We report the clinical and neuroimaging features of a disorder that occurs in women with OT and associates with subacute psychiatric symptoms, seizures, decrease level of consciousness, and frequent central hypoventilation. CSF findings suggest an encephalitic process that in prior accounts was suspected to be immune mediated. This hypothesis is now strongly supported by the detection in patients' sera and CSF of an immune response to antigens that colocalize with EFA6A, a brain-specific protein involved in the regulation of dendritic development of hippocampal neurons.19
Clinical recognition of this syndrome is important for two reasons: (1) the subacute presentation of the indicated symptoms in a young woman often leads to an initial diagnosis of acute psychosis, malingering, or drug abuse, and (2) despite the severity of symptoms patients usually recover. Because the symptom presentation is usually acute with prominent confusion, delusional thinking, agitation, and low level of consciousness, the recognition of the short-term memory deficits characteristic of limbic dysfunction can be difficult. Furthermore, the neuroimaging findings may differ from classic LE: combining MRI, FDG-PET, and SPECT only one third of the patients had involvement of the limbic system (cingulum, orbitofrontal region, temporal lobes), and none had the characteristic FLAIR or T2 medial temporal abnormalities seen in LE; the other two thirds of the patients had abnormalities in other areas of the central nervous system (CNS) or normal MRI.
Central hypoventilation has been reported in a few patients with paraneoplastic disorders involving brainstem, hypothalamus, or both.20-22
The high frequency of this complication in our group of patients (young women without a history of respiratory problems) is remarkable and suggests a pathogenic mechanism in common with the associated encephalitis. The development of hypoventilation was unrelated to seizures or antiepileptic medication; all patients required prolonged ventilatory support (median, 49 days) until recovery or in one case until death (Patient 3). The autopsy of this patient showed nonspecific microglial activation throughout gray and white matter of the brain, brainstem, and cerebellum, with scattered monocytes in perivascular spaces.11
Involvement of the brainstem was also shown by FDG-PET or MRI in three other patients (Patients 1, 4, and 5). Overall, these findings and other transient symptoms in some patients (ie, aphasia, diplopia, myelopathy) suggest that teratoma-associated encephalitis is an extensive or multifocal encephalitis, in which brainstem involvement is frequent and may be the cause of hypoventilation.
Despite the severity of the clinical features, three of our four patients and five patients reported by others survived, four with total recovery and four with substantial improvement. All but one patient had tumor resection, and six received immunosuppressants or chemotherapy. Because this is a small series, the contribution of each therapy to the neurological improvement is unclear. The close temporal association between presentation of teratoma-associated encephalitis and tumor diagnosis (median, 3 months) and the striking correlation in one case (Patient 4) between three episodes of neurological relapse and tumor recurrence suggest that the tumor plays a role in triggering the immune response. Frozen tumor tissue (not available from our patients) is needed to determine whether immunological activation depends on the tumor expression of neuronal proteins.
What is the cause of teratoma-associated encephalitis? The identification in four of four patients of antibodies with a unique pattern of reactivity with the CNS provides strong evidence that the disorder is immune mediated. Several features put these antibodies in a recently reported category12
that is different from the group of classic paraneoplastic antibodies to intracellular neuronal antigens (ie, HuD, CRMP5, or Ma2): (1) the immunohistochemical reactivity is highly restricted to the neuropil of rat hippocampus, sparing neuronal cell bodies; (2) the reactivity is missed by immunoblot analysis (data not shown) or if rat brain tissue is fixed with acetone or methanol-acetone; (3) studies with hippocampal neurons demonstrate that the immunolabeling occurs at the plasma membrane and dendritic processes suggesting that the antigens are on the cell surface; and (4) serum antibody titers become undetectable after neurological improvement.
To identify the autoantigens of the disorder, we probed a cDNA library of rat hippocampus with patients' sera, resulting in the isolation of EFA6A. When each individual serum was reacted with EFA6A expressed in bacteriophage plaques, only one recognized the protein, but affinity-purified EFA6A antibodies reproduced the same unique pattern of hippocampal reactivity as that of all patients' antibodies, with striking colocalization of neuronal immunolabeling. This finding indicates that the disorder's autoantigen is EFA6A itself but some epitopes are conformational or absent in the isolated partial sequence or that there is an additional coantigen that forms part of the complex of proteins that bind EFA6A. When EFA6A interacts with ARF6 at the plasma membrane,23
it binds to TWIK-1,24
a member of a new family of K(+) channels that is composed at least of 14 related subunits, each characterized by two-pore domains.25,26
In preliminary studies, we have found that ARF6 is not a coautoantigen, but the immunolabeling pattern of patients' antibodies resembles that of commercially available antibodies to some members of the two-pore-domain K(+) channels (ie, TASK-3; data not shown). These channels (also known as “leak K+ channels”) differ in structure and function from the shaker
channels recently reported as autoantigens of nonparaneo-plastic LE.16,27
A general property of the two-pore K+ channels is that they show little time and voltage dependence and play critical roles in controlling the resting membrane potential.
The ARF6/EFA6A/TWIK-1 complex is important for endocytosis, channel internalization, and recycling.17,24
Because the reactivity of patients' antibodies colocalizes with EFA6A, it is reasonable to speculate that an immune-mediated disruption of the ARF6/EFA6A/TWIK-1 complex could result in severe deficits, similar to those of our patients. These would have broad effects on the limbic system where EFA6A is enriched and other areas of the CNS where at least three EFA6 isoforms (B, C, D) are also expressed.18,24,28
Pertinent to the frequent hypoventilation and low level of consciousness of our patients, the function of at least two-pore channels (TASK-1 and TASK-3) is important for control of breathing and arousal.29,30
A next step to determine other autoantigens is to precipitate EFA6A-interacting proteins using patients' antibodies.
On the clinical side, this study has the following implications: (1) when confronted with a young woman with a new-onset psychiatric syndrome associated with seizures, memory loss, decreased level of consciousness or central hypoventilation physicians should suspect a paraneoplastic disorder; (2) the tumor search should focus in the ovaries, and if a tumor is detected (even with a benign appearance) removal is recommended; (3) clinical improvement is slow; the development of central hypoventilation does not necessarily imply an ominous prognosis, but patients usually require extended ventilatory support; (4) based in a recently reported animal model demonstrating CNS dysfunction caused by antibodies to membrane-associated antigens,31
the finding that the antigens of teratoma-associated encephalitis are readily accessed by antibodies in live neurons suggests that early use of corticosteroids and IgG-depleting strategies (IVIg or plasma exchange) may improve outcome. This could not be determined in our patients because immunotherapies were used empirically when the patients were critically ill. The hippocampal antibodies were retrospectively identified in archived sera and CSF examined after recovery in three patients, and after death in one case.
Analysis for EFA6A-specific antibodies by immuno-precipitation or affinity purification from patient's serum is complex, and we consider the use of these methods as a diagnostic test premature. However, immunohistochemical analysis for the antibodies reported here (anti-EFA6A or anti-EFA6A–interacting proteins) is simple, and the pattern of reactivity characteristic enough to allow its use to identify the disorder. Analysis of the functional effects of the antibodies in live neurons is in progress.