Targeted Next-Generation Sequencing Reveals Divergent Clonal Evolution in Components of Composite Pleomorphic Xanthoastrocytoma-Ganglioglioma

Abstract

Composite pleomorphic xanthoastrocytoma-ganglioglioma (PXA-GG) is an extremely rare central nervous system neoplasm with 2 distinct but intermingled components. Whether this tumor represents a "collision tumor" of separate neoplasms or a monoclonal neoplasm with divergent evolution is poorly understood. Clinicopathologic studies and capture-based next generation sequencing were performed on extracted DNA from all available PXA-GG at 2 medical centers. Five PXA-GG were diagnosed in 1 male and 4 female patients ranging from 13 to 25 years in age. Four arose within the cerebral hemispheres; 1 presented in the cerebellar vermis. DNA was sufficient for analysis in 4 PXA components and 3 GG components. Four paired PXA and GG components harbored BRAF p.V600E hotspot mutations. The 4 sequenced PXA components demonstrated CDKN2A homozygous deletion by sequencing with loss of p16 (protein product of CDKN2A) expression by immunohistochemistry, which was intact in all assessed GG components. The PXA components also demonstrated more frequent copy number alterations relative to paired GG components. In one PXA-GG, shared chromosomal copy number alterations were identified in both components. Our findings support divergent evolution of the PXA and GG components from a common BRAF p.V600E-mutant precursor lesion, with additional acquisition of CDKN2A homozygous deletion in the PXA component as is typically seen in conventional PXA.

Keywords: Collision tumor; Ganglioglioma; Intratumoral heterogeneity; Molecular neuropathology; Neurooncology; Pleomorphic xanthoastrocytoma; Precision medicine.

FIGURE 1.

Composite pleomorphic xanthoastrocytoma-ganglioglioma may present as a solid or mixed solid/cystic mass on preoperative imaging. Magnetic resonance imaging of patient #1 showed a solitary 1.2 cm T2-hyperintense heterogeneously enhancing mass within the right superior temporal gyrus involving the posterior sylvian fissure (top row). In contrast, magnetic resonance imaging of patient #5 demonstrated a solitary 4.5 cm T2-hyperintense enhancing solid and cystic mass centered within the corpus callosum body and extending into the paramedian left frontal lobe (bottom row).

FIGURE 2.

Composite pleomorphic xanthoastrocytoma-ganglioglioma are composed of 2 morphologically distinct but geographically juxtaposed solid components. Low-magnification microscopy of tumor #2 (center column) demonstrates intermingling of the 2 tumor components at the component interface. The pleomorphic xanthoastrocytoma component (left) is composed of large pleomorphic multinucleated glial cells within a fibrillar background. In contrast, the ganglioglioma component (right) is composed of variably sized dysmorphic ganglion cells admixed with round to mildly irregular glial cells. Both components demonstrate immunoreactivity for BRAF V600E mutant protein (center row) but show differential expression of p16 (bottom row)—loss of expression in the pleomorphic xanthoastrocytoma component and overexpression in the ganglioglioma component, suggestive of a related origin but divergent evolution of the 2 components.

FIGURE 3.

Histologically distinct components of composite pleomorphic xanthoastrocytoma-ganglioglioma demonstrate divergent molecular evolution from a common BRAF-mutant precursor lesion. Next-generation sequencing performed in cases #1, #2, and #3 demonstrated divergent copy number alterations in matched pleomorphic xanthoastrocytoma (PXA) and ganglioglioma (GG) components of each PXA-GG (left). In tumor #3, shared copy number alterations between the PXA and GG components included gain of whole chromosomes 7, 12, and 20 (bottom). Notably, the identical BRAF p.V600E activating hotspot mutation was identified in the PXA and GG components of all 3 cases. Immunohistochemistry for BRAF V600E mutant protein was positive in all assessed components (right). In addition to divergent copy number alterations at the whole chromosome level, CDKN2A homozygous deletion was limited to the PXA components. Immunohistochemistry for p16 demonstrated overexpression in the GG components and loss of expression (positivity only in rare nonneoplastic cells) in the PXA components (right).