PRAME Expression in Junctional Melanocytic Proliferations of the Conjunctiva: A Potential Biomarker for Primary Acquired Melanosis/Conjunctival Melanocytic Intraepithelial Lesions.

Conjunctival melanocytic proliferations are diagnostically challenging, often complicated by small specimen size, and are separated into 3 broad categories. The first group includes benign nevi and primary acquired melanosis (PAM) without atypia. The second group includes junctional melanocytic proliferations with a risk of progression to invasive melanoma (PAM with atypia). The last category is conjunctival melanoma, of which 65% of tumors arise in the setting of PAM with atypia. Preferentially expressed antigen in melanoma (PRAME) immunohistochemistry has been widely adopted to differentiate cutaneous nevi and melanoma. However, there are limited studies on its utility in the evaluation of conjunctival melanocytic proliferations with little data regarding its potential utility in stratifying PAM. Twenty-eight clinically annotated cases (14 PAM without atypia and 14 PAM with atypia) were retrospectively evaluated with PRAME/MART-1 duplex immunohistochemistry and were assigned the commonly used PRAME immunoreactivity score: 0 for no staining, 1+ for 1%-25% of cells positive, 2+ for 26%-50%, 3+ for 51%-75%, and 4+ for >75%. PAM without atypia showed low (0-3+) PRAME expression in 14 of 14 cases (100%). PAM with atypia showed strong and diffuse (4+) PRAME expression in 12 of 14 cases (86.7%). Seven of eight (87.5%) PAM with severe atypia, 4 of 4 PAM (100%) with moderate atypia, and 1 of 2 PAM (50%) with mild atypia showed 4+ PRAME expression. In addition, all 5 cases that recurred or progressed (all classified as PAM with atypia) showed 4+ PRAME expression. Although additional larger studies are needed, PRAME seems to be a useful adjunct in evaluating junctional melanocytic proliferations of the conjunctiva.