Transcobalamin II deficiency at birth

A novel TCN2 mutation with unusual clinical manifestations of hemolytic crisis and unexplained metabolic acidosis: expanding the genotype and phenotype of transcobalamin II deficiency

Background

Transcobalamin deficiency is a rare inborn metabolic disorder, characterized by pancytopenia, megaloblastic anemia, failure to thrive, diarrhea, and psychomotor retardation.

Case presentation

We describe a patient who first presented at 3 months of age, with pancytopenia, hepatosplenomegaly, recurrent infection, metabolic acidosis, and acute hemolytic crisis. Extensive hematologic and immunologic investigations did not identify inherited bone marrow failure syndrome, acute leukemia or its related disorders. Whole exome sequencing identified a novel homozygous TCN2 mutation, c.428-2A > G and mRNA study confirmed an aberrant transcription of exon 4 skipping. The mutant protein is predicted to have an in-fame 51 amino acids deletion (NP_000346:p.Gly143_Val193del). The patient exhibited marked clinical improvement following hydroxocobalamin treatment.

Conclusions

Transcobalamin deficiency should be investigated in infants with unexplained pancytopenia and acute hemolytic crisis with or without typical evidence of vitamin B12 deficiency.

Subclinical maculopathy and retinopathy in transcobalamin deficiency: a 10-year follow-up

INTRODUCTION

Transcobalamin (TC) transports cobalamin (vitamin B₁₂) from plasma into cells. Its congenital deficiency is a rare autosomal recessive disorder due to mutations in the TCN2 gene. It causes intracellular cobalamin depletion with early onset in the first months of life, failure to thrive with pallor due to megaloblastic anemia. It can be associated with pancytopenia, gastrointestinal symptoms with vomiting, diarrhea, and neurological complications with myelopathy. Aggressive vitamin B12 parenteral therapy must be instituted early and continuously. Retinopathy and maculopathy are rarely associated with this condition.

SUBJECT

We report the electrophysiological results of one TC-deficient patient diagnosed at the age of 4 months immediately and continuosly treated by hydroxocobalamin IM. Her visual function was followed by eight ophthalmological assessments, eight flash-ERG, six EOG, one mf-ERG, and seven P-ERG recordings over a 10-year period, between the age of 2y 9 m and 12y 6 m.

RESULTS

Her ophthalmological assessment including visual acuity, fundi, optical coherent tomography (OCT), and retinal nerve fiber layer (RNFL) remained normal. From the age of 2y 9 m to 5y, dark-adapted and light-adapted flash-ERGs, EOGs and pattern-ERG were normal. From the age of 6y 4 m to 12y 6 m, dark-adapted flash-ERGs and EOGs remained normal. Cone a-wave amplitudes remained normal, whereas cone b-wave and flicker-response amplitudes were decreased. At the age of 12y 6 m, mf-ERG N1P1 amplitudes on the central 30° were decreased. From the age of 7y 4 m to 12y 6 m, P-ERG P50 amplitudes were decreased with no N95.

COMMENTS

While clinical and anatomical assessments remained normal over a 10-year period, patient's electrophysiological results suggested the progressive onset of a subclinical retinopathy of inner-cone dystrophy type, and a subclinical maculopathy on the central 30° including the ganglion cell layer deficiency on the central 15°, despite continuous intramuscular treatment, RPE and scotopic system remaining normal. The origins of such subclinical retinopathy and maculopathy are unknown and independent of early disease identification and aggressive intramuscular hydroxocobalamin therapy.

Transcobalamin II deficiency in twins with a novel variant in the TCN2 gene: case report and review of literature

Objectives Transcobalamin II (TC) is an essential plasma protein for the absorption, transportation, and cellular uptake of cobalamin. TC deficiency presents in the first year of life with failure to thrive, hypotonia, lethargy, diarrhea, pallor, mucosal ulceration, anemia, pancytopenia, and agammaglobulinemia. Herein, we present TC deficiency diagnosed in two cases (twin siblings) with a novel variant in the TCN2 gene. Case presentation 4-month-old twins were admitted with fever, respiratory distress, vomiting, diarrhea, and failure to thrive. Physical examination findings revealed developmental delay and hypotonia with no head control, and laboratory findings were severe anemia, neutropenia, and hypogammaglobulinemia. Despite normal vitamin B12 and folate levels, homocysteine and urine methylmalonic acid levels were elevated in both patients. Bone marrow examinations revealed hypocellular bone marrow in both cases. The patients had novel pathogenic homozygous c.241C>T (p.Gln81Ter) variant in the TCN2 gene. In both cases, with intramuscular hydroxycobalamin therapy, laboratory parameters improved, and a successful clinical response was achieved. Conclusions In infants with pancytopenia, growth retardation, gastrointestinal manifestations, and immunodeficiency, the inborn error of cobalamin metabolism should be kept in mind. Early diagnosis and treatment are crucial for better clinical outcomes. What is new? In literature, to date, less than 50 cases with TC deficiency were identified. In this report, we presented twins with TCN2 gene mutation. Both patients emphasized that early and aggressive treatment is crucial for achieving optimal outcomes. In this report, we identified a novel variation in TCN2 gene.

Identification of transcobalamin deficiency with two novel mutations in the TCN2 gene in a Chinese girl with abnormal immunity: a case report

Background

Transcobalamin (TC) transports vitamin B12 from blood into cells. TC II deficiency is a rare autosomal recessive disorder. It is characterized by failure to thrive, diarrhoea, pallor, anaemia, pancytopenia or agammaglobulinemia. It is usually confirmed by molecular analysis of the TCN2 gene. We report a 2-month-old girl with two novel mutations, which were first reported in humans.

Case presentation

We present a 2-month-old Chinese girl with pancytopenia, severe combined immunodeficiency disease, and megaloblastic anaemia. Targeted next-generation sequencing (NGS) was performed, which detected compound heterozygous variants in exon 7 of the TCN2 gene (Mutation 1: c.1033 C > T; Mutation 2: c.1017-1031delinsGTAACAGAGATGGTT). These mutations result in stop codons in TCN2. The c.1033C > T mutation causes a stop at codon 345 (p.Gln345Ter), and the c.1017-1031delinsGTAACAGAGATGGTT mutation causes a stop at codon 340 (p.Leu340Ter). After being diagnosed, she was treated with intramuscular 1 mg hydroxycobalamin (OH-Cbl) every day for 2 months. The CBC value returned to normal after half a month. The peripheral blood lymphocyte subsets and immunoglobulin recovered after 2 months. Then, the dosage of OH-Cbl was gradually reduced.

Conclusions

TC II deficiency is a serious complication that requires lifelong treatment. Its diagnosis is difficult due to the lack of clearly identifiable symptoms. Genetic testing should be performed as early as possible if this disease is suspected. The specific observations of this case report make a considerable contribution to the literature and provide a reference for the diagnosis and treatment of future cases.

Combining newborn metabolic and DNA analysis for second-tier testing of methylmalonic acidemia

Purpose

Improved second-tier tools are needed to reduce false-positive outcomes in newborn screening (NBS) for inborn metabolic disorders on the Recommended Universal Screening Panel (RUSP).

Methods

We designed an assay for multiplex sequencing of 72 metabolic genes (RUSPseq) from newborn dried blood spots. Analytical and clinical performance was evaluated in 60 screen-positive newborns for methylmalonic acidemia (MMA) reported by the California Department of Public Health NBS program. Additionally, we trained a Random Forest machine learning classifier on NBS data to improve prediction of true and false-positive MMA cases.

Results

Of 28 MMA patients sequenced, we found two pathogenic or likely pathogenic (P/LP) variants in a MMA-related gene in 24 patients, and one pathogenic variant and a variant of unknown significance (VUS) in 1 patient. No such variant combinations were detected in MMA false positives and healthy controls. Random Forest–based analysis of the entire NBS metabolic profile correctly identified the MMA patients and reduced MMA false-positive cases by 51%. MMA screen-positive newborns were more likely of Hispanic ethnicity.

Conclusion

Our two-pronged approach reduced false positives by half and provided a reportable molecular finding for 89% of MMA patients. Challenges remain in newborn metabolic screening and DNA variant interpretation in diverse multiethnic populations.

Different Presentations of Patients with Transcobalamin II Deficiency: A Single-Center Experience from Turkey

Objective:

Transcobalamin II deficiency is a rare autosomal recessive disease characterized by decreased cobalamin availability, which in turn causes accumulation of homocysteine and methylmalonic acid. The presenting clinical features are failure to thrive, diarrhea, megaloblastic anemia, pancytopenia, neurologic abnormalities, and also recurrent infections due to immune abnormalities in early infancy.

Materials and Methods:

Here, we report the clinical and laboratory features of six children with transcobalamin II deficiency who were all molecularly confirmed.

Results:

The patients were admitted between 1 and 7 months of age with anemia or pancytopenia. Unexpectedly, one patient had a serum vitamin B12 level lower than the normal range and another one had nonsignificantly elevated serum homocysteine levels. Four patients had lymphopenia, four had neutropenia and three also had hypogammaglobulinemia. Suggesting the consideration of transcobalamin II deficiency in the differential diagnosis of immune deficiency. Hemophagocytic lymphohistiocytosis was also detected in one patient. Furthermore, two patients had vacuolization in the myeloid lineage in bone marrow aspiration, which may be an additional finding of transcobalamin II deficiency. The hematological abnormalities in all patients resolved after parenteral cobalamin treatment. In follow-up, two patients showed neurological impairments such as impaired speech and walking. Among our six patients who were all molecularly confirmed, two had the mutation that was reported in transcobalamin II-deficient patients of Turkish ancestry. Also, a novel TCN2 gene mutation was detected in one of the remaining patients.

Conclusion:

Transcobalamin II deficiency should be considered in the differential diagnosis of infants with immunological abnormalities as well as cytopenia and neurological dysfunction. Early recognition of this rare condition and initiation of adequate treatment is critical for control of the disease and better prognosis.

Long-term Outcome of 4 Patients With Transcobalamin Deficiency Caused by 2 Novel TCN2 Mutations

Cobalamin (vitamin B12 [Cbl]) is an essential cofactor for many biochemical pathways. Transcobalamin (TC) is required to internalize Cbl into the cells through membrane receptor-mediated endocytosis. Cbl is then processed in the cytoplasm and mitochondria by complementation factors leading to its active metabolites; methylcobalamin and 5-deoxyadenosyl-cobalamin. Deficiency of TC results in an elevation in methylmalonic acid and homocysteine. Patients usually present with macrocytic anemia, pancytopenia, failure to thrive, gastrointestinal symptoms, and neurological dysfunction. In this study, we report 4 patients from 2 unrelated families, with confirmed diagnosis of TC deficiency. Patients initially had a typical presentation of TC deficiency: severe diarrhea and vomiting, recurrent infections, stomatitis, macrocytic anemia, and neutropenia. Interestingly one of the patients was diagnosed at 3 months of age and developed ataxic gait related to cerebellar atrophy at the age of 14 months. His elder affected sibling was diagnosed at 5 months of age was completely normal. Two sibs, diagnosed at 2 months of age and immediately after birth, had autism spectrum disorder. Molecular investigations showed 2 novel mutations in TCN2 gene. Patients were treated and stayed stable on weekly injection of Cbl. In conclusion, TC deficiency has a wide heterogeneity in clinical phenotype, genotype, laboratory, and radiologic findings. Early detection of the disease and early initiation of aggressive parenteral treatment is probably associated with better prognosis and disease control.

Maternofetal transport of vitamin B12: role of TCblR/CD320 and megalin

Vitamin B₁₂ deficiency causes megaloblastic anemia and neurologic disorder in humans. Gene defects of transcobalamin (TC) and the transcobalamin receptor (TCblR), needed for cellular uptake of the TC-bound B₁₂, do not confer embryonic lethality. TC deficiency can produce the hematologic and neurologic complications after birth, whereas TCblR/CD320 gene defects appear to produce mild metabolic changes. Alternate maternofetal transport mechanisms appear to provide adequate B₁₂ to the fetus. To understand this mechanism, we evaluated the role of TC, TCblR/CD320, and megalin in maternofetal transport of B₁₂ in a TCblR/CD320-knockout (KO) mouse. Our results showed high expression of TCblR/CD320 in the labyrinth of the placenta, embryonic brain, and spinal column in wild-type (WT) mice. Megalin expression was about the same in both WT and KO mouse visceral yolk sac, brain, and spinal column. Megalin mRNA was down-regulated in the KO embryonic spinal cord (SC) and kidneys. Megalin expression remained unaltered in adult WT and KO mouse brain, SC, and kidneys. Injected dsRed-TC-B₁₂ and TC-⁵⁷CoB₁₂ accumulated in the visceral yolk sac of KO mice where megalin is expressed and provides an alternate mechanism for the maternofetal transport of Cbl during fetal development.-Arora, K., Sequeira, J. M., Quadros, E. V. Maternofetal transport of vitamin B₁₂: role of TCblR/CD320 and megalin.

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.

Seven Patients With Transcobalamin Deficiency Diagnosed Between 2010 and 2014: A Single-Center Experience

Transcobalamin deficiency (OMIM 275350) is a rare autosomal recessive disease presenting with nonspecific clinical features in early infancy. We report the clinical and laboratory manifestations of 7 children diagnosed with transcobalamin deficiency. All patients were admitted between 2 and 4 months of age with anemia, thrombocytopenia, and hyperhomocysteinemia. The most common complaints at admission were pallor, weakness, and poor feeding. Genetic analysis was performed in 5 patients and it revealed the same homozygous mutation. We initially treated all patients with intramuscular injections of a maximum of 1 mg cyanocobalamin (CN-Cbl) daily and with a final dose of 1 mg per week. Hemoglobin and platelet counts significantly decreased upon decrease or cessation of CN-Cbl therapy. The patients were reevaluated between 2 and 4 years of age and all had delay in speech and walking. In conclusion, 1 mg of intramuscular CN-Cbl every week suffices for hematological improvement but not for normal neurological development in patients who all had relapse due to decrease or cessation of treatment.

Transcobalamin II Deficiency in Four Cases with Novel Mutations

Objective:

Transcobalamin II deficiency is one of the rare causes of inherited vitamin B12 disorders in which the patients have characteristically normal or high vitamin B12 levels related to the transport defect of vitamin B12 into the cell, ending up with intracellular cobalamin depletion and high homocysteine and methylmalonic acid levels.

Materials and Methods:

Herein, we describe the findings at presentation of four patients who were diagnosed to have transcobalamin II deficiency with novel mutations.

Results:

These patients with transcobalamin II deficiency were found to have novel mutations, of whom 2 had the same large deletion (homozygous c.1106+1516-1222+1231del).

Conclusion:

Transcobalamin II deficiency should be considered in differential diagnosis of any infant with pancytopenia, failure to thrive, diarrhea, and vomiting.

Update on transcobalamin deficiency: clinical presentation, treatment and outcome

Transcobalamin (TC) transports cobalamin from blood into cells. TC deficiency is a rare autosomal recessive disorder usually presenting in early infancy with failure to thrive, weakness, diarrhoea, pallor, anemia, and pancytopenia or agammaglobulinemia. It can sometimes resemble neonatal leukemia or severe combined immunodeficiency disease. Diagnosis of TC deficiency is suspected based on megaloblastic anemia, elevation of total plasma homocysteine, and blood or urine methylmalonic acid. It is confirmed by studying the synthesis of TC in cultured fibroblasts, or by molecular analysis of the TCN2 gene. TC deficiency is treatable with supplemental cobalamin, but the optimal type, route and frequency of cobalamin administration and long term patient outcomes are unknown. Here we present a series of 30 patients with TC deficiency, including an update on multiple previously published patients, in order to evaluate the different treatment strategies and provide information about long term outcome. Based on the data presented, current practice appears to favour treatment of individuals with TC deficiency by intramuscular injections of hydroxy- or cyanocobalamin. In most cases presented, at least weekly injections (1 mg IM) were necessary to ensure optimal treatment. Most centres adjusted the treatment regimen based on monitoring CBC, total plasma homocysteine, plasma and urine methylmalonic acid, as well as, clinical status. Finally, continuing IM treatment into adulthood appears to be beneficial.

A novel mutation of the transcobalamin II gene in an infant presenting with hemophagocytic lymphohistiocytosis

Transcobalamin II (TC II) deficiency is a rare disorder of cobalamin (CBL, vitamin B12) metabolism that occurs due to mutations in transcobalamin gene (TCN2). Hemophagocytic lymphohistiocytosis (HLH) in contrast is a syndrome characterized by uncontrolled immune response with hyperinflammation. A 2-month-old male baby was admitted with complaints of fever, cough, diarrhea, and respiratory distress. The parents were first cousins. The baby exhibited five of the eight diagnostic criteria for HLH-2004 and was diagnosed as HLH. A second bone marrow aspiration demonstrated megaloblastic changes in the erythroid series. The patient's vitamin B12 level was normal; however, hyperhomocysteinemia was present. A genetic deficiency of TC II was suspected. The patient and his parents were tested for TCN2 mutation. He had a homozygote mutation that was not included in Human 'Gene Mutation Database Cardiff'. The patient was treated with intramuscular vitamin B12, which was followed by improvement in both clinical and laboratory findings. He was 12 months old at the time of this report, with normal physical and neuromotor development. In this case presenting with the clinical and laboratory findings of HLH, TC II deficiency was diagnosed. A new mutation was found that was not reported before. Potential causative mechanisms of HLH induced by defects of cobalamin synthesis merit further investigation.

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.

Mapping the functional domains of TCblR/CD320, the receptor for cellular uptake of transcobalamin-bound cobalamin

The membrane receptor TCblR/CD320 binds transcobalamin (TC) saturated with vitamin B12 [cobalamin (Cbl)] and mediates cellular uptake of the vitamin. The specificity of TC for Cbl and of the receptor for TC-Cbl ensures efficient uptake of Cbl into cells. The high-affinity interaction of TCblR with TC-Cbl (Ka=10 nM(-1)) was investigated using deletions and mutations of amino acid sequences in TCblR. Only the extracellular region (aa 32-229) is needed for TC-Cbl binding, but the N-glycosylation sites (N126, N195, and N213) are of no importance for this function. Deleting the cysteine-rich region (aa 95-141) that separates the two low-density lipoprotein receptor type A (LDLR-A) domains does not affect TC-Cbl binding (Ka = 19-24 nM(-1)). The two LDLR-A domains (aa 54-89 and 132-167) with the negatively charged acidic residues involved in Ca(2+) binding are critical determinants of ligand binding. The cytoplasmic tail is apparently crucial for internalization of the ligand. Within this region, the RPLGLL motif and the PDZ binding motifs (QERL/KESL) appear to be involved in initiating and completing the process of ligand internalization. Mutations and deletions of these regions involved in binding and internalization of TC-Cbl are likely to produce the biochemical and clinical phenotype of Cbl deficiency.

Laboratory evaluation for vitamin B12 deficiency: the case for cascade testing

OBJECTIVE

Potential vitamin B(12) deficiency is a common clinical diagnostic problem, and many providers have a low threshold for initiating therapy. The goal of this study was to systematically evaluate current practice patterns regarding the laboratory evaluation of suspected vitamin B(12) deficiency.

METHODS

This retrospective study reviewed the electronic medical records of 192 patients initiated on intramuscular vitamin B(12) injections.

RESULTS

Only 12 patients had objectively documented hematologic responses: decrease of mean corpuscular volume by ≥5 fL with stable or improved hemoglobin. Another 5 patients had equivocal hematologic responses. There was one plausible neurologic response. Thus, only 18 (9.4%) of 192 patients had data supportive of a clinical response. In these 18 patients, the baseline serum B(12) level was ≤107 pg/mL; only 3 patients also had a baseline serum methylmalonic acid level, which was ≥1.29 μmol/L in all 3 patients.

CONCLUSIONS

Currently, only a small minority of patients initiated on intramuscular vitamin B(12) supplementation derive any meaningful clinical benefit. Furthermore, current testing recommendations for vitamin B(12) deficiency are usually not followed. Up-front ordering of a diagnostic testing cascade is recommended to improve compliance; an example is presented with decision points chosen to improve specificity for clinically evident vitamin B(12) deficiency without loss of sensitivity. Ultimately, a better understanding of vitamin B(12) physiology is needed to develop and evaluate laboratory tests that more accurately reflect true intracellular vitamin B(12) status.

Transcobalamin deficiency caused by compound heterozygosity for two novel mutations in the TCN2 gene: a study of two affected siblings, their brother, and their parents

Transcobalamin (TC) deficiency (OMIM# 275350) is a rare, autosomal recessive disorder that presents in early infancy with a broad spectrum of symptoms, including failure to thrive, megaloblastic anemia, immunological deficiency, and neurological symptoms. Here we report a study of a family (parents and three children) with two children suffering from TC deficiency caused by two different mutations in the TCN2 gene. Initially, molecular genetic analysis of genomic DNA revealed a heterozygous mutation in the +1 position of exon 7 (c.1106+1 G > A) in the father and all three children. Bioinformatic analysis indicates that this mutation causes exon skipping, and further experiments supported this hypothesis and suggested that the mutant allele undergoes nonsense-mediated messenger RNA (mRNA) decay. We did not identify further mutations in genomic DNA that could explain TC deficiency in the two children. However, further efforts using complementary DNA (cDNA) derived from RNA from blood leukocytes identified a large deletion removing the entire exon 8, resulting in a frameshift and a premature stop codon (p.E371fsX372) in the mother and the two affected children. Our data indicate that if exon-by-exon DNA sequencing of genomic DNA does not uncover mutations corresponding to the phenotype, a systematic search for other mutations should be initiated by sequencing cDNA or using semiquantitative methods to detect large deletions in TCN2.

Positive newborn screen for methylmalonic aciduria identifies the first mutation in TCblR/CD320, the gene for cellular uptake of transcobalamin-bound vitamin B(12)

Elevated methylmalonic acid in five asymptomatic newborns whose fibroblasts showed decreased uptake of transcobalamin-bound cobalamin (holo-TC), suggested a defect in the cellular uptake of cobalamin. Analysis of TCblR/CD320, the gene for the receptor for cellular uptake of holo-TC, identified a homozygous single codon deletion, c.262_264GAG (p.E88del), resulting in the loss of a glutamic acid residue in the low-density lipoprotein receptor type A-like domain. Inserting the codon by site-directed mutagenesis fully restored TCblR function.

Should transcobalamin deficiency be treated aggressively?

Transcobalamin (transcobalamin II, TC) transports plasma vitamin B(12) (cobalamin, Cbl) into cells. TC deficiency is a rare autosomal recessive disorder causing intracellular Cbl depletion, which in turn causes megaloblastic bone marrow failure, accumulation of homocysteine and methylmalonic acid, and methionine depletion. The clinical presentation reflects intracellular Cbl defects, with early-onset failure to thrive with gastrointestinal symptoms, pancytopenia, and megaloblastic anemia, sometimes followed by neurological complications. We report the clinical, biological, and molecular findings and the outcome in five TC-deficient patients. The three treated early had an initial favorable outcome, whereas the two treated inadequately had late-onset severe neuro-ophthalmological impairment. Even if the natural course of the disease over time might also result in late-onset symptoms in the aggressively treated patients, these data emphasize that TC deficiency is a severe disorder requiring early detection and probably long-term aggressive therapy. Mutation analysis revealed six unreported mutations in the TCN2 gene. In silico structural analysis showed that these mutations disrupt the Cbl-TC interaction domain and/or the putative transcobalamin-transcobalamin receptor interaction domain.