‘Medusa head ataxia’: the expanding spectrum of Purkinje cell antibodies in autoimmune cerebellar ataxia. Part 1: Anti-mGluR1, anti-Homer-3, anti-Sj/ITPR1 and anti-CARP VIII

Jarius, S.; Wildemann, B.

doi:10.1186/s12974-015-0356-y

Review
Open Access
Published: 17 September 2015

‘Medusa head ataxia’: the expanding spectrum of Purkinje cell antibodies in autoimmune cerebellar ataxia. Part 1: Anti-mGluR1, anti-Homer-3, anti-Sj/ITPR1 and anti-CARP VIII

S. Jarius¹ &
B. Wildemann¹

Journal of Neuroinflammation volume 12, Article number: 166 (2015) Cite this article

5958 Accesses
67 Citations
3 Altmetric
Metrics details

Abstract

Serological testing for anti-neural autoantibodies is important in patients presenting with idiopathic cerebellar ataxia, since these autoantibodies may indicate cancer, determine treatment and predict prognosis. While some of them target nuclear antigens present in all or most CNS neurons (e.g. anti-Hu, anti-Ri), others more specifically target antigens present in the cytoplasm or plasma membrane of Purkinje cells (PC). In this series of articles, we provide a detailed review of the clinical and paraclinical features, oncological, therapeutic and prognostic implications, pathogenetic relevance, and differential laboratory diagnosis of the 12 most common PC autoantibodies (often referred to as ‘Medusa-head antibodies’ due to their characteristic somatodendritic binding pattern when tested by immunohistochemistry). To assist immunologists and neurologists in diagnosing these disorders, typical high-resolution immunohistochemical images of all 12 reactivities are presented, diagnostic pitfalls discussed and all currently available assays reviewed. Of note, most of these antibodies target antigens involved in the mGluR1/calcium pathway essential for PC function and survival. Many of the antigens also play a role in spinocerebellar ataxia. Part 1 focuses on anti-metabotropic glutamate receptor 1-, anti-Homer protein homolog 3-, anti-Sj/inositol 1,4,5-trisphosphate receptor- and anti-carbonic anhydrase-related protein VIII-associated autoimmune cerebellar ataxia (ACA); part 2 covers anti-protein kinase C gamma-, anti-glutamate receptor delta-2-, anti-Ca/RhoGTPase-activating protein 26- and anti-voltage-gated calcium channel-associated ACA; and part 3 reviews the current knowledge on anti-Tr/delta notch-like epidermal growth factor-related receptor-, anti-Nb/AP3B2-, anti-Yo/cerebellar degeneration-related protein 2- and Purkinje cell antibody 2-associated ACA, discusses differential diagnostic aspects and provides a summary and outlook.

Introduction

Autoimmune cerebellar ataxia (ACA) is an important differential diagnosis in patients presenting with signs and symptoms of cerebellar disease. Alongside multiple sclerosis and acute disseminated encephalomyelitis, autoantibody-associated disorders of the CNS are the most common cause of ACA. While ACA is a rare manifestation in some of these disorders, e.g. aquaporin-4 (AQP4) antibody-associated neuromyelitis optica (NMO), it is the most frequent or exclusive presentation in others. To date, around 30 different autoantibodies targeting brain antigens have been reported in patients with ACA, many of which are of paraneoplastic origin (Table 1).

Table 1 Selected antibodies to cerebellar antigens reported in patients with cerebellar ataxia

Full size table

When tested by immunohistochemistry (IHC) using cerebellum tissue sections, some of these antibodies (anti-metabotropic glutamate receptor 1 (mGluR1), anti-Homer protein homolog 3 (Homer-3), anti-Sj/inositol 1,4,5-trisphosphate receptor (ITPR1), anti-carbonic anhydrase-related protein VIII (CARP VIII), anti-protein kinase C gamma (PKCγ), anti-Ca/RhoGTPase-activating protein 26 (ARHGAP26), anti-glutamate receptor delta 2 (GluRδ2), anti-Tr/delta notch-like epidermal growth factor (EGF)-related receptor (DNER), voltage-gated calcium channels (VGCC) antibodies, anti-Nb/AP3B2, anti-Yo/cerebellar degeneration-related protein 2 (CDR2) and Purkinje cell antibody 2 (PCA-2)) show a staining pattern resembling a Gorgon’s head, caused by binding of IgG to Purkinje cell (PC) somata and dendrites and are therefore often referred to as ‘Medusa head’ antibodies (Fig. 1).

Due to their similar binding patterns, it can be very difficult to differentiate the members of the expanding family of somatodendritic or ‘Medusa head’ PC antibodies. Here, we show exemplary IHC findings for each of these antibodies, review the currently available diagnostic assays, and discuss diagnostic pitfalls. In addition, we provide a comprehensive summary of the clinical, paraclinical and epidemiological features associated with these antibodies, briefly review the available knowledge regarding their pathophysiological relevance and discuss their oncological and prognostic implications.

The present, first article in this series will review the current knowledge on anti-mGluR1-, anti-Homer-3-, anti-Sj/ITPR1- and anti-CARP VIII-positive ACA.

Antibodies targeting antigens involved in the glutamate/calcium pathway

Interestingly, most of the antigens so far identified in patients with ‘Medusa head’ antibodies—namely mGluR1, Homer-3, ITPR1, CARP VIII, PKCγ, GluRδ2, VGCC, CDR2-like (CDR2L), neuronal adaptin-like protein (beta-NAP) and possibly also ARHGAP26 and CDR2—are functionally and structurally related in that all are involved in the PC phosphatidylinositol-calcium second messenger system or, more generally, in maintaining intracellular calcium homeostasis: In the cerebellum, release of glutamate, the major excitatory neurotransmitter in the CNS, by parallel fibre (PF) (and possibly climbing fibre (CF)) [1] synapses stimulates postsynaptic mGluR1, which is the main metabotropic receptor on PCs. At the PC/PF synapse, this results in cleavage of phosphatidyl 4,5-bisphosphate (PIP₂) by phospholipase Cβ3 (PLCβ3), the target of the G proteins of mGluR1, into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). IP3 acts as a second messenger for ITPR1, a calcium channel mainly located in the membrane of the smooth endoplasmic reticulum (ER) and physically linked to mGluR1 via Homer-3 [1]. Upon activation by IP3, ITPR1 mediates intracellular Ca²⁺ release from the ER calcium storage [1]. CARP VIII, on the other hand, limits Ca2+ efflux from the ER by reducing the affinity of ITPR1 for IP3 [2]. Intracellular calcium together with DAG activates PKCγ [3], a protein kinase involved in calcium regulation by its capability to phosphorylate and thus inactivate the DAG-activated canonical transient receptor potential (TRPC) type 3 cation channel 3 [4–6], resulting in reduced influx of calcium ions [7]. GluRδ2 has been recently shown to associate with mGluR1, PKCγ and TRPC3 [5, 6, 8–10] and to regulate mGluR1-mediated synaptic transmission in PCs. ARHGAP26 has been found to precipitate with dynamin, which is involved in mGluR1 internalisation [11, 12]. Blockade of CDR2 by anti-Yo autoantibodies has been reported to induce the expression of PKCγ and the pore- and gating apparatus-forming VGCC protein Ca_v2.1, which is an autoantigen in ACA itself [13, 14], and also to regulate the expression of several other calcium-related proteins [15]. Finally, mGluR1a interacts directly with Ca_v2.1, forming a heteromeric protein complex [16–18] and has been shown to inhibit Ca_v2.1-mediated Ca2+ currents [16, 19, 20].

Of note, mutations in almost all of the components of the mGluR1 cascade have been demonstrated to cause cerebellar ataxia either in humans or in animal models. Homozygous mutations in the GRM1 gene encoding mGluR1 underlie spinocerebellar ataxia (SCA) 13 [21]. G_q mutant PCs remain multiply innervated by CFs and are associated with impaired motor coordination in adult mice [22, 23]. Phospholipase C mutant mice show deficient long-term synaptic depression and impaired motor learning [24]. Mutations in the ITPR1 gene have been found to cause SCA15 and 21 [25]. Missense mutations in the PKCγ gene have been found in SCA14 [26], and loss of PKCγ also seems to play a role in SCA1 [27]. Mice deficient in TRPC3 exhibit impaired walking behaviour [5]. Mutations in beta-3 spectrin influencing glutamate receptor GluRδ2 expression as well as deletions in the GRID2 gene itself have recently been discovered in SCA5 and other forms of hereditary cerebellar ataxia in humans [28, 29]. Finally, mutations in the Ca_v2.1 gene cause SCA6 [30–32].

All sections dealing with individual antibody reactivities are structured uniformly to improve accessibility of the information provided. Each section consists of an identically headed set of subsections dealing with (1) clinical, paraclinical and epidemiological features associated with the respective antibody; (2) associated tumours; (3) syndrome outcome and prognosis; (4) target antigen structure and function; (5) diagnostic IHC findings; (6) antigen-specific assays; (7) relevance of CSF testing; (8) association with other autoantibodies; (9) evidence for a pathogenic role of the antibody; and (10) molecular genetics, inasmuch as they corroborate a potential role of the target antigen in cerebellar ataxia.

Anti-mGluR1

Clinical, paraclinical and epidemiological features

Since the first description of anti-mGluR1 in 2000, five patients have been reported (three female, two male; median age 50 years, range 19–69) [33–36], all of whom presented with cerebellar gait ataxia (partly unable to walk without help) and limb ataxia (dysmetria of arms and/or legs, intention tremor). Further symptoms included trunk ataxia (partly unable to sit without help, head titubation, truncal sway) in four, dysarthria in four, and ocular symptoms (nystagmus, oscillopsia, diminution in speed of saccades, impaired adaptation of saccadic eye movements, difficulty directing and maintaining fixation of gaze, slight opsoclonus) in all cases. While a subacute onset was noted in two, symptoms worsened slowly in two other patients (no data in one). Magnetic resonance imaging (MRI) showed cerebellar atrophy in two patients [34, 36] and diffuse abnormal hyperintensity in the whole cerebellum present only on fluid-attenuated inversion recovery and diffusion sequences in another case [35], but was normal in the remaining two (follow-up for up to 6 months). Lumbar puncture revealed mononuclear pleocytosis in three patients (9, 28, and 190 cells/μl) and was normal in one (no data in one case); signs of intrathecal IgG synthesis were present in one of two patients examined. Additional cases of ACA and mGluR1 have been identified at the authors’ institution and elsewhere, but no additional clinical information is currently available.

Given that mGluR1 is expressed widely throughout the CNS, it is not surprising that two patients developed signs of encephalitis in addition to ataxia, including mild cognitive decline in one case and short-term memory loss in the other.

Association with tumours

In three out of five cases, anti-mGluR1 autoantibodies were associated with malignant tumours. The two index patients had a history of nodular sclerosing Hodgkin’s disease (HD) but had been in remission for 2 and 9 years, respectively, at the time of onset of anti-mGluR1-associated ACA; however, mGluR1 was not detected in a tumoral lymph node from one of those patients, and no tumour specimen was analysed in the second case, rendering it unclear whether the two conditions were pathophysiologically related [33]. Sera from patients with Hodgkin’s lymphoma but no cerebellar ataxia did not show anti-mGluR1 [33]. A third patient had an adenocarcinoma of the prostate, which was only discovered 20 months after onset of the cerebellar ataxia, as well as a history of a successfully treated cutaneous T cell lymphoma. In contrast to the index cases, mGluR1 was found to be abundantly present in the tumour tissue and binding of the patient’s IgG to tumoral mGluR1 could be demonstrated [34]. Two patients did not show any evidence of a tumour up to 40 months after onset [35, 36].

Outcome and prognosis

While treatment with steroids, plasma exchange (PEX), intravenous immunoglobulins (IVIG) and oral steroids was followed by slow yet complete recovery in index patient 1; PEX did not result in significant improvement in the second patient, who remained unable to walk without support. Hodgkin’s disease remained in complete remission in both cases [33]. In a third patient, commencement of treatment with steroids, IVIG and mycophenolate mofetil early in the disease course led to continuous clinical improvement and a drop in anti-mGluR1 serum titres (1:20,000 to 1:500). At last follow-up, 40 months after onset, the patient was still able to walk [35]. In patient 4, a transient improvement was noted after intravenous methylprednisolone (IVMP); however, subsequent courses of IVMP were not followed by further improvement, and severe and disabling ataxia and dysarthria were present at last follow-up [36]. In patient 5, treatment of the prostate carcinoma was associated with severe neurological deterioration; later on, sustained improvement was achieved after treatment with IVIG and low-dose steroids [34].

Antigen

MGluR1 (encoded by GRM1) is a cell surface receptor belonging to the guanine nucleotide-binding protein (G-protein)-activating receptor 3 family [37]. Its natural ligand is the excitatory neurotransmitter L-glutamate. Glutamate produces fast excitation through activation of ionotropic glutamate receptors (GluRs, including N-methyl D-aspartate (NMDA) receptors, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and kainate (KA) receptors) and slower actions through metabotropic receptors (mGluRs). To date, eight mGluRs are known (mGluR1-8). Together with mGluR5, mGluR1 forms group I of the metabotropic glutamate receptors. So far, five isoforms of mGluR1 have been described [37–39] with the canonical isoform alpha being a disulphide-linked homodimer primarily coupled to G_q/G₁₁ [40] by which it is linked to the inositol phospholipid metabolism, i.e. it elicits an increase in the PIP₂ turnover by activating PLCβ to hydrolyse PIP₂ to IP3 and DAG, which results in intracellular calcium release from intracellular stores and activation of PKCγ. Besides classical, glutamate-stimulated activation, also agonist-independent, ‘constitutive’ activity of mGluR1 (and mGluR5) occurs, modulated by intracellular proteins including Homer-3 and Homer-1a [41].

The protein comprises an extracellular N-terminus containing the glutamate binding site, seven alpha-helical transmembrane domains and an (isoform-specific) cytoplasmic C-terminus—with the exception of isoform 1e, which is truncated before the first transmembrane domain. The G-protein-binding C-terminus contains domains that regulate mGluR1 function/signalling as well as its localisation and subcellular distribution in the dendritic membrane, its trafficking and its internalisation. Importantly, via a cytoplasmic Homer-binding PPxxFR motif, the receptor (more specifically, the long 1194-amino acid isoform 1a [42, 43]) binds Homer-3 (as well as Homer-1 and -2), another autoantigen in ACA, which regulates the postsynaptic localisation of mGluR1 as well as its activity [44].

In common with other mGluRs, the postsynaptic group I mGluRs transduce stimulatory signals at excitatory synapses. MGluR1 is present in the highest concentrations at the PF/PC synapse. Upon stimulation, the receptor modulates neuronal excitability by controlling ion channels. Modifications in the subcellular expression and distribution of mGluR1, together with changes induced by stimulation of mGluR1, participate in the long-term synaptic plasticity involved in memory formation and learning [37]. MGluR1 regulation is believed to have an important role in both types of long-term synaptic plasticity: while it has been implicated in long-term depression (LTD) of synaptic efficacy in the cerebellum, it is involved in long-term potentiation (LTP) in the hippocampus [45–47].

The proteins with which mGluR1 is associated or interacts in the cerebellum, hippocampus or cerebral cortex include, among others, TRPC, a cation channel involved in slow excitatory cation conductance [5, 48], the P/Q-type voltage-gated calcium channel (VGCC) (Ca_v2.1) [16] and gamma-aminobutyric acid type B receptors [49], which are both known target autoantigens in patients with ACA themselves [13, 50–52], NMDA receptors [53, 54], an important autoantigen in autoimmune encephalitis [55], and adenosine A1 receptor [56].

Outside the cerebellum, mGluR1 has been found in mitral and tufted cells of the olfactory bulb [57] and, at lower levels, in the hippocampus [58], the amygdala, the hypothalamus, where they take part in regulating circadian rhythms [59] and hormone secretion [60], the basal ganglia including the subthalamic nucleus, the thalamus, where it is involved in processing of nociception and pain and of other sensory information [61–64], and the ventral horn, central grey, substantia gelatinosa and sensory trigeminal nuclei of the spinal cord, where it is also implicated in nociception, as well as in the cerebral cortex and brainstem [38, 65, 66]; mGluR1 has important physiological roles also in motor neurons [67–69].

Immunohistochemistry

As indicated above, mGluR1 is widely expressed throughout the central nervous system, mostly postsynaptically in neuronal dendrites and somata [38, 70, 71]. Within the cerebellum, it is enriched in PCs, with the highest levels in the dendritic spines (Fig. 2) [38, 58, 65]; in addition, granular cells and other interneurons seem to express mGluR1 at lower RNA and protein levels [58, 66, 72, 73]. Makoff et al. [39] found mGluR1c exclusively in granule cells by in situ hybridisation, while an mGLUR1a/mGLUR1b probe reacted in addition with PCs and basket cells. Mateos et al. [71] found both mGluR1a and mGluR1b by immunogold labelling in the dendritic spines of PCs receiving PF synaptic terminals and reported additional peri-extrasynaptic mGluR1a/b expression.

Anti-mGluR1 autoantibodies were originally detected by avidin-biotin peroxidase and avidin Texas red IHC, respectively, using formalin-perfused sections of mouse and human cerebellum [33]. The patients’ sera strongly stained PC bodies and dendrites (but not the PC axons). Using confocal microscopy, a strong punctate staining in the molecular layer of the cerebellum was observed, indicative of labelling of the PC spines. Later studies used mouse, rat or primate brain sections (formalin fixed in three studies, not specified in another one) and either conventional IHC or indirect immunofluorescence (IIF) and reported a similar binding pattern [34–36, 74]. The punctate staining seen with anti-mGluR1-positive sera was considered different from that reported for anti-Tr [35]. Outside the cerebellum, strong staining of neurons and neuropil was observed in the glomeruli of the olfactory bulb, the olfactory tubercle (including the islands of Calleja), the superficial layer of the cerebral cortex, the thalamus, the superior colliculus, the spinal trigeminal nucleus and the CA3 area [33] and dentate gyrus [34] of the hippocampus. See Fig. 3 for typical IHC findings.

Antigen-specific assays

A cell-based assay (CBA) employing human embryonic kidney 293 (HEK293) cells transfected with human mGluR1a (Euroimmun, Luebeck, Germany) is available at the authors’ institution for use in scientific studies. Several other HEK293 or Chinese hamster ovary (CHO) in-house CBAs employing rat, mouse or human mGluR1 have been reported [33–36]. The patient antibodies were shown to bind to isoform a in at least two studies (not specified in the remaining studies) [33, 34]. Other approaches to demonstrate specificity for mGluR1 included the use of cerebellum sections obtained from mGluR1-knockout mice, resulting in abolition of the typical staining pattern [33, 35], and an mGluR1 inhibition assay based on measurement of the glutamate-stimulated formation of inositol phosphates in CHO-transfected cells before and after incubation with patient serum [33].

CSF testing

The two index patients were positive for mGluR1 both in serum and in the CSF. However, titres per unit of IgG were 31 and 36 times as high in the CSF as in serum, indicating intrathecal synthesis [33]. In a third patient, a serum titre of 1:20,000 and a CSF titre of 1:500 were found in a fluorescence-based IHC assay [35]. CSF samples also tested positive in four studies of mGluR1-specific CBAs [33–36].

Association with other autoantibodies

No association of anti-mGluR1 antibodies with other anti-neural antibodies (including anti-Hu, -Yo, -Ri, -Tr, -CV2/CRMP5, -Ma/Ta, -glutamic acid decarboxylase (GAD), -NMDAR, -AMPAR, -GABABR, -glycine receptors, leucine-rich, glioma inactivated 1 (LGI1), contactin-associated protein-2 (CASPR2), -amphiphysin, -Homer-3 and –Tr/DNER) has been found so far [33–36]. Cross-reactivity with the structurally closely related mGluR5 receptor was excluded in three patients using HEK293- or CHO-based CBAs [33, 36].

Pathogenetic relevance

As a plasma membrane protein with a large extracellular domain [38, 39], mGluR1 is accessible to circulating IgG. Three independent studies consistently showed that anti-mGluR1 indeed targets the N-terminal, ligand-binding extracellular domain of native mGluR1a as indicated by their binding to living, i.e. unfixed, CHO or HEK cells expressing mGluR1 and by the lack of effect of anti-mGluR1 on PC function if injected intracellularly instead of being added extracellularly [33, 34, 75].

In contrast to most other antibodies discussed here, passive transfer experiments have been performed and strongly indicated a direct pathogenic effect of the antibody. Transfer of anti-mGluR1 into the subarachnoid space of normal mice, near the cerebellum, causes increasing ataxia with a wide and uncoordinated, irregular gait and a pathological rotorod test. The most strongly affected mice could hardly stand up owing to severe truncal ataxia [33]. Post-mortem analysis showed IgG deposits mainly in the cerebellum, including the cerebellar cortex [33]. The fact that antibodies eluted from mGluR1a-expressing CHO cells incubated with patient serum caused similar ataxic symptoms, while sera preadsorbed with such cells did not, proves that the effects were elicited by anti-mGluR1 and not by other antibodies potentially present in the patient serum [33].

As no freshly frozen human serum was co-injected as complement source and as the effect set in after a very short time and spontaneously subsided after 24 h, it is likely that ataxia was caused by functional blocking of mGluR1. Evidence for a functional impact of anti-mGluR1 on the receptor also comes from the demonstration of a dose-dependent decrease in glutamate-induced inositol phosphate formation following incubation of mGluR1a-expressing CHO cells with patient (but not control) IgG [33]. Similarly, IgG from anti-mGluR1-positive serum acutely reduced the holding inward current of PC in a slice culture model and markedly suppressed the inward current induced by (RS)-3,5-dihydroxyphenylglycine (DHPG), a selective agonist for group I mGluRs. The latter effect was reversible by a 20- to 40-min wash [75]. When incubated with spontaneously (unstimulated) firing PCs, a slight hyperpolarisation and thus hypoexcitability and a significant reduction in the action potential firing rate was noted [75]. Moreover, when applied in vivo directly to the flocculus of mice by a minipump, anti-mGluR1 (but not control IgG) strongly and acutely disturbed the visual component of compensatory eye movements as indicated by a reduction in the amplitude of the optokinetic reflex as well as the vestibulo–ocular reflex response in light; the effect was reversible by removal of the pump [75]. Finally, application of mGluR1 to cultured embryonic mouse PC during LTD induction strongly attenuated the LTD-defining decrease in the amplitude of the excitatory postsynaptic current following glutamate/depolarisation conjunctive stimulation at the PF/PC synapse [75]. While the calcium influx was unaltered, calcium mobilisation was significantly reduced, in line with the reduction in mGluR1-mediated inward current and the reduction of phosphatidylinositol turnover measured in mGluR1-expressing CHO cells [75].

Whether complement- or cell-mediated, antibody-related cytotoxicity is involved in the pathogenesis, in vivo has not been investigated thus far. Considering the prominent role of group I mGluRs in neuroprotection, blockade of the receptor might result in PC cell loss also in the absence of a strong immune reaction [76–78]. Coesmans et al. [75] indeed found an (up to two third) decrease in the density of PCs in all parts of the cerebellar hemispheres and vermis in a post-mortem analysis of a patient, who had died from cardiac infarction. Of note, no signs of an ongoing inflammation (including cytotoxic CD8+ T lymphocytes previously reported in other types of ACA) were noted despite severe persisting ataxia at the time of death. In areas with PC loss, reactive Bergmann gliosis was present [75]. Moreover, PC morphology was affected with the dendritic trees of the remaining PCs being severely amputated [75]. In accordance with that finding, cerebellar atrophy indirectly indicating cell loss was detected by MRI in two further patients [34, 36].

Indirect evidence for a pathogenic role of the antibody comes from the demonstration of mGluR1-specific plasma cell clones within the CNS [33, 35] and from the fact that fading of ataxia after immunotherapy was paralleled by disappearance of the antibody [33]; by contrast, persistence of ataxia was accompanied by persisting serum and CSF anti-mGluR1 in a second patient [33].

Considering that only a subset of patients with mGluR1 reported so far had an accompanying tumour and that the receptor was not detectable in tumour samples renders a simple paraneoplastic aetiology caused to ectopic protein expression unlikely.

Molecular genetics

A pathogenic impact of anti-mGluR1 is also supported by molecular genetic findings linking mGluR1 dysfunction to cerebellar ataxia. Most importantly, autosomal recessive spinocerebellar ataxia-13 (SCA13) has been found to be caused by a complex homozygous mutation in the GRM1 gene encoding mGluR1 that results in aberrant transcripts lacking important functional domains [21]. SCA13 is a slowly progressive CNS disorder with onset in infancy that is characterised by moderate to severe gait, stance and limb ataxia with dysmetria, tremor, dysdiadochokinesia and dysarthria, and generalised cerebellar atrophy on MRI with small inferior vermis and retrocerebellar cysts, eye movement abnormalities (horizontal nystagmus, hypometric saccades, abduction deficits, esotropia, ptosis), mild to profound mental retardation with ventriculomegaly and/or generalised brain atrophy, poor or absent speech, and, in some, hyperreflexia and/or seizures [21].

Mutations in the mGluR1 gene cause cerebellar ataxia also in mice: A spontaneous mutation in the ligand-binding region of mGluR1 has been found to underlie ataxia in the recoil wobbler (rcw) strain of ataxic mouse [79]. Disruption of mGluR1 in mice by homologous recombination-mediated gene targeting was associated with atactic gait and intention tremor, although the gross anatomy of the cerebellum was widely normal, as was the excitatory synaptic transmission from PFs and CFs to PCs [46]. However, LTD (but not short-term synaptic plasticity) is clearly impaired in mGluR^−/− mice [46], and multiple (instead of single) innervation of CFs to PCs was observed [46, 80–82]. Similarly, no basic anatomical abnormalities were found in the hippocampus in mGluR1-deficient mice [45]; in contrast to the cerebellum, LTD was intact, but (mossy fibre) LTP and learning were impaired [45, 80, 83]. In rescue mice, all effects could be reversed in a dose-dependent manner by reconstitution of mGluR1 signalling [84].

Testing for spinocerebellar ataxia (SCA; types 1, 2, 3, 6, 7 and 17), Friedreich’s ataxia (FRDA), and fragile-X tremor-ataxia syndrome (FXTAS) has been carried out in one patient with mGluR1 antibodies and was negative [36].

Anti-Homer-3

Clinical, paraclinical and epidemiological features

The index patient (65/F) presented with vertigo, vomiting, dysarthria and severe subacute limb and gait ataxia. Ataxia was irreversible [85]. A second patient (38/M) also presented with nausea, vomiting and a pancerebellar syndrome but, in addition, developed signs of encephalitis including drowsiness, confusion and complex partial seizures. In this patient, elevated opening pressure and papilloedema was noted [86]. CSF analysis revealed lymphocytic pleocytosis in both cases (29 and 60 cells/μl); signs of intrathecal IgG synthesis were present in one patient. Brain MRI was normal at first examination in both patients, with no available follow-up in patient 1 and mild atrophy of the vermis and cerebellar hemispheres in patient 2 after 10 months. Onset of disease was subacute in both cases. Two additional (as yet unpublished) cases of ACA and Homer-3 antibodies have recently been diagnosed by us. No evidence has been found for a role of anti-Homer-3 in patients with chronic cerebellar ataxia (n = 27), patients with opsoclonus–myoclonus syndrome (n = 20) or healthy subjects (n = 20) [85, 86].