Loss of 11p11 is a frequent and early event in sporadic nonfunctioning pancreatic neuroendocrine neoplasms

The Molecular and Clinical Landscape of Pancreatic Neuroendocrine Tumors

Pancreatic neuroendocrine tumors are rare tumors of the pancreas originating from the islets of the Langerhans. These tumors comprise 1% to 3% of all newly diagnosed pancreatic cancers every year and have a unique heterogeneity in clinical presentation. Whole-genome sequencing has led to an increased understanding of the molecular biology of these tumors. In this review, we will summarize the current knowledge of the signaling pathways involved in the tumorigenesis of pancreatic neuroendocrine tumors as well as the major studies targeting these pathways at preclinical and clinical levels.

Inherent variability of cancer-specific aneuploidy generates metastases

Background

The genetic basis of metastasis is still unclear because metastases carry individual karyotypes and phenotypes, rather than consistent mutations, and are rare compared to conventional mutation. There is however correlative evidence that metastasis depends on cancer-specific aneuploidy, and that metastases are karyotypically related to parental cancers. Accordingly we propose that metastasis is a speciation event. This theory holds that cancer-specific aneuploidy varies the clonal karyotypes of cancers automatically by unbalancing thousands of genes, and that rare variants form new autonomous subspecies with metastatic or other non-parental phenotypes like drug-resistance – similar to conventional subspeciation.

Results

To test this theory, we analyzed the karyotypic and morphological relationships between seven cancers and corresponding metastases. We found (1) that the cellular phenotypes of metastases were closely related to those of parental cancers, (2) that metastases shared 29 to 96% of their clonal karyotypic elements or aneusomies with the clonal karyotypes of parental cancers and (3) that, unexpectedly, the karyotypic complexity of metastases was very similar to that of the parental cancer. This suggests that metastases derive cancer-specific autonomy by conserving the overall complexity of the parental karyotype. We deduced from these results that cancers cause metastases by karyotypic variations and selection for rare metastatic subspecies. Further we asked whether metastases with multiple metastasis-specific aneusomies are assembled in one or multiple, sequential steps. Since (1) no stable karyotypic intermediates of metastases were observed in cancers here and previously by others, and (2) the karyotypic complexities of cancers are conserved in metastases, we concluded that metastases are generated from cancers in one step – like subspecies in conventional speciation.

Conclusions

We conclude that the risk of cancers to metastasize is proportional to the degree of cancer-specific aneuploidy, because aneuploidy catalyzes the generation of subspecies, including metastases, at aneuploidy-dependent rates. Since speciation by random chromosomal rearrangements and selection is unpredictable, the theory that metastases are karyotypic subspecies of cancers also explains Foulds’ rules, which hold that the origins of metastases are “abrupt” and that their phenotypes are “unpredictable.”

Upregulation of INS-IGF2 read-through expression and identification of a novel INS-IGF2 splice variant in insulinomas

Fusion transcripts arising from the combination of exons residing on neighboring genes on the same chromosome may give rise to chimeric or novel proteins. Such read-through transcripts have been detected in different cancers where they may be of pathogenetic interest. In this study, we describe for the first time the expression of a read-through transcript in insulinomas, a functioning neuroendocrine pancreatic neoplasm. The read-through transcript INS-IGF2, composed of exons from the two genes proinsulin precursor (INS) and insulin‑like growth factor 2 (IGF2), both mapping to chromosomal subband 11p15.5, was highly expressed in the two insulinomas analyzed. More precisely, version 2 of the INS-IGF2 transcript was expressed, indicating possible expression of the chimeric INS-IGF2 protein. We further identified a novel splice variant of the INS-IGF2 read-through transcript in one of the insulinomas, composed of exon 1 of INS3 and exons of IGF2. In the same tumor, we found high expression of INS3 and the presence of the A allele at SNP rs689. SNP rs689 has been previously described to regulate splicing of the INS transcript, indicating that this regulatory mechanism also affects splicing of INS-IGF2. The identification of the INS-IGF2 read-through transcript specifically in tumor tissue but not in normal pancreatic tissue suggests that high expression of INS-IGF2 could be neoplasia‑specific. These results may have potential clinical applications given that the read-through transcript could be used as a biomarker in insulinoma patients.

Transcriptomic Profiling of Tumor Aggressiveness in Sporadic Nonfunctioning Pancreatic Neuroendocrine Neoplasms

OBJECTIVES

The aim of the study was to compare RNA sequencing data of sporadic nonfunctioning pancreatic neuroendocrine neoplasms (PNENs) to identify gene expression patterns that may be important for molecular differentiation of tumor aggressiveness.

METHODS

RNA sequencing was performed on samples of sporadic nonfunctioning PNENs, grouped as tumors with mild behavior (nonmetastatic and Ki67 < 5%) or aggressive behavior (metastatic and Ki67 ≥ 5%), on an Illumina Genome Analyzer II platform. Bioinformatic analyses were performed on the resulting data.

RESULTS

Of 22,810 identified transcripts from protein-coding genes, a set of 309 genes were significantly differentially expressed between the 2 groups, of which 166 were upregulated and 143 downregulated in the aggressive disease group. Among the top protein-coding upregulated genes, we found genes encoding proteins involved in DNA packaging, ability to taste, chromosome structuring, cytoskeleton structuring, and cell-cell signaling. Among the top protein-coding downregulated genes, we found genes encoding proteins involved in neuronal differentiation, cytoskeleton structuring, cell-cell signaling, and immunological processes.

CONCLUSIONS

A higher degree of tumor aggressiveness in sporadic nonfunctioning PNENs seems to be associated with upregulation of genes involved in regulation of the cell cycle and cell division. Small sample size and lack of a replication set are limitations of this study.