Today, I review, link to, and excerpt from the preprint (not peer-reviewed) PDF article Transcobalamin Receptor Autoantibodies in Central Vitamin B12 Deficiency because the published article is behind a paywall. The published article is available at PubMed Abstract and link to the peer-reviewed full text article is available here. Sci Transl Med. 2024 Jun 26;16(753):eadl3758. doi: 10.1126/scitranslmed.adl3758. Epub 2024 Jun 26.
All that follows in this post (except for the abstract which is from the published article) is from the preprint (not peer-reviewed) PDF.
Abstract
Vitamin B12 is critical for hematopoiesis and myelination. Deficiency can cause neurologic deficits including loss of coordination and cognitive decline. However, diagnosis relies on measurement of vitamin B12 in the blood, which may not accurately reflect the concentration in the brain. Using programmable phage display, we identified an autoantibody targeting the transcobalamin receptor (CD320) in a patient with progressive tremor, ataxia, and scanning speech. Anti-CD320 impaired cellular uptake of cobalamin (B12) in vitro by depleting its target from the cell surface. Despite a normal serum concentration, B12 was nearly undetectable in her cerebrospinal fluid (CSF). Immunosuppressive treatment and high-dose systemic B12 supplementation were associated with increased B12 in the CSF and clinical improvement. Optofluidic screening enabled isolation of a patient-derived monoclonal antibody that impaired B12 transport across an in vitro model of the blood-brain barrier (BBB). Autoantibodies targeting the same epitope of CD320 were identified in seven other patients with neurologic deficits of unknown etiology, 6% of healthy controls, and 21.4% of a cohort of patients with neuropsychiatric lupus. In 132 paired serum and CSF samples, detection of anti-CD320 in the blood predicted B12 deficiency in the brain. However, these individuals did not display any hematologic signs of B12 deficiency despite systemic CD320 impairment. Using a genome-wide CRISPR screen, we found that the low-density lipoprotein receptor serves as an alternative B12 uptake pathway in hematopoietic cells. These findings dissect the tissue specificity of B12 transport and elucidate an autoimmune neurologic condition that may be amenable to immunomodulatory treatment and nutritional supplementation.
Case Report
A woman in her 60s presented with 5 weeks of progressive bilateral lower extremity pain, difficulty speaking, ataxia, and tremor (Case 1; Fig. 1A). Her neurologic exam was notable
for scanning speech, a rubral tremor of the head and hands, diminished vibratory sensation in her feet, dysmetria, and truncal ataxia. MRI of the brain revealed bilateral symmetric T2-weighted hyperintensities in the dorsal brainstem with gadolinium enhancement of the superior cerebellar peduncles (Fig. 1B). There were no lesions on MRI of the cervical spine. Lumbar puncture yielded non-inflammatory CSF (2 white blood cells, 2 red blood cells, total protein of 54 mg/dL, glucose of 91 mg/dL, zero oligoclonal bands, and a normal IgG index of 0.6; Fig. 1C; Table S1). Clinical autoimmune encephalopathy antibody testing* was negative, including testing for AQP4 antibodies. MOG antibody testing** 4 years later was also negative. A broad rheumatologic workup demonstrated elevated titers of antinuclear, antiphospholipid, and antidouble stranded DNA antibodies. A repeat MRI 11 days later demonstrated progression of disease, and she was empirically treated with pulse dose glucocorticoids.*Update on the diagnosis and management of autoimmune encephalitis [PubMed Abstract] [Full-Text HTML] [Full-Text PDF]. Clin Med (Lond). 2020 Jul; 20(4): 389–392. doi: 10.7861/clinmed.2020-0241
**MOG Antibody Disease. Last updated: December 07, 2021. National Organization for Rare Disorders. NORD gratefully acknowledges Michael Levy, MD, PhD, Neuroimmunologist, Director, NMO Clinic and Research Laboratory, Massachusetts General Hospital and the Siegel Rare Neuroimmune Association for the preparation of this report.
Her neurologic exam incrementally improved over the subsequent 3 months, and a repeat brain MRI showed an interval decrease in gadolinium enhancement. Eighteen months
after her initial presentation, she was formally diagnosed with systemic lupus erythematosus (SLE) and treated with hydroxychloroquine (HCQ). Her functional status slowly recovered over the next 3 years: her speech improved, her handwriting became legible, and she returned to playing the piano. Surveillance brain MRI showed resolution of contrast-enhancement but persistent T2 hyperintensities. Although her initial neurologic deficits stabilized, she experienced
gradually worsening word-finding difficulty. Formal neuropsychologic assessment demonstrated mild impairments in processing speed, fluency, and naming. Blood tests for reversible causes of dementia were normal, including serum vitamin B12 levels between 614 and 1162 pg/mL and methylmalonic acid (MMA, a metabolite that accumulates downstream of B12 deficiency) levels between 170 and 240 nM (Fig. 1D)Methods
The patient was enrolled in a research study to detect novel autoantibodies in suspected neuroinflammatory disease (IRB 13-12236).
Results
We screened for candidate autoantigens using PhIP-Seq. Among enriched antigens meeting a false discovery rate cutoff of <5%, CD320 was the top cell surface hit, a characteristic consistent with a pathogenic autoantibody (Table S2).8 . . . These findings confirm that CD320 is the cell-surface target of an autoantibody present in the serum and CSF.
CD320, also known as the transcobalamin receptor, is enriched in endothelial cells at the blood-brain barrier (Fig. S2A-C) and mediates the cellular uptake and transcytosis of transcobalamin-bound vitamin B12 (holotranscobalamin) into the CNS.11-12 . . .These findings demonstrate patient immunoglobulins inhibit the function of CD320 and suggest that anti-CD320 may impair vitamin B12 transport across the blood-brain barrier by depleting cell-surface receptor availability.
To interrogate a possible blood-brain barrier transport defect in this patient, we measured vitamin B12 in paired serum and CSF samples. In three healthy controls, the mean serum concentration of vitamin B12 was 439 pg/mL and the mean CSF concentration was 9.0 pg/mL, yielding a CSF to serum vitamin B12 ratio of 0.02, concordant with prior studies (Fig. 2F).13,14 In contrast, the patient’s serum vitamin B12 concentration was 617 pg/mL and CSF concentration was 1.6 pg/mL, yielding a CSF to serum vitamin B12 ratio of 0.003, nearly one order of magnitude lower than healthy controls. High-dose oral vitamin B12 supplementation was initiated, and the patient’s CSF B12 level increased to 4.8 pg/mL (Fig. S4E). After 9 months
of treatment, she reported subjective improvements in mood and cognitive function.Discussion
We identified an autoantibody targeting the transcobalamin receptor in a patient with progressive tremor, ataxia, and scanning speech. Anti-CD320 impaired cellular uptake of
holotranscobalamin and was associated with low levels of vitamin B12 in the CSF despite normal levels in the blood. We retrospectively found autoantibodies reactive to the same epitope of CD320 in seven individuals enrolled in a neuroinflammatory disease study and in 6 percent of
a healthy control cohort. Detection of anti-CD320 in the blood predicted vitamin B12 deficiency in the CSF. [Emphasis Added] These findings support a model in which anti-CD320 impairs transport of vitamin B12 across the blood-brain barrier, leading to ABCD with varied clinical manifestations (Fig. S6A).The neurologic spectrum of traditional systemic vitamin B12 deficiency is wide, ranging from peripheral neuropathy to encephalomyelopathy. Prior to the onset of these symptoms,
individuals progress from replete to deficient through a state of inadequacy known as subclinical vitamin B12 deficiency.16 We detected transcobalamin receptor autoantibodies in a wide range of cases, including healthy controls, patients with other neurologic conditions (MS), and a patient with classic signs of vitamin B12 deficiency despite a normal serum B12 level (Case 8). This heterogeneity may reflect differences in anti-CD320 antibody avidity, the degree and duration of CSF vitamin B12 deficiency, and/or host factors conferring differential susceptibility to disease (Fig. S6B). Although not directly addressed in the present study, the high specificity
and positive predictive value of anti-CD320 in the blood for detecting early markers of B12 deficiency in the CSF suggest that seropositive healthy controls may be at risk for ABCD. In
contrast to neuronal surface autoantibodies in autoimmune encephalitides (e.g. NMDA receptor antibody or LGI1 antibody) that are directly neuromodulatory,17,18,19 CD320 autoantibodies may exert a second order effect on brain function by first decreasing CSF levels of vitamin B12 that
then leads to insidious neurologic sequelae. This study suggests that the measurement of vitamin B12 metabolites in CSF should be considered for anti-CD320 seropositive patients with
unexplained neurologic deficits.Several patients improved after immunosuppression (Cases 1, 2, 3, and 8), and two of these patients (Cases 1 and 8) received concurrent high-dose systemic B12 supplementation. It is unclear whether immunosuppression mediated improvement in these cases via dampening of alternative neuroinflammatory mechanisms or via mitigation of anti-CD320 autoantibody
production. The increased B12 concentration in post-treatment CSF from Case 1 suggests that high serum B12 levels may suffice to overcome BBB transport defects caused by anti-CD320.This study has several limitations. First, without prospective recruitment and empiric B12 targeted treatment of seropositive patients, it is not possible to determine whether anti-CD320
autoantibodies primarily drive disease pathogenesis, exacerbate underlying pathology (“second hit”), or passively spectate. Second, anti-CD320 sensitivity and specificity calculations were
based on measurements in a cohort of patients with other neurologic diseases (primarily MS) which may not accurately reflect the clinical relevance of the autoantibody in healthy individuals. Future studies will be necessary to evaluate whether anti-CD320 autoantibodies modulate clinical outcomes in the general population and in other neurologic diseases like MS. Finally, this study should not be used as a comprehensive clinical characterization of ABCD given the heterogeneity of disease and atypical features (e.g. cerebellar peduncle involvement in Cases 1 and 3) that distinguish it from classic systemic vitamin B12 deficiency.These findings parallel cerebral folate deficiency syndrome*, in which autoantibodies targeting folate receptors at the choroid plexus are associated with low 5-methyltetrahydrofolate levels in the CSF despite normal levels in the blood.24 Although it is unclear whether transcobalamin receptor autoantibodies contribute to the primary pathophysiology of neurologic deterioration in these patients, correction of central vitamin B12 deficiency may provide clinical benefit with minimal risk. Future studies will be necessary to determine the therapeutic effect of high dose vitamin B12 supplementation and/or immunomodulatory therapy.
*Cerebral Folate Deficiency Syndrome: Early Diagnosis, Intervention and Treatment Strategies. [PubMed Abstract] [Full-Text HTML] [Full-Text PDF]. Nutrients. 2022 Jul 28;14(15):3096. doi: 10.3390/nu14153096.
