A retrospective multicenter study on clinical and serological parameters in patients with MuSK myasthenia gravis with and without general immunosuppression.

Introduction: Muscle-specific kinase (MuSK)- myasthenia gravis (MG) is caused by pathogenic autoantibodies against MuSK that correlate with disease severity and are predominantly of the IgG4 subclass. The first-line treatment for MuSK-MG is general immunosuppression with corticosteroids, but the effect of treatment on IgG4 and MuSK IgG4 levels has not been studied. Methods: We analyzed the clinical data and sera from 52 MuSK-MG patients (45 female, 7 male, median age 49 (range 17-79) years) from Italy, the Netherlands, Greece and Belgium, and 43 AChR-MG patients (22 female, 21 male, median age 63 (range 2-82) years) from Italy, receiving different types of immunosuppression, and sera from 46 age- and sex-matched non-disease controls (with no diagnosed diseases, 38 female, 8 male, median age 51.5 (range 20-68) years) from the Netherlands. We analyzed the disease severity (assessed by MGFA or QMG score), and measured concentrations of MuSK IgG4, MuSK IgG, total IgG4 and total IgG in the sera by ELISA, RIA and nephelometry. Results: We observed that MuSK-MG patients showed a robust clinical improvement and reduction of MuSK IgG after therapy, and that MuSK IgG4 concentrations, but not total IgG4 concentrations, correlated with clinical severity. MuSK IgG and MuSK IgG4 concentrations were reduced after immunosuppression in 4/5 individuals with before-after data, but data from non-linked patient samples showed no difference. Total serum IgG4 levels were within the normal range, with IgG4 levels above threshold (1.35g/L) in 1/52 MuSK-MG, 2/43 AChR-MG patients and 1/45 non-disease controls. MuSK-MG patients improved within the first four years after disease onset, but no further clinical improvement or reduction of MuSK IgG4 were observed four years later, and only 14/52 (26.92%) patients in total, of which 13 (93.3%) received general immunosuppression, reached clinical remission. Discussion: We conclude that MuSK-MG patients improve clinically with general immunosuppression but may require further treatment to reach remission. Longitudinal testing of individual patients may be clinically more useful than single measurements of MuSK IgG4. No significant differences in the serum IgG4 concentrations and IgG4/IgG ratio between AChR- and MuSK-MG patients were found during follow-up. Further studies with larger patient and control cohorts are necessary to validate the findings.

Epidemiology of seropositive myasthenia gravis in Sardinia: A population-based study in the district of Sassari.

INTRODUCTION/AIMS: The global incidence and prevalence of myasthenia gravis (MG) range between 6-31/million and 10-37/100,000, respectively. Sardinia is a high-risk region for different immune-mediated disorders, but the epidemiology of MG remains unclear. We determined the epidemiology of MG with acetylcholine receptor (AChR)-immunoglobulin G (IgG) and muscle-specific tyrosine kinase (MuSK)-IgG in the district of Sassari (North-Western Sardinia; population, 325,288). METHODS: From the laboratory of the University Hospital of Sassari (reference for AChR/MuSK-IgG testing in Sardinia since 1998) and the main neurology units in Sardinia, we retrospectively identified MG patients with (1) AChR-IgG and/or MuSK-IgG positivity by radioimmunoprecipitation assay; and (2) residency in the district of Sassari. Incidence (January 2010-December 2019) and prevalence (December 31, 2019) were calculated. RESULTS: A total of 202 patients were included (incident, 107; prevalent, 180). Antibody specificities were AChR (n = 187 [93%]) and MuSK (n = 15 [7%]). The crude MG incidence (95% confidence interval) was 32.6 (26.8-39.2)/million, while prevalence was 55.3 (47.7-63.9)/100,000. After age-standardization to the world population, incidence decreased to 18.4 (14.3-22.5)/million, while prevalence decreased to 31.6 (26.1-37.0)/100,000. Among incident cases, age strata (years) at MG onset were: <18 (2%), 18-49 (14%), 50-64 (21%), and ≥65 (63%). DISCUSSION: Sardinia is a high-risk region for MG, with a prevalence that exceeds the European threshold for rare disease. Identification of the environmental and genetic determinants of this risk may improve our understanding of disease pathophysiology.

National Multicenter Study on the Comparison of Robotic and Open Thymectomy for Thymic Neoplasms in Myasthenic Patients: Surgical, Neurological and Oncological Outcomes.

Thymectomy is the gold standard in the treatment of thymic neoplasm and plays a key role in the therapeutic path of myasthenia gravis. For years, sternotomy has been the traditional approach for removing anterior mediastinal lesions, although the robotic thymectomy is now widely performed. The literature is still lacking in papers comparing the two approaches and evaluating long-term oncological and neurological outcomes. This study aims to analyze the postoperative results of open and robotic thymectomy for thymic neoplasms in myasthenic patients. Surgical, oncological and neurological data of myasthenic patients affected by thymic neoplasms and surgically treated with extended thymectomy, both with the open and the robotic approach, in six Italian Thoracic Centers between 2011 and 2021 were evaluated. A total of 213 patients were enrolled in the study: 110 (51.6%) were treated with the open approach, and 103 (48.4%) were treated with robotic surgery. The open surgery, compared with the robotic, presented a shorter operating time (p < 0.001), a higher number of postoperative complications (p = 0.038) and longer postoperative hospitalization (p = 0.006). No other differences were observed in terms of surgical, oncological or neurological outcomes. The robotic approach can be considered safe and feasible, comparable to the open technique, in terms of surgical, oncological and neurological outcomes.

Somatic mutations of thymic epithelial tumors with myasthenia gravis.

Background: Thymic epithelial tumors are rare malignant neoplasms that are frequently associated with paraneoplastic syndromes, especially myasthenia gravis. GTF2I is an oncogene mutated in a subgroup of thymomas that is reputed to drive their growth. However, for GTF2I wild-type tumors, the relevant mutations remain to be identified. Methods: We performed a meta-analysis and identified 4,208 mutations in 339 patients. We defined a panel of 63 genes frequently mutated in thymic epithelial tumors, which we used to design a custom assay for next-generation sequencing. We sequenced tumor DNA from 67 thymomas of patients with myasthenia gravis who underwent resection in our institution. Results: Among the 67 thymomas, there were 238 mutations, 83 of which were in coding sequences. There were 14 GTF2I mutations in 6 A, 5 AB, 2 B2 thymomas, and one in a thymoma with unspecified histology. No other oncogenes showed recurrent mutations, while sixteen tumor suppressor genes were predicted to be inactivated. Even with a dedicated assay for the identification of specific somatic mutations in thymic epithelial tumors, only GTF2I mutations were found to be significantly recurrent. Conclusion: Our evaluation provides insights into the mutational landscape of thymic epithelial tumors, identifies recurrent mutations in different histotypes, and describes the design and implementation of a custom panel for targeted resequencing. These findings contribute to a better understanding of the genetic basis of thymic epithelial tumors and may have implications for future research and treatment strategies.

Thymectomy in severe (Myasthenia Gravis Foundation of America classes IV-V) generalized myasthenia gravis: is the game really worth the candle?

OBJECTIVES: Total thymectomy in addition to medical treatment is an accepted standard therapy for myasthenia gravis (MG). Patients with severe generalized MG present life-threatening events, poor prognosis and higher risk of postoperative myasthenic crisis. The aim of our study is to investigate neurological and surgical results in patients with Myasthenia Gravis Foundation of America (MGFA) class IV and V MG following thymectomy. METHODS: Data on 76 MG patients with preoperative MGFA classes IV and V who underwent thymectomy were retrospectively collected. Primary end points included short-term surgical outcomes and long-term neurological results including the achievement of complete stable remission and any improvement as defined by MGFA Post-Intervention Status criteria. RESULTS: There were 27 (35.5%) males and 49 (64.5%) females; 53 (69.7%) were classified as MGFA class IV and 23 (30.3%) as class V. Thymectomy was performed through sternotomy in 25 (32.9%) patients, Video-assisted thoracic surgery (VATS) in 5 (6.6%) and Robot-assisted thoracic surgery (RATS) in 46 (60.5%). The median operative time was 120 (interquartile range: 95; 148) min. In-hospital mortality was observed in 1 (1.3%) patient and postoperative complications in 14 (18.4%) patients. The median postoperative hospital stay was 4 (interquartile range: 3; 6) days. Pathological examination revealed 31 (40.8%) thymic hyperplasia/other benign and 45 (59.2%) thymomas. Cumulative complete stable remission and improvement probabilities were 20.6% and 83.7% at 5 years and 66.9% and 97.6% at 10 years, respectively. A significant improvement rate was found in patients with age at the time of thymectomy of ≤50 years (P = 0.0236), MGFA class V (P = 0.0154) and acetylcholine receptor antibodies positivity (P = 0.0152). CONCLUSIONS: Thymectomy in patients with severe MG yields good perioperative outcomes and satisfactory long-term neurological improvement, especially for patients younger than 50 years, with MGFA class V and anti-AChR+ MG.

Preferential X Chromosome Inactivation as a Mechanism to Explain Female Preponderance in Myasthenia Gravis.

Myasthenia gravis (MG) is a neuromuscular autoimmune disease characterized by prevalence in young women (3:1). Several mechanisms proposed as explanations for gender bias, including skewed X chromosome inactivation (XCI) and dosage or sex hormones, are often involved in the development of autoimmunity. The skewed XCI pattern can lead to an unbalanced expression of some X-linked genes, as observed in several autoimmune disorders characterized by female predominance. No data are yet available regarding XCI and MG. We hypothesize that the preferential XCI pattern may contribute to the female bias observed in the onset of MG, especially among younger women. XCI analysis was performed on blood samples of 284 women between the ages of 20 and 82. XCI was tested using the Human Androgen Receptor Assay (HUMARA). XCI patterns were classified as random (XCI < 75%) and preferential (XCI ≥ 75%). In 121 informative patients, the frequency of skewed XCI patterns was 47%, significantly higher than in healthy controls (17%; p ≤ 0.00001). Interestingly, the phenomenon was observed mainly in younger patients (<45 years; p ≤ 0.00001). Furthermore, considering the XCI pattern and the other clinical characteristics of patients, no significant differences were found. In conclusion, we observed preferential XCI in MG female patients, suggesting its potential role in the aetiology of MG, as observed in other autoimmune diseases in women.

Identification of genetic risk loci and prioritization of genes and pathways for myasthenia gravis: a genome-wide association study.

Myasthenia gravis is a chronic autoimmune disease characterized by autoantibody-mediated interference of signal transmission across the neuromuscular junction. We performed a genome-wide association study (GWAS) involving 1,873 patients diagnosed with acetylcholine receptor antibody-positive myasthenia gravis and 36,370 healthy individuals to identify disease-associated genetic risk loci. Replication of the discovered loci was attempted in an independent cohort from the UK Biobank. We also performed a transcriptome-wide association study (TWAS) using expression data from skeletal muscle, whole blood, and tibial nerve to test the effects of disease-associated polymorphisms on gene expression. We discovered two signals in the genes encoding acetylcholine receptor subunits that are the most common antigenic target of the autoantibodies: a GWAS signal within the cholinergic receptor nicotinic alpha 1 subunit (CHRNA1) gene and a TWAS association with the cholinergic receptor nicotinic beta 1 subunit (CHRNB1) gene in normal skeletal muscle. Two other loci were discovered on 10p14 and 11q21, and the previous association signals at PTPN22, HLA-DQA1/HLA-B, and TNFRSF11A were confirmed. Subgroup analyses demonstrate that early- and late-onset cases have different genetic risk factors. Genetic correlation analysis confirmed a genetic link between myasthenia gravis and other autoimmune diseases, such as hypothyroidism, rheumatoid arthritis, multiple sclerosis, and type 1 diabetes. Finally, we applied Priority Index analysis to identify potentially druggable genes/proteins and pathways. This study provides insight into the genetic architecture underlying myasthenia gravis and demonstrates that genetic factors within the loci encoding acetylcholine receptor subunits contribute to its pathogenesis.

Robotic thymectomy for thymoma in patients with myasthenia gravis: neurological and oncological outcomes.

OBJECTIVES: The goal of this study was to analyse the outcomes in 53 patients with thymoma, 34 of whom had myasthenia gravis (MG), who were treated with robotic surgery. The oncological outcomes of the whole series of patients were analysed. Furthermore, because consistent data are not yet available in the literature, the main focus was the analysis of the neurological results of the patients affected by MG and thymoma. METHODS: The clinical outcomes of 53 patients with a diagnosis of thymoma who underwent robotic thymectomy between January 2014 and December 2019 in our institution were collected and evaluated; 34 of these patients had a concomitant diagnosis of MG. The neurological status of the patients was determined from a clinical evaluation according to the Osserman classification and on pre- and post-surgery Myasthenia Gravis Composite scores, whereas neurological clinical outcomes were assessed using the Myasthenia Gravis Foundation of America Post-Intervention Score. Reduction of steroid therapy was also considered. The recurrence rate, adjuvant radiotherapy and overall survival of the patients with a thymoma were evaluated. RESULTS: Neurological outcomes: improvement of the clinical conditions was obtained in 26 patients (76.5%) following the operation: complete stable remission was observed in 5 patients (14.7%), pharmacological remission in 10 (29.4%) and minimal manifestation in 11 (32.3%). Four patients (11.8%) exhibited no substantial change from the pretreatment clinical manifestations or reduction in MG medication and 4 (11.8%) patients experienced worsening of clinical conditions. In 21 patients (61.7%) a reduction of the dosage of steroid therapy was obtained. Oncological outcomes: at an average follow-up of 36 months, the overall survival was 96%, 4 patients (7.5%) had pleural relapses and 12 patients (22.6%) underwent postoperative radiotherapy, according to their stage. In accordance with Masaoka staging, 34% were in stage I, 56.6% in stage II and 9.4% in stage III. CONCLUSIONS: Our results suggest that robotic surgical treatment of patients with thymoma and concomitant MG is effective in improving the neurological outcomes. Moreover, the oncological results obtained in this series confirm the efficacy of robotic surgery for the treatment of thymic malignancies, with results in line with those of open surgery. However, due to the indolent growth of thymomas, further observations with longer follow-up are necessary.

Disappearance of Anti-Thyroid Autoantibodies following Thymectomy in Patients with Myasthenia Gravis.

OBJECTIVES: The thymus plays a central role in immune tolerance, which prevents autoimmunity. Myasthenia gravis (MG) is commonly associated with thymoma or thymus hyperplasia, and it can coexist with autoimmune thyroid diseases. However, the role of the thymus in thyroid autoimmunity remains to be clarified, which we investigated here. STUDY DESIGN: The study design entailed the inclusion of consecutive MG patients and the measurement of anti-thyroid autoantibodies at baseline and, limited to autoantibody-positive patients, also at 24 and 48 weeks. One hundred and seven MG patients were studied. The main outcome measure was the behaviour of anti-thyroglobulin autoantibodies (TgAbs) and anti-thyroperoxidase autoantibodies (TPOAbs) over time in relation to thymectomy. RESULTS: Serum TgAbs and/or TPOAbs were detected in ∼20% of patients in the absence of thyroid dysfunction. The prevalence of positive serum TgAbs and/or TPOAbs decreased significantly (p = 0.002) over the follow-up period in patients who underwent thymectomy, but not in patients who were not thymectomized. When the analysis was restricted to TgAbs or TPOAbs, findings were similar. On the same line, there was a general trend towards a reduction in the serum concentrations of anti-thyroid autoantibodies in patients who underwent thymectomy, which was significant for TPOAbs (p = 0.009). CONCLUSIONS: Our findings suggest a role of the thymus in the maintenance of humoral thyroid autoimmunity.

Anti-Neuronal IgG4 Autoimmune Diseases and IgG4-Related Diseases May Not Be Part of the Same Spectrum: A Comparative Study.

Background: IgG4 is associated with two emerging groups of rare diseases: 1) IgG4 autoimmune diseases (IgG4-AID) and 2) IgG4-related diseases (IgG4-RLD). Anti-neuronal IgG4-AID include MuSK myasthenia gravis, LGI1- and Caspr2-encephalitis and autoimmune nodo-/paranodopathies (CNTN1/Caspr1 or NF155 antibodies). IgG4-RLD is a multiorgan disease hallmarked by tissue-destructive fibrotic lesions with lymphocyte and IgG4 plasma cell infiltrates and increased serum IgG4 concentrations. It is unclear whether IgG4-AID and IgG4-RLD share relevant clinical and immunopathological features. Methods: We collected and analyzed clinical, serological, and histopathological data in 50 patients with anti-neuronal IgG4-AID and 19 patients with IgG4-RLD. Results: A significantly higher proportion of IgG4-RLD patients had serum IgG4 elevation when compared to IgG4-AID patients (52.63% vs. 16%, p = .004). Moreover, those IgG4-AID patients with elevated IgG4 did not meet the diagnostic criteria of IgG4-RLD, and their autoantibody titers did not correlate with their serum IgG4 concentrations. In addition, patients with IgG4-RLD were negative for anti-neuronal/neuromuscular autoantibodies and among these patients, men showed a significantly higher propensity for IgG4 elevation, when compared to women (p = .005). Last, a kidney biopsy from a patient with autoimmune paranodopathy due to CNTN1/Caspr1-complex IgG4 autoantibodies and concomitant nephrotic syndrome did not show fibrosis or IgG4+ plasma cells, which are diagnostic hallmarks of IgG4-RLD. Conclusion: Our observations suggest that anti-neuronal IgG4-AID and IgG4-RLD are most likely distinct disease entities.

Stereotactic body radiation therapy for the treatment of pleural metastases in patients with thymoma: a retrospective review of 22 patients.

BACKGROUND: Thymomas can benefit of cytoreductive surgery even if a complete resection is not feasible. The pleural cavity is the most common site of progression and the resection of pleural metastases can be performed in selected patients. We evaluated the results of stereotactic body radiation therapy for the treatment of pleural metastases in patients not eligible for surgery. METHODS: We retrospectively selected 22 patients treated with stereotactic body radiation therapy for pleural metastases between 2013 and 2019. According to RECIST criteria 1.1 modified for thymic epithelial tumors, time to local failure and progression free survival were calculated using Kaplan-Meier method. RESULTS: The median age was 40 years (range, 29-73 years). There were 1 A, 3 AB, 3 B1, 3 B2, 3 B2/B3 and 9 B3 thymomas. Pleural metastases and primary tumor were synchronous in 8 patients. Five patients had a single pleural metastatic site and 17 presented multiple localizations. Sixteen patients received stereotactic body radiation therapy on multiple sites of pleural metastases. The median dose of radiation was 30 Gy (range, 24-40 Gy). With a median follow-up of 33.2 months (95% CI: 13.1-53.3 months), ten patients experienced disease progression with a median progression free survival was 20.4 months (95% CI: 10.7-30.0 months). The disease control rate was 79% and 41% after 1 and 2 years, respectively. Local disease control rate was 92% and 78% after 1 and 2 years, respectively. There were not significant differences in progression free survival between patients diagnosed with synchronous and metachronous metastases (P=0.477), across those treated or not with chemotherapy (P=0.189) and between those who received or not a previous surgical resection of the pleural metastases (P=0.871). There were not grade 3-4 toxicities related to the treatment. CONCLUSIONS: Stereotactic body radiation therapy of pleural metastases is feasible and offers a promising local control of diseases. The impact of this treatment on patients' survival is hardly predictable because of the heterogeneous clinical behavior of thymomas.

Thymoma-associated myasthenia gravis: Clinical features and predictive value of antiacetylcholine receptor antibodies in the risk of recurrence of thymoma.

BACKGROUND: Thymoma-associated myasthenia gravis (TAMG) is one of the subtypes of myasthenia gravis with autoantibodies against the acetylcholine receptor (AChR-Ab). We analyzed the clinical features of our cohort of TAMG patients and the changes in AChR-Ab titer before and after thymectomy in order to identify factors predicting thymoma relapses. METHODS: We retrospectively assessed: age of MG onset, MG clinical status according to MGFA (Myasthenia Gravis Foundation of America), epoch of thymectomy, post-thymectomy status, oncological features and surgical approach. AChR-Ab dosages were measured both before and after thymectomy. Linear regression models were applied to identify clinical determinants of AChR-Ab titers and the Cox regression model was fitted to estimate the factors associated with the risk of thymoma recurrence. RESULTS: The study sample included 239 MG patients, 27 of whom experienced one or more recurrences (median follow-up time: 4.8 years). The AChR-Ab titers decreased after first thymectomy (P < 0.001); the decrease was more pronounced in female patients (P = 0.05), in patients diagnosed with MG at an older age (P = 0.003), and in those who had lower MG stage before surgery (P = 0.02) or higher Masaoka-Koga stage (P = 0.005). The risk of relapse was closely linked with the age of the patient, the Masaoka-Koga stage and the surgical approach. CONCLUSIONS: Presurgery levels of AChR-Ab or their change after surgery were not associated with thymoma recurrence. The reduction of AChR-Ab titers after thymectomy confirms an immunological role of thymoma in the pathogenesis of MG. KEY POINTS: Significant findings of the study: Young MG patients with an advanced Masaoka staging score of the primary tumor who underwent thymectomy with approaches different from sternotomy and VATS should be monitored for high risk of recurrence. WHAT THIS STUDY ADDS: No other study has ever investigated the changes in AChR-Ab titers before and after thymectomy in a large cohort of TAMG patients. The reduction of AChR-Ab titers after thymectomy suggests an immunological role of thymoma in the pathogenesis of MG.

Investigation of MLH1, MGMT, CDKN2A, and RASSF1A Gene Methylation in Thymomas From Patients With Myasthenia Gravis.

A feature of thymomas is their frequent association with myasthenia gravis (MG), an autoimmune disease characterized by the production of autoantibodies directed to different targets at the neuromuscular junction. Indeed, almost 30-40% of thymomas are found in patients with a type of MG termed thymoma-associated MG (TAMG). Recent studies suggest that TAMG-associated thymomas could represent a molecularly distinct subtype of thymic epithelial tumors (TETs), but few data are still available concerning the epigenetic modifications occurring in TAMG tissues. The promoter methylation levels of DNA repair (MLH1 and MGMT) and tumor suppressor genes (CDKN2A and RASSF1A) have been frequently investigated in TETs, but methylation data in TAMG tissues are scarce and controversial. To further address this issue, we investigated MLH1, MGMT, CDKN2A, and RASSF1A methylation levels in blood samples and surgically resected thymomas from 69 patients with TAMG and in the adjacent normal thymus available from 44 of them. Promoter methylation levels of MLH1, MGMT, CDKN2A, and RASSF1A genes were not increased in cancer with respect to healthy tissues and did not correlate with the histological or pathological features of the tumor or with the MG symptoms. The present study suggests that hypermethylation of these genes is not frequent in TAMG tissues.

Investigation of GHSR methylation levels in thymomas from patients with Myasthenia Gravis.

BACKGROUND: Hypermethylation of the growth hormone secretagogue receptor gene (GHSR) is increasingly observed in human cancers, suggesting that it could represent a pan-cancer biomarker of clinical interest. However, little is still known concerning GHSR methylation levels in thymic epithelial tumors, and particularly in thymomas from patients with Myasthenia Gravis (TAMG). MATERIAL AND METHODS: In the present study we collected DNA samples from circulating lymphocytes and surgically resected tumor tissues of 65 TAMG patients, and from the adjacent healthy thymic tissue available from 43 of them. We then investigated GHSR methylation levels in the collected tissues searching for correlation with the clinical characteristics of the samples. RESULTS: GHSR hypermethylation was observed in 18 thymoma samples (28%) compared to the healthy thymic tissues (P < 1 × 10-4), and those samples were particularly enriched in advanced disease stages than stage I (94% were in stage II or higher). GHSR was demethylated in the remaining 47 thymomas, as well as in all the investigated healthy thymic samples and in circulating lymphocytes. CONCLUSIONS: GHSR hypermethylation is not a pan-cancer marker or an early event in TAMG, but occurs in almost 1/4 of them and mainly from stage II onward. Subsequent studies are required to clarify the molecular pathways leading to GHSR hypermethylation in TAMG tissues and their relevance to disease progression.

The thymidylate synthase enhancer region (TSER) polymorphism increases the risk of thymic lymphoid hyperplasia in patients with Myasthenia Gravis.

BACKGROUND: Myasthenia Gravis (MG) is caused, in approximately 80% of the patients, by autoantibodies against the nicotinic acetylcholine receptor (AChR). The disease is often associated with pathological changes of the thymus: thymic epithelial tumours are present in about 10-20% of the patients, while up to 80% of the patients with early disease onset have thymic hyperplasia. Folate metabolism is required for the production of DNA precursors and for proper DNA methylation reactions, and impaired folate metabolism has been often associated with cellular growth and cancer. METHODS: We investigated if major polymorphisms of folate-related genes, namely MTHFR c.677C>T, MTR c.2756A>G, MTRR c.66A>G and TYMS TSER (a 28-bp tandem repeat in the 5' promoter enhancer region of TYMS) increase the risk of pathological changes of the thymus in AChR+ MG patients. A total of 526 AChR+ MG patients, including 132 patients with normal (involuted) thymus, 146 patients with thymic hyperplasia, and 248 patients with a thymoma were included in the study. Allele and genotype comparisons were performed among the three study groups, after correcting for multiple testing. RESULTS: The frequency of the TYMS TSER 3R allele was significantly higher in MG patients with thymic hyperplasia (P=0.004), and the TYMS TSER 3R3R genotype was significantly associated with increased risk of thymic hyperplasia [OR 2.71 (95% CI: 1.34-5.47)]. CONCLUSIONS: The 3R allele in the thymidylate synthase promoter enhancer region results in increased protein production, required for the synthesis of DNA precursors. The present study suggests that the TYMS TSER 3R allele increases the risk of thymic lymphoid hyperplasia in AChR+ MG patients.