Concordant loss of MTAP and p16/CDKN2A expression in pancreatic intraepithelial neoplasia: evidence of homozygous deletion in a noninvasive precursor lesion

Abstract

The p16INK4A/CDKN2A (p16) gene on chromosome 9p21 is inactivated in 490% of invasive pancreatic cancers. In 40% of pancreatic cancers the p16 gene is inactivated by homozygous deletion, in 40% by an intragenic mutation coupled with loss of the second allele, and in 10-15% by hypermethylation of the p16 gene promoter. Immunohistochemical labeling for the p16 gene product parallels gene status, but does not provide information of the mechanism of p16 gene inactivation. The methylthioadenosine phosphorylase gene (MTAP) gene also resides on chromosome 9p21, approximately 100 kb telomeric to the p16 gene. The MTAP gene is frequently contained within p16 homozygous deletions, producing concordant loss of both p16 and MTAP gene expression. Concordant loss of both p16 and MTAP protein expression can therefore be used as a surrogate marker for p16 homozygous deletion. Here we immunolabeled a series of pancreatic intraepithelial neoplasia (PanIN) lesions of various histologic grades for the p16 and MTAP gene products using a high-throughput PanIN tissue microarray (TMA) format. We demonstrate concordant loss of p16 and MTAP protein expression in 6/73 (8%) PanINs, including five high-grade lesions and one low-grade lesion. Immunolabeling for both p16 and MTAP protein expression provides a tool to evaluate tissues with intact morphology for p16 gene homozygous deletions. The concordant loss of expression of both genes in PanIN lesions demonstrates that homozygous deletions of the p16 tumor suppressor gene can occur in noninvasive precursor lesions. Keywords: homozygous deletion; precursor lesion; pancreatic intraepithelial neoplasia; p16; MTAP Pancreatic intraepithelial neoplasia (PanIN) is a powerful system to study noninvasive precursors of an infiltrating cancer. First, the disease is important. The infiltrating cancer associated with PanINs, infiltrating adenocarcinoma of the pancreas, is the fourth leading cause of cancer death. 1 This year approximately 31,000 Americans will be diagnosed with PanINs infiltrating adenocarcinoma of the pancreas and 31 000 will die from it. Second, PanINs are histologically well-defined. An international consensus has been developed for the classification and grading of PanINs, allowing investigators at one institution to compare their results directly with findings from another institution. 2 Third, the genetics of PanINs are well described, and a progression model for the occurrence of genetic alterations in PanINs has been developed. 3 Telomere shortening and activating point mutations in the KRAS2 oncogene occur early in PanIN-1 lesions, the p16INK4A/CDKN2A (henceforth referred to as p16) gene is inactivated in intermediate and late lesions (PanINs 2 and 3), and the TP53, MADH4, and BRCA2 genes are inactivated late, in PanIN-3 lesions. [4][5][6][7][8][9][10]

Commonly dysregulated genes in murine APL cells

“…47 Hif1␣ has been implicated in the pathogenesis of many tumors (including AML), and was expressed in all murine APL samples tested. [48][49][50] Mtap1a, a tumor suppressor implicated in several solid tumor syndromes, [51][52][53][54][55] was normally expressed in murine promyelocytes, but was expressed at very low levels in all APL samples tested. Much additional work will be required to fully understand the relationship between these dysregulated genes, and their respective roles in the pathogenesis of this disorder.…”

Precursors to Pancreatic Cancer

“…This has been clearly demonstrated at the molecular level for IPMN, MCN and PanIN (15)(16)(17). For example, Moskaluk et al microdissected paired PanIN lesions and infiltrating cancers from patients with pancreatic cancer, and in one case, were able to demonstrate identical p16/CDKN2A gene mutations in a PanIN lesion as in the patient's paired infiltrating cancer(17).…”

Inactivation of Smad4 Accelerates KrasG12D-Mediated Pancreatic Neoplasia

“…Although loss of p16 INK4A tumor suppressor expression can be detected in PanIN-1A and PanIN-1B lesions, it is more frequently associated with PanIN-2, wherein the ductal epithelium exhibits nuclear atypia and papillary architecture (15). In invasive PDAC, more than 90% of cases show loss of p16 INK4A function (12,(16)(17)(18)(19). In mice, loss of either p16…”