Glycoprotein nonmetastatic melanoma protein b immunohistochemistry can be a useful ancillary tool to diagnose subependymal giant cell astrocytoma

Subependymal giant cell astrocytoma (SEGA) is a World Health Organization Central Nervous System grade 1 tumor, strongly associated with tuberous sclerosis complex (TSC). Recent research indicates that Glycoprotein Nonmetastatic Melanoma Protein B (GPNMB), regulated by microphthalmia (MiT) family transcription factors may also be modulated by loss-of-function mutations in TSC1/2. We evaluated GPNMB as a diagnostic marker of subependymal giant cell astrocytoma (SEGA). A total of 11 patients with SEGA were included in the study. The control group comprised 185 primary central nervous system tumors, including high-grade and low-grade gliomas and glioneuronal/neuronal tumors. Strong and diffuse (≥ 50% of tumor cells) GPNMB expression was present in all SEGAs. In contrast, TTF-1 expression was detected in nine SEGAs, resulting in a sensitivity of 81.8%. Among the control group, 77 cases (41.6%) were negative for GPNMB and 102 (55.1%) cases were scored as > 1% < 50% positive. Only six control tissues (3.2%) showed diffuse and strong GPNMB expression. Among the tumors with strong GPNMB expression, there were three glioblastomas (GBMs) with morphology potentially mimicking SEGA but lacking TSC1, TSC2, or MTOR mutations. Using a cutoff of diffuse (≥ 50%) and strong positivity, GPNMB demonstrated 100% sensitivity (95% confidence interval: 74.1%-100%) and 96.8% specificity (95% confidence interval: 93.1%-98.5%) for diagnosing SEGA.