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Arch Pathol Lab Med. Author manuscript; available in PMC Jun 1, 2012.
Published in final edited form as:
PMCID: PMC3107523
NIHMSID: NIHMS278085

Molecular Signatures of Pancreatic Cancer

Abstract

Context

The introduction of genome- and epigenome-wide screening techniques has dramatically improved our understanding of the molecular mechanisms underlying the development of pancreatic cancer. There are now 3 recognized histologic precursors of pancreatic cancer: pancreatic intraepithelial neoplasia, intraductal papillary mucinous neoplasm, and mucinous cystic neoplasm. Each of these precursor lesions is associated with specific molecular alterations.

Objective

To understand the molecular characteristics of pancreatic ductal adenocarcinoma and its precursor lesions.

Data Sources

PubMed (US National Library of Medicine).

Conclusions

In this review, we briefly summarize recent research findings on the genetics and epigenetics of pancreatic cancer. In addition, we characterize these molecular alterations in the context of the histologic subtypes of pancreatic cancer.

Pancreatic cancer is the fourth leading cause of cancer death in both men and women in the United States. In 2010, it is estimated that 43 140 Americans will be diagnosed and 36 800 patients will die of pancreatic cancer.1 Most pancreatic cancers are pancreatic ductal adenocarcinomas and the 5-year survival rate for patients with localized disease after surgical resection is 20% and for those with metastatic disease, the survival is only 2%.1 The poor survival rate is attributed to the late detection of pancreatic cancers; 85% of patients present with advanced disease that is unresectable. Although significant resources have been committed to improving the survival of patients with pancreatic cancer in the past decades, there has been no significant improvement in survival.1 Research into the molecular mechanisms of pancreatic cancer has revealed that the disease is due to both genetic and epigenetic changes. The introduction of genome- and epigenome-wide screening techniques has expanded the numbers of genes linked to pancreatic cancer.26 In this review, we briefly summarize recent research findings on genetics and epigenetics of pancreatic cancer in the context of histologic variants, precursor lesions, and familial pancreatic cancer.

GENETICS OF PANCREATIC CANCER

A recent comprehensive study of the pancreatic cancer genome profiled the genetic abnormalities of pancreatic ductal adenocarcinomas. In this study, Jones and colleagues7 sequenced 20 661 protein-coding genes in 24 ductal adenocarcinomas and demonstrated an average of 48 nonsilent mutations, 6 amplifications, and 8 homozygous deletions per pancreatic cancer. These mutations were associated with 12 core signaling pathways.7 Based on the frequency of genetically affected genes in pancreatic cancers, a genetic “topographic map” of the pancreatic cancers can be generated in which the most frequent mutations are represented as 4 “mountains” (high-frequency driver genes) involving KRAS2, CDKN2A/p16, SMAD4/DPC4, and TP53, with numerous “hills” (low-frequency driver genes) involving SMARC4A, CDH1, EPHA3, FBXW7, EGFR, IDH1, and NF1.7

1. Oncogenes and Pancreatic Cancer

The most frequently mutated oncogene in pancreatic cancers is KRAS2 (mutated in >95% of pancreatic cancers), which is activated by point mutations, most often in codon 12.7,8 The KRAS2 gene is located on chromosome arm 12p and encodes a membrane-bound guanosine triphosphate (GTP)–binding protein. This GTP-binding protein mediates various cellular functions, such as proliferation, cellular survival, motility, and cytoskeletal remodeling. Activating KRAS gene mutations abolish the regulated GTPase activity of the Kras protein, which results in constitutive signaling.9 Mutations in the KRAS2 gene are observed in the earliest _ENREF_10pancreatic intraepithelial neoplasia (PanIN) lesions and are considered to be one of the earliest genetic events in pancreatic tumorigenesis.7,10,11 Several additional signaling pathways downstream from KRAS2, including BRAF-MAPK and PI3K-AKT, may also be activated by mutations. The BRAF pathway is activated by a point mutation at V600E. BRAF gene mutations are observed in 5% of pancreatic cancers that do not possess a KRAS2 mutation.12 These cancers are often microsatellite unstable. Similarly, amplifications in the AKT2 gene are seen in 10% to 20% of pancreatic cancers.13,14 Amplifications of other oncogenes such as C-MYC,7,15 KRAS, and GATA6,16, 15 are less frequent.

2. Tumor Suppressor Genes in Pancreatic Cancer

Three tumor suppressor genes, CDKN2A/p16, TP53, and SMAD4/DPC4, are commonly inactivated in pancreatic cancers.7,1720 CDKN2A/p16 on chromosome arm 9p is inactivated in more than 95% of pancreatic cancers by several different mechanisms, such as homozygous deletion of both alleles of the gene; intragenic mutation in 1 allele, coupled with loss of the other allele; or promoter hypermethylation.17,21,22 The p16 protein inhibits progression of the cell cycle at the G1-S checkpoint binding of cyclin-dependant kinases (CDKs), including CDK4 and CDK6.23 The TP53 gene on chromosome arm 17p is inactivated in 50% to 75% of pancreatic cancers.7,19,24,25

p53 proteins play several key roles including maintaining G2-M arrest, regulating G1-S checkpoint, inducing apoptosis, regulating senescence, repairing DNA, and changing cellular metabolism.26 Inactivation of the TP53 gene typically occurs through intragenic mutations of 1 allele, accompanied with loss of the other allele.19 Functional loss of the p53 protein enables cellular survival and division in the presence of DNA damage; this facilitates the accumulation of further genetic abnormalities.26

SMAD4/DPC4 on chromosome arm 18q is inactivated in 55% of pancreatic cancers.27,28 SMAD4/DPC4 is inactivated by homozygous deletion and by intragenic mutations accompanied by loss of the other allele.28,29 Smad4 (dpc4) protein has a critical function in the signal transduction cascade that involves transforming growth factor β (TGF-β) and multiple targets in the TGF-β pathway. Binding of the TGF-β ligand to its receptor triggers a series of reactions including binding of the transcription factor smad2/3 to smad4. Through multiple target genes, the TGF-β pathway normally regulates cellular growth. Loss of smad4 (dpc4) function abolishes the smad4-dependant TGF-β pathway and gives rise to unregulated cellular proliferation.30 Loss of smad4 nuclear labeling by immunohistochemistry is generally observed late in pancreatic carcinogenesis, such as in PanIN-3 precursor lesions and infiltrating adenocarcinomas.31 Both SMAD4/DPC4 mutation and loss of smad4 expression are markers of poor prognosis in pancreatic cancers.32,33 In contrast to SMAD4/DPC4 mutation, mutations in TP53 and CDKN2A/p16 have not been shown to predict survival.33 Loss of smad4 protein expression can also be used in the differential diagnosis of carcinomas of unknown primary tumor; SMAD4/DPC4 mutations with loss of nuclear smad4 labeling frequently occur in pancreatic adenocarcinomas, but not in extrapancreatic malignancies.34 SMAD4 mutations have recently been associated with poor prognosis and with the development of widespread metastases in pancreatic cancer.33,35

An additional tumor suppressor pathway that can be altered in pancreatic cancers involves STK11/LKB1 on chromosome arm 19p. Germline mutations of STK11/LKB1 are responsible for Peutz-Jeghers syndrome and are associated with intraductal papillary mucinous neoplasms (IPMNs) and invasive pancreatic cancer. In addition to germline mutations, somatic mutations of STK11/LKB1 are observed in 5% of patients with sporadic IPMNs and pancreatic cancers.36,37 Other tumor suppressor genes, including TGFBR2,38 MAP2K4/MKK4,39,40 FBXW7,12 and ACVR1B4, are inactivated in a small subset of pancreatic cancers. The genetically altered genes involved in pancreatic cancer are summarized in Table 1.

Table 1
List of Selected Genes That Are Genetically Altered in Pancreatic Cancer

3. Genetics of Precursor Lesions

There are 3 histologically recognized precursor lesions of pancreatic cancer: PanINs, IPMNs, and mucinous cystic neoplasms (MCNs).4245 Pancreatic intraepithelial neoplasia lesions are microscopic papillary or flat noninvasive epithelial neoplasms (<0.5 mm) arising in pancreatic ducts characterized by mucin-containing cuboidal to columnar cells.

Pancreatic intraepithelial neoplasia lesions can be further classified according to the degree of cytologic and architectural atypia as PanIN-1, PanIN-2, and PanIN-3.43,44 Two distinct genetic events occur in early low-grade PanIN lesions (PanIN-1): telomere shortening and KRAS2 gene mutations.8,10,11,46,47 Activating point mutations of KRAS2 occur in approximately 45% of PanIN-1 lesions.8,10,11,47 Telomere shortening is found in approximately 90% of PanIN-1 lesions and may contribute to global chromosomal abnormalities in PanINs.46 Inactivating mutations of CDKN2A/p16 begin to occur in PanIN-2 lesions, while inactivation of TP53, SMAD4/DPC4, and BRCA2 are generally associated with higher-grade PanIN lesions(PanIN-3).31,48

Intraductal papillary mucinous neoplasms are mucin-producing epithelial neoplasms, usually with papillary architecture; they arise from the main pancreatic duct or branch ducts.44 These neoplasms are larger lesions than PanINs (≥1 cm) and therefore can be detected by imaging.44 Activating point mutations of KRAS2 occur in approximately 50% of IPMNs with low-grade dysplasia, and the prevalence of KRAS mutations increases with the degree of dysplasia.4951 Inactivating mutations of CDKN2A/p16 and TP53 are found in IPMNs with high-grade dysplasia.52 Loss of smad4 expression is observed in only a small subset of IPMNs (3%), whereas smad4 loss in PanIN3 occurs in approximately 30% of cases.53 As described above, somatic mutations of STK11/LKB1, with loss of the wild-type allele and corresponding inactivation of stk11 protein, occur in a small proportion of IPMNs.36,37,53

Mucinous cystic neoplasms occur predominantly in women.42 In contrast to IPMNs, MCNs do not have a connection with the pancreatic duct. In addition, MCNs are unique among pancreatic precursor lesions because of an associated ovarian-type stroma.42 As compared to PanINs and IPMNs, the genetic alterations of MCNs have not been well defined. Studies of MCNs5456 have reported a range in the prevalence of KRAS2 mutations and p53 overexpression, with the prevalence of abnormalities increasing with increasing degrees of dysplasia. One observation is that Smad4 mutation and loss of nuclear expression do not occur in most noninvasive MCNs. As with cancers arising from PanIN-3 lesions, smad4 expression is lost when infiltrating cancers arise from MCNs.29 This suggests that inactivation of SMAD4/DPC4 occurs in the late stages of neoplastic progression from MCNs.29

4. Genetics of Histologic Variants of Pancreatic Cancer

Several histologic variants of pancreatic cancer have been described, which include adenosquamous carcinoma, colloid carcinoma, medullary carcinoma, signet ring cell carcinoma, undifferentiated carcinoma, and undifferentiated carcinoma with osteoclast-like giant cells.42 Recognition of these variants is clinically important. Indeed, colloid and medullary carcinomas typically have better prognoses than the typical infiltrating ductal adenocarcinomas, and adenosquamous and undifferentiated carcinomas have worse prognoses than the typical ductal adenocarcinomas.5759 Furthermore, medullary carcinomas have distinct mechanisms of pathogenesis. We will briefly describe the genetic characteristics of these histologic variants, but we recommend a more comprehensive review for more in-depth discussion.60

Adenosquamous carcinomas contain both glandular and squamous components.42 The squamous component, by definition, comprises at least 30% of the neoplasm. Adenosquamous carcinomas share similar genetic features with ductal adenocarcinomas, including KRAS2 mutations and inactivation of CDKN2A/p16, SMAD4/DPC4, and/or TP53.58 The squamous component expresses p63, which is a helpful finding for identifying squamous components. Recognition of adenosquamous carcinoma is clinically important because it is associated with worse survival than adenocarcinomas.58

Medullary carcinomas are characterized by well-defined pushing border, syncytial growth pattern, and poorly differentiated cancer cells.59,61,62 Similar to medullary carcinomas of the colorectum, medullary carcinomas of the pancreas are often microsatellite unstable; this is caused either by germline or somatic mutation of the mismatch repair genes MHL1 and MSH2 or by epigenetic silencing of MLH1 by promoter methylation.22,59,61,62 Medullary carcinomas are associated with a better prognosis than ductal adenocarcinomas. Medullary colorectal cancers (with microsatellite instability) respond poorly to 5-fluorouracil–based chemotherapy, but it is not known if this 5-fluorouracil resistance applies to medullary carcinoma of the pancreas.63

Colloid carcinomas are characterized by well-differentiated neoplastic cells floating in pools of extracellular mucin; by definition, the mucin pools should comprise at least 80% of the tumor.57 The neoplastic cells have intestinal differentiation and label with antibodies to MUC2 and/or CDX2.64,65 Colloid carcinomas are associated with a better prognosis than ductal adenocarcinomas.57

Undifferentiated carcinomas lack histologic features of differentiation.42,5961 The median survival time for patients with undifferentiated pancreatic adenocarcinoma is only 5 months after surgical resection.66 Undifferentiated carcinomas are noncohesive cancers characterized by the loss of E-cadherin protein expression.67 The expression of L1CAM, COX2, and EGFR proteins in undifferentiated carcinomas have been noted as possible future targets of inhibitor-based treatments.68

Undifferentiated carcinomas with osteoclast-like giant cells are composed of cytologically benign, multinucleated, osteoclast-like giant cells admixed with atypical pleomorphic mononuclear cells.42 Frequently, undifferentiated carcinomas with osteoclast-like giant cells occur in association with noninvasive precursor lesions and share mutations with the associated noninvasive precursor lesions.6973

5. Genetics of Familial Pancreatic Cancer

Up to 10% of pancreatic cancers have a familial basis.74 Several cohort and case-control studies75,76 report that individuals with first-degree relatives who have pancreatic cancer are at significantly greater risk for pancreatic cancer, a risk that increases with the number of affected relatives. Thus, the risk for pancreatic cancer in individuals with 1 first-degree relative with pancreatic cancer is 2-fold higher than that for an individual without an affected first-degree relative; persons with 2 affected first-degree relatives have a 6-fold increased risk; and persons with 3 or more affected first-degree relatives have a 14- to 32-fold increased risk for pancreatic cancer.75,76

Several genetic syndromes are linked to the development of familial pancreatic cancer. Hereditary breast and ovarian cancer syndrome is an autosomal, dominantly inherited disease characterized by early development of breast and ovarian cancer and germline mutation of BRCA2 and BRCA1.74 Germline mutation of BRCA2 increases risk for pancreatic cancer 3.5- to 10-fold.7779 BRCA2 is a member of the Fanconi anemia gene family, and the function of the BRCA2 gene product is to repair DNA interstrand cross-links and double-strand breaks.80 Pancreatic cancer cells with BRCA2 mutation are hypersensitive to DNA interstrand cross-linking agents, including mitomycin C, cisplatin, and poly(ADP-ribose) polymerase inhibitors.8183 Peutz-Jeghers syndrome is an autosomal, dominantly inherited disease characterized by hamartomatous polyps of the gastrointestinal tract and pigmented macules of the lips and buccal mucosa.84 Germline mutations of STK11/LKB1 are responsible for Peutz-Jeghers syndrome, and patients with this syndrome have a very high lifetime risk for pancreatic cancer (up to 132-fold).84,85 As we described above, pancreatic cancers from patients with Peutz-Jeghers syndrome develop as IPMNs.

Familial atypical multiple mole melanoma (FAMMM) is an autosomal, dominantly inherited disorder characterized by multiple nevi and atypical nevi and an increased risk for malignant melanoma.86,87 Germline mutations of CDKN2A/p16 cause FAMMM, and patients with FAMMM and mutated CDKN2A/p16 have a 47-fold increased risk for pancreatic cancer.88 Hereditary pancreatitis is characterized by recurrent attacks of pancreatitis at a young age. Germline mutations of PRSS1 are associated with a markedly increased risk for hereditary pancreatitis and a 53-fold increased risk for pancreatic cancer.8992 Variants in SPINK1 are associated with a moderate increased risk for pancreatitis.

Hereditary nonpolyposis colorectal cancer syndrome (HNPCC) is an autosomal, dominantly inherited disease characterized by early onset of right-sided colon cancer as well as an increased risk for endometrial cancer and carcinomas of the small intestine, stomach, endometrium, ovary, bile duct, and kidney.93 Germline mutations of mismatch repair genes, including MLH1, MSH2, PMS1, PMS2, and MSH6, are associated with HNPCC. When pancreatic cancers arise in patients with HNPCC, they usually have a characteristic medullary phenotype.

Familial adenomatous polyposis (FAP) syndrome is an autosomal, dominantly inherited disease characterized by the presence of more than hundreds of polyps in the colon at an early age.94,95 Germline mutation of APC is linked with FAP. Patients with FAP have a 4-fold increased risk for pancreatic cancer.96

Genetic syndromes associated with familial pancreatic cancer are summarized in Table 2.

Table 2
Genetic Syndromes Associated With Familial Pancreatic Cancer

EPIGENETICS OF PANCREATIC CANCER

Epigenetics is defined as heritable changes in gene expression without accompanying changes in DNA sequence.97 The main epigenetic mechanisms that may affect gene expression include DNA methylation, histone modification, and microRNA expression.

1. DNA Methylation

DNA methylation is the covalent binding of a methyl group (CH3-) to the 5-carbon of cytosine residues. This methyl-group binding is catalyzed and maintained by a family of enzymes, DNA methyltransferases (DNMTs), including DNMT1, DNMT3A, and DNMT3B. DNMT1 is involved in preserving parental methylation patterns and transferring these patterns to offspring. DNMT3A and DNMT3B are involved in de novo methylation.98100 Approximately 80% of pancreatic cancers overexpress dnmt1 protein.101

A major pattern of DNA methylation occurs in CpG islands. CpG islands are stretches of DNA with a high CG nucleotide content (>50%).102 The CpG islands are frequently located near the transcriptional start sites of genes. About 60% of human genes have associated CpG islands; for many years CpG islands were thought to be unmethylated except during genomic imprinting and X-chromosome inactivation,103 but more recent evidence indicates that some CpG islands are methylated in a tissue-specific manner,104 and CpG island methylation increases with age at many loci.105,106 Aberrant hypermethylation of promoter CpG islands is tightly associated with gene silencing and may be associated with loss of tumor suppressor function in cancer.107

Aberrant Hypermethylation in Pancreatic Cancer

Several classic tumor suppressor genes, as well as increasing numbers of functionally important genes, show aberrant promoter CpG island hypermethylation in a subset of pancreatic cancers. The first tumor suppressor gene that was shown to undergo promoter hypermethylation and silencing in pancreatic cancer was CDKN2A/p16.21 Other genetically inactivated tumor suppressor genes in pancreatic cancers, including TP53, MADH4/DPC4, and STK11/LKB1, have not been shown to undergo epigenetic silencing by DNA methylation.

MLH1 on chromosome arm 3p undergoes DNA methylation in pancreatic cancers and is associated with microsatellite instability in medullary carcinomas.22,108,109 The CDH1 gene on chromosome arm 16q, which encodes E-cadherin protein, shows aberrant methylation in a small fraction of pancreatic cancers.22

SPARC, located on chromosome arm 5q, encodes a calcium-binding protein that interacts with extracellular matrix.110 Sparc has effects on cellular migration, proliferation, angiogenesis during wound healing, cell-matrix adhesion, and tissue remodeling.110 In pancreatic and other cancers, Sparc expression is usually lost through abnormal DNA methylation.110 Pancreatic cancer–associated peritumoral fibroblasts often express Sparc, and patients with pancreatic cancer and sparc-expressing peritumoral fibroblasts were reported to have a poorer survival in 1 study.111

Other cancer-related genes that have been shown to undergo abnormal methylation and induced gene silencing include RELN,112 CCND2,105 TFPI2,113 RUNX3,114 SOCS-1,115 and TSLC1/IGSF4.116

Genome-wide screening has made it possible to identify epigenetic alterations in novel genes within the setting of pancreatic cancer. Ueki and colleagues4 used methylated CpG island amplification with representational difference analysis to identify differentially methylated CpG islands in pancreatic cancer. PENK (preproenkephalin) on chromosome arm 8q was one of the genes identified by this method, and more than 90% of pancreatic cancers in this study had aberrantly methylated PENK.4,117 Using oligo-nucleotide microarrays, Sato and colleagues5 identified a total of 475 candidate genes that were induced by a DNMT inhibitor (5-aza-2′-deoxycytidine) in 4 pancreatic cancer cell lines, but not in HPDE (a nonneoplastic pancreatic ductal epithelial cell line). Of these 475 genes, UCHL1 on chromosome arm 4p was methylated in all 42 pancreatic cancers studied.5 RPRM on chromosome arm 2q was methylated in 80% of pancreatic cancers studied and was associated with a worse prognosis.118 More recently, Omura and colleagues3 applied the methylated CpG island amplification technique to an Agilent 44K promoter microarray (Agilent Technologies, Santa Clara, California) and identified 606 differentially methylated genes in pancreatic cancer cell lines compared to normal pancreas.

A selected list of genes that are aberrantly hypermethylated in pancreatic cancer is summarized in Table 3.

Table 3
List of Selected Genes That Are Aberrantly Methylated in Pancreatic Cancer

Aberrant Methylation in Precursor Lesions

The discovery of abnormal methylation in pancreatic cancer has been followed by the investigation of methylation in precursor lesions. Many genes that are epigenetically silenced in pancreatic cancers also are silenced or have reduced expression in precursor lesions of pancreatic cancer. For example, global gene expression profiles of IPMN were compared with those of normal pancreatic ductal epithelial samples.119 CDKN1C/p57KIP2 on chromosome arm 11p codes for an inhibitor of cyclin/CDK complexes and negative regulator of cellular proliferation.120,121 Partial methylation of the CDKN1C/p57KIP2 promoter CpG islands in IPMNs and pancreatic cancer cell lines was correlated with a corresponding decrease in cdkn1c protein expression.119

Other genes identified in precursor lesions include PENK, CDKN2A/p16, STK11/LKB1, SPARC, SFRP1/SARP2 (chromosome arm 8p), TSLC1, RELN (chromosome arm 7q), TFPI2, CLDN5 (chromosome arm 22q), and UCHL1 in IPMNs37,122,123; PENK, CDKN2A/p16, CLDN5, NPTX2, RPRM, SFRP1/SARP2, and LHX1 (chromosome arm 11p) in PanINs117,118,124; and CDKN2A/p16 in mucinous cystic neoplasms.56 A selected list of genes that are aberrantly hypermethylated in pancreatic precursor lesions is summarized in Table 4.

Table 4
List of Selected Genes That Are Aberrantly Hypermethylated in Pancreatic Precursor Lesions

The degree of methylation for these genes positively correlates with the degree of cytologic and architectural atypia. These findings suggest that aberrant CpG island methylation begins in the earliest stages of precursor lesions, such as PanINs, IPMNs, and MCNs, and their prevalence progressively increases during pancreatic carcinogenesis.

Aberrant Hypomethylation in Pancreatic Cancer

In addition to hypermethylation as a mechanism of carcinogenesis, aberrant loss of methylation (hypomethylation of DNA) is also common in pancreatic adenocarcinomas. Hypomethylation can be detected at the genomic scale (global hypomethylation) and at the sequence-specific level (regional hypomethylation). Although global DNA hypomethylation associated with cancer was firstly described in the early 1980s,125,126 its significance is not known, but it may contribute to genomic instability. Folate and vitamin B12 deficiency can cause global DNA hypomethylation, which is associated with decreased levels of the methyl-group donor S-adenosylmethionine. Decreased DNA methylation results in decreased thymidine synthesis from uracil.127 Misplacement of uracil into thymidine leads to an imbalance of nucleotide pools and an increased frequency of DNA strand breaks; this can lead to genomic instability that can promote the development of cancer.128,129 Pancreatic cancers with defective methylenetetrahydrofolate reductase genotypes have more DNA hypomethylation, which is associated with increased chromosomal loss and genomic instability.130

DNA hypomethylation occurs at the 5′ regions of certain genes in pancreatic cancer and is associated with overexpression of the encoded protein. Thus, whereas hypermethylation results in overregulation and silencing of gene and protein expression, hypomethylation can result in loss of regulation and the promotion of gene and protein expression. S100A4 is linked with hypomethylation at specific CpG sites within the first intron and is associated with protein overexpression.131,132 Other frequently hypomethylated genes, including CLDN4 (chromosome arm 7q, encoding claudin-4), LCN2 (chromosome arm 9q, encoding lipocalin-2), SFN/14-3-3σ (chromosome arm 18q), TFF2 (chromosome arm 21q, encoding trefoil factor 2), MSLN (chromosome arm 16p, encoding mesothelin), and PSCA (8q, encoding prostate stem cell antigen), are overexpressed in pancreatic cancer cells in comparison with normal pancreatic duct.132 With oligo-nucleotide microarray technologies, 2 additional genes, S100P (chromosome arm 4p) and SERPINB5 (chromosome arm 18q, encoding maspin), have been identified as being hypomethylated and are overexpressed.6 A selected list of genes that are aberrantly hypomethylated in pancreatic cancer is summarized in Table 2.

2. MicroRNAs

Aberrant MicroRNA Expression in Pancreatic Cancers and Precursor Lesions

MicroRNAs (miRNAs) are a recently described family of small, nonprotein-coding RNA molecules (18 to 24 nucleotides) that regulate transcription of target messenger RNAs.133 More than 400 miRNAs in the human genome have been described and many are implicated in the regulation of cellular differentiation, proliferation, and apoptosis.23 Aberrant miRNA expression has been described in many types of cancers.134,135 Several mechanisms are involved in aberrant miRNA expression, including genetic (amplification and deletion)136138 and epigenetic (chromatin modification, DNA methylation) alterations139141 and transcription factor regulation.142,143

Pancreatic ductal adenocarcinomas have been shown to aberrantly express numerous miRNAs, including miR-200, miR-34, miR-21, miR-155, miR-221, and miR-222.144151 For example, Li and colleagues152 have demonstrated hypomethylation and overexpression of miR-200a and miR-200b. Aberrant expression of some of these miRNAs is evident in PanINs. For example, miR-155 overexpression is evident in PanIN-2 lesions and aberrant miR-21 expression is evident in PanIN-3 lesions.153 Similarly, Habbe et al154 have reported abnormal miR-21 and miR-155 expression in IPMN lesions.

CONCLUSIONS

Pancreatic ductal adenocarinoma continues to be a fatal cancer that is difficult to treat. In the past decade, major advances have been made in the understanding of the earliest histologic and molecular changes that occur in precursor lesions and cancers of the pancreas. Subclassification of pancreatic adenocarcinomas according to its histologic features and molecular alterations could have important therapeutic and prognostic importance. In addition, the identification of molecular signatures that identify the earliest changes of carcinogenesis may lead to the earlier detection of pancreatic cancer. The survival data for pancreatic cancer clearly illustrate that patients do much better with earlier detection and surgical resection regardless of adjuvant chemotherapy or radiotherapy intervention. Understanding the signature of molecular alterations that occur before the development of invasive pancreatic cancer may lead to improved detection and survival in pancreatic cancer.

Acknowledgments

This work was supported by National Institutes of Health (NIH) grants (P50-CA62924, R01-CA120432, RO1-CA97075) and the Michael Rolfe Foundation.

Footnotes

The authors have no relevant financial interest in the products or companies described in this article.

Presented at the 9th Spring Seminar of the Korean Pathologists Association of North America (KOPANA), Washington, District of Columbia, March 18–20, 2010, in conjunction with the 99th Annual Meeting of the United States and Canadian Academy of Pathology (USCAP).

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