Pancreatic cancer continues to be one of the most lethal gastrointestinal malignancies 31
. Currently, surgical resection of lesions detected at the earliest possible stage represents the greatest chance for cure. However, very few cancers are diagnosed at an early stage, and known precursor lesions of pancreatic cancer, such as IPMN or Mucinous Cystic Neoplasms (MCNs) do not harbor the same malignant potential in every single case, rendering their appropriate management difficult to assess. Therefore, there is a great need to better characterize the molecular biology of cystic precursor lesions such as IPMNs, in order to better stratify the management for individual patient needs.
Recently, high throughput sequencing of various human cancers have uncovered recurrent mutations in genes involved in chromatin remodeling 32;33
. Vogelstein B. et al., have demonstrated that mutations of MEN1
, which encodes the histone methyltransferase component menin, occur at a frequency of 44% in pancreatic neuroendocrine tumors (PanNETs). Another 43% of PanNETs harbor mutations in DAXX
(death-domain–associated protein) and ATRX
(a thalassemia/mental retardation syndrome X-linked) whose protein products are also involved in chromatin remodeling 34
Brg1 is an ATPase/helicase and as such constitutes the catalytic subunit of the SWI/SNF chromatin remodeling complex. This complex disrupts the adhesion of histone components to DNA thereby giving transcription factors access to their target genes 35;36
. In mice, developmental homozygous deletion of BRG1
results in peri-implantation lethality, whereas BRG1
heterozygotes are predisposed to exencephaly and apocrine like tumors 37
Abnormalities in the expression of BRG1
have been reported in a number of different cancer types such as lung, breast, brain, colon, ovarian, and less frequently, pancreas 18;20;21;23;24;38
. Functional studies have reiterated the putative tumor suppressive effects of Brg1 protein in human cancer cells. For example, knock-in of functional BRG1
in a breast cancer cell line with a mutant BRG1
allele induces growth inhibition. This inhibition appears to be at least partly mediated by the suppression of cyclin E as an E2F target gene, and the overexpression of transcripts of cyclin-dependent kinase inhibitors p21 and p15 36
. It has also been shown that Brg1 itself interacts with tumor suppressor proteins (e.g. RB1, BRCA1) and components involved in Wnt signaling 24;39–45
, suggesting a role for this gene not only in chromatin remodeling, but also in cell-cycle regulation and in the activity of tumor-suppressor factors.
The role of BRG1
in pancreatic carcinogenesis is poorly defined. The recent sequencing of the pancreatic cancer genome elucidated that somatic mutations of BRG1
occur in ~2% of pancreatic cancers 26
. Rosson et al., found that reduction of BRG1
expression in pancreatic cancer cell lines by lentiviral short hairpin shRNA mediated RNA interference resulted in morphologic changes; however, no significant impact on in vitro
growth of cultured cell lines could be observed 46
In this study, we investigated the putative role of BRG1 in IPMNs, the most common cystic precursor lesion of pancreatic cancer, by performing immunohistochemical evaluation of BRG1 expression on IPMN tissue microarrays.
We found a progressive loss of Brg1 expression associated with increasing degrees of dysplasia in IPMNs. Only 28% of low-grade IPMNs showed loss of Brg1 expression, compared to 76% of high-grade lesions (p=0.03) (). Intermediate-grade IPMNs displayed an intermediate percentage of loss (52%), suggesting a progressive decrease of expression from low- towards high-grade lesions. Loss of Brg1 expression was found in 2 out of 8 invasive cancer present in our TMA (25%) and this pattern was highly concordant with their non-invasive counterpart. This finding substantiates the progressive loss of Brg1 during tumor progression from lesions with mild dysplasia to invasive cancer. Interestingly we did not observe a difference in Brg1 expression among different histologic subtypes nor among different locations within the ductal system. It is known that main duct IPMNs are more likely to exhibit pancreaticobiliary or intestinal features and high-grade dysplasia and are more commonly associated with an invasive carcinoma compared to branch duct lesions. However, loss of Brg1 does not appear to be involved in a phenotype differentiation nor to a preferential location within the ductal tree.
In order to extrapolate the tissue studies to cultured cell lines, especially in a recently described in vitro
IPMN model 29
, we assessed Brg1 expression in IPMN and PDAC cells. When compared with non-neoplastic HPNE cells, IPMN 1T showed a marked decrease in Brg1 expression, which was comparable to the “positive” control Pa03C cells, that are known to harbor a somatic mutation of BRG1
. This finding corroborates our results and provides additional support to the role of Brg1 in IPMN development and progression. Further, the Pa03C and IPMN 1T cells provide a novel in vitro
platform to study the functional consequences of BRG1
restitution in pancreatic neoplasia for the first time.