An update on malignant tumor-related stiff person syndrome spectrum disorders: clinical mechanism, treatment, and outcomes

Stiff person syndrome (SPS) is a rare central nervous system disorder associated with malignancies. In this review, we retrieved information from PubMed, up until August 2023, using various search terms and their combinations, including SPS, stiff person syndrome spectrum disorders (SPSSDs), paraneoplastic, cancer, and malignant tumor. Data from peer-reviewed journals printed in English were organized to explain the possible relationships between different carcinomas and SPSSD subtypes, as well as related autoantigens. From literature searching, it was revealed that breast cancer was the most prevalent carcinoma linked to SPSSDs, followed by lung cancer and lymphoma. Furthermore, classic SPS was the most common SPSSD subtype, followed by stiff limb syndrome and progressive encephalomyelitis with rigidity and myoclonus. GAD65 was the most common autoantigen in patients with cancer and SPSSDs, followed by amphiphysin and GlyR. Patients with cancer subtypes might have multiple SPSSD subtypes, and conversely, patients with SPSSD subtypes might have multiple carcinoma subtypes. The first aim of this review was to highlight the complex nature of the relationships among cancers, autoantigens, and SPSSDs as new information in this field continues to be generated globally. The adoption of an open-minded approach to updating information on new cancer subtypes, autoantigens, and SPSSDs is recommended to renew our database. The second aim of this review was to discuss SPS animal models, which will help us to understand the mechanisms underlying the pathogenesis of SPS. In future, elucidating the relationship among cancers, autoantigens, and SPSSDs is critical for the early prediction of cancer and discovery of new therapeutic modalities.


. Introduction
Stiff person syndrome (SPS) is a rare chronic central nervous system (CNS) disorder (1).The clinical manifestations of SPS encompass a wide range of symptoms, including muscle rigidity, sporadic muscle spasms, and chronic muscle pain.It is also characterized by psychiatric symptoms, such as depression and anxiety, and also other neurological
To date, SPSSDs include the following: (1) partial SPS, limited to extremities and often only one limb (stiff limb syndrome, SLS) or the torso; (2) SPS-plus, with classic SPS symptoms that exist in combination with cerebellar and/or brainstem findings; (3) PERM; and (4) some overlapping syndromes, such as classic SPS with epilepsy or limbic encephalitis (LE) (7), classic SPS with myasthenia gravis (8), classic SPS with anti-N-methyl-Daspartate receptor (NMDAR) encephalitis (NMDARE-SPS) (9), classic SPS with central sleep apnea (10), and classic SPS with pure red blood cell aplasia (11).Most patients with SPSSD are middle-aged females; however, some patients with SPSSD are either pediatric individuals or adult males.For example, among a total of 22 patients, eight older male patients with SPSSD showed early prominent vestibular and ocular motor dysfunction (4, 12).

. . Lymphoma and similar hematological carcinomas
Lymphoma and similar hematological carcinomas have been reported to be associated with SPSSD.In total, 10 studies involving three SPSSD subtypes, such as classic SPS (21, 34, 77-80), SLS (81), and PERM (82-84), have been reported (Table 3).Some authors have reported the coexistence of thymoma and breast cancer with lymphoma and SPSSD (34, 80).Table 3 shows that GlyR is the most commonly reported autoantigen in patients with lymphoma and similar hematological carcinomas and SPSSD, followed by GAD65, PCA-1, PCA-Tr, and striational antibodies.

. Possible mechanisms of paraneoplastic SPSSD
As we believe that autoantigens might be good candidates for determining the possible mechanism underlying paraneoplastic SPSSD, we have summarized the detailed information on autoantigens, including GAD and amphiphysin, followed by GlyR, gephyrin, anti-islet cell, and LGI1 (please see Supplementary Table 1).

. . GAD . . . GAD isoform
GAD is predominantly expressed in neurons, which might be linked to SPSSD, and insulin-secreting pancreatic β cells, which might be linked to type I diabetes (107).GAD regulates the decarboxylation of glutamate to gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter within the CNS, and is related to SPSSD (108,109).There are two GAD isoforms-GAD65 and GAD67.GAD65 is expressed in the presynaptic end of nerve terminals in its inactive form and is converted to its active form at the post-natal stage to rapidly synthesize GABA for synaptic transmission (110).GAD65 is also responsible for packaging GABA after its synthesis (111).Additionally, GAD67 is expressed in the cell body and dendrites and is responsible for synthesizing basal levels of GABA (100).
GAD65 antibody (Ab) titers and epitope specificities are present in different diseases and different subtypes of SPSSD (112, 113).For example, the GAD65 Ab titer is 348 U/mL in  114).A high range of GAD65 Ab levels is associated with SPS, whereas a lower one is associated with type I diabetes (13).A possible mechanism underlying this distinction could be that the GAD Ab in type I diabetes primarily reacts with conformational epitopes, whereas GAD antibodies in SPS recognize linear epitopes (115)(116)(117).Furthermore, GAD Ab-positive type I diabetes or SPS, CA, and LE are associated with different HLA class II haplotypes (118-121).
. . .Decreased GABAergic activity is the major physiopathological mechanism of SPSSD GABAergic neurons are responsible for inhibitory signals in the CNS and express high levels of GAD65.They are mainly located in the hippocampus, cerebellum, basal ganglia, brainstem nuclei, and spinal gray matter (122,123).GABA binds GABAA and GABAB receptors to mediate the hyperpolarization of postsynaptic neurons, comprising an inhibitory signal (124, 125).The GAD Ab inhibits GAD65 to block GABA synthesis, thereby reducing the uptake of newly synthesized GABA in synaptic vesicles and its synaptic release (111,(126)(127)(128).
Inhibiting GABA synthesis results in decreased GABAergic transmission, which is linked to neuronal hyperexcitability and is the core pathophysiological mechanism in SPS (129, 130).For example, the possible mechanism underlying SPS might be mediated by the inhibition of GABAergic neurons in the spinal cord, resulting in a state of motor neuron hyperexcitability, ultimately causing the simultaneous contraction of agonist and antagonist muscles (100,131).GABAergic interneurons are located at different levels of the CNS, other than the spinal cord, leading to other subtypes of SPSSD, such as PERM (128), which has also been supported by animal studies (132,133).However, this is not the case for LE and temporal lobe epilepsy, owing to insufficient data.

. . . Titer of anti-GAD Ab in the serum vs. the cerebrospinal fluid of patients with SPSSD
One report showed that the median concentration of anti-GAD65 Ab, measured via ELISA, is 30-fold higher in the serum (74,700 IU/mL) than in the cerebrospinal fluid (CSF) (2,430 IU/mL).However, these data were from 34 patients with classical anti-GAD65-associated syndromes, including SPS, CA, chronic epilepsy, and LE, with overlapping syndromes in some of the cases (138).The serum/CSF ratio of anti-GAD65 Ab was reported to be approximately 20 in patients with SPS (138).Moreover, serum and CSF anti-GAD65 Ab titers decreased, with those of CSF decreasing more rapidly than serum titers after patients with SPS received immunotherapy (138). . . .Amphiphysin . . .Amphiphysin superfamily Amphiphysins are members of the Bin-Amphiphysin-Rvsp (BAR) family of proteins, which includes the mammalian bridgingintegrators (Bin1 and Bin2), amphiphysins, and yeast Rvs161p and Rvs167p (139).Some members of the amphiphysin superfamily have conserved BAR domains, mainly in the N-terminus, and an SH3 domain in the C-terminus (139).Amphiphysin I is expressed in chicken and mammalian brains (140) and is associated with SPS and breast cancer (49,55).Two members of amphiphysin II are also expressed in the brain.Amphiphysin II, also known as BIN1 (MYC box-dependent interacting protein-1 or bridging integrator-1) or SH3P9, is associated with cancer progression, several myopathies, heart failure, and late-onset Alzheimer's disease (141).Amphiphysin IIa shares a brain-specific domain with amphiphysin I (142, 143), and amphiphysin IIb has a skeletal muscle-specific domain with a tumor suppressor that interacts with the c-Myc oncoprotein (142, 144).In several cancers, such as breast, colon, prostate, and lung cancers, as well as hepatocarcinoma and neuroblastoma, the expression of amphiphysin II is reduced or altered (145)(146)(147)(148).In addition, the ablation of amphiphysin II is linked to a poor cancer prognosis and increased metastasis (145,(148)(149)(150)(151). Amphiphysin II can also inhibit Myc-dependent transformation and tumorigenesis (145,(148)(149)(150)(151).
There are four α subunits and one β unit in GlyR, and these are expressed in the spinal cord and retina, respectively (158-160).Microglia secrete glycine, enhance NMDA receptormediated responses (161), and express GlyR to induce membrane depolarization, increasing intracellular calcium and proliferation (162).In addition, glial cells modulate synaptic development by participating in the induction of the action potential conduction in white matter via GlyRs (163).Importantly, glycine has also been linked to rapid cancer cell proliferation due to glycine metabolism (164).For example, α1 and α3 GlyR subunits were found to be expressed in human brain tumor biopsies, and the lack of α1 GlyR protein expression resulted in inhibition of the self-renewal capacity and tumorigenicity of GL261 glioma cells (165).GlyR knockdown can increase P53 tumor suppressor protein expression (166, 167).

. . Lymphoma and similar hematological carcinomas
Lymphoma and similar hematological carcinomas have been reported to be associated with SPSSD.In total, 10 studies involving three SPSSD subtypes, such as classic SPS (21, 34, 77-80), SLS (81), and PERM (82-84), have been reported (Table 3).Some authors have reported the coexistence of thymoma and breast cancer with lymphoma and SPSSD (34, 80).Table 3 shows that GlyR is the most commonly reported autoantigen in patients with lymphoma and similar hematological carcinomas and SPSSD, followed by GAD65, PCA-1, PCA-Tr, and striational antibodies.

. Treatment and outcomes of paraneoplastic SPSSD
For patients with paraneoplastic SPSSD, the carcinoma is typically detected and identified prior to treatment while concurrently managing and addressing symptoms.

. . GABAergic therapy
In patients with SPSSD, antibodies attack the GAD enzyme, which is essential for GABA production.Therefore, drugs targeting GABAergic neurons can be effective in treating SPSSD; by inhibiting the attack on GAD, GABA levels are reduced (168).

. . . Benzodiazepines
Benzodiazepines are the first-line treatment for patients with SPS.These drugs enhance the neurotransmitter effect of GABA at its receptor.Furthermore, benzodiazepines are widely used for their sedative, muscle-relaxant, and anticonvulsant effects (21).Long-term benzodiazepine therapy has been shown to benefit patients with classic or partial SPS and reduce GAD-65positive Ab-mediated stiffness and spasm symptoms; however, this improvement might also be due to other adjunct medications.
The major drug for SPSSD treatment is diazepam, which results in a good response in most patients at high doses of up to 60 mg daily (169).However, owing to concerns about withdrawal from long-term use and high doses of diazepam therapy, tizanidine has emerged as a good candidate for alternative therapy.As an NMDAR, tizanidine is an α2 inhibitor that inhibits glutamate release and prevents glutamatergic hyperactivity, thereby resolving convulsions in patients with SMS.Nevertheless, the dose of tizanidine should be individualized (21, 169).

. . . Baclofen
Baclofen is an agonist of GABA type B receptors that inhibits reflexive muscle contraction by blocking the release of excitatory neurotransmitters through voltage-gated calcium channels (170).It is also a second-line therapy for patients with SPS.However, to date, the use of oral baclofen therapies is still being debated.In one report, high doses (however, the dose is unknown) of oral baclofen therapy were found to result in serious side effects, such as sedation and respiratory depression (171).However, oral baclofen had good effects on SPS patients without serious side effects.For example, oral baclofen therapy (5 mg, three times per day) plus clonazepam resulted in improvements in a 69-year-old man with SPS and amphiphysin antibodies (172).Symptomatic treatment initiated with oral clonazepam and baclofen (5 mg Bid), followed by intravenous immunoglobulin (IVIG) resulted in improvements in a 60-year-old man with SPS associated with critical illness polyneuropathy (173).Baclofen (30 mg/day) combined with oral diazepam and steroids resulted in improvements in a 55-year-old GAD-Ab-positive female patient with SLS and breast carcinoma (57).For childhood-onset SMS, three SMS patients had good clinical responses with oral baclofen (dose range, 60-80 mg) Frontiers in Neurology frontiersin.orgcombined with diazepam, IVIG, plasma exchange or dantrolene, and botulinum toxin (174).Alternatively, intrathecal therapy is an effective route for baclofen treatment (2).The chronic infusion of intrathecal baclofen can improve SPS patient outcomes, including the pain Numeric Rating Scale, Spasm Frequency Scale, and lower extremity Modified Ashworth Scale (171).Intrathecal baclofen (100 µg) followed by a rehabilitation program resulted in substantial clinical and functional improvements in a 59-year-old female SPS patient, who had no therapeutic response with oral benzodiazepines and botulinum toxin injections (175).In addition, intrathecal baclofen (started from 50 µg/d up to 100 µg/d) improved motor functions in a 48-year-old male GAD-negative SPS patient (176).Baclofen can be used to effectively treat SPS because it is a direct agonist of GABA-B receptors and does not require endogenous GABA to induce presynaptic inhibition (176).

. . . Levetiracetam
Levetiracetam binds to synaptic vesicle glycoprotein 2A (SV2A), resulting in the release of the neurotransmitter stored within the vesicle, rapidly inhibiting firing neurons and potassium and N-type calcium channels (177,178).In a previous study, three patients with high anti-GAD65 Ab levels did not respond satisfactorily to IVIG and diazepam treatment with or without plasmapheresis (179).These patients were treated with 500 mg oral levetiracetam twice daily, which improved axial rigidity and the disappearance of paroxysmal respiratory arrest within 3 days of therapy initiation, with markedly reduced leg stiffness and ameliorated walking difficulties (179).However, to date, there is no evidence of the effects of long-term levetiracetam therapy.The possible mechanism by which levetiracetam achieves its effects could be by stabilizing and strengthening GABAA and decreasing hyperexcitability in spinal cord neurons (179).

. . . Pregabalin
Structurally, pregabalin is classified as a GABA analog or gabapentinoid (180).In a previous study, a female patient with SMS who did not respond to diazepam treatment, owing to excessive sedation, was successfully treated with a 3-month pregabalin regimen (181).The possible mechanism underlying the effects of pregabalin might be the inhibition of calcium influx and subsequent release of excitatory neurotransmitters, including glutamate and norepinephrine, resulting in compensation for the imbalance between inhibitory and excitatory intracortical circuits (181).

. . . Propofol
The mechanism of action of propofol in the CNS is unclear.Propofol might enhance the function of GABA receptors, evoking the chloride current in central neurons at clinically relevant concentrations, ultimately activating the GABA receptor-chloride ionophore complex (182).Notably, a low dose of propofol improves symptoms in patients with SPS who do not respond to high-dose benzodiazepines, baclofen, corticosteroids, levetiracetam, IVIG, or IV ethanol.Furthermore, propofol is effective for patients with SMS that is refractory to therapy (183).Unfortunately, longterm propofol therapy has unsatisfactory effects in patients with SPS (184).

. . Immunotherapy . . . Rituximab
Rituximab binds to the CD20 antigen on mature B cells, leading to B cell lysis, while sparing precursor B cells.Rituximab improves SPS and other neurological autoimmune disorders, such as Devic's disease, myasthenia gravis, autoimmune neuropathies, and inflammatory myopathies (185).SPS is associated with elevated titers of anti-GAD65 Abs and glycine receptor α-subunits in patients (186).Four reports have demonstrated the benefits of rituximab for patients with SPS (186)(187)(188)(189).Although rituximab improved the clinical conditions of patients, the decrease in the anti-GAD titer was inconsistent in different reports.Some reports demonstrated that after rituximab treatment, the anti-GAD titer was rapidly (17 days, from positive to undetectable) or slowly (1 year, from 1,000 to 400 U/mL) reduced (187).However, another case report showed that the anti-GAD Ab titer remained elevated, even during treatment with rituximab (188).

. . . Tacrolimus
Tacrolimus inhibits the calcium calcineurin pathway and exerts its immunosuppressive effect by reducing the proliferation of activated T cells (190).Furthermore, tacrolimus decreases IL-2 levels and impairs T-helper cell functions, finally reducing the activation of B cells to produce antibodies.It also suppresses the function of anti-GAD Abs, thereby blocking GABAergic neurotransmission and interfering with GABA synthesis (191).Tacrolimus directly blocks calcineurin in the GABAergic inhibitory system.Nonetheless, the neuroprotective effect of tacrolimus therapy on SPS demonstrated based on the reduced density of neurons with somal areas and improved pathological conditions, remains debatable (192); evidence that macrolide antibiotics inhibit the function of immunophilins and provide neuroprotective and neuroregenerative effects contradicts this assertion (184).Tacrolimus combined with IVIG or prednisone treatment greatly improved symptoms and reduced Ab titers in two patients who showed no response to other medicines (192).After 4 weeks of treatment with tacrolimus, serum anti-GAD Ab titers in patients with SPS were decreased, with an increase in motor ability, and the patients became completely self-dependent (191).

. . IVIG therapy
IVIG is the initial immunomodulator for patients with SPS with severe symptoms or unsatisfactory symptom improvements on other medications (193).IVIG therapy for patients with SPS partially improves symptoms (193) or the patient quality of life (194).It is also safe, with the duration of improvement being 6 weeks to 1 year (2).

. . Plasma exchange (plasmapheresis) therapy
Plasma exchange therapy is an option for patients with SPS who have failed to respond to other treatments (194).Plasmapheresis is usually conducted in one cycle with five sessions of plasma exchange.In a previous study, plasma exchange was used to treat two patients with SPS who had failed to respond to other treatments, resulting in improved symptoms and increased anti-GAD levels (195).Albahra et al. reported that among 10 patients with SPS, three had completely resolved symptoms, whereas seven had only partially relieved symptoms (196).
The outcomes of SPS treatment were reported to vary, resulting in a large range of improvements and moderate walking disability (21, 118).Limited reports have shown that patients with CA undergoing treatment have exhibited considerable improvements when assessed using the modified Rankin score.However, walking disability was still observed (197).Unfortunately, there were only modest outcomes for patients with LE following treatment (110,138,(198)(199)(200), with symptoms, such as seizures and cognitive impairment, remaining (199).

. . Changes in autoantibody titers after treatment . . . Anti-GAD
After immune globulin therapy, 11 patients with SPS showed improvements in their movement disorder and decreased serum anti-GAD65 Ab titers (169).As we previously described, serum and CSF anti-GAD65 Ab titers were found to decrease, with those of CSF decreasing more rapidly than those of serum after patients with SPS received immunotherapy (138).

. . . Ovarian teratoma
A 26-year-old woman with anti-NMDAR encephalitis and SPS with an ovarian teratoma was successfully treated via laparoscopic removal of the ovarian tumor.She received immune-suppressant medications (methylprednisolone followed by a combination with baclofen) preoperatively and postoperatively, and her symptoms were gradually resolved (9).

. . . Breast cancer
A 53-year-old male patient had anti-amphiphysin-positive SPS and breast cancer, as previously mentioned.After undergoing surgery to excise the cancer, he received adjuvant chemotherapy with cyclophosphamide, methotrexate, and 5-fluorouracil, followed by post-mastectomy radiation and adjuvant endocrine therapy with tamoxifen.After 1 year of surgery, the stiffness in his upper extremity, but not his lower extremities, greatly improved (42).However, for a 30-year-old female patient with anti-amphiphysin, GAD Ab-negative SPS, and breast cancer symptoms were not alleviated following surgery (41).

. . . Lung cancer
A 56-year-old woman with anti-amphiphysin-positive SPS associated with small-cell lung cancer received treatment with benzodiazepines and corticosteroids, followed by cancer therapy with cisplatin/etoposide and radiotherapy.Following treatment, she exhibited signs of improved stiffness and was able to walk independently for short distances (72).

. . In vitro animal tissue studies
In vitro SPSSD studies are usually divided into three assays, enzymatic assays, whole-cell patch clamp recordings, and immunofluorescence-using cultures.The major samples in studies using enzymatic assays have been rat pancreatic islet extracts (201), crude rat cerebellar extracts (202), and recombinant human GAD65 (113).These studies demonstrated that high titers of GAD Abs are associated with SPS, whereas few cases (2/12) of high GAD Ab titers were reported in type I diabetes (202).Furthermore, the studies revealed that GAD65 can recognize conformational epitopes in the C-terminus (113).
The major samples for studies using whole-cell patch clamp recordings have been rat cerebellar slices (203,204), rat hippocampal neurons (205), mouse hippocampal neurons (206), and rat hippocampal slices (207).These studies revealed that presynaptic GABAergic transmission is inhibited by GAD Abs in the CSF of patients with SPS and selectively suppressed (203,204).In addition, these studies demonstrated that post-synaptic inhibitory potentials are increased by GAD-positive epileptic serum (205) but not by serum from patients with GAD65 Ab-associated LE (206) or with GAD65 Ab-associated epilepsy (206,207).The major sample for studies using immunofluorescence based on cultures has been rat hippocampal neurons (114,208,209).These studies found that GAD Abs from some patients with SPS do not bind to the neuronal surface or that GAD Abs are not internalized by live neurons, suggesting the presence of other Abs specific to unknown antigens, rather than GAD (13).

. . In vivo animal model
The two major reported types of in vivo SPSSD animal models are passive transfer animal models, where transfer is induced using the serum or CSF antibodies from patients with SPS, and active immunized animal models induced using the human GAD65 protein (13).

. . . Passive transfer animal model
The main reported methods for passive transfer animal models using rats or mice are single cerebellar or paraspinal injections (132) and intrathecal (210,211) or intraperitoneal injections (208,211).Unfortunately, these animal models do not effectively mimic the clinical symptoms of SPS.However, some symptoms, such as paraspinal electrophysiological evidence of continuous motor activity (132), increased anxiety-like behavior (212), worsened rotarod results, and deficits in postural control (211), were partially matched.

. . . Animal model of active immunization
Active immunization using the human GAD65 protein has been effectively performed in type I diabetes studies; however, it has failed for neurologic diseases, including SPS, despite the high titers of GAD Abs (213) developing in these studies.This suggests that the GAD65 protein is also important for the pathogenesis of SPS; however, it is regulated by other autoantigens that contribute to the pathogenesis of SPS.

. Conclusion
This review demonstrated that the relationship among cancers, autoantigens, and SPSSDs is complicated, and new information in this field is still being revealed globally.Our findings would facilitate the development of an open-minded approach to updating information on novel cancer subtypes, autoantigens, and SPSSDs to renew our database.Future investigations are urgently required to reveal the mechanism by which cancers, autoantigens, and SPSSDs interact, which will facilitate the early prediction of cancer outcomes and the discovery of new therapeutic modalities.
TABLE Other carcinomas associated with SPSSD.