Association of somatic mutations and histologic subtype/grade on prognosis and PD-L1 expression in mesothelioma

8507 Background: Mesothelioma includes epithelioid, sarcomatoid, and biphasic histologic subtypes, and the epithelioid subtype is histologically graded based on mitotic count and nuclear atypia. Correlation between the most common somatic mutations ( BAP1, NF2, TP53, CDKN2A), and overall survival is poorly understood. High PD-L1 expression is a poor prognostic factor for survival and may be a predictor of response to immunotherapy. Few studies have evaluated the relationships between these factors. Methods: We performed a retrospective study in 217 patients with pleural, peritoneal and bicavitary mesothelioma as part of an IRB-approved biobanking protocol. Histologic subtype, grade of epithelioid mesothelioma, tumor stage, and overall survival time were determined. Next-generation DNA sequencing (NGS) was used to identify the prevalence of somatic mutations. PD-L1 immunohistochemistry was obtained on 184 samples and quantified as percentage of tumor cells. Histologic subtype, nuclear grade, clinical stage, PD-L1 expression, survival time, and the prevalence of each mutation were compared using chi-squared, Kruskal-Wallis, or Cox proportional hazard tests. All p-values were determined in R version 4.2.0 using a two-sided alpha error of 0.05 to mark statistical significance. Results: The most frequent somatic mutations were found in BAP1 (99/217, 45.6%), CDKN2A (47/217, 21.7%), TP53 (37/217, 17.1%), and NF2 (31/217, 14.3%). Patients with CDKN2A-mutated mesothelioma experienced shorter survival (p=0.032). TP53 mutations were associated with higher nuclear grade (p=0.015), higher mitotic count (p=0.013), and greater nuclear atypia (p=0.015). TP53 mutations were more prevalent in pleural than peritoneal mesothelioma (p=0.016) and in tumors without lymph node metastasis (p=0.05). As expected, patients with epithelioid mesothelioma had superior overall survival (p=0.02) and their tumors had lower PD-L1 expression (p=0.007) than either sarcomatoid or biphasic mesotheliomas. Neither PD-L1 expression nor mutations in BAP1, NF2, CDKN2A, or TERT were associated with nuclear grade, atypia, or mitotic count. No association was found between BAP1 or NF2 mutations and tumor stage, lymph node metastasis, or PD-L1 expression. Conclusions: To our knowledge, this is the first study of this scale to correlate PD-L1 expression, somatic mutation data, histologic features, and survival in mesothelioma patients. The most significant findings were that patients with CDKN2A mutations had shorter survival and TP53 mutations, which were more prevalent in pleural mesotheliomas, were associated with higher nuclear grade, higher mitotic count, and greater nuclear atypia. An ongoing analysis evaluates the contribution of germline mutations, which occurred in a significant proportion (15.7%) of this mesothelioma patient cohort.