Transcobalamin receptor antibodies in autoimmune vitamin B12 central deficiency, 2024, John V. Pluvinage et al

[Mij]

Editor’s summary
Vitamin B12 deficiency can lead to neurological deficits, sometimes in the absence of anemia, and mice and patients with mutations in the transcobalamin receptor CD320 do not develop anemia, suggesting that the mechanisms governing B12 uptake in different tissues are not fully understood.
Here, Pluvinage and colleagues identified an autoantibody against CD320 in a patient with progressive neurological symptoms that blocked B12 uptake in endothelial cells and was associated with central but not peripheral vitamin B12 deficiency. The authors identified additional individuals with anti-CD320 autoantibodies and also found an association with neuropsychiatric systemic lupus erythematosus. The low-density lipoprotein receptor was identified as a noncanonical transcobalamin receptor in the periphery only, potentially explaining why anti-CD320 autoantibodies lead to central but not peripheral vitamin B12 deficiency. —Melissa L. Norton

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.

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[Hutan]

Posting on behalf of a member:
Medscape article about this study
Form of B12 Deficiency Affecting the Central Nervous System May Be New Autoimmune Disease

Researchers have identified a form of B12 deficiency caused by autoantibodies that specifically affects the central nervous system.

Discovered while studying a puzzling case of one patient with inexplicable neurological systems, the same autoantibody was detected in a small percentage of healthy individuals and was nearly four times as prevalent in patients with neuropsychiatric systemic lupus erythematosus (SLE).

"It's a beautifully done study," added Betty Diamond, MD, director of the Institute of Molecular Medicine at the Feinstein Institutes for Medical Research in Manhasset, New York, commenting on the research for Medscape Medical News. It uncovers "yet another example of a disease where antibodies getting into the brain are the problem."

 

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The investigation began in 2014 with a 67-year-old woman presenting with difficulty speaking, ataxia, and tremor. Her blood tests showed no signs of B12 deficiency, and testing for known autoantibodies came back negative.

Solving this mystery required a more exhaustive approach. The patient enrolled in a research study focused on identifying novel autoantibodies in suspected neuroinflammatory disease, using a screening technology called phage immunoprecipitation sequencing.

"We adapted this technology to screen for autoantibodies in an unbiased manner by displaying every peptide across the human proteome and then mixing those peptides with patient antibodies in order to figure out what the antibodies are binding to," explained Pluvinage.

Using this method, he and colleagues discovered that this woman had autoantibodies that target CD320 — a receptor important in the cellular uptake of B12. While her blood tests were normal, B12 in the patient's cerebral spinal fluid (CSF) was "nearly undetectable," Pluvinage said. Using an in vitro model of the blood-brain barrier (BBB), the researchers determined that anti-CD320 impaired the transport of B12 across the BBB by targeting receptors on the cell surface.

In a group of healthy controls, anti-CD320 seropositivity was 6%, similar to the positivity rate in 132 paired serum and CSF samples from a cohort of patients with multiple sclerosis (5.7%)....

"The clinical relevance of anti-CD320 in healthy controls remains uncertain," the authors wrote. However, it is not uncommon to have healthy patients with known autoantibodies.

"There are always people who have autoantibodies who don't get disease, and why that is we don't know," said Diamond. Some individuals may develop clinical symptoms later, or there may be other reasons why they are protected against disease.

Pluvinage is eager to follow some seropositive healthy individuals to track their neurologic health overtime, to see if the presence of anti-CD320 "alters their neurologic trajectories."

Lastly, Pluvinage and colleagues set out to explain why patients with anti-CD320 in their blood did not show any signs of B12 deficiency. They hypothesized that another receptor may be compensating and still allowing blood cells to take up B12. Using CRISPR screening, the team identified the low-density lipoprotein receptor as an alternative pathway to B12 uptake.

"These findings suggest a model in which anti-CD320 impairs transport of B12 across the BBB, leading to autoimmune B12 central deficiency (ABCD) with varied neurologic manifestations but sparing peripheral manifestations of B12 deficiency," the authors wrote.