P18.06.b evaluation of p16 and mtap immunohistochemical expression as surrogate markers of cdkn2a/b deletion in meningiomas

Abstract BACKGROUND Homozygous deletion (HD) of CDKN2A/B genes (chr. 9p21) is a molecular hallmark for meningiomas grading. However, testing is expensive and time-consuming. Immunohistochemical (IHC) evaluation of the CDKN2A (p16) and adjacent MTAP gene products has been proposed as a diagnostic surrogate of CDKN2A HD, with unsatisfactory or inconclusive results. MATERIAL AND METHODS CDKN2A status was assessed by FISH with the Zytolight SPEC CDKN2A/CEN 9 Dual Color probe (Zytovision©) in 68 meningiomas from 56 patients. A 15% cut-off of HD cells was used to define a sample as deleted. Negative (<10% positive cells) p16 (Ventana) and MTAP (Clone 2G4; Abnova) IHC were considered as a surrogate for CDKN2A HD. Since CDKN2A/B deletion in meningiomas is characterized by spatial and longitudinal heterogeneity, the same area selected for FISH analysis was assessed by IHC. Pearson Chi-square test for categorical variables and survival analysis with log-rank test and the Kaplan-Meier method were performed using the Stata/MP 17.0 software. RESULTS In our series, 29/56 (51.7%) patients were male, and median age at diagnosis was 69 years. Median overall survival (OS) from diagnosis was 37.9 months (27 months for HD cases and 39.3 months for non-HD). 51 (75%) samples were diagnosed as WHO grade 3 meningiomas, 11 (16.2%) as grade 2 and 6 (8.8%) as grade 1. 11 cases (16.2%) were CDKN2A-deleted by FISH. p16 IHC was negative in 25 (36.8%) cases, while MTAP IHC was negative in 24 (35.3%) cases. No correlation between these markers was observed (Cohen’s κ=-0.0202, p=0.7158). 5/11 (45.4%) deleted samples were negative for p16 staining, while 37/57 (64.9%) non-deleted samples displayed p16 positivity (sensitivity: 20%; specificity: 86.1%). MTAP IHC was negative in 8/11 (72.7%) deleted samples, whereas 41/57 (71.9%) non-deleted samples retained MTAP expression (sensitivity: 33.3%; specificity: 93.2%). Unlike p16 IHC, MTAP loss correlated with CDKN2A HD (p=0.005). To address intratumoral heterogeneity, FISH analysis was repeated on 2 samples with markedly variable p16 and MTAP stainings. For each sample, one positive and one negative area were analyzed, observing a correlation between IHC and HD FISH status. CONCLUSION MTAP IHC showed a higher correlation with CDKN2A HD compared to p16 IHC in meningiomas, even though the low sensitivity hinders its use in routine diagnostics. Nevertheless, the use of p16/MTAP IHC to select areas for FISH analysis could be considered to address intratumoral heterogeneity of CDKN2A status.