GABA(B) receptor antibodies in limbic encephalitis and anti-GAD-associated neurologic disorders

Abstract

Background: γ-Aminobutyric acid-B receptor antibodies (GABA(B)R-ab) were recently described in 15 patients with limbic encephalitis (LE), associated with small-cell lung cancer (SCLC) or with concurrent glutamic acid decarboxylase (GAD) antibodies. We analyzed the frequency of GABA(B)R-ab in 147 patients with LE or neurologic syndromes associated with GAD-ab.

Methods: We examined the presence of GABA(B)R-ab in 70 patients with LE (33 paraneoplastic with onconeural antibodies, 18 paraneoplastic without onconeural antibodies [5 with Gad-ab], and 19 idiopathic with either GAD-ab [5 patients] or seronegative) and 77 patients with GAD-ab-associated neurologic syndromes other than LE (29 stiff-person syndrome, 28 cerebellar ataxia, 14 epilepsy, and 6 with diverse paraneoplastic neurologic syndromes). GABA(B)R-ab were analyzed in serum or CSF by indirect immunofluorescence on HEK293 cells transfected with GABA(B1) and GABA(B2) receptor subunits.

Results: GABA(B)R-ab were detected in 10 of the 70 patients with LE (14%). Eight had SCLC and 2 were idiopathic. One of the 8 patients with LE with SCLC had an additional onconeural antibody (Hu) and 2 GAD-ab. GABA(B)R-ab were identified in 7 (70%) of the 10 patients with LE and SCLC without onconeural antibodies. GABA(B)R-ab antibodies were not found in patients with GAD-ab and stiff-person syndrome, idiopathic cerebellar ataxia, or epilepsy. However, one patient with GAD-ab, paraneoplastic cerebellar ataxia, and anaplastic carcinoid of the thymus also presented GABA(B)R-ab.

Conclusions: GABA(B)R-ab are the most common antibodies found in LE associated with SCLC previously considered "seronegative." In patients with GAD-ab, the frequency of GABA(B)R-ab is low and only observed in the context of cancer.

Figure 1. Detection of γ-aminobutyric acid-B receptor antibodies (GABA_BR-ab) using a HEK293 cell–based assay

HEK293 cells were transfected to express GABA_B1/B2 receptor and incubated live, not permeabilized, with a patient's CSF. Afterwards, cells were fixed, permeabilized, and incubated with a polyclonal antibody against an intracellular epitope of the B1 subunit of the GABA_B receptor. Note that patient's CSF stains the cell surface of cells that specifically express GABA_B receptors (A), as demonstrated by the intracellular reporter antibody (B). Both reactivities are shown merged in C. Nuclei counterstained with DAPI. Scale bar = 20 μm.

Figure 2. Primary culture of rat hippocampal neurons incubated in vivo with CSF of a patient with γ-aminobutyric acid-B receptor antibodies

There is an intense punctate reactivity in the neuronal membrane consistent with immunoreactivity against a surface antigen. Scale bar = 20 μm.