An immunohistochemical study of XIAP expression in pleomorphic adenoma and carcinoma ex pleomorphic adenoma

X-linked inhibitor of apoptosis protein (XIAP) is a member of the inhibitor of apoptosis proteins family of caspase inhibitors. Expression of XIAP in various neoplasms has been associated with aggressive behavior. The biological progression from pleomorphic adenoma (PA) to carcinoma ex pleomorphic adenoma (CXPA) has been poorly understood. We studied XIAP expression by immunohistochemistry in PA and CXPA.

Formalin-fixed, paraffin-embedded representative sections of 14 cases of PA and seven cases of CXPA (four invasive and three intracapsular) were stained with anti-XIAP (# 610763; BD Biosciences, San Jose, CA, USA) following citrate-based antigen retrieval. Granular cytoplasmic staining was considered positive and intensity was assessed from weak (1+) to strong (3+). PAs were morphologically evaluated for cellularity, cytological atypia and mitotic activity.

Of the seven PAs composed mostly of myxohyaline stroma with scattered ductal elements, two tumors showed no staining and five showed rare (<1%) 1+ positive cells. Of seven more cellular PAs, five had sheets of tumor cells comprising more than 50% of the tumor and two had sheets comprising more than 80% of the tumor (cellular PA), focal to diffuse 2+ to 3+ staining was observed. Tumor cells with strong staining often exhibited cytological atypia in the form of nuclear enlargement and contour irregularity, prominent nucleoli and eosinophilic cytoplasm. Mitotic activity was occasionally seen in cellular areas expressing XIAP. All cases of CXPA demonstrated diffuse 3+ staining in the carcinomatous component and 1+ to focally 3+ staining in cellular areas of the underlying PA.

Increased expression of XIAP from PA to cellular PA to CXPA and in atypical cells within cellular areas of PA adds to our growing understanding of defective apoptotic pathways in malignant transformation in this group of salivary gland tumors and suggests an adenoma to adenocarcinoma model of progression. Further correlation with other oncogene expression may provide insight into the multiple molecular pathways that are affected in these tumors. Targeted therapy of XIAP may play a future role in the management of CXPA.