Thymomas were neoplasms arising from or exhibiting differentiation toward thymic epithelial cells. It has been reported that different subtypes of thymoma have different genetic characteristics, recent studies indicated that chromosomal 1 gain plays an important role in molecular genetic mechanism of thymic epithelium tumors [10, 23–25].
C-Jun (cellular Jun), a member of nucleus transcription factor, is an oncogene locating on chromosome 1. It was indicated that the expression of c-Jun immunohistochemistry can reveal the mRNA level of c-Jun . In this project, expression of c-Jun in 22 from 60 (36.7%) TETs were observed, which specifically located on cell nucleus. Statistical test showed that the abnormal expression of c-Jun was significantly higher in thymic epithelium tumors than that in normal tissue controls. There were also statistical differences of c-Jun positive expression in different subtypes of thymoma. Among those TETs, including thymic carcinomas, type B3, and type B2 thymomas took the higher percentage immune reaction (more than 40%) of c-Jun. Our results were consistent with Sasaki’s research , indicating a strong positive expression of c-Jun might correlate with high grade TETs. Therefore, we speculated that c-Jun might be regard as a potential positive regulator of cell reproduction [26, 27], or might play an important role in the process of tumor differentiation. Besides, proliferation index Ki-67 was increase in type B3 thymoma cells. In our project, there were statistical differences of c-Jun expression in different clinical stages of thymic epithelium tumors: advanced thymomas (III + IV) were significantly higher than those of the early thymomas (I + II); and there was no statistical difference between stage I and II; neither between stage III and stage IV. This research showed that the expression of c-Jun increased in invasive thymic tumors, which also suggested that c-Jun might be used to help judging the biological behaviors, clinical stage, and prognosis of tumors.
N-ras is one of the ras gene family members locating on chromosome 1. N-ras function as an important factor in the process of cell proliferation, senescence, immortalization and carcinogenesis. N-ras can also inhibit the cancer cells proliferation by Suv39h1 and H3K9 methylation. Mutational ras protein can affect cell proliferation, cell cycle regulation and anti-apoptotic signal by decreasing the activity of endogenous GTPase, or transcriptional decreasing the expression of Fas receptor and regulating the last time of the p38 activity of Jun N-terminal protein kinase (JNK) by Ral-GEF (Ras related GTPase-guanine exchange factor) pathway  Meanwhile, N-ras has different function to the generation of cancer in different individuals[30–32]. In our study, N-ras expression was found in 12 cases of TETs, however, no significant difference was observed between thymic epithelium and normal tissue controls, similar results existed between the subtypes of thymic epithelium tumors. The results suggested that N-ras might play a role in the generation of thymic epithelium tumors. However, it will be hard to indicate the relationship of N-ras expression in different subtypes of thymic epithelium tumors due to the small sample size of the current study. Further studies are needed to confirm these observations and to determine the mechanism of N-ras in the origin and development of TETs. No statistical differences were detected in N-ras expression of different clinical stages of thymic epithelium tumors.
Caspase-9 gene locates on 1p36.3-p36.1. It precipitates in mitochondria induced cell senescence pathway . Several studies indicated that decreasing Caspase-9 transcription and translation are detected in head and neck squamous cell carcinoma , and leukemia . In our project, 30 out of 60 (50%) thymic epithelial neoplasms have positive Caspase-9 expression, which was slightly lower than the Caspase-9 expression in normal control tissues (7/11, 63.6%). But the difference was not statistically significant. The result indicated that there were significant differences among Caspase-9 over expressions in different subtypes of thymic epithelium tumors. In different subtype, the expression of Caspase-9 in thymic epithelium tumors mainly existed in thymomas constructed by bland epithelial cells, including type A and metaplastic thymoma. Caspase-9 presented a lower expression in type B thymoma and thymic carcinoma than in type A and metaplastic thymoma, which was consistent with previous research [33, 34]. The decreasing tendency of Caspase-9 transcription and translation indicated that the interruption of Caspase-9 related apoptosis signaling pathways might promote the generation of type B thymoma and thymic carcinoma. However, in the current study, we first divided TETs into two groups as described above (low-grade TETs and high-grade TETs), and found that Caspase-9 presented relatively lower expression in low-grade TETs when compared with that in high-grade TETs. The mechanism was not clear, and need more future studies to validate our results. No statistical differences are detected in Caspase-9 expression of different clinical stages of thymic epithelium tumors.
p73 gene locates on human chromosome 1p36.33. Many isomers of p73 were identified, and the expression and interaction of those different isomers involved the process of regulate transcription and growth inhibition [35, 36]. The fact that p73 abnormal expression was observed more common in cancer tissues than in normal tissue indicated that p73 might be an oncogene . In this project, over-expression of p73 in thymic epithelium tumors was significantly higher than that in normal control tissue. This result suggested that the expression of p73 increased in thymic epithelium neoplasm, which was similar with the previous research on digestive system tumor . The p73 protein detected by immunohistochemical methods were probably wild type. The positive expression of p73 in different subtypes of thymic epithelium tumors existed statistical differences. In addition, our results indicated that p73 presented with similar positive expression levels in type B2, B3 thymoma and thymic carcinoma, which were significantly higher than other subtypes of thymoma. This conclusion suggested that p73 might play an important role in type B2, B3 thymoma and thymic carcinoma. It was also revealed that the molecular change of type B2, B3 thymoma might be similar with thymic carcinoma, and differ from other types of thymoma. Considering previous research results of our group that p53 protein positive expression increased in type B3 thymoma , it can be inferred that p73 and p53 protein mutants might embrace a synergistic effect in thymoma. However, the limitation of the follow-up data was too small to make such conclusion, the further analysis between p73 expression and prognosis need more data from the future follow-up data.
In summary, the results indicated that c-Jun and p73 expressed significantly higher in thymic epithelium tumors than in normal control tissues. c-Jun and p73 also had similar positive expression level in high-grade TETs, which is significantly higher than low-grade TETs. In addition, Caspase-9 expression was relative lower in type B thymoma and thymic carcinoma. However, no significant difference of N-ras expression among different tissues of the thymus and different thymic epithelium tumors was observed. What we observed suggested that different genes on chromosome 1 might employ different functions in the generation and development of thymic epithelium tumors. c-Jun and p73 may promote the tumor formation. Previous studies of our group also suggested that chromosome 1 gain was significantly higher in thymic epithelium tumors than normal thymus tissue, and it was also higher in type B3 thymic epithelium tumors than other subtypes of thymoma . It is highly possible that type B3 thymoma has a different molecular change with other types of thymoma, and similar with thymic carcinoma. Those evidences suggested that type B3 thymoma should be distinguished from other subtypes of thymoma and might be classified as a intermediate malignant tumor, which needs more future studies to validate our results before they can have widespread application. Meanwhile, The use of a combination of c-Jun,p73 and Caspase-9 could help differential diagnosing.