Strong nuclear expression of HOXB13 is a reliable surrogate marker for DNA methylome profiling to distinguish myxopapillary ependymoma from spinal ependymoma

Spinal ependymoma and myxopapillary ependymoma are the two most common spinal ependymal tumor types that feature distinct histological characteristics, genetic alterations and DNA methylation profiles. Their histological distinction may be difficult in individual cases and molecular diagnostic assessment, in particular DNA methylome profiling, may then be required to assign the correct diagnosis. Expression of the homeobox gene HOXB13 at the mRNA and protein levels has been reported as a frequent finding in myxopapillary ependymoma that may serve as a diagnostic marker for these tumors. Here, we evaluated the diagnostic role of HOXB13 immunostaining in 143 spinal neoplasms, comprising 54 histologically classified myxopapillary ependymomas, 46 histologically classified spinal ependymomas, and various other tumor types. Immunohistochemical results for HOXB13 protein were compared to molecular findings obtained by bead array-based DNA methylation and DNA copy number profiling, as well as next generation gene panel sequencing-based mutational analysis. Our findings indicate strong nuclear HOXB13 expression as a reliable diagnostic marker for molecularly confirmed myxopapillary ependymoma. Moreover, we provide evidence that differential HOXB13 protein expression is related to differential HOXB13-associated CpG site methylation in myxopapillary vs. spinal ependymomas, which can be assessed by targeted DNA methylation analysis. Taken together, immunohistochemistry for HOXB13 protein expression and targeted DNA methylation analysis of HOXB13 represent useful surrogate approaches that may substitute for DNA methylome profiling in routine diagnostics and facilitate precise classification of spinal ependymal tumors. In particular, strong nuclear HOXB13 immunoreactivity may serve as a novel diagnostic criterion for the classification of myxopapillary ependymoma.