Intraductal papillary mucinous neoplasms (IPMN) are benign pancreatic cysts found in the ducts of the pancreas that have the potential to become malignant. Identifying IPMNs that have high potential to become pancreatic cancer may help prevent unnecessary surgery which is the definitive treatment of IPMNs. Whole exome and targeted sequencing were utilized to better characterize the genetic alterations in IPMNs. The most commonly mutated gene in IPMNs is KRAS with 50–80% of IPMNs harboring a KRAS mutation.
1. Introduction
Pancreatic ductal adenocarcinoma (PDAC), currently the third leading cause of cancer-related mortality, is projected to be the second most common cause of death due to malignancies in the United States by 2030
[1]. This poor clinical outcome can be partly attributed to the often vague or absent clinical symptoms associated with pancreatic ductal adenocarcinoma. Thus, many patients with PDAC present with advanced stages of the disease. While early PDAC, which constitutes <15% of all new diagnoses, are surgically resectable and associated with a better outcome, the mainstay treatment of advanced pancreatic cancer is limited to chemotherapy. Despite extensive research, attempts to develop targeted therapies were largely unsuccessful
[2]. A better understanding of the development of PDAC may provide insight into the discovery of new diagnostic tools and therapeutics. PDAC commonly arises from precursor lesions including pancreatic intraepithelial neoplasia (PanIN), intraductal papillary mucinous neoplasm (IPMN), and mucinous cystic neoplasm (MCN). Among the cystic precursors, IPMN is the most prevalent.
These cysts are mucin-producing and arise from the main pancreatic or the branch duct. Main duct lesions (MD-IPMN) has a higher risk of malignancy compared to branch duct (BD-IPMN). There is a third type that shares features of MD-IPMN and BD-IPMN and is referred to as mixed type (MT-IPMN). MT-IPMN and MD-IPMN tend to be more symptomatic. Symptomatic patients tend to progress to malignancy more than those who are asymptomatic, regardless of subtype. While they all arise from cells which produce mucin, four histopathological IPMN types are distinguished by the specific mucin(s) they produce: gastric (49–63%), intestinal (18–36%), pancreaticobiliary (7–18%), and oncocytic (1–8%). Of these, gastric is the most common and rarely progresses to malignancy. While pancreaticobiliary is less common, it is commonly associated with aggressive PDAC
[3]. The degree of cytological atypia and crowding of the epithelium allows for classification of the IPMNs into low, intermediate, and high grade dysplasia
[4]. In addition to MD-IPMN carrying a larger risk for malignant progression, other high risk features inform malignant potential including the size of the lesion, rate of growth of the lesion, presence of solid components within the lesion, the presence of high grade dysplasia, main duct dilation on imaging, and the presence of symptoms including jaundice, new onset diabetes, or pancreatitis
[3].
The primary management of IPMN is the surgical resection of high-risk lesions that can progress to malignant disease
[5]. Risk stratification of these cystic lesions is largely based on imaging and clinical characteristics of the cyst including size, location, and grade of dysplasia
[5]. The exact molecular mechanisms driving IPMN progression to PDAC are unclear; however, genetic, metabolic, immune, and inflammatory changes appear to play a role in this process.
2. Genetic Alterations
Whole exome and targeted sequencing were utilized to better characterize the genetic alterations in IPMNs. The most commonly mutated gene in IPMNs is
KRAS with 50–80% of IPMNs harboring a
KRAS mutation
[6][7]. The most common location of the
KRAS mutation is in codon 12, resulting in a G12D mutation. This
KRAS mutation encodes a constitutively active GTP-binding protein that regulates various signaling cascades including cell growth and proliferation. Furthermore,
KRAS mutations are found in >90% of PDACs and, hence, are thought to be essential and an early event for tumorigenesis
[8]. Mutation in the
GNAS gene is another common alteration in IPMNs and was identified in 40–70% of lesions
[9].
GNAS encodes the stimulatory alpha subunit of the guanine nucleotide-binding protein which activates cyclic adenosine monophosphate (cAMP), leading to the activation of multiple effectors including protein kinase A and EPAC (Exchange Protein directly activated by cAMP). Although the relevance of these pathways in pancreatic cancer was not fully established, these pathways were implicated in other cancer types
[10][11]. The
GNAS mutation is predominantly located at codon 201, commonly leading to R201H or R201C alterations
[9]. This is thought to be an activating mutation that results in the constitutive activation of
GNAS. In contrast to
KRAS,
GNAS mutations are not commonly observed in non-IPMN PDACs but were found in 25–61% of IPMN- derived PDACs
[7][12][13].
KRAS and/or
GNAS mutations are found in >90% of IPMNs, both in those with advanced neoplasia (high-grade dysplasia or adenocarcinoma) and low-grade dysplasia
[7]. The prevalence of their alteration suggests that they play an important role in IPMN development. These mutations were found to coexist, suggesting that they are not mutually exclusive. Interestingly, transgenic mouse models of IPMNs required synergistic mutation between
GNAS and
KRAS to generate cystic lesions that are histologically similar to human IPMNs
[9][14]. Although there are no significant differences in
KRAS and
GNAS alterations in IPMNs with and without advanced neoplasia, a difference in mutation frequency were observed when analyzing the histological subtypes
[7].
IPMNs can be divided into histological subtypes including gastric, intestinal, and pancreatobiliary. The oncocytic subtype is now considered a separate entity, i.e., “Intraductal Oncocytic Papillary Neoplasm”, by WHO’s classification of tumors, due to its different molecular and clinical features. These subtypes can give rise to different types of invasive carcinoma
[15]. In general, intestinal IPMN gives rise to colloid carcinoma while tubular carcinoma can arise from gastric or pancreatobiliary subtype. Interestingly, there is a higher frequency of
GNAS mutation in IPMN-associated colloid carcinoma and
KRAS mutations in IPMN-associated tubular carcinoma
[7]. This difference in mutational frequency may be clinically relevant, given that tubular adenocarcinoma is associated with worst clinical outcomes, and thus, it may offer prognostic value.
Inactivating mutation in the
RNF43 gene is another common genetic alteration found in IPMNs, occurring in 10–75% of cystic lesions
[7]. RNF43 encodes a transmembrane E3 ubiquitin ligase that negatively regulates the Wnt pathway. Thus, the loss of function of RNF43 confers Wnt pathway activation, which was shown to play a potential role in mediating PDAC progression
[16][17]. Other loss of function mutations found in IPMNs include
TP53, CDKN2A, and
SMAD4. These genetic alterations occur at a lower rate in IPMNs with low-grade dysplasia compared to those with advanced neoplasia, suggesting that these genes may play a role in mediating progression to malignancy
[7]. Furthermore, these genetic alterations are also commonly detected in invasive pancreatic cancer.
More recently, Noe et al. performed whole exome sequencing (WES) of IPMNs and MCNs and their associated invasive carcinomas to better understand the genetic alteration driving tumorigenesis in these cystic precursors
[18]. WES analysis revealed the high prevalence of previously identified drivers of pancreatic tumorigenesis including
KRAS,
GNAS,
RNF43,
CDKN2A,
TP53, and
SMAD4. In addition, the use of WES revealed novel mutations in IPMN tumorigenesis. Somatic mutations in the
ATM gene were found in 17% of lesions.
ATM encodes a serine/threonine kinase that is involved in DNA double-strand break repair as well as other cellular processes including metabolism and chromatin remodeling. Moreover, individuals with germline pathogenic
ATM variants appear to have an increased lifetime risk of pancreatic cancer
[19][20]. In addition, loss of ATM expression in a mouse model of PDAC resulted in a greater number of proliferative precursor lesions
[21]. Somatic mutation in
GLI3 gene was also observed in 8% of samples.
GLI3 gene encodes a transcription factor that is a member of the Hedgehog signaling pathway and is involved in normal cellular processes including tissue and immune cell development. Furthermore, studies implicated GLI3 in mediating pro-tumorigenesis pathways in various cancers, including pancreatic cancer
[22][23][24].