PRAME (EPR20330) in dermatopathology

PRAME is involved in the regulation of diverse cell biological processes (Fig.1). Based on its categorization as Cancer/Testis Antigen (CTA), it is found almost exclusively in testis, endometrium, placenta, adrenal gland and ovary, while it is absent in other organs. In the pathological context, however, it is found in primary and metastatic melanoma, with the exception of desmoplastic melanoma. [3] Immunohistochemical staining shows mostly diffusely positive tumor tissue. Since normal skin is usually negative for PRAME, detection of PRAME protein is useful as an additional tool to determine the tumor margin. In addition, PRAME is an excellent and reliable marker to differentiate between melanocytic nevi and melanoma. The more dedifferentiated (from atypical nevus to metastatic melanoma) the melanocytes are, the lower the p16 expression rate and the higher the PRAME expression. Thus, approximately 90% of primary invasive melanoma are positive for PRAME, while only 9% are p16 negative. Thus, a cocktail of both markers is ideal for more precise diagnosis. [3] Also, in differentiating nodal nevi from nodal melanoma in sentinel lymph nodes, detection of PRAME shows a significantly higher sensitivity and specificity than the loss of p16. [4] Kaczorowski et al., observe in their study that PRAME is expressed in some melanoma that contain the typical BRAF or NRAS mutations but are negative for the classical markers such as S100 and SOX10. [10] In addition, there is a high level of agreement between results from PRAME IHC and corresponding cytogenetic studies. [5] In mucosal melanoma, strong expression of the marker correlates with poor prognosis.[6] PRAME is an independent prognostic biomarker in uveal melanoma. Here, it is used to identify a higher risk of metastasis in patients with grade 1 categorized tumors. [7, 8] Data suggest that PRAME may represent a potential target for immunotherapy. [9] Recently, data from an ongoing phase 1 cell therapy trial was published (Immatics Press Release, 10/10/2022).

PRAME is not exclusive to melanocytic tumors, but it is also expressed in several other tumor entities, such as rhabdomyosarcoma, breast carcinoma, and Hodgkin's/non-Hodgkin's lymphoma. [9] Only recently data has been published on the expression of PRAME in normal tissue and in more than 5,800 different tumors in the American Journal of Surgical Pathology. [10] In contrast to previous mRNA-based studies, this work is based on immunohistochemical experiments. Because immunohistochemistry is a reliable and comparatively inexpensive method, the authors discuss the detection of PRAME in diverse tumor entities with regard to potential immunotherapy.

A paper by Takata et al., 2022 demonstrates a dual function of PRAME in the pathogenesis of Diffuse Large B-Cell Lymphoma (DLBCL). [11] The absence of PRAME resulted in T cell immune escape. This can be reversed by inhibition of the repression complex with restoration of the PRAME protein. [11] This mechanism is discussed in terms of potential therapeutic approaches in certain lymphoma patients.