Loss of H3K27me3 Expression Is a Highly Sensitive Marker for Sporadic and Radiation-induced MPNST

Abstract

Most malignant peripheral nerve sheath tumors (MPNSTs) exhibit combined inactivation of NF1, CDKN2A, and polycomb repressive complex 2 component genes (Embryonic Ectoderm Development [EED] and Suppressor of Zeste 12 [SUZ12]). Mutations in EED and SUZ12 induce loss of trimethylation at lysine 27 of histone 3 (H3K27me3), with subsequent aberrant transcriptional activation of polycomb repressive complex 2-repressed homeobox master regulators. These findings prompted us to investigate the performance of an anti-H3K27me3 monoclonal antibody clone C36B11 as an immunohistochemical marker for MPNSTs. We assessed the C36B11 reactivity pattern in a pathologically and genetically well-characterized cohort of 68 MPNSTs, spanning various clinical presentations, such as type 1 neurofibromatosis (NF1), radiotherapy, and sporadic MPNSTs. We found that 69% (n=47) of all MPNSTs demonstrated loss of H3K27me3 expression, with 42 (61%) showing complete loss and 5 (7%) showing partial loss, whereas 31% (n=21) retained H3K27me3 expression. Among the NF1-related high-grade MPNSTs, 60% demonstrated loss of expression. In contrast, the majority of both sporadic (95%) and radiotherapy-related (91%) MPNSTs showed loss of H3K27me3 expression. Two of the 3 low-grade MPNSTs and all neurofibromas showed retained expression. Furthermore, all 5 epithelioid MPNSTs retained H3K27me3 labeling. The specificity of H3K27me3 loss as a marker for MPNSTs was studied by testing a large spectrum of lesions included in MPNST differential diagnosis, such as spindle/desmoplastic melanomas, synovial sarcomas, myoepithelial tumors, and other mesenchymal neoplasms, all of which retained expression of H3K27me3. We conclude that immunohistochemical analysis of H3K27me3 has good sensitivity and robust specificity for the diagnosis of MPNST, particularly outside of NF1 clinical history, which represents the most challenging diagnostic setting.

Figure 1

H3K27me3 immunostaining in morphologic variants of MPNSTs. Classic HG MPNST features with intersecting fascicles of monomorphic spindle cells shown here in an NF1-related tumor (A, B; MPNST03), with an adjacent plexiform NF (B, MPNST03, right side). H3K27me3 immunolabeling showed retained expression in NF (C), whereas the adjacent MPNST exhibited complete loss of expression (C, arrows). Note the internal positive controls: vessels (C, arrowheads), and inflammatory cells retaining staining. An RT-associated HG MPNST (MPNST48.1) with pleomorphic features (D, E) showing loss of H3K27me3 immunostaining (F, note internal control in vessels, arrowheads).

Figure 2

Additional patterns of H3K27me3 immunolabeling in MPNST. Partial loss or mosaic pattern of staining seen in 1 NF1-related tumor (A–C, MPNST12). Note the focal pleomorphism in MPNST12 (A, B). An epithelioid MPNST composed of solid nests of large polygonal cells with eosinophilic cytoplasm and vesicular nuclei with macronucleoli (D, E), showing diffuse immunolabeling (F).

Figure 3

Morphologic and H3K27me3 IHC intertumoral heterogeneity in NF1-related MPNST. This patient (MPNST15) developed 3 different MPNST primaries, which showed variable morphology: HG with focal pleomorphism (MPNST15.1) (A), HG classic (C) arising with an adjacent plexiform NF (MPNST15.3), and HG with pleomorphic features (MPNST15.4) (E). Only the MPNST with pleomorphic features showed retained H3K27me3 (F), whereas the others lost expression (B, D).

Figure 4

H3K27me3 staining in MPNSTs with heterologous components. A sporadic case (MPNST35) with discordant H3K27me3 expression between the 2 types of divergent differentiation components, rhabdomyosarcomatous (A, B, RMS) and glandular (B): lost expression in the homologous and RMS element (C, RMS) and retained expression in the glandular component (C, arrowheads). H3K27me3 differential expression in another sporadic MPNST (MPNST30) with 2 types of divergent differentiation components: loss of expression in the homologous (small cell) component (D, F right side and inset) and retained expression in both heterologous elements, leiomyosarcoma (E, F left side LMS) and glandular (not shown). LMS indicates leiomyosarcoma; RMS, rhabdomyosarcoma.

Figure 5

Morphologic mimics of MPNST showed retained H3K27me3 expression. An SCM with long intersecting fascicles of bland monomorphous spindle cells (A) showing diffuse and strong immunoreactivity for H3K27me3 (B). Selected cases of other entities included in the control group with H3K27me3-retained expression: monophasic SS (C) and GIST (D).