Expression of DLL3 and SEZ6 in the Spectrum of Neuroendocrine Neoplasia

Delta-like protein 3 (DLL3), a Notch ligand, has been identified in high-grade small- and large-cell lung carcinomas and prostate neuroendocrine carcinomas (NECs). SEZ6 (Seizure-related 6 homolog), a membrane-associated protein, has also been identified neuroendocrine neoplasms (NENs). Both DLL3 and SEZ6 are targets of novel antibody-drug conjugates (ADCs). Their expression in the broader family of NENs remains to be clarified. We examined a series of NENs of all types as well as several non-neuroendocrine neoplasms using immunohistochemistry for DLL3 and SEZ6. Staining was scored with the semi-quantitative assessment of intensity and percentage of stained cells that yields a histoscore (H-score from 0 to 300). We identified strong expression of DLL3 in all lung NECs (average H-score 180) and SEZ6 in 1 of 3 (H-score 70). Merkel cell carcinomas (n = 13) expressed DLL3 strongly and diffusely (H-score 178, range 10-300) and 10 of 13 had positivity for SEZ6 (H-score 128). Both DLL3 and SEZ6 were expressed in medullary thyroid carcinomas (10 of 11 cases, H-scores 199 for DLL3 and 224 for SEZ6). Two thymic neuroendocrine tumors (NETs) had weak expression of DLL3 (H-score 20) and SEZ6 (H-score 70). Nine of 11 lung NETs expressed DLL3 (H-score 191), while only two had focal weak staining for SEZ6 (H-score 45). DLL3 was negative in nine of 11 pancreatic NETs; two grade 3 pancreatic NETs had variable weak positivity (H-score 5). In contrast, seven of 11 pancreatic NETs expressed SEZ6 (H-score 45). DLL3 was positive in two pancreatic NECs (H-score 197), and SEZ6 was positive in 1 (H-score 60). One of 13 gastric NETs, a metastatic grade 3 tumor, expressed both DLL3 and SEZ6 (H-score 200 for each) and one other expressed SEZ6 at lower levels. A gastric NEC was weakly positive for both markers (H-scores 50 and 40). All 10 duodenal, 10 ileal, and nine rectal NETs were negative for DLL3; eight duodenal, six ileal, and four rectal NETs expressed SEZ6 (average H-scores 206, 78 and 45, respectively). Among 10 appendiceal NETs, two expressed DLL3 focally and weakly (H-score 45); eight were positive for SEZ6 (H score 109). Duodenal NECS (n = 2) were negative for DLL3; one duodenal NEC expressed SEZ6 (H-score 110). Among five colonic NECs, two expressed DLL3 (H-score 50) and one expressed SEZ6 (H-score 40). Pituitary NETs also expressed DLL3 with eight of 18 positive (H-scores from 10 to 180), and 11 of 18 expressed SEZ6 (average H-score 65). Three of 20 paragangliomas expressed DLL3 weakly (H-score 43), and six expressed SEZ6 (H-score 73). One of four parathyroid carcinomas expressed DLL3 weakly (H-score 30), and all four were negative for SEZ6; five parathyroid adenomas were negative for both. In 43 non-neuroendocrine neoplasms of the GI tract, pancreas, and liver and 10 non-neuroendocrine thyroid carcinomas, there was only weak focal reactivity for DLL3 in three and two cases, respectively, and for SEZ6 in one case each. These results suggest that expression of DLL3 and SEZ6 varies among NENs of almost all types. Expression appears to be limited primarily to NENs and is rarely seen only focally and weakly in non-NENs. The presence of one or both of these antigens offers a novel approach to the treatment of patients with neuroendocrine neoplasms across the entire spectrum.