Solid-pseudopapillary tumors of the pancreas are genetically distinct from pancreatic ductal adenocarcinomas and almost always harbor beta-catenin mutations

Abstract

Solid-pseudopapillary tumors (SPTs) are unusual pancreatic neoplasms of low malignant potential that most frequently affect young women. Genetic events contributing to the development of SPTs are unknown. Whereas the more common ductal adenocarcinomas of the pancreas essentially never harbor beta-catenin or APC gene mutations, we have recently identified alterations of the APC/beta-catenin pathway in other nonductal pancreatic neoplasms including pancreatoblastomas and acinar cell carcinomas. We analyzed a series of 20 SPTs for somatic alterations of the APC/beta-catenin pathway using immunohistochemistry for beta-catenin protein accumulation, direct DNA sequencing of beta-catenin exon 3, and direct DNA sequencing of the mutation cluster region in exon 15 of the APC gene in those SPTs that did not harbor beta-catenin mutations. Immunohistochemical labeling for cyclin D1 was performed to evaluate the overexpression of this cell-cycle protein as one of the putative downstream effectors of beta-catenin dysregulation. In addition, we analyzed the SPTs for genetic alterations commonly found in pancreatic ductal adenocarcinomas, including mutations in the K-ras oncogene and p53 and DPC4 tumor suppressor genes, using direct DNA sequencing of K-ras and immunostaining for p53 and Dpc4. Almost all SPTs harbored alterations in the APC/beta-catenin pathway. Nuclear accumulation of beta-catenin protein was present in 95% (19 of 20), and activating beta-catenin oncogene mutations were identified in 90% (18 of 20) of the SPTs. Seventy-four percent (14 of 19) showed overexpression of cyclin D1, ranging from 10 to 70% of tumor nuclei. In contrast, no K-ras mutations were present in any of the 20 SPTs, and Dpc4 expression was intact in all 16 SPTs for which immunohistochemical labeling was successful. Overexpression of p53 was limited to only 3 of 19 (15.8%) SPTs. These results emphasize the two distinct, divergent genetic pathways of neoplastic progression in pancreatic ductal and nonductal neoplasms.

Figure 1.

Histological appearance of SPTs. A: In solid areas, patternless sheets of uniform epithelial cells are punctuated by numerous small vascular channels. B: Degenerative changes and the discohesive nature of the epithelial cells give rise to characteristic pseudopapillae in which residual tumor cells appear to rosette around vascular cores.

Figure 2.

β-catenin oncogene mutations in SPTs. Representative DNA sequencing chromatograms demonstrate a GAC (aspartic acid)→AAC (asparagine) mutation in codon 32 of case S11, a GAC (aspartic acid)→GTC (valine) mutation in codon 32 of S9, a GGA (glycine)→AGA (arginine) mutation in codon 34 of S15, a GGA (glycine)→GTA (valine) mutation in codon 34 of S4, and a TCT (serine)→TTT (phenylalanine) mutation in codon 37 of S5. In each case, a mixture of the mutant and wild-type peaks is present because of the dominant-positive nature of β-catenin mutations.

Figure 3.

Downstream effects of β-catenin dysregulation in SPTs. A: Nuclear translocation and accumulation of β-catenin protein is seen in neoplastic epithelial cells that have infiltrated into normal pancreas at the edge of the tumor. Nonneoplastic acini show only normal membranous β-catenin labeling, but are negative for nuclear accumulation. B: Overexpression of cyclin D1, a downstream transcriptional target of nuclear β-catenin-Tcf/Lef complex. Cyclin D1 labeling is seen in the nuclei of neoplastic epithelial cells of this SPT, but not in the foamy histiocytes at center (arrow).

Figure 4.

SPTs rarely harbor alterations in the genes commonly involved in ductal adenocarcinomas. A: Only wild-type K-ras sequences were present at codons 12 and 13 in all SPTs. B: Intact Dpc4 protein expression was present in all SPTs. C: Only 3 of 19 (15.9%) SPTs showed p53 protein overexpression, as in this case (S6) that contained a distinct patch of neoplastic epithelial cells with nuclear labeling.