Editors' Note: In reference to “Aquaporin-4 antibody–positive cases beyond current diagnostic criteria for NMO spectrum disorders,” Blum et al. describe 2 cases of patients with neuromyelitis optica spectrum disorder detected by cell-based assay but negative on ELISA. Authors Sato et al. answer their questions about aquaporin-4 antibody testing. Verghese presents a case of myoclonus in a patient with type III Gaucher disease. —Megan Alcauskas, MD, and Robert C. Griggs, MD
Sato et al.1 described 13 cases of neuromyelitis optica spectrum disorder (NMO-SD) and 3 of those cases had brainstem involvement in which antibodies (Ab) to the M1 isoform of aquaporin-4 (AQP4) were detected by a cell-based assay (CBA) but not by an ELISA.
We saw 2 patients with NMO-SD who had lesions in the posterior pons adjacent to the floor of the fourth ventricle. Both had a similar clinical presentation of nystagmus and intranuclear ophthalmoplegia. One subject had further relapses with intractable nausea, hiccups, and gait ataxia and developed bilateral thalamic lesions. Neither had optic neuritis or spinal cord disease and both responded well to aggressive immunosuppressive therapy.
Similar to Sato et al., sera from these subjects were positive for AQP4-Ab on a commercial CBA (Euroimmun, Luebeck, Germany) but negative by indirect immunofluorescence on unfixed rodent brain tissue, indicating a lack of binding to the native protein. It is possible that these antibodies detected an epitope exposed on the cells transfected with M1 but not on native AQP4 or in the ELISA assay. Different structural arrangement of AQP4 in cell membranes could lead to a different epitope.2,3
Author Response
We thank Blum et al. for their comments. Their 2 cases show that AQP4-Ab seropositivity is critical for the diagnosis of the NMO-SD without typical attacks of optic neuritis, longitudinally extensive myelitis, or both.
Blum et al. also questioned the AQP4 epitopes and differences on the assay sensitivities using nonhuman AQP4, human AQP4 isoforms, or fixed material. There are differences on the rodent and the human AQP4 proteins, so some patients' AQP4-Ab may not recognize rodent epitopes, providing negative results.4 In addition, the human AQP4-M23 used in our CBA1 was able to form orthogonal array of particles (OAP) in the cell membrane5 that are not observed using either AQP4-M1 or linearized AQP4 (ELISA). These OAPs increased the assay sensitivity, as antibodies are more likely to recognize this large 3D structure. We have not seen a sample that has been positive on AQP4-M1 and not on AQP4-M23, and this has been confirmed.6,7 Finally, fixation prior to exposure (commercial CBA) to human sera may reduce assay sensitivity.7 In our study, the cells were only fixed after the secondary antibody.
- © 2014 American Academy of Neurology
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