Background
GIST are the most frequent non-epithelial tumors of the GI tract. They originate from the interstitial cells of Cajal and are strongly dependent on signaling from the receptor tyrosine kinase (RTK) KIT and the KIT ligand, stem cell factor. Activating mutations in KIT and platelet derived growth factor receptor-A (PDGFRA) result in uncontrolled proliferation and ultimately trigger the onset of GIST [
1,
2]. Mutations in the
kit gene in exons 11, 9, 13 and 17 represent early events in GIST onset and have been associated with malignancy [
1,
3]. Constitutive activation of RTKs induces expression and/or phosphorylation of a variety of intracellular proteins involved in cell survival, proliferation, and motility and adhesion signals, including components of the PTEN/PI3K/AKT pathway [
4], which modulates cell cycle progression by down-regulation of p27
Kip1 in various neoplasms. Reduced expression of PTEN due to mutation is predictive of aggressive disease in human tumors [
5]. Low or absent expression of PTEN in GISTs has been found to be associated with clinical progression and poor prognosis [
6].
Tumour size and mitotic index play a key role in the grading system for GIST [
1,
2], although their value in predicting the biological behaviour is limited and all GIST, of any size, can potentially metastasize. To date no single morphologic, immuno-phenotypic or genetic marker can predict the aggressiveness of GIST [
1].
The risk classification derived from a 2002 NIH consensus conference provided a means for hierarchical classification of aggressiveness into four categories: very low, low, intermediate and high risk, based on tumour size and mitotic rate [
1]. This classification, while conferring essential information on prognostic criteria, still needs to be confirmed by using real follow-up data. In addition, the consensus criteria over-estimate the biologic potential of gastric GIST, in particular that of large tumors with low mitotic rate, as recently reviewed by Miettinen and Lasota [
2]. However in general, gastric tumours have a more favourable prognosis than intestinal ones with similar parameters [
7].
The proliferation marker Ki67 has proved useful in assessing the rate of tumour cell proliferation in GIST, although it does not seem to be more reliable than mitotic count [
8]. Uncontrolled cell growth caused by a reduction of levels of cyclin-dependent kinase inhibitors (CKI) is a key event in human tumour evolution [
9]. Loss of p27
Kip1 correlates with aggressive potential in human carcinomas [
10], and sarcomas [
11], and is an independent prognostic factor in other human tumours [
12]. Published evidence suggests a more general deregulation of the cell cycle in GIST, and we have recently reported that p16
INK4A loss can identify high risk GIST [
13], in agreement with data from a separate study [
14]. p27
Kip1 loss has been associated with malignant potential in some reports on GIST [
15‐
17], although its value as a prognostic marker has not been conclusively determined in this tumour type [
18]. The human Jun activation-binding protein 1 (Jab1) has been identified as a p27
Kip1 interacting protein, that induces nuclear export of p27
Kip1 to the cytoplasm, thus permitting its degradation via the ubiquitin-proteosome pathway [
19], and in particular by the activation of the SCF-Skp2/E3 ubiquitin protein ligase pathway [
20]. The F-Box protein Skp2 functions as the substrate recognition factor of the SCF complex, which recognizes and binds to phosphorylated p27
Kip1 [
21]. Recent findings indicate that p27
Kip1 contributes to Skp2 inhibition via a mechanism that involves repression of target gene promoter activity, suggesting a reciprocal regulation between the two proteins [
22]. Skp2 overexpression plays a role as an independent prognosticator, stronger than p27
Kip1 and Ki67, in soft tissues sarcomas [
23]. To the best of our knowledge, its significance as a prognostic marker in GIST has not been investigated.
The role of Jab1 in human oncogenesis is currently under investigation. Tomoda et al. showed that Jab1 physically interacted with p27
Kip1 and enhanced its cytoplasmic translocation, which resulted in accelerated degradation of p27
Kip1 via the ubiquitin/proteasome pathway [
19]. Data on human tumours demonstrate that p27
Kip1 expression is inversely correlated to Jab1 expression [
24], and recently Jab1 expression has been demonstrated to induce p27
Kip1 degradation by a Skp2 independent mechanism in pancreatic carcinoma cells [
25].
In this study we have evaluated the significance of histologic parameters and expression of cell cycle regulators in 94 GISTs, 59 of which with clinical information. Our data confirm the importance of using a mixture of histologic and immunohistochemical parameters in order to predict potential benign or malignant behavior in a given GIST at diagnosis. They also point out to a role for Skp2 and p27Kip1 as prognostic indicators, in GIST, of good or bad behavior respectively.
Discussion
Our results, on a TMA containing 94 primaries GIST, show that morphological parameters and immunohistochemical markers, alone and in combination, may be useful in predicting benign or malignant biologic behavior of GIST at diagnosis. In particular, our analysis of 59 cases, for which clinical information was available, suggests 1) an independent role for high cellularity in predicting reduced OS; 2) a significant correlation of cellularity, mitosis, tumour size, high risk, Ki67 expression, and p27Kip1 expression with RFS; 3) a direct correlation of p27Kip1 loss as well as of Ki67 and Skp2 over expression with high risk; and 4) a significant correlation between Ki67 over expression and OS.
High cellularity, mitosis, and tumour size have been demonstrated to be prognostic factors in separate studies [
2,
8,
27]. Our results confirm the validity of the above classical morphological parameters in attributing potentially malignant behaviour to a given tumour.
We also analyzed the tumours for the presence of TIL. We found a trend toward an inverse correlation between the presence of TIL and shortened RFS. TIL are the primary immune component infiltrating solid tumours and are considered to be a manifestation of the host anti-tumour reaction. To the best of our knowledge this feature, only analyzed in one recent report [
28], has not been identified as a histologically significant indicator in GIST. Our data not only suggest a role for TIL in an anti-tumour reaction, but also suggest further investigation on additional series, into whether GIST patients might be identified who could benefit from immunotherapy.
In our study, Ki67 over-expression significantly correlated with high risk and with both OS and RFS. These data are in apparent contradiction with a study attributing a stronger potential to mitotic rate than Ki67 index as a prognosticator in GIST [
15], however they are consistent with other studies in which Ki67 labeling index (LI) discriminated between potentially benign and malignant GIST better than counts of mitotic figures [
8]. The direct correlation between expression of Ki67 and Skp2 was significant, in agreement with what has been found in several tumour types, including mesenchymal neoplasms [
23,
29,
30].
In this study p27
Kip1 loss was significantly correlated with the degree of malignancy of GIST. The role of p27
Kip1 expression as a diagnostic or prognostic marker for a broad series of human tumours is currently under intense investigation. Our results are in agreement with a number of published studies [
16,
31,
32]. In particular Gelen et al. demonstrated that p27
Kip1 expression is correlated to RFS [
31]. p27
Kip1 loss has not been conclusively associated with biological aggressiveness of GIST, and is not used as a predictor of survival [
15,
17,
18]. Our results, obtained on a larger series, show an association between p27
Kip1 loss and RFS, suggesting an important role for p27
Kip1 in the progression of GIST, and urging further investigation of its potential as a prognostic marker.
Cell cycle progression in mammalian cells is finely regulated by the sequential activation of a cascade of cyclin-dependent kinases, which are modulated by a complex series of interactions between cyclins and cyclin-dependent kinases inhibitors (CKIs). p27
Kip1 is a specific CKI present at high abundance in quiescent cells and is a putative tumour suppressor [
11,
33]. While loss of function of tumour suppressors occurs frequently at the genetic level, p27
Kip1 mutations represent an extremely rare event, consistent with data that p27
Kip1 expression is regulated at the post-transcripitonal level [
34].
In our study, Skp2 expression significantly correlated with cellularity, high risk, tumour size, mitotic count, and Ki67 expression. To our knowledge, the significance of Skp2 over-expression as a prognostic marker in GIST has not been previously investigated. We found an inverse correlation between levels of Skp2 and p27
Kip1, which approached the statistical significance. These data are in agreement with studies on lymphomas [
35] and colorectal cancer [
10] and suggest that over-expression of Skp2 contributes to p27
Kip1 degradation in GIST. Our data are in contrast with findings by Oliveira et al [
23] in soft tissues sarcomas, which suggest the possibility of an alternative mechanism for p27
Kip1 degradation in sarcomas. Skp2 over-expression plays a role in aggressiveness of soft tissues sarcomas, where it has been found to be an independent prognosticator, stronger than p27
Kip1 and Ki67 [
23]. In our study, Skp2 over-expression did not correlate with OS and RFS. However, the correlation of Skp2 expression with several parameters of malignant potential (cellularity, high risk, tumour size, mitotic count, and Ki67 expression) suggests an important auxiliary role of Skp2 in predicting the aggressive potential of GIST.
The role of Jab1 in human oncogenesis is currently under investigation. Recently Jab1 expression, inversely correlated to p27
Kip1 expression in pancreatic carcinoma and has been demonstrated to induce p27
Kip1 degradation by a Skp2-independent mechanism [
25]. In our series, Jab1 expression did not correlate with any of the other parameters we analyzed, including p27
Kip1 expression, suggesting that degradation of p27
Kip1 in GIST might be Jab1-independent. It would be interesting to test this hypothesis using in vitro systems.
Competing interests
The study was funded by Novartis-Stiftung für medizinisch-biologische Forschung.
The authors declare that there are no other competing interests
Authors' contributions
DDV designed the study, performed the immunohistochemical assay, analyzed the histologic and immunohistochemical results, and helped to draft the manuscript. FD performed the statistical analysis, and along with SS, helped to draft the manuscript. LTe, GP, LTo and MRF participated in the design of the study and MRF edited the manuscript. LI conceived the study, analyzed the histologic and immunohistochemical results, and helped to draft the manuscript. All authors read and approved the final manuscript.