Pancreatic cancer is a deadly disease. Discovery of the mutated genes that cause the inherited form(s) of the disease may shed light on the mechanism(s) of oncogenesis. Previously we isolated a susceptibility locus for familial pancreatic cancer to chromosome location 4q32–34. In this study, our goal was to discover the identity of the familial pancreatic cancer gene on 4q32 and determine the function of that gene.
Methods and Findings
A customized microarray of the candidate chromosomal region affecting pancreatic cancer susceptibility revealed the greatest expression change in palladin (PALLD), a gene that encodes a component of the cytoskeleton that controls cell shape and motility. A mutation causing a proline (hydrophobic) to serine (hydrophilic) amino acid change (P239S) in a highly conserved region tracked with all affected family members and was absent in the non-affected members. The mutational change is not a known single nucleotide polymorphism. Palladin RNA, measured by quantitative RT-PCR, was overexpressed in the tissues from precancerous dysplasia and pancreatic adenocarcinoma in both familial and sporadic disease. Transfection of wild-type and P239S mutant palladin gene constructs into HeLa cells revealed a clear phenotypic effect: cells expressing P239S palladin exhibited cytoskeletal changes, abnormal actin bundle assembly, and an increased ability to migrate.
These observations suggest that the presence of an abnormal palladin gene in familial pancreatic cancer and the overexpression of palladin protein in sporadic pancreatic cancer cause cytoskeletal changes in pancreatic cancer and may be responsible for or contribute to the tumor's strong invasive and migratory abilities.
The presence of abnormalpalladin in familial pancreatic cancer and its overexpression in sporadic pancreatic cancer leads to cytoskeletal changes and may be responsible for the tumor's invasive and migratory abilities.
Pancreatic cancer is a leading cause of cancer-related death in the US. Because it causes few symptoms in its early stages, pancreatic cancer is rarely detected until it has spread (metastasized) around the body. Pancreatic tumors can occasionally be removed surgically but the usual treatment is radio- or chemotherapy, and neither of these is curative; most patients die within a year of diagnosis. As in other cancers, the cells in pancreatic tumors have acquired genetic changes (mutations) that allow them to divide uncontrollably (normal cells divide only to repair damaged tissue). Other mutations alter the shape of the cells and allow them to migrate into (invade) other areas of the body. These mutations usually arise randomly—the cells in the human body are bombarded by chemicals and other agents that can damage their DNA—and cause “sporadic” pancreatic cancer. But some people inherit mutated genes that increase their susceptibility to pancreatic cancer. These people are recognizable because pancreatic cancer is more common in their families than in the general population.
Why Was This Study Done?
The identification of the genes that are mutated in familial pancreatic cancer might provide insights into how both inherited and sporadic cancer develops in the pancreas. Such information could suggest ways to detect pancreatic cancers earlier than is currently possible and could identify new therapeutic targets for this deadly disease. Previous work by the researchers who did this study localized a gene responsible for inherited pancreatic cancer to a small region of Chromosome 4 in a family in which pancreatic cancer is very common (Family X). In this study, the researchers identified which of the genes in this region is likely to be responsible for the susceptibility to pancreatic cancer of Family X.
What Did the Researchers Do and Find?
The researchers made a DNA microarray (a small chip spotted with DNA sequences) of the 243 genes in the chromosomal region linked to pancreatic cancer in Family X. They used this to examine gene expression in dysplastic pancreatic tissue from a Family X member (pancreatic dysplasia is a precancerous lesion that precedes cancer), in normal pancreatic tissue, and in samples from sporadic pancreatic cancers. The most highly overexpressed (compared to normal tissue) gene in both the Family X tissue and the sporadic cancers encoded a protein called palladin. Palladin is a component of the cytoskeleton (a structure that helps to control cell shape and motility) and it organizes other cytoskeletal components. Next, the researchers quantified the expression of palladin RNA in an independent set of normal and cancerous pancreatic samples, and in precancerous pancreatic tissue taken from Family X members and from people who inherit pancreatic cancer but who were not in Family X. This analysis indicated that palladin was overexpressed early in sporadic and inherited pancreatic cancer development. Sequencing of the palladin gene then uncovered a mutation in palladin that was present in Family X members with pancreatic cancer or precancerous lesions but not in unaffected members. This specific mutation, which probably affects palladin's interaction with another cytoskeletal protein called alpha-actinin, was not found in sporadic cancers although many sporadic cancer cell lines had abnormal expression of alpha-actinin protein in addition to palladin protein. Finally, the researchers showed that the introduction of mutated palladin into a human cell line growing in the laboratory increased its migration rate and disrupted its cytoskeleton.
What Do These Findings Mean?
These results strongly suggest that mutated palladin is involved in the development of familial pancreatic cancer. Because genes tend to be inherited in groups, there is still chance that a mutation in a nearby gene could be responsible for the increased susceptibility to pancreatic cancer in Family X. However, the data showing palladin overexpression in sporadic tumors and alterations of cell behavior in the laboratory after introduction of the mutated gene make this unlikely. To prove the involvement of palladin in pancreatic cancer, palladin mutations must now be identified in other familial cases and the overexpression of palladin in sporadic cancers must be explained. The results here nevertheless provide an intriguing glimpse into a potential new mechanism for cancer development in the pancreas and possibly other tissues, one in which abnormalities in palladin function or expression (or in the proteins with which it associates) drive some of the changes in cell migration, shape, and size that characterize cancer cells.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0030516
US National Cancer Institute, information on pancreatic cancer for patients and health professionals
MedlinePlus encyclopedia entry on pancreatic carcinoma
Cancer Research UK, information for patients about pancreatic cancer
Johns Hopkins University, information on pancreatic cancer that includes details on familial cancer
CancerQuest, information provided by Emory University about how cancer develops