Immunohistochemical p16INK4a analysis of archival tumors with deletion, hypermethylation, or mutation of the CDKN2/MTS1 gene. A comparison of four commercial antibodies.

The MTS1/CDKN2/p16 gene encoding the p16INK4a tumor-suppressor protein is commonly inactivated by homozygous deletion or hypermethylation of the promoter in a wide range of human malignancies. In select tumor types, including pancreatic adenocarcinomas, intragenic mutations are found in a significant percentage of cases. The immunoreactivity of mutant p16 proteins has not been comprehensively studied. Moreover, the immunohistochemical properties of commercially available antibodies have not been described in detail. We studied 35 pancreatic adenocarcinomas with a molecularly defined p16 status (16 homozygous deletions, 3 hypermethylated cases, and 16 tumors with an intragenic mutation in one allele associated with loss of the second allele). In addition, we studied nine cell lines (three homozygous deletions, three hypermethylated lines, and three intragenic mutations). Paraffin sections of the tumors and cell blocks were reacted with four different anti-p16 antibodies: polyclonal and monoclonal (clone G175-405) antibodies from PharMingen, monoclonal antibody DCS-50 from Oncogene Science, and monoclonal antibody ZJ11 from Neo-Markers. Optimal staining conditions were established for each antibody. The pancreatic carcinomas with homozygous p16 deletions were largely devoid of nuclear staining (admixed nonneoplastic cells served as internal positive controls); only one adenocarcinoma each reacted with DCS-50 and the polyclonal antibody, and five were positive with ZJ11, suggesting that nonspecific nuclear staining can occur under certain conditions. Antibody DCS-50 produced nuclear staining in all three hypermethylated carcinomas, whereas G175-405 stained none of them. Three of the four antibodies produced nuclear immunoreactivity in 7 to 14 of the 16 carcinomas carrying p16 mutations; G175-405 showed only weak reactivity in one case. Cytoplasmic staining was present in all carcinomas and cell lines and with all antibodies and therefore cannot be considered specific; it was strongest with G175-405. Thus, we found antibody G175-405 to be the most specific, and monoclonals DCS-50 and ZJ11 the least specific for wild-type p16. However, the former tends to give stronger cytoplasmic background staining. For tumor types in which p16 mutations are uncommon, the PharMingen polyclonal antibody may be a suitable alternative.