PRAME protein expression in DICER1-related tumours

Abstract

DICER1 syndrome is an autosomal dominant tumour predisposition syndrome usually affecting persons under 30 years of age. Many of the associated benign and malignant lesions occur almost exclusively in DICER1 syndrome. One such tumour, pituitary blastoma (pitB), overexpresses PRAME 500x above control levels. PRAME (PReferentially expressed Antigen in MElanoma) is expressed in malignancies that are not DICER1-related (e.g. melanoma). To address whether PRAME expression is part of the DICER1 phenotype, or simply a feature of pitB, a series of 75 DICER1-mutated specimens and 33 non-mutated specimens was surveyed using immunohistochemistry for PRAME, together with EZH2, which complexes with PRAME. In DICER1-mutated specimens, positive staining for PRAME was only seen in malignant tumours; 7 of 11 histological types and 34/62 individual tumours were positive, while non-tumourous lesions were always negative. Pleuropulmonary blastoma (PPB) showed a continuum in staining, with type I lesions being PRAME negative (n = 7) but all type II and type III lesions PRAME positive (n = 7). Similarly, cystic nephroma (CN) was negative (n = 8), with anaplastic sarcoma of the kidney being positive (n = 2). However, one atypical CN with mesenchymal cell proliferation was PRAME-positive. Embryonal rhabdomyosarcoma (RMS) with DICER1 pathogenic variants (PVs) was positive for PRAME (5/6), but the same tumour type without DICER1 PVs was also positive (9/15). Staining for EZH2 corresponded to that seen with PRAME, validating the latter. This study leads us to conclude that (1) PRAME expression occurs in two-thirds of DICER1-related malignancies; (2) PRAME may be a marker for the progression that certain DICER1-related lesions are thought to undergo, such as PPB and CN; and (3) PRAME expression in some tumours, such as RMS, appears to be an intrinsic feature of the tumour, rather than specifically related to DICER1 PVs. Therapy directed against PRAME may offer novel treatment options in patients with the DICER1 syndrome.

Keywords: DICER1; PRAME; anaplastic sarcoma of the kidney; cystic nephroma; pleuropulmonary blastoma.

Figure 1

PRAME and EZH2 staining in PPB. All cases shown had DICER1 PVs. For each case, fields are matched for the two antibodies. PPB type I was usually negative for PRAME (PPB‐I 4) but occasionally showed focal staining of epithelial cells (PPB‐I 3). In contrast, type II (PPB‐II 1, PPB‐II 2, and PPB‐II 3) and type III (PPB‐III 2, PPB‐III 3, and PPB‐III 4) lesions were PRAME‐positive, with diffuse staining of mesenchymal cells. Immunostaining for EZH2 generally matched that of PRAME, but with greater staining of epithelial cells in PPB type 1 (PPB‐I 4 and PPB‐I 3) and more intense staining in some cases (e.g. PPB‐III 3 and PPB‐III 4) (original magnifications ×200). Specimen numbers correspond to cases listed in Table 1 and supplementary material, Tables S1 and S2.

Figure 2

PRAME and EZH2 staining in CN and ASK. All cases shown had DICER1 PVs. For each case, fields are matched for the two antibodies. Most cases of CN were PRAME‐negative (CN 3) or showed focal staining of some epithelial cells (CN 5). One CN was unusual with proliferation of mesenchymal cells in the septa; these cells were PRAME‐positive (CN 9). ASK was strongly PRAME‐positive, particularly in the undifferentiated areas (ASK 1) and rhabdomyoblastic areas (ASK 2). Immunostaining for EZH2 generally matched that of PRAME except for additional staining of some epithelial and stromal cells in CNs (CN 3 and CN 5) (original magnifications ×200). Specimen numbers correspond to cases listed in Table 1 and supplementary material, Tables S1 and S2.

Figure 3

PRAME and EZH2 staining in embryonal rhabdomyosarcoma (ERMS). For each case, fields are matched for the two antibodies (PRAME left, EZH2 right). In DICER1‐mutated cases, two‐thirds of cases were PRAME‐positive (cERMS 2, cERMS 4, and cERMS 1) with the others showing only focal staining (cERMS 3). In cases without DICER1 PVs, two‐thirds of cases were positive (pERMS B, pERMS C, and pERMS D) with the others showing focal (pERMS A) or negative staining (original magnifications ×200). Specimen numbers for RMS specimens with PVs correspond to cervical (c) cases listed in Table 1 and supplementary material, Tables S1 and S2. RMS without PVs are paratesticular (p), and only appear in Table 1 as a group; they are therefore not numbered.