Decreased expression of cyclin G2 is significantly linked to the malignant transformation of papillary carcinoma of the thyroid.

Background: Cyclin G2 is a novel cyclin negatively regulating the cell cycle progression, contrary to the characteristics of conventional cyclins. However, little is known about the cyclin G2 expression in human carcinomas. We thus investigated cyclin G2 expression in human thyroid neoplasms.

Materials And Methods: We immunohistochemically examined cyclin G2 expression in 40 normal thyroids and 80 thyroid neoplasms.

Results: Normal thyroids expressed cyclin G2 in more than 5% of follicular cells. Of 30 papillary carcinomas including 6 microcarcinoma, cyclin G2 expression was not, or only occasionally, observed in carcinoma cells, indicating its expression decreased in all these cases. On the other hand, in 16 of the 24 follicular adenomas (66.7%) and 5 of the 23 follicular carcinomas (21.7%), cyclin G2 expression was retained (more than 5% of neoplastic cells were positive), and adenomas more frequently (p = 0.0032) retained cyclin G2 expression than carcinomas.

Conclusion: Our results suggest that lack of cyclin G2 plays an important role in the malignant transformation of papillary carcinoma. Also, it may play an adjuvant role in the transformation of follicular adenoma to carcinoma. This is the first study of the expression of cyclin G2, a novel cyclin having a role opposite to that of conventional cyclins, in human carcinoma.