Degeneration of retinal on bipolar cells induced by serum including autoantibody against TRPM1 in mouse model of paraneoplastic retinopathy

Broad locations of antigenic regions for anti-TRPM1 autoantibodies in paraneoplastic retinopathy with retinal ON bipolar cell dysfunction

PURPOSE

Cancer-associated retinal ON bipolar cell dysfunction (CARBD), which includes melanoma-associated retinopathy (MAR), has been reported to be caused by autoantibodies against the molecules expressed in ON bipolar cells, including TRPM1. The purpose of this study was to determine the antigenic regions of the autoantibodies against TRPM1 in the sera of CARBD patients, in whom we previously detected anti-TRPM1 autoantibodies.

METHODS

The antigenic regions against TRPM1 in the sera of eight CARBD patients were examined by Western blots using HEK293T cells transfected with the plasmids expressing FLAG-tagged TRPM1 fragments. The clinical course of these patients was also documented.

RESULTS

The clinical course differed among the patients. The electroretinograms (ERGs) and symptoms were improved in three patients, deteriorated in one patient, remained unchanged for a long time in one patient, and were not followable in three patients. Seven of the eight sera possessed multiple antigenic regions: two sera contained at least four antigen recognition regions, and three sera had at least three regions. The antigen regions were spread over the entire TRPM1 protein: five sera in the N-terminal intracellular domain, six sera in the transmembrane-containing region, and six sera in the C-terminal intracellular domain. No significant relationship was observed between the location of the antigen epitope and the patients' clinical course.

CONCLUSIONS

The antigenic regions of anti-TRPM1 autoantibodies in CARBD patients were present not only in the N-terminal intracellular domain, which was reported in an earlier report, but also in the transmembrane-containing region and in the C-terminal intracellular domain. In addition, the antigenic regions for TRPM1 were found to vary among the CARBD patients examined, and most of the sera had multiple antigenic regions.

Gene Therapy for Rdh12-Associated Retinal Diseases Helps to Delay Retinal Degeneration and Vision Loss

Purpose

The aim of study was to establish Rdh12-associated inherited retinal disease (Rdh12-IRD) mouse model and to identify the best timepoint for gene therapy.

Methods

We induced retinal degeneration in Rdh12^-/- mice using a bright light. We clarified the establishment of Rdh12-IRD mouse model by analyzing the thickness of retinal layers and electroretinography (ERG). Rdh12-IRD mice received a subretinal injection of adeno-associated virus 2/8-packaged Rdh12 cDNA for treatment. We evaluated the visual function and retinal structure in the treated and untreated eyes to identify the best timepoint for gene therapy.

Results

Rdh12-IRD mice showed significant differences in ERG amplitudes and photoreceptor survival compared to Rdh12^+/+ mice. Preventive gene therapy not only maintained normal visual function but also prevented photoreceptor loss. Salvage gene therapy could not reverse the retinal degeneration phenotype of Rdh12-IRD mice, but it could slow down the loss of visual function.

Conclusion

The light-induced retinal degeneration in our Rdh12^-/- mice indicated that a defect in Rdh12 alone was sufficient to cause visual dysfunction and photoreceptor degeneration, which reproduced the phenotypes observed in RDH12-IRD patients. This model is suitable for gene therapy studies. Early treatment of the primary Rdh12 defect helps to delay the later onset of photoreceptor degeneration and maintains visual function in Rdh12-IRD mice.

CLINICAL COURSE OF PARANEOPLASTIC RETINOPATHY WITH ANTI-TRPM1 AUTOANTIBODY IN JAPANESE COHORT

PURPOSE

To report the clinical course of eyes with paraneoplastic retinopathy caused by an autoantibody against transient receptor potential cation channel, subfamily M, member 1 (TRPM1).

METHODS

Ten paraneoplastic retinopathy patients with retinal ON-bipolar cell dysfunction, including six melanoma-associated retinopathy, from eight institutions in Japan were evaluated for the presence of an anti-TRPM1 antibody. The results of ophthalmic examinations and the presence of anti-TRPM1 antibody were analyzed.

RESULTS

Five patients were positive for the anti-TRPM1 antibody. These patients had similar clinical findings in both eyes at the time of diagnosis; relatively preserved best-corrected visual acuity, absence of fundus and optical coherence tomography abnormalities, and specific abnormalities of the electroretinography (ERG); and negative-type ERGs with bright stimulus flashes. One patient whose retinal ON-bipolar cells remained dysfunctional for the entire testing period, although the anti-TRPM1 antibody had disappeared. On the other hand, the ERGs recovered in 2 cases within 2 years after the onset. One case progressed to additional impairment of the photoreceptors with deterioration of ERGs. One case died and the clinical course was unavailable.

CONCLUSION

Paraneoplastic retinopathy patients with retinal ON-bipolar cell dysfunction possess autoantibodies against TRPM1 at the onset of the disease process; however, the clinical course of these eyes can be different.

Identification and characterization of novel TRPM1 autoantibodies from serum of patients with melanoma-associated retinopathy

Melanoma-associated retinopathy (MAR) is a rare paraneoplastic retinal disorder usually occurring in the context of metastatic melanoma. Patients present with night blindness, photopsias and a constriction of the visual field. MAR is an auto-immune disorder characterized by the production of autoantibodies targeting retinal proteins, especially autoantibodies reacting to the cation channel TRPM1 produced in melanocytes and ON-bipolar cells. TRPM1 has at least three different isoforms which vary in the N-terminal region of the protein. In this study, we report the case of three new MAR patients presenting different anti-TRPM1 autoantibodies reacting to the three isoforms of TRPM1 with variable binding affinity. Two sera recognized all isoforms of TRPM1, while one recognized only the two longest isoforms upon immunolocalization studies on overexpressing cells. Similarly, the former two sera reacted with all TRPM1 isoforms on western blot, but an immunoprecipitation enrichment step was necessary to detect all isoforms with the latter serum. In contrast, all sera labelled ON-bipolar cells on Tprm1+/+ but not on Trpm1-/- mouse retina as shown by co-immunolocalization. This confirms that the MAR sera specifically detect TRPM1. Most likely, the anti-TRPM1 autoantibodies of different patients vary in affinity and concentration. In addition, the binding of autoantibodies to TRPM1 may be conformation-dependent, with epitopes being inaccessible in some constructs (truncated polypeptides versus full-length TRPM1) or applications (western blotting versus immunohistochemistry). Therefore, we propose that a combination of different methods should be used to test for the presence of anti-TRPM1 autoantibodies in the sera of MAR patients.

Electrically Evoked Potentials Are Reduced Compared to Axon Numbers in Rhodopsin P347L Transgenic Rabbits With Severe Photoreceptor Degeneration

Purpose

To determine the relationship between the amplitudes of the electrically evoked potentials (EEPs) and the number of optic nerve axons at a late stage of retinal degeneration in rhodopsin P347L transgenic (Tg) rabbits, a model of retinitis pigmentosa.

Methods

Six eyes of six wild-type (WT) (43.8 ± 7.5 months of age) and six eyes of six Tg (40.3 ± 2.6 months of age) rabbits were studied. The EEPs were elicited by 1 to 5 mA of transcorneal electrical stimulation. The first positive wave, the P1 component, was analyzed. After euthanasia, the number of axons in the optic nerve was counted.

Results

The threshold current to elicit a P1 was significantly higher in Tg rabbits than WT rabbits. The amplitude of P1 elicited by 5 mA in Tg rabbits was about 24% of that in WT rabbits (P < 0.01). The number of axons in the optic nerve of Tg rabbits was reduced to about 59% of that of WT rabbits (P < 0.01). The correlation between the axon number and the amplitude of the P1 in Tg and WT rabbits was not significant. The mean ratio of the P1 amplitude/axon in Tg rabbits was decreased to 53% of that in WT rabbits (P < 0.05).

Conclusions

The degree of reduction in the EEP in Tg rabbits is more severe than the reduction in the number of optic nerve axons. The use of transcorneal electrical stimulation to determine the suitable candidates for prosthesis at the end-stage of retinitis pigmentosa may underestimate the condition of the optic nerves.

Paraneoplastic Syndromes in Neuro-ophthalmology

PURPOSE OF REVIEW

This article discusses the varied types of paraneoplastic syndromes that commonly have neuro-ophthalmologic manifestations. Diagnostic considerations and therapeutic options for individual diseases are also discussed.

RECENT FINDINGS

Paraneoplastic syndromes can affect the afferent and efferent visual systems. Paraneoplastic syndromes may result in reduced visual acuity from retinal degeneration, alterations in melanocyte proliferation and uveal thickening, or acquired nystagmus. Ocular motor abnormalities related to paraneoplastic syndromes may present with symptoms from opsoclonus or from neuromuscular junction disease. Diagnosis remains challenging, but serologic identification of some specific antibodies may be helpful or confirmatory. Treatment, in addition to directed therapies against the underlying cancer, often requires systemic corticosteroids, plasma exchange, or immunosuppression, but some specific syndromes improve with use of targeted pharmacologic therapy.

SUMMARY

Diagnosis and therapy of paraneoplastic syndromes presenting with neuro-ophthalmic symptoms remain a challenge, but strategies are evolving and new approaches are on the horizon.

Are Anti-Retinal Autoantibodies a Cause or a Consequence of Retinal Degeneration in Autoimmune Retinopathies?

Autoantibodies (AAbs) against various retinal proteins have been associated with vision loss in paraneoplastic and non-paraneoplastic autoimmune retinopathies (AR). There are two major paraneoplastic syndromes associated anti-retinal AAbs, cancer-associated retinopathy (CAR), and melanoma-associated retinopathy. Some people without a cancer diagnosis may present symptoms of CAR and have anti-retinal AAbs. The etiology and pathogenesis of those entities are not fully understood. In this review, we provide evidence for the role of AAbs in retinal death and degeneration. Studies of epitope mapping for anti-recoverin, anti-enolase, and anti-carbonic anhydrase II revealed that although patients' AAbs may recognize the same retinal protein as normal individuals they bind to different molecular domains, which allows distinguishing between normal and diseased AAbs. Given the great diversity of anti-retinal AAbs, it is likely some antibodies have greater pathogenic potential than others. Pathogenic, but not normal antibodies penetrate the target cell, reach their specific antigen, induce apoptosis, and impact retinal pathophysiology. Photoreceptors, dying by apoptosis, induced by other than immunologic mechanisms produce substantial amounts of metabolic debris, which consequently leads to autoimmunization and enhanced permeability of the blood-retinal barrier. AAbs that were made as a part of anti-cancer response are likely to be the cause of retinal degeneration, whereas others, generated against released antigens from damaged retina, contribute to the progression of retinopathy. Altogether, AAbs may trigger retinal degeneration and may also exacerbate the degenerative process in response to the release of sequestered antigens and influence disease progression.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

Pikachurin Protein Required for Increase of Cone Electroretinogram B-Wave during Light Adaptation

In normal eyes, the amplitude of the b-wave of the photopic ERGs increases during light adaptation, but the mechanism causing this increase has not been fully determined. The purpose of this study was to evaluate the contribution of receptoral and post-receptoral components of the retina to this phenomenon. To accomplish this, we examined the ERGs during light adaptation in Pikachurin null-mutant (Pika -/-) mice, which have a misalignment of the bipolar cell dendritic tips to the photoreceptor ribbon synapses. After dark-adaptation, photopic ERGs were recorded from Pika -/- and wild type (WT) mice during the first 9 minutes of light adaptation. In some of the mice, post-receptoral components were blocked pharmacologically. The photopic b-waves of WT mice increased by 50% during the 9 min of light adaptation as previously reported. On the other hand, the b-waves of the Pika -/- mice decreased by 20% during the same time period. After blocking post-receptoral components, the b-waves were abolished from the WT mice, and the ERGs resembled those of the Pika -/- mice. The extracted post-receptoral component increased during light adaptation in the WT mice, but decreased for the first 3 minutes to a plateau in Pika -/- mice. We conclude that the normal synaptic connection between photoreceptor and retinal ON bipolar cells, which is controlled by pikachurin, is required for the ERGs to increase during light-adaptation. The contributions of post-receptoral components are essential for the photopic b-wave increase during the light adaptation.

Properties and functions of TRPM1 channels in the dendritic tips of retinal ON-bipolar cells

An increase in light intensity induces a depolarization in retinal ON-bipolar cells via a reduced glutamate release from presynaptic photoreceptor cells. The underlying transduction cascade in the dendritic tips of ON-bipolar cells involves mGluR6 glutamate receptors signaling to TRPM1 proteins that are an indispensable part of the transduction channel. Several other proteins are recognized to participate in the transduction machinery. Deficiency in many of these leads to congenital stationary night blindness, because rod bipolar cells, a subgroup of ON-bipolar cells, constitute the main route for sensory information under scotopic conditions. Here, we review the current knowledge about TRPM1 ion channels and how their activity is regulated within the postsynaptic compartment of ON-bipolar cells. The functional properties of TRPM1 channels in the dendritic compartment are not well understood as they differ substantially from those of recombinant TRPM1 channels. Critical evaluation of possible explanations of these discrepancies indicates that some key components of this transduction pathway might still not be known. The continued exploration of this pathway will yield further clinically useful insights.

Case of paraneoplastic retinopathy with retinal ON-bipolar cell dysfunction and subsequent resolution of ERGs

PURPOSE

To report a patient with cancer-associated retinopathy and retinal ON-bipolar cell dysfunction who had a resolution of the electroretinograms (ERGs) after a resection of an ovarian cancer and chemotherapy.

CASE REPORT

A 71-year-old Japanese female patient visited us complaining of night blindness and photopsia in both eyes for 6 months. Her visual acuity was 20/20 in both eyes, and fundus examination, fluorescence angiography, and optical coherence tomography showed no abnormalities in both eyes. The rod responses of the ERGs were absent and bright-flash ERGs were electronegative. The ON responses of the focal macular ERGs and full-field long-flash ERGs were absent. These ERG findings indicate an ON-bipolar cell dysfunction. A general physical examination revealed the presence of ovarian cancer. After resection of the ovarian cancer and adjuvant chemotherapy, the ERGs of the left eye completely recovered within 2 years and those of right eye recovered subsequently. The autoantibody against transient receptor potential melastatin 1 (TRPM1) was not detected in the serum.

CONCLUSION

Our case demonstrates that retinal ON-bipolar dysfunction can be caused by ovarian cancer. Our case indicates that some autoantibodies against other than TRPM1 might cause transient dysfunction of retinal ON-bipolar cells.