Distant metastasis occurs late during the genetic evolution of pancreatic cancer

Abstract

Metastasis, the dissemination and growth of neoplastic cells in an organ distinct from that in which they originated, is the most common cause of death in cancer patients. This is particularly true for pancreatic cancers, where most patients are diagnosed with metastatic disease and few show a sustained response to chemotherapy or radiation therapy. Whether the dismal prognosis of patients with pancreatic cancer compared to patients with other types of cancer is a result of late diagnosis or early dissemination of disease to distant organs is not known. Here we rely on data generated by sequencing the genomes of seven pancreatic cancer metastases to evaluate the clonal relationships among primary and metastatic cancers. We find that clonal populations that give rise to distant metastases are represented within the primary carcinoma, but these clones are genetically evolved from the original parental, non-metastatic clone. Thus, genetic heterogeneity of metastases reflects that within the primary carcinoma. A quantitative analysis of the timing of the genetic evolution of pancreatic cancer was performed, indicating at least a decade between the occurrence of the initiating mutation and the birth of the parental, non-metastatic founder cell. At least five more years are required for the acquisition of metastatic ability and patients die an average of two years thereafter. These data provide novel insights into the genetic features underlying pancreatic cancer progression and define a broad time window of opportunity for early detection to prevent deaths from metastatic disease.

Fig. 1. Summary of somatic mutations in metastatic pancreatic cancers

a. Histopathology of primary infiltrating pancreatic cancer and metastatic pancreatic cancer to the peritoneum, liver and lung. In addition to infiltrating cancer cells in each lesion (arrows), non-neoplastic cell types are abundant. b. Total mutations representing parental clones (founder mutations), and clonal evolution (progressor mutations) within the primary carcinoma based on comparative lesion sequencing. Mutations common to all samples analyzed were the most common category identified.

Fig. 2. Geographic mapping of metastatic clones within the primary carcinoma and proposed clonal evolution of Pa08

a. Illustration of the pancreatic specimen removed from Pa08 at rapid autopsy, and the planes of sectioning of the specimen. b. Mapping of the parental clone and subclones identified in a by comparative lesion sequencing within serial sections of the infiltrating pancreatic carcinoma. Metastatic subclones giving rise to liver and lung metastases are nonrandomly located within slice 3, indicated by blue circles. These clones are both geographically and genetically distinct from clones giving rise to peritoneal metastases in this same patient, indicated by green. c. Proposed clonal evolution based on the sequencing data. In this model, after development of the parental clone, ongoing clonal evolution continues within the primary carcinoma (yellow rectangle), and these subclones seed metastases in distant sites. * indicates two mutations were found in the TTN gene.

Fig. 2. Geographic mapping of metastatic clones within the primary carcinoma and proposed clonal evolution of Pa08

a. Illustration of the pancreatic specimen removed from Pa08 at rapid autopsy, and the planes of sectioning of the specimen. b. Mapping of the parental clone and subclones identified in a by comparative lesion sequencing within serial sections of the infiltrating pancreatic carcinoma. Metastatic subclones giving rise to liver and lung metastases are nonrandomly located within slice 3, indicated by blue circles. These clones are both geographically and genetically distinct from clones giving rise to peritoneal metastases in this same patient, indicated by green. c. Proposed clonal evolution based on the sequencing data. In this model, after development of the parental clone, ongoing clonal evolution continues within the primary carcinoma (yellow rectangle), and these subclones seed metastases in distant sites. * indicates two mutations were found in the TTN gene.

Fig. 3. Schema of the genetic evolution of pancreatic cancer

Tumorigenesis begins with an initiating mutation in a normal cell that confers a selective growth advantage. Successive waves of clonal expansion occur in association with the acquisition of additional mutations, corresponding to the progression model of pancreatic intraepithelial neoplasia (PanIN) and time T₁. One founder cell within a PanIN lesion will seed the parental clone and hence initiate an infiltrating carcinoma (end of T₁ and beginning of T₂). Eventually, the cell that will give rise to the index lesion will appear (end of T₂ and beginning of T₃). Unfortunately, most patients are not diagnosed until well into time interval T₃ when cells of these metastatic subclones have already escaped the pancreas and started to grow within distant organs. The average time for intervals T_1, T₂ and T₃ for all seven patients is indicated in the parentheses at left (see also Supplementary Table 6).