metabolism, inactivate MS and MCM leading to the accumulation of homocysteine (Hcy) and methylmalonic acid (MMA), respectively. In conjunction with total B12 and its bioactive protein-bound form, holo-transcobalamin (holo-TC), Hcy, and MMA are the preferred serum biomarkers utilized to determine B12 status. Clinically, vitamin B12 deficiency leads to neurological deterioration and megaloblastic anemia, and, if left untreated, to death. Subclinical vitamin B12 deficiency (usually defined as a total serum B12 of <200 pmol/L) presents asymptomatically or with rather subtle generic symptoms that oftentimes are mistakenly ascribed to unrelated disorders. Numerous studies have now established that serum vitamin B12 has limited diagnostic value as a stand-alone marker. Low serum levels of vitamin B12 not always represent deficiency, and likewise, severe functional deficiency of the micronutrient has been documented in the presence of normal and even high levels of serum vitamin B12. This review discusses the usefulness and limitations of current biomarkers of B12 status in newborn screening, infant and adult diagnostics, the algorithms utilized to diagnose B12 deficiency and unusual findings of vitamin B12 status in various human disorders.

Added value of next generation gene panel analysis for patients with elevated methylmalonic acid and no clinical diagnosis following functional studies of vitamin B12 metabolism

Next generation sequencing (NGS) based gene panel testing is increasingly available as a molecular diagnostic approach for inborn errors of metabolism. Over the past 40 years patients have been referred to the Vitamin B12 Clinical Research Laboratory at McGill University for diagnosis of inborn errors of cobalamin metabolism by functional studies in cultured fibroblasts. DNA samples from patients in which no diagnosis was made by these studies were tested by a NGS gene panel to determine whether any molecular diagnoses could be made. 131 DNA samples from patients with elevated methylmalonic acid and no diagnosis following functional studies of cobalamin metabolism were analyzed using the 24 gene extended cobalamin metabolism NGS based panel developed by Baylor Miraca Genetics Laboratories. Gene panel testing identified two or more variants in a single gene in 16/131 patients. Eight patients had pathogenic findings, one had a finding of uncertain significance, and seven had benign findings. Of the patients with pathogenic findings, five had mutations in ACSF3, two in SUCLG1 and one in TCN2. Thus, the NGS gene panel allowed for the presumptive diagnosis of 8 additional patients for which a diagnosis was not made by the functional assays.

Regulation of Hematopoiesis and Methionine Homeostasis by mTORC1 Inhibitor NPRL2

Nitrogen permease regulator-like 2 (NPRL2) is a component of a conserved complex that inhibits mTORC1 (mammalian Target Of Rapamycin Complex 1) in response to amino acid insufficiency. Here, we show that NPRL2 is required for mouse viability and that its absence significantly compromises fetal liver hematopoiesis in developing embryos. Moreover, NPRL2 KO embryos have significantly reduced methionine levels and exhibit phenotypes reminiscent of cobalamin (vitamin B12) deficiency. Consistent with this idea, NPRL2 KO liver and mouse embryonic fibroblasts (MEFs) show defective processing of the cobalamin-transport protein transcobalamin 2, along with impaired lysosomal acidification and lysosomal gene expression. NPRL2 KO MEFs exhibit a significant defect in the cobalamin-dependent synthesis of methionine from homocysteine, which can be rescued by supplementation with cyanocobalamin. Taken together, these findings demonstrate a role for NPRL2 and mTORC1 in the regulation of lysosomal-dependent cobalamin processing, methionine synthesis, and maintenance of cellular re-methylation potential, which are important during hematopoiesis.

Methionine synthase reductase deficiency (CblE): A report of two patients and a novel mutation

IMPORTANCE

Functional methionine synthase reductase deficiency, also known as cobalamin E disorder, is a rare autosomal recessive inherited disease that results in an impaired remethylation of homocysteine to methionine. It presents with macrocytic anemia, hyperhomocysteinemia, and hypomethioninemia, and may also be accompanied with neurological impairment.

CLINICAL PRESENTATION

We describe two new cases of unrelated girls with megaloblastic anemia misclassified at first as congenital dyserythropoietic anemia with development of neurologic dysfunction in one of them.

INTERVENTION

The posterior finding of biochemical features (hyperhomocysteinemia and hypomethioninemia) focused the diagnosis on the inborn errors of intracellular vitamin B12. Subsequent molecular analysis of the methionine synthase reductase (MTRR) gene revealed compound heterozygosity for a transition c.1361C > T (p.Ser454Leu) and another, not yet described in literature, c.1677-1G > A (p.Glu560fs) in one patient, and a single homozygosis mutation, c.1361C > T (p.Ser545Leu) in the other one. These mutations confirmed the diagnosis of cobalamin E deficiency.

CONCLUSION

Treatment with hydroxocobalamin in combination with betaine appears to be useful for hematological improvement and prevention of brain disabilities in CblE-affected patients. Our study widens the clinical, molecular, metabolic, and cytological knowledge of deficiency MTRR enzyme.

Cobalamin coenzyme forms are not likely to be superior to cyano- and hydroxyl-cobalamin in prevention or treatment of cobalamin deficiency

Methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl) are coenzymes for methionine synthase and methylmalonyl-CoA mutase, respectively. Hydroxylcobalamin (HOCbl) and cyanocobalamin (CNCbl) are frequently used for supplementation. MeCbl and AdoCbl have recently emerged as alternative forms in supplements. In the light of metabolic transformation of Cbl into its cofactor forms, this review discusses current evidence on efficacy and utility of different Cbl forms in preventing or treating Cbl deficiency. Cbl-transporting proteins bind and mediate the uptake of all aforementioned forms of Cbl. After internalization and lysosomal release, Cbl binds to the cytosolic chaperon MMACHC that is responsible for (i) flavin-dependent decyanation of [CN-Co³⁺]Cbl to [Co²⁺]Cbl; (ii) glutathione-dependent dealkylation of MeCbl and AdoCbl to [Co^2+/1+]Cbl; and (iii) glutathione-dependent decyanation of CNCbl or reduction of HOCbl under anaerobic conditions. MMACHC shows a broad specificity for Cbl forms and supplies the Cbl²⁺ intermediate for synthesis of MeCbl and AdoCbl. Cobalamin chemistry, physiology, and biochemistry suggest that MeCbl and AdoCbl follow the same route of intracellular processing as CNCbl does. We conclude that supplementing MeCbl or AdoCbl is unlikely to be advantageous compared to CNCbl. On the other hand, there are obvious advantages of high parenteral doses (1–2 mg) of HOCbl in treating inborn errors of Cbl metabolism.

Fresh and vitrified bovine preantral follicles have different nutritional requirements during in vitro culture

The aim of this study was to compare the efficiency of different media for the in vitro culturing of fresh and vitrified bovine ovarian tissues. Fragments of the ovarian cortex were subjected to vitrification and histological and viability analyses or were immediately cultured in vitro using the alfa minimum essential medium, McCoy's 5A medium (McCoy), or medium 199 (M199). Samples of different culture media were collected on days 1 (D1) and 5 (D5) for quantification of reactive oxygen species and for hormonal assays. In non-vitrified (i.e., fresh) ovarian tissue cultures, the percentage of morphologically normal follicles was significantly greater than that recorded for the other media (e.g., M199). In the case of previously vitrified tissues, the McCoy medium was significantly superior to the other media in preserving follicular morphology up until the last culture day (i.e., D5), thus maintaining a similar percentage from D1 to D5. Reactive oxygen species levels were higher in D1 vitrified cultured tissues, but there were no differences in the levels among the three media after 5 days. The hormonal assays showed that in the case of previously vitrified tissues, at D5, progesterone levels increased on culture in the M199 medium and estradiol levels increased on culture in the McCoy medium. In conclusion, our results indicate that the use of M199 would be recommended for fresh tissue cultures and of McCoy for vitrified tissue cultures.

Perturbation of neuronal cobalamin transport by lysosomal enzyme inhibition

Cbl (cobalamin) utilization as an enzyme cofactor is dependent on its efficient transit through lysosomes to the cytosol and mitochondria. We have previously proposed that pathophysiological perturbations in lysosomal function may inhibit intracellular Cbl transport with consequences for down-stream metabolic pathways. In the current study, we used both HT1080 fibroblasts and SH-SY5Y neurons to assess the impact that protease inhibitors, chloroquine and leupeptin (N-acetyl-L-leucyl-L-leucyl-L-argininal), have on the distribution of [⁵⁷Co]Cbl in lysosomes, mitochondria and cytosol. Under standard cell culture conditions the distribution of [⁵⁷Co]Cbl in both neurons and fibroblasts was ~5% in lysosomes, 14% in mitochondria and 81% in cytosol. Treatment of cells with either 25 μM chloroquine or 40 μM leupeptin for 48 h significantly increased the lysosomal [⁵⁷Co]Cbl levels, by 4-fold in fibroblasts and 10-fold in neurons, and this was associated with reduced cytosolic and mitochondrial [⁵⁷Co]Cbl concentrations. Based on Western blotting of LAMP2 in fractions recovered from an OptiPrep density gradient, lysosomal Cbl trapping was associated with an expansion of the lysosomal compartment and an increase in a subpopulation of lysosomes with increased size and density. Moreover, the decreased mitochondrial Cbl that was associated with lysosomal Cbl trapping was correlated with decreased incorporation of [¹⁴C] propionate into cellular proteins/macromolecules, indicating an inhibition of Cbl-dependent Mm-CoA (methylmalonyl-coenzyme A) mutase activity. These results add support to the idea that lysosomal dysfunction may significantly impact upon Cbl transport and utilization.