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).
"I didn't think this single investigation was going to yield a broader phenomenon with other patients," said lead author John Pluvinage, MD, PhD, a neurology resident at the University of California San Francisco, in an interview with Medscape Medical News. "It started as an N-of-one study just based on scientific curiosity."
"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."
The research was published in Science Translational Medicine on June 26.
The Patient
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.
Treating the patient with a combination of immunosuppressant medication and high-dose B12 supplementation increased B12 levels in the patient's CSF and improved clinical symptoms.
Identifying More Cases
Pluvinage and colleagues tested the 254 other individuals enrolled in the neuroinflammatory disease study and identified seven participants with CSF anti-CD320 autoantibodies — four of whom had low B12 in the CSF.
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%). In this group of patients with multiple sclerosis, anti-CD320 presence in the blood was highly predictive of high levels of CSF methylmalonic acid, a metabolic marker of B12 deficiency.
Researchers also screened for anti-CD320 seropositivity in 408 patients with non-neurologic SLE and 28 patients with neuropsychiatric SLE and found that the autoantibody was nearly four times as prevalent in patients with neurologic symptoms (21.4%) compared with in those with non-neurologic SLE (5.6%).
"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."
Alternative Pathways
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.
The work was supported by the National Institute of Mental Health, National Center for Chronic Disease Prevention and Health Promotion, Department of Defense, UCSF Helen Diller Family Comprehensive Cancer Center Laboratory for Cell Analysis Shared Resource Facility, National Multiple Sclerosis Society, Valhalla Foundation, and the Westridge Foundation. Pluvinage is a co-inventor on a patent application related to this work. Diamond had no relevant disclosures.