Role of CDKN2A deletion in grade 2/3 IDH-mutant astrocytomas: need for selective approach in resource-constrained settings

Molecular neuropathology: an essential and evolving toolbox for the diagnosis and clinical management of central nervous system tumors

Molecular profiling has transformed the diagnostic workflow of CNS tumors during the last years. The latest WHO classification of CNS tumors (5th edition), published in 2021, pushed forward the integration between histopathological features and molecular hallmarks to achieve reproducible and clinically relevant diagnoses. To address these demands, pathologists have to appropriately deal with multiple molecular assays mainly including DNA methylation profiling and DNA/RNA next generation sequencing. Tumor classification by DNA methylation profiling is now a critical tool for many diagnostic tasks in neuropathology including the assessment of complex cases, to evaluate novel tumor types and to perform tumor subgrouping in hetereogenous entities like medulloblastoma or ependymoma. DNA/RNA NGS allow the detection of multiple molecular alterations including single nucleotide variations, small insertions/deletions (InDel), and gene fusions. These molecular markers can provide key insights for diagnosis, for example, if a tumor-specific mutation is detected, but also for treatment since targeted therapies are progressively entering the clinical practice. In the present review, a brief, but comprehensive overview of these tools will be provided, discussing their technical specifications, diagnostic value, and potential limitations. Moreover, the importance of molecular profiling will be shown in a representative series of CNS neoplasms including both the most frequent tumor types and other selected entities for which molecular characterization plays a critical role.

CDKN2A/B deletion in IDH-mutant astrocytomas: An evaluation by Fluorescence in-situ hybridization

INTRODUCTION

CDKN2A/B homozygous deletion is one of the defining features of grade 4 in IDH-mutant astrocytic tumours.

AIM

To evaluate CDKN2A/B-deletion in IDH-mutant astrocytic tumours and its clinicopathological impact.

MATERIALS AND METHODS

CDKN2A/B-deletion was evaluated by Fluorescence in-situ hybridisation (FISH) and interpreted by two recently accepted methods.

RESULTS

Eighty-three out of 94 cases (histologically-grade 2: 3, grade 3: 46, grade 4: 34) were interpretable on FISH. Concordant CDKN2A/B-deletion was observed in 71% (27/38) of lower-grade tumours (n = 49) and 90% (27/30) of histological grade 4 tumours (n = 34). Both the interpretation methods showed good agreement (Kappa = 0.75). CDKN2A/B-deletion showed an inverse correlation for < 10% MIB-1 labeling index (p = 0.01) while that by method-2 showed a significant correlation for grade 4 (p = 0.02). No significant correlation was observed for any other clinicopathological parameters. Twenty-four patients showed progression/recurrence (including deaths), and no significant difference in frequency of CDKN2A/B deletion was observed among cases with disease progression across different histological grades.

CONCLUSIONS

CDKN2A/B-deletion was observed across all the histological grades of IDH-mutant astrocytic tumours, expectedly more in the higher grade. FISH, as a method, can be used for the detection of CDKN2A/B homozygous deletion, when there is concordant interpretation.

Molecular prognostication in grade 3 meningiomas and p16/MTAP immunohistochemistry for predicting CDKN2A/B status

Background

The World Health Organization 2021 classification introduces molecular grading criteria for anaplastic meningiomas, including TERT promoter (TERTp) mutations and CDKN2A/B homozygous deletion. Additional adverse prognostic factors include H3K27me3 and BAP1 loss. The aim of this study was to explore whether these molecular alterations stratified clinical outcomes in a single-center cohort of grade 3 meningiomas. Additionally, we examined whether p16 and MTAP immunohistochemistry can predict CDKN2A/B status.

Methods

Clinical and histopathological information was obtained from the electronic medical records of grade 3 meningiomas resected at a tertiary center between 2007 and 2020. Molecular testing for TERTp mutations and CDKN2A/B copy-number status, methylation profiling, and immunohistochemistry for H3K27me3, BAP1, p16, and methylthioadenosine phosphorylase (MTAP) were performed. Predictors of survival were identified by Cox regression.

Results

Eight of 15 cases demonstrated elevated mitotic index (≥20 mitoses per 10 consecutive high-power fields), 1 tumor exhibited BAP1 loss, 4 harbored TERTp mutations, and 3 demonstrated CDKN2A/B homozygous deletion. Meningiomas with TERTp mutations and/or CDKN2A/B homozygous deletion showed significantly reduced survival compared to anaplastic meningiomas with elevated mitotic index alone. Immunohistochemical loss of p16 and MTAP demonstrated high sensitivity (67% and 100%, respectively) and specificity (100% and 100%, respectively) for predicting CDKN2A/B status.

Conclusions

Molecular alterations of grade 3 meningiomas stratify clinical outcomes more so than histologic features alone. Immunohistochemical loss of p16 and MTAP show promise in predicting CDKN2A/B status.

P16 immunohistochemistry is a sensitive and specific surrogate marker for CDKN2A homozygous deletion in gliomas

Molecular characterization of gliomas has uncovered genomic signatures with significant impact on tumor diagnosis and prognostication. CDKN2A is a tumor suppressor gene involved in cell cycle control. Homozygous deletion of the CDKN2A/B locus has been implicated in both gliomagenesis and tumor progression through dysregulated cell proliferation. In histologically lower grade gliomas, CDKN2A homozygous deletion is associated with more aggressive clinical course and is a molecular marker of grade 4 status in the 2021 WHO diagnostic system. Despite its prognostic utility, molecular analysis for CDKN2A deletion remains time consuming, expensive, and is not widely available. This study assessed whether semi-quantitative immunohistochemistry for expression of p16, the protein product of CDKN2A, can serve as a sensitive and a specific marker for CDKN2A homozygous deletion in gliomas. P16 expression was quantified by immunohistochemistry in 100 gliomas, representing both IDH-wildtype and IDH-mutant tumors of all grades, using two independent pathologists' scores and QuPath digital pathology analysis. Molecular CDKN2A status was determined using next-generation DNA sequencing, with homozygous CDKN2A deletion detected in 48% of the tumor cohort. Classifying CDKN2A status based on p16 tumor cell expression (0-100%) demonstrated robust performance over a wide range of thresholds, with receiver operating characteristic curve area of 0.993 and 0.997 (blinded and unblinded pathologist p16 scores, respectively) and 0.969 (QuPath p16 score). Importantly, in tumors with pathologist-scored p16 equal to or less than 5%, the specificity for predicting CDKN2A homozygous deletion was 100%; and in tumors with p16 greater than 20%, specificity for excluding CDKN2A homozygous deletion was also 100%. Conversely, tumors with p16 scores of 6-20% represented gray zone with imperfect correlation to CDKN2A status. The findings indicate that p16 immunohistochemistry is a reliable surrogate marker of CDKN2A homozygous deletion in gliomas, with recommended p16 cutoff scores of ≤ 5% for confirming and > 20% for excluding biallelic CDKN2A loss.