(P 176) LOSS OF PTEN EXPRESSION IN MINIGISTS AND CLINICALLY RELEVANT GISTS

Objective: Gastrointestinal stromal tumors (GIST) are the most prevalent mesenchymal tumors of the gastrointestinal tract. To date, there are no widely accepted prognostic or predictive molecular biomarkers for GIST, other than mutation status of the KIT and PDGFRA genes. The Phosphatase and tensin homolog (PTEN), a tumor suppressor and a regulator of the PI3K/Akt/mTOR pathway downstream of KIT and PDGFRA, has been previously identified as a candidate molecular prognostic biomarker in GISTs. In our previous study, we observed heterozygous loss of the PTEN gene in 10% of tumors, and we demonstrated that both genomic loss and decreased expression of the PTEN gene were associated with unfavorable outcome (PMID: 36001861). Two previously published studies that included 84 and 112 patients, reported a complete loss of PTEN protein expression in 13% and 9% of cases, respectively (PMIDs: 24712384, 24743220). One of these studies also pointed to the association between PTEN protein loss and unfavorable outcome. However, none of these studies examined the frequency of PTEN loss in miniGISTs (GISTs smaller than 1cm), which are considered as precursor lesions for malignant GISTs. Here, we present the first analysis of PTEN expression in miniGISTs and the validation of prognostic value of PTEN expression in the largest cohort of clinically annotated primary untreated tumors from patients with localized disease at the time of diagnosis.

Methods: PTEN protein expression was evaluated by immunohistochemistry in two tissue microarrays. One tissue microarray included duplicate cores from 142 GIST specimens deposited in the Life Raft Group tumor bank, including 97 primary untreated tumors from patients with localized disease at the time of diagnosis. We also examined a second tissue microarray that consisted of single cores of 18 miniGISTs.
PTEN expression was scored as negative (a complete absence of staining in tumor cells) or positive (nuclear and/or cytoplasmic staining in >10% of tumor cells). Next, we analyzed the association between loss of PTEN expression and clinicopathological features. Kaplan-Meier analysis was used for survival analysis.

Results: We observed loss of PTEN protein expression in 15/142 (11%) of GISTs in the Life Raft Group cohort (13 patients diagnosed with localized disease and 2 patients diagnosed with metastatic disease). Eleven tumors showed homogeneous loss of PTEN expression in two representative cores, and four tumors showed loss of PTEN only in one of the two cores. Association between PTEN loss and clinicopathological features was performed in 97 primary untreated tumors from patients with localized disease at diagnosis. In this group, we observed PTEN loss in 10/97 (10%) of patients. Recurrence-free survival analysis showed a trend towards unfavorable outcome associated with PTEN loss (log rank p = 0.14). We also observed that PTEN loss was significantly associated with a larger tumor size (Mann-Whitney test p = 0.003, median size: 11.5 cm vs. 6.5 cm). While PTEN loss was not significantly associated with other clinicopathological features, it was observed mostly in high-risk cases (according to the modified NIH risk criteria). Interestingly, in the second cohort composed of miniGISTs, we observed loss of PTEN expression in 2/18 (11%) of these tumors, which indicates that this may be an early event in GIST development.

Conclusion: Our study confirms loss of PTEN protein expression in a subset of GISTs using the largest cohort examined up to date. We also confirm that PTEN loss may be associated with unfavorable recurrence-free survival. In addition, we show for the first time that loss of PTEN expression can be detected in miniGISTs, indicating that it can occur early in GIST tumorigenesis. Our findings warrant studies on the loss of PTEN as a biomarker for stratification of high-risk patients to participate in clinical trials of mTOR inhibitors, and low-/intermediate-risk patients to undergo intense surveillance.