Abstract
Nodular fasciitis (NF) is a self-limited tumorous lesion occurring in the upper as well as lower extremities. NF is composed of a proliferation of "primary culture"-like myofibroblastic cells with nuclear atypia and large nucleoli, thus mimicking sarcoma. NF harbors a promoter-swapping fusion gene containing the entire coding region of USP6 gene. Therefore, NF is a tumor with a fusion oncogene but self-limited. In order to explore why NF is self-limited, we examined whether myofibroblastic cells in NF express p16 protein, a gene product of CDKN2A gene and an inhibitor of cyclin-dependent kinase 4 (CDK4) as well as one of the hallmarks of cellular senescence. We immunohistochemically demonstrated strong and diffuse expression of p16 in myofibroblastic cells in 11 out of 15 cases of NF, and strong but partial expression in the remaining 4 of the cases. We also showed that 15 out of 15 cases of NF were immunohistochemically negative or only showed focal and faint immunopositivity for CDK4, murine double minute 2 (MDM2), and TP53 proteins. Furthermore, there were no significant changes in the copy number of CDKN2A, CDK4 and MDM2 genes, and no significant mutations in TP53, RB1, and CDKN2A genes in 1 case of NF selected. These data suggest a possible involvement in cell cycle arrest and presumed cellular senescence by p16 in myofibroblastic cells in NF. This may explain the self-limited as well as inflammatory nature of NF as a senescence-associated secretory phenotype.