December 2002 Vol 2 No 12

Nature Reviews Cancer

2 , 897-909 (2002); doi:10.1038/nrc949

December 2002 Vol 2 No 12 preface

Epidemiology of pancreatic cancer

Molecular genetics of pancreatic adenocarcinoma

Pancreatic cancer biology

Future directions boxes links glossary references about the authors

REVIEW

Nature Reviews Cancer

2 , 897-909 (2002); doi:10.1038/nrc949

PANCREATIC CANCER BIOLOGY AND GENETICS

Nabeel Bardeesy & Ronald A. DePinho

about the authors

Department of Adult Oncology, Dana–Farber Cancer Institute and Departments of Medicine and

Genetics, Harvard Medical School, Boston, Massachusetts 02115, USA.

correspondence to: Ronald A. DePinho Correspondence to: Ron_Depinho@dfci.harvard.edu

Pancreatic ductal adenocarcinoma is an aggressive and devastating disease, which is characterized by invasiveness, rapid progression and profound resistance to treatment. Advances in pathological classification and cancer genetics have improved our descriptive understanding of this disease; however, important aspects of pancreatic cancer biology remain poorly understood. What is the pathogenic role of specific gene mutations? What is the cell of origin? And how does the stroma contribute to tumorigenesis? A better understanding of pancreatic cancer biology should lead the way to more

effective treatments.

PANCREATIC ADENOCARCINOMA

States 1

is the fourth leading cause of cancer death in the United

. With a 5-year survival rate of only 3% and a median survival of less than 6 months, a diagnosis of pancreatic adenocarcinoma carries one of the most dismal prognoses in all of medicine 2 . Due to a lack of specific symptoms and limitations in diagnostic methods, the disease often eludes detection during its formative stages.

Whipple and colleagues reported the first PANCREATICODUODENECTOMY in 1935 and surgery has since offered the only possibility of cure, although surgical intervention alone rarely achieves a curative end point 2 . For the 15–20% of patients who undergo potentially curative resection, the 5-year survival is only 20% (Ref. 3 ). Some improvements in surgical outcome occur in patients who also receive chemotherapy and/or radiotherapy, although the impact on long-term survival has been minimal owing to the intense resistance of pancreatic adenocarcinoma to all extant treatments. So, management of most patients focuses on palliation.

A recent National Cancer Institute 'think tank' (known as a Progress Review Group) articulated the crucial questions and challenges facing the field and provided recommendations to address key unmet needs in the clinical and basic research arenas 4

(see Pancreatic Cancer — Agenda for Action in online links box). Many hurdles and opportunities were noted, and among these were the overarching needs to conduct a more penetrating analysis of pancreatic cancer biology and to develop refined animal models of pancreatic adenocarcinoma. The identification of signature gene mutations in pancreatic adenocarcinoma was recognized as a valuable starting point, providing a conceptual framework to guide the future analysis of complex aspects of this disease.

How these genetic changes translate into the classical biological features of pancreatic cancer cells stands as a key area for increased active investigation. It is anticipated that more sophisticated genetic-engineering strategies will generate refined mouse models for the systematic dissection of the pathophysiological impact of various genetic alterations, and elucidation of the complexities of intersecting signalling pathways that control cellular growth, survival and differentiation. So how can information and technological advances be integrated to create a 'roadmap' for an improved understanding of pancreatic cancer biology, and how might such systems lead to more effective treatments?

Epidemiology of pancreatic cancer

Pancreatic adenocarcinoma is generally thought to arise from pancreatic ductal cells

( Fig. 1 ; Table 1 ); however, this remains an area of ongoing study (see below) 14 , 147 . The aetiology of pancreatic adenocarcinoma remains poorly defined, although important clues of disease pathogenesis have emerged from epidemiological and genetic studies.

Pancreatic adenocarcinoma is a disease that is associated with advancing age 5 — rare before the age of 40, it culminates in a 40-fold increased risk by the age of 80.

Environmental factors might modulate pancreatic adenocarcinoma risk 5 . On the genetic level, numerous studies have documented an increased risk in relatives of pancreatic adenocarcinoma patients (approximately threefold), and it is estimated that 10% of pancreatic cancers are due to an inherited predisposition 6 . As with most cancer types, important insights have emerged from the study of rare kindreds that show an increased incidence of pancreatic adenocarcinoma. However, unlike familial cancer syndromes for breast, colon and melanoma, pancreatic adenocarcinoma that is linked to a familial setting has a lower penetrance (<10%) and maintains a comparable age of onset to sporadic cases in the general population. Among the genetic lesions that are linked to familial pancreatic adenocarcinoma are germline mutations in CDKN2A (which encodes two tumour suppressors — INK4A and ARF ), BRCA2 , LKB1 and MLH1 (Ref. 7 ). The low penetrance of pancreatic adenocarcinoma that is associated with these germline mutations might point to a role in the malignant progression of precursor lesions rather than in the limiting events that control initiation of neoplastic growth from normal pancreatic cells. With respect to CDKN2A and BRCA2, this notion gains experimental support from the observation that inactivation of these genes is not detected in premalignant ductal lesions that are thought to represent early stages of pancreatic tumorigenesis (see below).

Figure 1 | Anatomy of the pancreas.

The pancreas is comprised of separate functional units that regulate two major physiological processes: digestion and glucose metabolism 136 . The exocrine pancreas consists of acinar and duct cells. The acinar cells produce digestive enzymes and constitute the bulk of the pancreatic tissue. They are organized into grape-like clusters that are at the smallest termini of the branching duct system. The ducts, which add mucous and bicarbonate to the enzyme mixture, form a network of increasing size, culminating in main and accessory pancreatic ducts that empty into the duodenum. The endocrine pancreas, consisting of four specialized cell types that are organized into compact islets embedded within acinar tissue, secretes hormones into the bloodstream. The - and> -cells regulate the usage of glucose through the production of glucagon and insulin, respectively. Pancreatic polypeptide and somatostatin that are produced in the PP and> cells modulate the secretory properties of the other pancreatic cell types.>a | Gross anatomy of the pancreas. b | The exocrine pancreas. c | A single acinus. d | A pancreatic islet embedded in exocrine tissue.

Table 1 | Types of pancreatic neoplasms

Beyond the above classical tumour-suppressor mutations, additional genetic defects seem to be operative in rare families in which pancreatic cancer is inherited as an autosomal-dominant trait with very high penetrance 6 . A pancreatic cancer syndrome — so far identified in a single family — has been linked to chromosome 4q32-34 (Ref. 8 ) and is associated with diabetes, pancreatic exocrine insufficiency and pancreatic adenocarcinoma, with a penetrance approaching 100%. Patients with hereditary pancreatitis , which is associated with germline mutations in the cationic trypsinogen gene PRSS1 , experience a 53-fold increased incidence of pancreatic adenocarcinoma 9 , 10 .

Mutations in PRSS1 cause the encoded enzyme either to be more effectively autoactivated or to resist inactivation and, consequently, to display deregulated proteolytic activity. It is assumed that the resulting inflammation promotes tumorigenesis, in part, by producing growth factors, cytokines and reactive oxygen species (ROS), thereby inducing cell proliferation, disrupted cell differentiation and selecting for oncogenic mutations. We speculate that increased cell turnover and ROS might also lead to increased telomere attrition and dysfunction, setting the stage for potentially oncogenic genomic instability (see below).

Molecular genetics of pancreatic adenocarcinoma

Signalling pathways and tumour progression. A careful molecular and pathological analysis of evolving pancreatic adenocarcinoma has revealed a characteristic pattern of genetic lesions. The field is now faced with the challenge of understanding how these signature genetic lesions — mutations of KRAS , CDKN2A, TP53 , BRCA2 and

SMAD4 /DPC4 — contribute to the biological characteristics and evolution of this disease.

The progression model for colorectal cancer has served as a template for relating sequential, defined mutations to increasingly atypical growth states 11 the years ahead.

. Whether such a progression series exists for pancreatic adenocarcinoma will draw continued scrutiny in

The pancreatic-duct cell is generally believed to be the progenitor of pancreatic adenocarcinoma. As defined in Cubilla and Fitzgerald's classic study neoplasia, PanIN) 5 , 15

12 , the increased incidence of abnormal ductal structures (now designated pancreatic intraepithelial

in patients with pancreatic adenocarcinoma, and the similar spatial distribution of such lesions to malignant tumours, are consistent with the hypothesis that such lesions might represent incipient pancreatic adenocarcinoma. Histologically,

PanINs show a spectrum of divergent morphological alterations relative to normal ducts that seem to represent graded stages of increasingly dysplastic growth 14 ( Fig. 2 ). Cell proliferation rates increase with advancing PanIN stages, which is consistent with the idea that these are progressive lesions 15 . A growing number of studies have identified common mutational profiles in simultaneous lesions, providing supportive evidence of

the relationship between PanINs and the pathogenesis of pancreatic adenocarcinoma.

Specifically, common mutation patterns in PanIN and associated adenocarcinomas have been reported for KRAS and for CDKN2A

HETEROZYGOSITY

16 . In addition, similar patterns of LOSS OF

(LOH) at chromosomes 9q, 17p and 18q (harbouring CDKN2A, TP53 and SMAD4, respectively) have been detected in coincident lesions (see below), and studies have consistently shown an increasing number of gene alterations in highergrade PanINs 17-20 . Intriguingly, there seems to be an ordered series of mutational events in association with specific neoplastic stages 7 .

Figure 2 | Genetic progression model of pancreatic

adenocarcinoma.

Pancreatic intraepithelial neoplasias (PanINs) seem to represent progressive stages of neoplastic growth that are precursors to pancreatic adenocarcinomas. The genetic alterations documented in adenocarcinomas also occur in PanIN in what seems to be a temporal sequence, although these alterations have not been correlated with the acquisition of specific histopathological features. The stage of onset of these lesions is depicted. The thickness of the line corresponds to the frequency of a lesion.

The temporal alterations in telomerase activity and telomere length are by inference from

Refs 62 , 139 and need further substantiation in PanIN. Normal duct, PanIN-1A/PanIN-1B and

PanIN-3 images reproduced with permission from ref. 14 (2001) Lippincott Williams &

Wilkins; PanIN-2 and adenocarcinoma images kindly provided by Dr Ralph Hruban, Johns

Hopkins University (http://pathology.jhu.adu/pancreas/panin/).

KRAS. The earliest ductal lesions do not usually display genetic alterations. Activating

KRAS mutations are the first genetic changes that are detected in the progression series, occurring occasionally in histologically normal pancreas and in about 30% of lesions that show the earliest stages of histological disturbance ( Fig. 2 ; see Ref. 13 and references therein). KRAS mutations increase in frequency with disease progression, and are found in nearly 100% of pancreatic adenocarcinomas; they seem to be a virtual rite of passage for this malignancy coordinately induced with the onset of KRAS mutations, perhaps due to activation of the mitogen-activated protein kinase

21 . WAF1 (also known as p21 and CIP1) seems to be

(MAPK) PATHWAY 22 .

Activating mutations of RAS-family oncogenes produce a remarkable array of cellular effects, including induction of proliferation, survival and invasion through the stimulation of several effector pathways (reviewed in Ref. 23 ). Although the roles of specific KRAS effector pathways in pancreatic cancer pathogenesis have not been resolved, there is evidence for an important contribution of AUTOCRINE epidermal growth-factor ( EGF )family signalling 24-27 , 31 . This autocrine loop and resulting stimulation of the phosphatidylinositol 3-kinase (PI3K) PATHWAY is required for transformation of several cell lineages by RAS-family oncogenes 28 . Consistent with the existence of such an autocrine loop, pancreatic adenocarcinomas overexpress EGF-family ligands (such as transforming growth factor- ( TGF) and EGF) and receptors ( EGFR , ERBB2 (also known as HER2/neu) and ERBB3 ) 24 , 26 , 28 . EGFR and ERBB2 induction occurs in low-grade

PanINs, indicating that autocrine EGF-family signalling might be operative at the earliest stages of pancreatic neoplasia 29 . The functional importance of this pathway is illustrated by the growth inhibition of pancreatic adenocarcinoma cell lines in vitro and in xenografts following attenuation of EGFR signalling by blocking antibodies or expression of dominant-negative EGFR alleles pathogenic role of activated KRAS, and a deeper understanding of this oncogenic programme will be vital for the development of new treatment approaches towards this disease.

27 , 30 , 31 . Many pathways are likely to contribute to the

CDKN2A. Germline mutations in the CDKN2A tumour-suppressor gene are associated with the familial atypical mole-malignant melanoma (FAMMM) syndrome . In addition to a very high incidence of melanoma, the inheritance of mutant CDKN2A alleles confers a

13-fold increased risk of pancreatic cancer 32 , 33 . Although pancreatic adenocarcinoma arises in some, but not all, FAMMM kindreds with CDKN2A mutations, there are no clear genotype–phenotype associations, indicating a modulating role for environmental factors in disease penetrance 34 , 35 . FAMMM kindreds that harbour mutant loci other than

CDKN2A, such as cyclin-dependent kinase 4 ( CDK4 ) alleles that abrogate INK4A binding

or other uncharacterized loci, do not have increased incidence of pancreatic adenocarcinoma 33 , 36 .

Loss of CDKN2A function — brought about by mutation, deletion or promoter hypermethylation — also occurs in 80–95% of sporadic pancreatic adenocarcinomas 21 .

CDKN2A loss is generally seen in moderately advanced lesions that show features of dysplasia ( Fig. 2 ). The dissection of the role of CDKN2A has been a fascinating story as this tumour-suppressor locus, at 9q21, encodes two tumour suppressors — INK4A and

ARF — via distinct first exons and alternative reading frames in shared downstream exons 37 . Given this physical juxtaposition and frequent homozygous deletion of 9p21 (in

40% of tumours), many pancreatic cancers sustain loss of both the INK4A and ARF transcripts, thereby disrupting both the retinoblastoma ( RB ) and p53 tumoursuppression pathways. INK4A inhibits CDK4/ CDK6 -mediated phosphorylation of RB, thereby blocking entry into the S (DNA synthesis) phase of the cell cycle; ARF stabilizes p53 by inhibiting its> MDM2 -dependent proteolysis. INK4A seems to be the more important pancreatic cancer suppressor at this locus, as germline and sporadic mutations have been identified that target INK4A, but spare AR> 21 , 38 , 39 .

The INK4A transcript displays a highly restricted expression pattern and is dispensable for normal development and homeostasis 40-43 . When primary cells are placed into culture, INK4A-transcript expression is induced, and this seems to be a stress response to the inappropriate growth environment that is associated with in vitro culture 44 , 45 ; similar INK4A induction is observed in vivo in association with reactive processes 43 . This induction by environmental stress and aberrant proliferative signals provides a plausible basis for the tumour-suppression function of INK4A, although the relationship of this phenomenon to cancer suppression in vivo is not established. Recent studies have also implicated INK4A in the cellular response to DNA damage in vivo 46 , so the absence of

INK4A might also contribute to the chemoresistance of pancreatic adenocarcinoma.

In human fibroblasts, high levels of activated RAS genes induce INK4A, which, in turn, results in premature senescence 47-49 — a presumed defence mechanism against oncogenic stress. This had been thought to contribute to the coincident mutations of

RAS and CDKN2A in cancer, but numerous observations now indicate that this simple model might require reconsideration. Notably, KRAS mutations are often detected in non-neoplastic states, such as chronic pancreatitis, and possibly in normal pancreas lesions

50 .

Furthermore, several different KRAS mutations might be detected in individual PanIN

16 , 51 . Loss of INK4A usually occurs only in later stages of pancreatic neoplasia.

These data indicate that, rather than inducing premature senescence, KRAS mutations confer a survival advantage in vivo. This is corroborated by studies in mouse cells in which an activated KRAS allele, which is expressed at physiological levels, provokes immortalization of fibroblasts in vitro or adenoma formation of lung epithelial cells in vivo 52 (D. Tuveson and T. Jacks, personal communication). Although loss of INK4A probably facilitates the oncogenicity of activated RAS alleles, as shown in animal models 53 , 54 , its occurrence later in pancreatic tumour progression indicates that the intersection of these pathways might require other events, such as disrupted contacts with the extracellular matrix or elevations in the level of activated KRAS.

TP53. The TP53 tumour-suppressor gene is mutated, generally by missense alterations of the DNA- binding domain, in more than 50% of pancreatic adenocarcinomas 21 . TP53 mutations arise in later-stage PanINs that have acquired significant features of dysplasia, reflecting the function of TP53 in preventing malignant progression. In contrast to many other cancer types, there does not seem to be a reciprocal relationship in the loss of CDKN2A and TP53 (Refs 22 , 55 ), which points to non-overlapping functions for ARF and p53 in pancreatic cancer suppression. TP53 loss probably facilitates the rampant genetic instability that characterizes this malignancy. These tumours have profound aneuploidy and complex cytogenetic rearrangements, as well as intratumoral heterogeneity, which is consistent with ongoing genomic rearrangements 57 , 144 .

Cytogenetic studies have provided evidence that telomere dynamics might contribute to this genomic instability. Although reactivation of telomerase is crucial to the emergence of immortal cancer cells, a preceding and transient period of telomere shortening and dysfunction might also contribute to carcinogenesis by leading to the formation of chromosomal rearrangements through breakage–fusion–bridge cycles 56 , 58 ( Fig. 3 ). The survival of cells with critically short telomeres (crisis), which continue to go through

breakage–fusion–bridge events, is enhanced by inactivation of the p53-dependent DNAdamage response 59 , allowing the acquisition of oncogenic chromosomal alterations 61 .

Studies in the telomerase-knockout mouse support this model, as telomere dysfunction and p53 loss cooperate to promote the development of carcinomas in multiple tissues 58 .

An analysis of a large series of human pancreatic cancer cell lines revealed that telomeres were frequently lost from chromosome ends and that anaphase bridging occurred, indicating that persistent genomic instability is associated with critically short telomeres 60 , 61 . As these features were observed in both low- and high-grade tumours, the authors conclude that telomere dysfunction was an early step in the pathogenic process. Moreover, studies of pancreatic adenocarcinomas revealed that tumours have shortened telomere length and that the activation of telomerase is a late event 61 , 62 , 139 .

Telomere attrition due to increased cell turnover might contribute to the high incidence of pancreatic adenocarcinomas that are associated with hereditary pancreatitis. To establish more conclusively the role of crisis in the genesis of pancreatic adenocarcinoma, it will be of interest to specifically correlate telomere length, p53 status and the onset of genomic instability in PanINs, and to develop pancreatic cancer models in mouse strains with telomere dysfunction.

Figure 3 | Telomere attrition and genomic instability.

Most human somatic cells lack telomerase activity, hence telomeres are eroded as cells proliferate. If the proliferative stimulus is maintained, such as in cells that have sustained oncogenic mutations, progressive telomere shortening activates

DNA-damage responses, resulting in growth arrest. Loss of these checkpoint responses, such as by mutation of TP53, allows cells to continue proliferating, leading to telomere dysfunction and genomic instability (crisis). Chromosome breakage–fusion cycles produce severe aneuploidy and chromosomal translocations that contribute to tumour progression. Telomerase reactivation subsequently stabilizes the genome and facilitates the immortal growth of the tumour cells. Independent processes also lead to genomic instability — centrosome defects result in a disorganized mitotic spindle, which leads to aneuploidy or chromosome breakage; BRCA2 mutations produce genomic instability by disabling the homologous recombination-based DNA-repair pathway.

BRCA2. Inherited BRCA2 mutations are typically associated with familial breast and ovarian cancer syndrome, but also carry a significant risk for the development of pancreatic cancer. Approximately 17% of pancreatic cancers that occur in a familial setting harbour mutations in this gene 64 , 65 . As is the case for those individuals with germline CDKN2A mutations, the penetrance of pancreatic adenocarcinoma in BRCA2mutation carriers is relatively low ( 7%) and the age of onset is similar to that of patients with the sporadic form of the disease. Familial breast cancer alleles other than

BRCA2 do not seem to predispose to pancreatic adenocarcinoma.

Loss of wild-type BRCA2 seems to be a late event in those individuals who inherit germline heterozygous mutations of BRCA2, which is restricted to severely dysplastic

PanINs and adenocarcinomas 65 . Although the numbers are small, these patients do not show an elevated incidence of PanINs. These data are consistent with the possibility that

BRCA2 loss promotes the malignant progression of existing lesions in pancreatic neoplasia. BRCA2 is necessary for the maintenance of genomic stability by regulating the homologous-recombination-based DNA-repair processes; consequently, BRCA2 deficiency in normal cells results in the accumulation of lethal chromosomal aberrations 63 . The fact that BRCA2 is selectively mutated late in tumorigenesis probably reflects the need for DNA-damage-response pathways to be inactivated first — for example, by TP53 mutation — so that the damage incurred can be tolerated.

Chromosomal instability. Defects in the mitotic-spindle apparatus conferred by centrosome abnormalities might also contribute to the aneuploidy and genomic instability of pancreatic adenocarcinomas. Centro-some abnormalities are detected in

85% of pancreatic adenocarcinomas, and there is a correlation between levels of such abnormalities and the degree of chromosomal aberrations 66 . Overall, the loss of TP53 and BRCA2, and the detection of abnormal mitosis and severe nuclear abnormalities in

PanIN-3 lesions, indicate that genomic instability is initiated at this stage of tumour

progression.

These observations have several implications. First, the detection of clonal genetic alterations in PanINs and synchronous adenocarcinomas is consistent with the concept that PanINs are, indeed, neoplastic growths that are precursors to adenocarcinomas.

Although KRAS mutations are an early, and probably necessary, event in the development of pancreatic adenocarcinoma, their absence in the earliest lesions indicates that KRAS activation is not responsible for neoplastic initiation. This notion is supported by the observation of different KRAS mutations between PanINs of the same individual. One possibility is that the earliest lesions might be non-clonal areas of aberrant proliferation and altered states of differentiation that are associated with the replacement of damaged cells and with inflammatory processes. These disruptions in tissue architecture and induction of cell proliferation could produce a FIELD DEFECT in which there is significant selection for cells that sustain activating KRAS mutations.

Along these lines, inflammatory stimuli promote the expression of both TGF- and EGFR in the pancreatic ducts, providing a pathway that could synergize with activated KRAS 25 .

In addition to the extreme aneuploidy of pancreatic adenocarcinomas, there is a high degree of genetic heterogeneity within these tumours. For instance, different KRAS mutations and 9q, 17p and 18q LOH patterns have been observed in adjacent PanINs, and several KRAS mutations have been detected in the same adenocarcinomas 16-18 .

Importantly, it seems that there is spatial distribution of genetic heterogeneity 17 .

Neoplastic foci from adjacent regions tend to show similar mutation patterns, whereas increasing genetic divergence has been documented in more geographically distant foci.

It seems likely that adenocarcinomas can develop from the clonal progression of one of several related but divergent lesions. These features might indicate that a key event beyond the initiation of PanINs is the acquisition of a mutator state that allows initiated cells to acquire progression-associated genetic lesions. It is tempting to speculate that this tremendous degree of heterogeneity and ongoing instability lies at the heart of the resistance of pancreatic tumours to chemotherapy and radiotherapy.

The marked chromosomal abnormalities and the disruptions in DNA-repair processes in pancreatic adenocarcinoma might reflect the existence of additional loci, the genomic alterations of which contribute to the malignant progression. This is supported by the detection of recurrent chromosomal amplifications and deletions by COMPARATIVE

69 , 144 GENOMIC HYBRIDIZATION (CGH) and other cytogenetic methods . In addition to the signature losses of 17p, 9p and 18q, deletions of chromosomes 8p and 6q and 4q, and amplifications of chromosomes 8q, 3q, 20q and 7p, have been consistently reported.

Microsatellite instability. Microsatellite instability is a second mode of genomic instability that, in contrast to the large-scale alterations that are associated with chromosomal instability, is characterized by very high mutation rates at small DNA repeat sequences. This phenotype is caused by mutations in DNA mismatch-repair genes, including MLH1, MSH2 and MSH6 (Ref. 70 ) and is associated with hereditary non-polyposis colon cancer (HNPCC) syndrome . There seems to be an elevated risk of pancreatic cancer in HNPCC families 67 , 71 . The pancreatic adenocarcinomas in HNPCC patients show distinct molecular genetic profiles, such as a lower rate of KRAS and TP53 mutation, frameshift mutations in BAX and TGF RII, characteristic histopathology and a less-aggressive clinical course compared with pancreatic adenocarcinomas that occur outside of this syndrome 68 , 72 , 73 .

SMAD4/DPC4. Another frequent alteration in pancreatic adenocarcinoma is the loss of

SMAD4/DPC4 (Ref. 74 ), which encodes a transcriptional regulator that is a keystone component in the TRANSFORMING GROWTH FACTOR- (TGF- )-FAMILY SIGNALLING

CASCADE 75 . This gene maps to chromosome 18q21 — a region that sustains deletion in approximately 30% of pancreatic cancers 74 . The pathogenic role of SMAD4 inactivation is strongly supported by the identification of inactivating intragenic lesions of SMAD4 in a subset of tumours. SMAD4 seems to be a progression allele for pancreatic adenocarcinoma, as its loss occurs only in later-stage PanINs 18 , 19 ; moreover, there does not seem to be a strong predisposition to pancreatic adenocarcinoma in patients that inherit a germline SMAD4 mutation (that is, in juvenile polyposis syndrome patients).

Loss of SMAD4 is a predictor of decreased survival in pancreatic adenocarcinoma 20 , which is consistent with a role for it in disease progression. The mechanism by which

SMAD4 loss contributes to tumorigenesis is likely to involve its role in TFG- -mediated

growth inhibition. TGF> inhibits the growth of most normal epithelial cells by either blocking the G1–S cell-cycle transition or by promoting apoptosis 75 . The cellular responses to TGF- are partially, but not exclusively, SMAD4-dependent 76 and, correspondingly, pancreatic adenocarcinomas show a degree of TGF- resistance. The roles of TGF- signalling in pancreatic adenocarcinoma pathogenesis are not well defined. Studies have shown inconsistent effects of this cytokine on cultured cell lines with respect to cell proliferation rates and dependency on SMAD4 status for TGF- responsiveness 77-80 . These ambiguous results probably stem from the heterogeneity that is associated with cancer cell lines and the non-physiological conditions that are encountered in vitro. It seems that in-depth investigational approaches will be required to understand the contribution of this signalling pathway to pancreatic cancer biology.

SMAD4 loss is also likely to contribute to tumour progression through effects on tumour– STROMA interactions (see below).

LKB1/STK11. The Peutz–Jeghers syndrome (PJS), which is caused by LKB1/STK11 mutations, is another familial cancer syndrome that is associated with an increased incidence of pancreatic adenocarcinoma 81 benign intestinal polyposis at a young age

. PJS patients are primarily afflicted with

85 , although advancing age carries an increased risk of developing gastrointestinal malignancies, including a more than 40-fold increase in pancreatic adenocarcinoma 145 pancreatic neoplasia 84 , 86 , 87 .

. PJS is associated with loci other than LKB1 in some families; it will be important to evaluate the relative role of the different PJS loci in

Pancreatic cancer biology

Molecular pathology and cancer genetic studies have provided an outline of the cellular perturbations that are associated with pancreatic adenocarcinoma; however, the picture remains static with only correlative links to underlying biology at present. A more direct mechanistic view of how classical lesions influence pancreatic cancer biology is needed and some key questions need to be answered. What is the CELL OF ORIGIN of pancreatic cancer, what are the roles of specific lesions in the tumorigenic programme and how does the cell(s) of origin's developmental state influence the activity of a cancerrelevant genetic lesion?

Cell lineage studies. Pancreatic adenocarcinoma cells show phenotypic resemblance to pancreatic-duct cells, displaying cuboidal shape, ductal antigen expression and growth into tubular structures 82 . The genetic progression model of pancreatic adenocarcinoma therefore provides strong support for a ductal origin of this malignancy. This model does not, however, establish whether the progenitors are a specific cell type that resides within the ducts. Moreover, the focal expression of non-ductal lineage markers, including endocrine factors and pancreatic enzymes, indicates that there might be developmental plasticity of the tumorigenic process (reviewed in Ref. 88 ). Indeed, observations from animal-model systems have indicated that there are several routes to pancreatic adenocarcinoma, indicating that putative pancreatic stem cells and transdifferentiated endocrine or exocrine cells might be involved.

Parallel observations have been made in rat and hamster carcinogen models

Trp53-mutant mice that express Tgf- in their acinar cells (Ela-Tgf- ) 90

83 , 89 and in

. In these models, acinar cells are lost — due to direct damage or apparent TRANSDIFFERENTIATION

— and duct-like tubular complexes emerge and proliferate; these eventually give rise to pancreatic ductal adenocarcinomas. But different progenitors are thought to give rise to the tubular complexes — duct cells in the rat model, islet cells or islet-associated stem cells in the hamster, and de-differentiated acinar cells in Tgf- -transgenic>Trp53mutant mice. A role for islet-associated cells in pancreatic adenocarcinoma is also indicated by the demonstration that mouse islet-cell cultures that express polyoma virus middle T oncogene proceed to form pancreatic ductal adenocarcinoma when transplanted into histocompatible mice 91 . The complexities in tracing the cell of origin of pancreatic neoplasia should not be surprising given the close developmental relationships of the pancreatic cell types and the known propensity of endodermal lineages to transdifferentiate in vitro and in vivo 92 ( Box 1 ).

These dynamic cell-lineage relationships fit well with emerging concepts of the plasticity of differentiation states 93 . In contrast to previous models that indicate that

differentiation is an irreversible process and that stem cells have a rigidly defined identity, this revised view proposes that differentiation involves a graded loss of stemcell propensity and that 'stem-cellness' represents a differentiation state rather than a discrete entity. Implicit in this model is the notion of facultative stem cells — differentiated cell lineages that have the potential to be stimulated to assume a stemcell role. Based on studies of cell renewal and differentiation, Bonner–Weir 94 has argued persuasively that all or nearly all of the pancreatic-ductal cells are potential facultative stem cells, with the capacity to differentiate into both endocrine and exocrine lineages.

The adult pancreas is a relatively quiescent organ, with low but measurable levels of cell division in the acini, ducts and islets 95 . Increases in proliferation rate can be induced by certain stimuli, demonstrating the regenerative capacity of this organ. In rats subjected to partial pancreatectomy, the exocrine and endocrine compartments are regenerated by both replication of differentiated cells and expansion of multipotent cells from the pancreatic ducts 96 , 97 . In the latter process (neogenesis), proliferating duct cells give rise to both endocrine and exocrine structures, recapitulating embryonic development ( Fig.

4 ). Re-entry into the cell cycle is immediately followed by transient induction of Pdx1 expression, a transcription factor that marks multipotent progenitors in the embryonic pancreas, and loss of expression of differentiated ductal markers. Virtually all the cells of the ductal epithelium undergo this process of dedifferentiation and expansion, indicating that the normal, differentiated cells of the ducts have the capacity to transiently become pluripotent. Comparable processes of pancreatic neogenesis from duct-cell progenitors are observed in various rodent models of pancreatic damage 98-100 . Neogenesis from ductal progenitors might also be relevant to human physiological processes, such as in new islet formation (nesidioblastosis) that is associated with type I diabetes. Overall, these observations seem consistent with a general role of the duct cells as facultative stem cells for pancreatic-cell replacement. Such a proliferating cell with unlimited replicative potential would be a prime candidate for oncogenic mutation. It is noteworthy that the paracrine signals that are implicated in the regulation of ductal proliferation in these models — TGF- /EGF and hepatocyte growth factor (HGF) as initiating factors and

TGF- as an inhibitor — are also engaged during tumorigenesis 26 , 96 , 101 .

Figure 4 | Model of relationship of pancreatic regeneration and

tumorigenesis.

a | There is a low rate of cell proliferation in the normal pancreas.

Injury to the organ by various insults results in a prominent proliferative response. b | This response might involve proliferation of the duct cells that are associated with disruptions in the basement membrane (for example, MMP and UPA activity), inflammatory responses due to cytokine release and reactive oxygen species (ROS) production, and autocrine/paracrine release of growth factors (for example, TGF- , HGF and KGF). c | The proliferative response might result in regeneration of pancreatic tissue and return to quiescence if the stimulus is attenuated. d | This proliferative arrest is likely to involve INK4A and TGF- /SMAD4 pathways. e | Alternatively, the proliferating ductal cells might incur oncogenic mutations, such as activated KRAS. f | In the face of this sustained proliferative stimulus, there is selective pressure for subsequent mutations in growth-inhibitory pathways (such as in

INK4A), leading to PanIN and g) metastatic cancer.

Although it seems that the ducts are a main source for pancreatic neogenesis, each pancreatic compartment shows the capacity for assuming a dedifferentiated, duct-like phenotype with multipotent differentiative capacity. Cultured acinar cells undergo transdifferentiation to ductal cells in the absence of cell division and with associated reactivation of Pdx1 (Ref. 102 ). The emerging ductal cells can subsequently redifferentiate into cells that show features of either exocrine or endocrine lineages that are consistent with a stem-cell character. Similarly, numerous experiments have shown the propensity of islet cells for facultative stem-cell behaviour. One prediction of this developmental plasticity is that tumours that show particular differentiation phenotypes might not have a unique cell of origin. Experimental studies in mice indicate that this model might apply to brain tumour pathogenesis, as mutations of Cdkn2a and Egfr in either neural stem cells or differentiated astrocytes gave rise to malignant gliomas with indistinguishable tumour phenotypes 103 . These data indicate that it is the specific genetic alterations rather than the identity of the target cell that defines the ensuing malignant

phenotype. The highly specific mutational profiles of the different types of pancreatic cancer indicate that this concept might be relevant to pancreatic neoplasia ( Table 1 ). An alternative possibility is that the pancreatic-duct cells (in the case of pancreatic adenocarcinoma) have a particular sensitivity to mutations in these genes. Mouse models and primary cultures of human pancreatic cells should allow a more direct experimental analysis of this question.

Tumour–stroma interactions. Heterotypic microenvironmental cellular interactions seem to be important in the pathogenesis of pancreatic adenocarcinoma. Notably, these tumours show a marked proliferation of stromal fibroblasts and deposition of extracellular matrix components such as matrix metalloproteinases and collagens

(desmoplasia) 82 . The role of this process in cancer pathogenesis remains uncertain, as it is not well established whether the response is part of the tumorigenic programme or whether it represents a form of host defence against the tumour. Recent evidence indicates that the carcinoma cells direct the desmoplastic response and that TGF- contributes to this process 104 . There are suggestions that SMAD4 loss might be permissive for these effects, as intestinal tumours in APC prominent desmoplastic reactions than those seen in Apc

Min/+ Smad4

Min/+

+/-

Smad4

mice show more-

+/+ animals — notably, Smad4-deficient tumours show increased growth and invasiveness in this model. Another role for SMAD4 in regulating heterotypic interactions is indicated by experiments in which Smad4 is reintroduced into some pancreatic adenocarcinoma cell lines. In these experiments, Smad4 blocks tumorigenic growth in immunodeficient mice by inhibiting angiogenesis, but does not affect tumour-cell sensitivity to Tgf105 . These concepts are consistent with recent studies showing that cancers 'programme' an oncogenic stroma that, in turn, contributes to tumour growth through paracrine signalling, angiogenesis and protection from immune attack 106 , 107 .

Mouse models. Mouse models provide tractable genetic systems to dissect the complexities of evolving cancers in a physiological context (for a review of other animal models of pancreatic cancer, see Ref. 108 ). Pancreatic adenocarcinoma is rarely observed spontaneously or following carcinogen administration in the laboratory mouse, but genetic engineering has allowed the generation of strains that harbour germline oncogenic lesions that are found in human pancreatic adenocarcinomas. Early attempts to model exocrine pancreatic cancer used acinar-specific transgene expression, because of the availability of promoters that are capable of directing transgene expression to this compartment. Transgenic mice that express SV40 large T antigen (T Ag) 109 , 110 , activated

HRAS 111 or c-Myc 112 , 113 in the acini using the elastase (Ela) promoter develop acinar-cell carcinomas, although Ela-Myc tumours progress to mixed acinar-ductal histology.

Transgenic mice expressing TGF- in the acinar cells (Ela-TGF- ) develop acinar-ductal metaplasia, accompanied by an induction in Egfr expression in the resulting metaplastic ducts that proliferate due to this autocrine loop. On a Trp53-deficient background, these metaplastic ducts give rise to pancreatic adenocarcinomas that recapitulate several features of the human disease, including expression of ductal markers, deletion of the

Cdkn2a locus, and the propensity for invasiveness and metastasis 90 . Although no Kras mutations are found in these tumours, the Ela-TGF- mice show constitutive activation of Ras proteins in their premalignant tubular complexes. The Ela-TGF- mice show progressive fibrotic lesions of the pancreas and so the relationship of this model to human pancreatic adenocarcinoma pathogenesis remains unclear. Metallo-thionein-TGF-

(MT-TGF- ) mice also have acinar TGF- expression and tubular metaplasia, but do not develop pancreatic adenocarcinoma even in the context of Trp53 and Cdkn2a deficiency. Instead, these tumour-suppressor mutations cooperate to promote benign serous cystadenomas (SCA) of the pancreas

Ela-TGF- Trp53 -/ and MT-TGF- Trp53 -/-

114 . The differences in tumour phenotype of

models could relate to the extinction of expression of the MT-TGF- transgene in progressive ductal lesions. The absence of a sustained oncogenic stimulus in the initiated lesions of the MT-TGF- Trp53 then result in a benign, rather than malignant, neoplastic course.

-/ mice might

Mouse strains harbouring mutant alleles of the tumour suppressors that are implicated in pancreatic cancer have been generated; however, pancreatic adenocarcinomas have not been reported in these animals ( Table 2 ). The rapid onset of sarcomas and lymphomas in the Trp53-, Ink4a-, Arf- and Ink4a/Arf-mutant mouse strains might obscure the development of other malignancies. Introduction of the Ink4a-insensitive mutant Cdk4 R24C allele into the mouse germline (knock-in) results in islet-cell

hyperplasia and pancreatic endocrine, but not exocrine, tumours 115 , 116 . A series of different Brca2-mutant alleles have been generated, resulting predominantly in embryonic lethality due to defects in DNA repair. In a heterozygous state, these alleles do not strongly predispose to cancer, and pancreatic adenocarcinomas have not been reported. The absence of Trp53 is likely to facilitate the cancer susceptibility of Brca2mutant strains by abrogating DNA-damage responses. Finally, heterozygosity of either

Smad4 (Refs 117 , 118 ) or Lkb1 (Refs 119–121 ) results in gastrointestinal polyps. Some mouse strains with compound mutations of tumour-suppressor genes develop pancreatic neoplasms of the islets or acini, but adenocarcinoma is not observed 146 .

Table 2 | Mouse knockouts of pancreatic adenocarcinoma tumour suppressors

The phenotypes of these mice do not accord with the susceptibility to pancreatic adenocarcinoma of humans with inherited mutations in homologous genes. This might reflect fundamental species-dependent differences in pancreatic tumour-suppression pathways and/or compensating events during mouse development. As an example of the 'compensation' phenomenon, Rb-heterozygous mutant mice and Rb -/ chimaeras do not develop retinoblastoma; however, chimaeras that are derived from Rb -/ p107 -/embryonic stem cells are highly prone to these tumours 122 . Despite these experimental issues, a large body of evidence indicates that tumour-suppressor gene function can be modelled in the mouse despite possible alterations in the 'wiring' of signalling pathways 108 . The construction of compound-mutant mice and conditional targeting (see below) should further enhance the use of the mouse in the dissection of key pancreatic tumour-suppressor mechanisms.

Future directions

Refined mouse models of pancreatic adenocarcinoma. The use of conditional targeting methods is expected to facilitate the generation of genetically accurate, pancreatic-cancer-prone mouse strains and allow greater insight into pancreatic cancer biology (for reviews, see Refs 108 , 123 ). Conditional tumour-suppressor alleles, in conjunction with cell-type-specific expression of CRE RECOMBINASE transgenes, should circumvent the development of competing malignancies, as well as allow a more-direct analysis of the cell-of-origin issue. The versatility of such systems can also be expanded by the generation of mouse strains with tissue-specific expression of the AVIAN

RETROVIRAL RECEPTOR , allowing somatic introduction of oncogenes, or by the use of the inducible expression systems. A prominent issue in these efforts will be the availability of suitable promoters to target transgene expression to specific compartments of the pancreas. Cre recombinase — under the control of the Pdx1 promoter — is effective in deleting loxp-flanked sequences from the entire pancreatic epithelium, while remaining inactive in nearly all other tissues 124 . For targeting of the pancreatic compartments, highly specific promoters exist for acini (elastase) and -cells >insulin), whereas a robust pancreatic-duct-specific promoter remains an unmet need in the field.

Inducible mouse cancer models might be well suited to the identification of crucial genes that regulate tumour maintenance; that is, initiating lesions might or might not be relevant for sustained tumorigenicity of advanced malignancies that have incurred numerous subsequent mutations during progression. Studies of transgenic melanoma and lung adenocarcinoma models directed by inducible HRAS and KRAS alleles, respectively, have shown that sustained mutant RAS activity is necessary for both the initiation of tumorigenesis and for maintenance of the transformed state 54 , 125 . In both systems, extinction of RAS expression results in rapid tumour regression. Expressionprofiling analysis of growing tumours and of those in stages of regression ('RAS-on' versus 'RAS-off') should give insight into the oncogenic programme that is controlled by mutant RAS. The application of such concepts to models of pancreatic adenocarcinoma might allow the elucidation of the molecular basis for oncogenic transformation of the pancreas and the identification and validation of novel molecular targets for therapy.

The availability of genetically accurate mouse models will afford the opportunity to evaluate chemotherapeutic agents in a relevant physiological context. The generation of

models with different mutant alleles should uncover the genetics behind chemotherapeutic responsiveness, as has been done for lymphomas 47 . Furthermore, such models will allow the systematic testing of chemopreventive protocols. Recent studies have indicated that pancreatic adenocarcinomas and PanIN have elevated expression of cyclooxygenases and lipoxygenases — enzymes that regulate inflammatory arachidonic-acid signalling 126-128 . Furthermore, it seems that use of nonsteroidal anti-inflammatory drugs (NSAIDs) reduces pancreatic cancer incidence 129

Mouse models of intestinal tumorigenesis have proven useful in demonstrating the

. chemopreventive efficacy of NSAIDs and have enabled insight into the mechanistic basis of these effects (reviewed in Ref. 130 ).

Gene discovery. Novel genomic and proteomic technologies for global expression analysis have shown promise in providing a molecular taxonomy of tumours (reviewed in Ref. 131 ). Signature profiles have allowed the improved classification of tumour types and the elucidation of prognostic markers. These methods have recently been used to study pancreatic adenocarcinoma and have revealed potential new diagnostic markers and therapeutic targets 132-135 . As discussed above, the identification of recurrent chromosomal amplifications and deletions in pancreatic adenocarcinomas indicates that there are numerous loci involved in the pathogenesis of this malignancy. High-resolution gene-discovery technologies, coupled with the validation potential of inducible mouse models, should expand the list of essential targets for more-productive drugdevelopment initiatives.

Another important avenue for pancreatic adenocarcinoma gene discovery might be from genetic mapping studies of pancreatic-adenocarcinoma-prone kindreds. The genetic lesions in most of these families have yet to be identified. Segregation analysis of a large number of kindreds has indicated that susceptibility might be due to autosomaldominant inheritance of a rare allele(s) the biology of the disease.

7 . The identification of such an allele would be of great potential use for the early identification of patients at risk and in understanding

The last decade has seen remarkable progress in fields that are relevant to pancreatic adenocarcinoma, including cancer genetics, development biology of the pancreas and genetic engineering in the mouse. There is now unprecedented commitment to understanding the pathogenesis of pancreatic cancer. The availability of powerful new technologies and continued contributions of investigators in many related disciplines provides a measure of optimism towards future progress in treating this disease.

Boxes

Box 1 | Cell differentiation programmes in the pancreas

Despite their distinct morphological and functional properties, all of the epithelial cells of the endocrine and exocrine pancreas share a common embryological origin — the gut endoderm 137 . The genetic programme for pancreatic development is characterized by a hierarchical and combinatorial network of transcription factors and by the inductive signalling from adjacent tissues 138 , 140 . The endodermal region that is marked for pancreatic differentiation is distinguished from adjacent endoderm by absence of sonic hedgehog (Shh) expression 141 . The Pdx1 transcription factor is required for the proliferation and branching of early pancreatic progenitors that give rise to all pancreatic epithelial cell types, hence the Pdx1-expressing pancreatic endoderm represents a multipotent pancreatic stem-cell population. Some of the transcription factors controlling differentiation to endocrine and exocrine lineages are indicated in the figure.

Many of the transcription factors that regulate pancreatic differentiation also contribute to the development of other endodermal tissues and/or of separate stages of pancreatic development 142 . Moreover, ectopic expression of some of these factors can reprogramme other endodermal tissues to a pancreatic differentiation phenotype or alter the differentiation of pancreatic compartments 143 . This illustrates the importance of combinatorial activities of a small number of transcription factors in endodermal-cell specification and reinforces the close interrelation among endodermal cell types. This conversion of cellular phenotype — metaplasia — also occurs spontaneously during development and in response to injury, whereby foci of misplaced hepatocytes arise in the regenerating pancreas 92 . In addition, transdifferentiation — phenotypic conversion of differentiated cells — of pancreatic cells in culture is well described. For example, appropriate culture conditions leads acinar–ductal, islet–ductal and ductal–islet transdifferentiation of purified cell populations.

Links

DATABASES

Cancer.gov: pancreatic adenocarcinoma

LocusLink: BRCA2 | CDK4 | CDK6 | CDKN2A | EGF | EGFR | ERBB2 | ERBB3 | KRAS |

LKB1 | MDM2 | MLH1 | MSH2 | MSH6 | Pdx1 | PRSS1 | RB | SMAD4 | TGF | TP53 |

WAF1

OMIM: FAMMM syndrome | hereditary pancreatitis | HNPCC syndrome | Peutz–

Jeghers syndrome

FURTHER INFORMATION

Pancreatic Cancer Agenda for Action

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Figure 1 | Anatomy of the pancreas. The pancreas is comprised of separate functional units that regulate two major physiological processes: digestion and glucose metabolism 136 . The exocrine pancreas consists of acinar and duct cells. The acinar cells produce digestive enzymes and constitute the bulk of the pancreatic tissue. They are organized into grape-like clusters that are at the smallest termini of the branching duct system. The ducts, which add mucous and bicarbonate to the enzyme mixture, form a network of increasing size, culminating in main and accessory pancreatic ducts that empty into the duodenum. The endocrine pancreas, consisting of four specialized cell types that are organized into compact islets embedded within acinar tissue, secretes hormones into the bloodstream. The - and> -cells regulate the usage of glucose through the production of glucagon and insulin, respectively. Pancreatic polypeptide and somatostatin that are produced in the PP and> -cells modulate the secretory properties of the other pancreatic cell types.>a |

Gross anatomy of the pancreas. b | The exocrine pancreas. c | A single acinus. d | A pancreatic islet embedded in exocrine tissue.

Figure 2 | Genetic progression model of pancreatic adenocarcinoma. Pancreatic intraepithelial neoplasias (PanINs) seem to represent progressive stages of neoplastic growth that are precursors to pancreatic adenocarcinomas. The genetic alterations documented in adenocarcinomas also occur in PanIN in what seems to be a temporal sequence, although these alterations have not been correlated with the acquisition of specific histopathological features. The stage of onset of these lesions is depicted. The thickness of the line corresponds to the frequency of a lesion. The temporal alterations in telomerase activity and telomere length are by inference from Refs 62,139 and need further substantiation in PanIN. Normal duct, PanIN-1A/PanIN-1B and PanIN-3 images reproduced with permission from ref. 14 (2001) Lippincott

Williams & Wilkins; PanIN-2 and adenocarcinoma images kindly provided by Dr Ralph Hruban, Johns Hopkins

University ( http://pathology.jhu.adu/pancreas/panin/ ).

Figure 3 | Telomere attrition and genomic instability. Most human somatic cells lack telomerase activity, hence telomeres are eroded as cells proliferate. If the proliferative stimulus is maintained, such as in cells that have sustained oncogenic mutations, progressive telomere shortening activates DNA-damage responses, resulting in growth arrest. Loss of these checkpoint responses, such as by mutation of TP53, allows cells to continue proliferating, leading to telomere dysfunction and genomic instability (crisis).

Chromosome breakage–fusion cycles produce severe aneuploidy and chromosomal translocations that contribute to tumour progression. Telomerase reactivation subsequently stabilizes the genome and facilitates the immortal growth of the tumour cells. Independent processes also lead to genomic instability — centrosome defects result in a disorganized mitotic spindle, which leads to aneuploidy or chromosome breakage; BRCA2 mutations produce genomic instability by disabling the homologous recombination-based

DNA-repair pathway.

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Figure 4 | Model of relationship of pancreatic regeneration and tumorigenesis. a | There is a low rate of cell proliferation in the normal pancreas. Injury to the organ by various insults results in a prominent

proliferative response. b | This response might involve proliferation of the duct cells that are associated with disruptions in the basement membrane (for example, MMP and UPA activity), inflammatory responses due to cytokine release and reactive oxygen species (ROS) production, and autocrine/paracrine release of growth factors (for example, TGF- , HGF and KGF). c | The proliferative response might result in regeneration of pancreatic tissue and return to quiescence if the stimulus is attenuated. d | This proliferative arrest is likely to involve INK4A and TGF- /SMAD4 pathways. e | Alternatively, the proliferating ductal cells might incur oncogenic mutations, such as activated KRAS. f | In the face of this sustained proliferative stimulus, there is selective pressure for subsequent mutations in growth-inhibitory pathways (such as in INK4A), leading to

PanIN and g) metastatic cancer.

Table 1 | Types of pancreatic neoplasms

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December 2002 Vol 2 No 12

Nature Reviews Cancer

REVIEW

Nature Reviews Cancer

PANCREATIC CANCER BIOLOGY AND GENETICS

Nabeel Bardeesy & Ronald A. DePinho

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