Molecular Mimicry may Underlie a Worm-Associated Epilepsy Syndrome

Commentary

Nodding Syndrome May Be an Autoimmune Reaction to the Parasitic Worm Onchocerca volvulus.

Johnson TP, Tyagi R, Lee PR, Lee MH, Johnson KR, Kowalak J, Elkahloun A, Medynets M, Hategan A, Kubofcik J, Sejvar J, Ratto J, Bunga S, Makumbi I, Aceng JR, Nutman TB, Dowell SF, and Nath A. Sci Transl Med 2017;15:eaaf6953.

Nodding syndrome is an epileptic disorder of unknown etiology that occurs in children in East Africa. There is an epidemiological association with Onchocerca volvulus, the parasitic worm that causes onchocerciasis (river blindness), but there is limited evidence that the parasite itself is neuroinvasive. We hypothesized that nodding syndrome may be an autoimmune-mediated disease. Using protein chip methodology, we detected autoantibodies to leiomodin-1 more abundantly in patients with nodding syndrome compared to unaffected controls from the same village. Leiomodin-1 autoantibodies were found in both the sera and cerebrospinal fluid of patients with nodding syndrome. Leiomodin-1 was found to be expressed in mature and developing human neurons in vitro and was localized in mouse brain to the CA3 region of the hippocampus, Purkinje cells in the cerebellum, and cortical neurons, structures that also appear to be affected in patients with nodding syndrome. Antibodies targeting leiomodin-1 were neurotoxic in vitro, and leiomo-din-1 antibodies purified from patients with nodding syndrome were cross-reactive with O. volvulus antigens. This study provides initial evidence supporting the hypothesis that nodding syndrome is an autoimmune epileptic disorder caused by molecular mimicry with O. volvulus antigens and suggests that patients may benefit from immunomodulatory therapies.

Nodding syndrome (NS) is an epilepsy syndrome with the curious feature of being geographically constrained to regions of Africa near Uganda, South Sudan, and Tanzania (1). The characteristic presentation of NS is a child who develops minutes-long spells of head bobbing associated with impairments in consciousness. EEG recordings during the spells show features that are consistent with atonic seizures, and at least a subset of children affected by NS show other seizure types as well, including generalized tonic–clonic episodes (2). In addition to the nodding spells, NS also features cognitive dysfunction and is anecdotally reported to lead to neurodegeneration (although a systematic study of the natural history of the disease is lacking). Prior to the current report by Johnson and colleagues, no mechanism had been proposed for the cause of NS, although multiple groups had arrived at the conclusion that NS was associated with infection with Onchocerca volvulus (1, 2), the filarial nematode carried by black flies and responsible for river blindness. Based on this epidemiologic association with O. volvulus infection, it has long been suspected that O. volvulus infection may lead to NS, either through direct CNS infection or through some inflammatory or autoimmune process (1).

In a fascinating account by Johnson et al., the group sets out to determine if there is an autoantibody produced by infection with O. volvulus that ultimately leads to NS. Their systematic approach to the problem is laudable and establishes a guide for how future studies may be conducted. First, the interdisciplinary team of researchers from the United States and Uganda went directly to the source and collected serum samples from villagers afflicted by NS as well as healthy control subjects. They then compared the antibodies present in the two groups using an antigen screening assay (with pooled sera from healthy villagers vs NS patients), which identified a handful of proteins recognized by antibodies found more frequently in NS patients than in healthy villagers. This screen identified leiomodin-1, an intracellular protein involved in actin filament assembly previously thought to only be expressed in smooth muscle and thyroid tissues (4). Because this pattern of expression would be unlikely to explain the molecule's role in epilepsy, the group next examined the expression of the protein in the central nervous system. They observed for the first time the expression of leiomodin-1 in cortical neurons, Purkinje cells, and pyramidal neurons of the hippocampal area CA3. Additional characterization experiments with appropriate negative controls will ultimately be needed to fully define leiomodin-1 expression in vivo. However, their experiments suggest a potential link between the antibody and neurologic sequelae. To further establish a mechanistic link between these antibodies and the development of NS, the authors next examined the effect of the antibody on cultured neurons in vitro and found that the leiomodin-1 antibodies derived from patient sera were neurotoxic.

Although the above discussion raises the possibility that the leiomodin-1 antibodies may lead to NS, it remains unclear why leiomodin-1 antibodies would arise in the setting of O. volvulus infection. To answer that question, the team completed a clever series of experiments in which they performed western blotting with antibodies derived from NS patients and unaffected villager controls. These antibodies were incubated with membranes in which O. volvulus proteins had been immobilized, and the bands recognized by patient sera were then sent for mass spectrometry (a proteomics method in which all the proteins in a sample are identified). This candidate list of proteins was compared with the structure of leiomodin-1, and in combination with computer modeling studies, the authors discovered that tropomodulin expressed by O. volvulus is structurally similar to leiomodin-1. As a result, the authors reason that cross reactivity between antibodies directed against O. volvulus tropomodulin and leiomodin-1 expression in humans is responsible for NS.

While the work of Johnson et al. is compelling, there are several caveats regarding this study that need resolution: First, it remains unclear if leiomodin-1 antibodies are the cause of NS. Leiomodin-1 is an intracellular antigen (3), and antibodies that react with it would first need to cross into a privileged immunologic space and then exert their effect across the cell membrane (4). Although it is possible this is indeed the case—especially if O. volvulus infection leads to blood–brain barrier compromise through some other mechanism—it might also be the case that neuronal damage caused by other processes leads to the production of antibodies against newly released antigens. Previous work on autoantibodies directed against intracellular antigens associated with epilepsy (such as glutamic acid decarboxylase [5]) has grappled with the same question, which remains largely unanswered. Disentangling the inciting insult that leads to NS will ultimately require additional experimentation (5–7).

A second limitation of the study is the relatively poor correlation between the presence of the antibody and NS. The authors report that a substantial proportion of unaffected villagers have the antibody in their serum (30.9%) and that it is found in only 52.7% of the patients with nodding syndrome. A host of factors could influence these numbers (including the latency period of the disease, the sensitivity/specificity characteristics of the assay, etc.), but an important consideration is the possibility that O. volvulus infection leads to the production of leiomodin-1 antibodies but that these antibodies themselves do not have a role in the etiology of NS. The authors also establish that in 8/16 NS patients examined, their CSF contained the leiomodin-1 antibody while 0/8 North American epilepsy patients had the leiomodin-1 antibody (although North American patients may be presumed to lack exposure to O. volvulus as well). While it is encouraging that the North American patients lack the antibody, the observation that just 50% of the NS patients has the antibody in the CSF suggests that other non–leiomodin-1 mechanisms are at work. The authors themselves acknowledge that there are likely other antigens involved.

Finally, despite the outstanding approach and technical expertise employed to test their hypothesis, the authors are not able to mechanistically link the leiomodin-1 antibodies to the production of epilepsy. Although the authors report an in vitro experiment showing neurotoxicity, this is not a standard metric for epileptogenic potential; animal studies will ultimately be needed (5–7). At a minimum, it is likely the case that some other neurologic insult precipitates leiomodin-1 antibodies crossing the blood–brain barrier to achieve a toxic effect (4, 8). The current study by Johnson et al. succeeds in generating a compelling hypothesis for the mechanism of NS, but there is still more work to be done. Their study is commendable for its innovative approach to the study of autoimmune disorders and should serve as a template for future work on NS and investigation of autoimmunity as a cause of neurological syndromes throughout the world.