The efficacy of DNA mismatch repair enzyme immunohistochemistry as a screening test for hypermutated gliomas

Abstract

A subset of gliomas has DNA repair defects that lead to hypermutated genomes. While such tumors are resistant to alkylating chemotherapies, they may also express more mutant neoantigens on their cell surfaces, and thus be more responsive to immunotherapies. A fast, inexpensive method of screening for hypermutated gliomas would therefore be of great clinical value. Since immunohistochemistry (IHC) for the DNA mismatch repair (MMR) proteins Msh2, Msh6, Mlh1, and Pms2 is already used to screen for hypermutated colorectal cancers, we sought to determine whether that panel might have similar utility in gliomas. MMR IHC was scored in 100 WHO grade I-IV gliomas (from 96 patients) with known tumor mutation burden (TMB), while blinded to TMB data. Cases included 70 grade IV GBMs, 13 grade III astrocytomas, 4 grade II astrocytomas (3 diffuse astrocytomas and 1 pleomorphic xanthoastrocytoma), 1 grade I pilocytic astrocytoma, 2 grade III oligodendrogliomas, 7 grade II oligodendrogliomas, and 3 grade I glioneuronal tumors. Eight of 100 tumors showed loss of one or more MMR proteins by IHC, and all 8 were hypermutated. Among the remaining 92 gliomas with intact MMR IHC, only one was hypermutated; that tumor had an inactivating mutation in another DNA repair gene, ATM. Overall accuracy, sensitivity, and specificity for DNA MMR IHC compared to the gold standard of TMB were 99, 89, and 100%, respectively. The strongest correlates with hypermutation were prior TMZ treatment, MGMT promoter methylation, and IDH1 mutation. Among the 8 MMR-deficient hypermutated gliomas, 4 (50%) contained both MMR-lost and MMR-retained tumor cells. Together, these data suggest that MMR IHC could be a viable front-line screening test for gliomas in which immunotherapy is being considered. They also suggest that not all cells in a hypermutated glioma may actually be MMR-deficient, a finding that might need to be considered when treating such tumors with immunotherapies.

Keywords: Glioma; Hypermutator; IDH1; MGMT; Mismatch repair; Temozolomide.

Fig. 1

MMR IHC in Tumor 1. The tumor was a recurrent GBM, post-TMZ therapy, in a 57 year-old woman (Table 2). Tumor cells showed loss of Msh2 (a) and Msh6 (b), and retention of Mlh1 (c) and Pms2 (d). Note the normal immunostaining within nonneoplastic cells scattered throughout the tumor in (a) and (b). Scale bar = 100 μm

Fig. 2

Heterogeneous MMR IHC in hypermutated gliomas. In tumor 7, which was a post-TMZ IDH1 mutant GBM in a 43 year-old woman (Table 2), clusters of tumor cells retained Msh2 and Msh6 positivity, but were surrounded by Msh2/6-deficient cells (a, b). Tumor 6 was a post-TMZ IDH1 wild-type GBM in a 65 year-old woman (Table 2). Msh6 was lost in many glioma cells (c), but under high power, it was apparent that a subset of cells with identical tumor nuclear morphology retained Msh6 (d, red asterisk). Also note the smaller rounded nuclei in (d), which are most likely lymphocytes and/or oligodendrocytes. Scale bar = 100 μm in (a, b, e), 50 μm in (c, d) and 25 μm in (f)

Fig. 3

Correlation matrix. Heatmap showing Spearman ρ (rho) correlation coefficients, with 1 = perfect direct correlation, 0 = no correlation, and − 1 = perfect inverse correlation. N = 93 tumors (cases without MGMT data were excluded, as well as hypermutated tumor #2 since it came from the same patient as tumor #1, see Additional file 1: Tables S1 and S2). TMZ = temozolomide