Paraneoplastic Kelch-like protein 11 antibody-associated cerebellar and limbic encephalitis caused by metastatic “burned-out” seminoma – A scar(r)y phenomenon

INTRODUCTION
The diagnosis of paraneoplastic neurologic syndromes is challenging when the primary tumor masquerades as scar tissue (i.e. "burned-out").


METHODS
Case report.


RESULTS
A 45-year-old male patient presented with progressive cerebellar symptoms and hearing loss. Initial screening for malignancy and extensive testing of paraneoplastic and autoimmune neuronal antibodies gave negative results. Repeated whole-body FDG-PET CT revealed a single paraaortic lymphadenopathy, metastasis of a regressed testicular seminoma. Anti-Kelch-like protein-11 (KLHL11) encephalitis was finally diagnosed.


CONCLUSION
Our case highlights the importance of continued efforts to find an often burned-out testicular cancer in patients with a highly unique clinical presentation of KLHL11 encephalitis.


Introduction
Paraneoplastic neurological syndromes (PNSs) are rare, immunemediated remote effects of cancer that can affect any part of the nervous system, often presenting with stereotyped clinical manifestations and frequently associated with specific neuronal antibodies. Recognition of a set of neurologic symptoms as paraneoplastic is often challenging and time consuming. Diagnosis therefore can be delayed, especially in cases when the suspected primary tumor shows complete spontaneous regression (i.e. "burn-out") and masquerades as scar tissue. <5% of all testicular germ cell tumors show spontaneous regression and the pathogenesis of this phenomenon is not understood (Astigueta et al., 2018).
Recently, anti-Kelch-like protein 11 (KLHL11) autoantibody has emerged as a high-risk paraneoplastic antibody associated with rhombencephalitis, discovered in patients with seminoma (Mandel-Brehm et al., 2019). A novel clinical scoring system has been developed  and later validated (Hammami et al., 2023) as a potent predictive tool for identification of patients with Kelch-like protein-11 autoantibodies.
Here we present a case of a "conventional" neuronal antibodynegative rapidly progressive cerebellar syndrome, brainstem and limbic encephalitis in a patient with previously unrecognized and spontaneously regressed ("burned-out") testicular seminoma with a single paraaortic lymph node metastasis. A thorough diagnostic workup eventually revealed anti-KLHL11 autoimmunity-associated paraneoplastic syndrome.

Case presentation
A previously healthy 45-year-old Caucasian male patient first presented with a few weeks' history of progressive gait disturbance, slurred speech, dizziness and left-sided tinnitus with moderate hearing loss, accompanied by a marked weight loss (approximately 20 kg in 1 month).
His first neurological examination revealed mild gait ataxia and backward falling. Audiometry found moderate left perceptual hearing loss. Initial brain magnetic resonance imaging (MRI) depicted an uncertain left temporal contrast-enhancing lesion apparently unrelated to his symptoms ( Fig. 1/a). Thoraco-abdomino-pelvic computerized tomography (TAP-CT) scan and whole-body 2-[18F]fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) scan did not show any evidence of malignancy.
He received 1 g intravenous methylprednisolone daily for 5 days assuming an autoimmune/inflammatory or paraneoplastic origin, but his symptoms did not improve.
After 9 months of gradual disease worsening, we noted pyramidal signs, generalized hypotonia, dysphagia and bilateral limb ataxia. He was only able to take a few steps. Neuropsychological examination revealed cognitive impairment with executive prefrontal dysfunction (Mini Mental State Examination: 25 points, Addenbrook Cognitive Inventory-III test: 80 points).
On follow-up brain MRI the left temporal contrast-enhancing lesion was no longer identifiable. Repeated TAP-CT scan was negative.
CSF cytology and flow cytometry was normal. Repeated serum and CSF paraneoplastic and autoimmune antibody panel (same as above) was negative.
In the 14th month of the disease course, additional symptoms developed: vertical nystagmus, cerebellar saccadic speech, severe dysarthria, severe limb and gait ataxia, emotional incontinence and obsessive crying. He was unable to walk, but he could use a wheelchair. 5 cycles of plasma exchange was performed, followed by minimal improvement in dysarthria, dysphagia, and incoordination.
On follow-up MRI, right hippocampal and significant cerebellar atrophy could be identified ( Fig. 1/c and 1/d).
Because of the high suspicion for a paraneoplastic disease, a third TAP-CT was performed, that depicted a pathologically sized (13 × 9 mm) paraaortic lymph node and a slightly enlarged left testis. With urological physical examination the testes and epididymides were intact. Ultrasound showed inhomogeneity of the entire left testis with microlithiasis. Left orchiectomy was performed and histological assessment reported scar tissue throughout without evidence of viable malignancy, but raised the possibility of a burned-out testicular tumor. Repeated FDG PET/CT scan showed increased FDG-uptake in the same solitary paraaortic retroperitoneal lymph node ( Fig. 1.b) as shown on TAP-CT. After laparoscopic removal of the lymph node, histology showed metastasis of seminoma (Fig. 2.).
Because of this highly unique clinical presentation, we considered anti-Kelch-like protein-11 (KLHL11) antibody associated rhombencephalitis, that was confirmed at the French Reference Center for Paraneoplastic Neurological Syndromes and Autoimmune Encephalitis (Lyon, France) revealing high titers of anti-KLHL11-antibodies in the CSF. KLHL11-antibody positivity was defined as a positive cell based assay (CBA) result using HEK293 cells transfected with a human KLHL11 clone. In order to establish if a characteristic staining was present, the patient's samples were also examined by indirect immunofluorescence assay (IFA) on rat brain sections. To assure that the immunostaining detected on IFA was related to the KLHL11 antibodies, we compared it to the one obtained using a commercial KLHL11-antibody Dubey et al., 2020;Maudes et al., 2020).
The patient received one cycle of bleomycin, etoposide, and cisplatin (BEP) as adjuvant chemotherapy. PET-CT 2 months after treatment did not show any signs of malignancy. He completed a 4-week long institutional rehabilitation program, but remained essentially wheelchairbound.

Discussion
Paraneoplastic neurological syndromes (PNSs) are rare and can precede the detection of the underlying malignancy posing a diagnostic challenge. The most common malignancies associated with neurological symptoms are lung cancer, breast cancer, thymoma, lymphoma and germ cell tumors (Vogrig et al., 2020;Shah et al., 2022). The so-called "classic" PNSs, like limbic encephalitis, rapidly progressive cerebellar syndrome, opsoclonus-myoclonus are considered high risk phenotypes (Graus et al., 2021), and warrant a thorough and repeated workup for malignancies including various imaging modalities (MRI, CT, FDG PET, US) and testing of the serum and CSF for paraneoplastic autoantibodies.
The presence of "high risk" or "intermediate risk" neuronal antibodies consistent with the neurological syndrome (Graus et al., 2021) are useful in indicating a specific occult tumor (e.g. Ma2 in testicular cancer) (Höftberger et al., 2015), but some cases are antibody-negative or require antibody testing in reference centres , which makes their diagnosis more challenging. Recently, a novel clinical score (MATCH score) has been established which helps in identifying a subgroup of conventional antibody-negative patients harboring anti-Kelch-like protein 11 autoantibodies . Indeed, our case perfectly fits this newly described paraneoplastic KLHL11rhombencephalitis, with a maximal MATCH score of 5 [M (male sex, 1 point), A (ataxia or other cerebellar signs, 1 point), T and C (testicular tumor 2 points or other types of tumors 1 point), H (hearing impairment, 1 point)]. Moreover, this novel PNS is highly associated with spontaneously regressed testicular germ cell tumor , as we also found in our case. Since its description, additional KLHL11 encephalitis cases have been published. With a high estimated prevalence, this disease now appears to be one of the most common PNS, that needs to be considered in patients with cerebellar symptoms, especially if there is additional hearing impairment (Delgado-García and Balint, 2021).
We highlight the importance of recognizing cochleovestibulopathy as an early sign of KLHL11 encephalitis. In line with our observations, progressive cochleovestibular dysfunction (rapid sensorineural hearing impairment, hearing loss, vertigo) was found to be the initial presentation in KLHL11 encephalitis before the onset of rhombencephalitis/ encephalomyelitis (Hammami et al., 2021;Fioretti et al., 2017;Krivitski et al., 2023;Kattah et al., 2021). The hearing loss in KLHL11 encephalitis involves initially high-frequency sounds, is often bilateral and is gender-specific to men. Moreover, progressive cochleovestibular impairment was found to be especially uncommon in other PNS (Kattah et al., 2021).
FDG-PET scan seems to have the most robust diagnostic performance for the detection of an occult malignancy with 87% sensitivity and 86% specificity (García Vicente et al., 2017;Salas Fragomeni et al., 2017). Yet, in our patient it was his second PET/CT that discovered the pathological lymph node. This highlights the importance of repeated investigations in suspicious cases even if initial results are negative.
The pathomechanism leading to the scarring or spontaneous regression of a primary testicular tumor and the exact pathogenesis of the associated PNS is not fully understood. 60% of PNS show positivity for autoantibodies (Delgado-García and Balint, 2021), signifying a role for immune mechanisms, but also highlighting that a significant proportion might have yet undiscovered autoantibodies or other underlying mechanisms. Although the pathophysiology of KLHL11 encephalitis is not yet fully understood, it is thought to involve antigen-specific T cellmediated cytotoxicity, similar to other classic PNSs, such as anti-Yo paraneoplastic cerebellar degeneration (Albert et al., 1998). Cytotoxic T lymphocytes appear to be essential in the immune-mediated primary tumor regression and CNS destruction in KLHL11 encephalitis. In addition to the histologically demonstrated CD8+ T cell infiltration of the CNS, autoantigen-specific activation of CD8+ T cells was also identified. When T cells were stimulated with KLHL11-specific antigens, their phenotypes were found to be predominantly effector or effector memory phenotypes. These findings point towards a primarily cytotoxic T cell-mediated disease pathogenesis in KLHL11 encephalitis, similarly to other classic PNSs (Dubey et al., 2020;Höftberger et al., 2015).
The effectiveness of antibody-eliminating therapy and the prognosis of patients with PNS is highly dependent on the type of the autoantibody: patients with intracellular autoantibodies -serving as surrogate markers for a cytotoxic T-cell-mediated injury -are poorly responsive and treatment can only achieve symptom stabilization or mild improvement; while patients with antibodies against cell surface antigens -that are directly pathogenic -can experience marked improvement or even full recovery (Höftberger et al., 2015). KLHL11 is an intracellular autoantibody, accordingly our patient has not shown functionally meaningful improvement after many cycles of plasma exchange and chemotherapy, mirroring what has been published in other KLHL11-PNS cases (Li et al., 2022). Rapid, early and sustained immunosuppression may have had provided benefit, but the diagnosis was delayed, and residual symptoms were observed.
Our report highlights the importance of detailed and repeated investigations for testicular cancer in male patients between the ages of 20-50 with suspected PNS, since it is the second most common malignancy in this patient cohort, and the first most common between 20 and 40 years of age (Ward et al., 2019). The number of reports describing PNS associated with burned-out testicular tumor is limited Chen et al., 2021;Guilmot et al., 2021;Maldonado Slootjes et al., 2022;Onishi et al., 2014;Freifeld et al., 2018;Ishikawa et al., 2016;van de Warrenburg et al., 2007), and it is tempting to speculate that at least some of them might have been associated with KLHL11 autoantibodies (Maldonado Slootjes et al., 2022).
Our present case confirms the high predictive value of the MATCH score, supports the association between KLHL11 autoimmunity and regressed seminoma and contributes to the growing body of literature on this very rare and scar(r)y phenomenon.

Ethics approval
The study has been approved by the Ethics Committee of Semmelweis University in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments.

Consent to publish
The patient has consented to the submission of this paper to the journal.

Funding
JH is funded by the BETPSY project, which is supported by a public grant overseen by the Agence Nationale de la Recherche (ANR) as part of the second Investissements d'Avenir program (ANR-18-RHUS-0012), and by the LABEX CORTEX (ANR-11-LABX-0042) of the Université de Lyon operated by the ANR.

Declaration of Competing Interest
The authors declare that they have no conflict of interest.

Data availability
Data will be made available on request.