Frequent mono-allelic loss associated with deficient PTEN expression in imatinib-resistant gastrointestinal stromal tumors

Abstract

Insufficiency of phosphatase and tensin homolog (PTEN) occurs in numerous tumor types and has been implicated as a resistance mechanism to receptor tyrosine kinase-targeted therapies in human cancer. In this study, we have performed a comprehensive molecular and immunohistochemical characterization of PTEN in 58 imatinib-naı¨ve and 54 imatinib-treated gastrointestinal stromal tumors (GISTs). The findings were correlated with clinicopathological data. At the genomic level, PTEN was affected mainly by mono-allelic loss, which was significantly less frequent in imatinib-naı¨ve vs imatinib-resistant tumors (9% vs 39%, Po0.001). Neither PTEN mutations nor PTEN promoter hyper-methylation were found. By immunohistochemistry, PTEN depletion was clearly related to GIST progression. Low PTEN protein expression was common (50%) and often paralleled with total immunonegativity in imatinib-resistant tumors. The abnormal PTEN protein expression correlated with PTEN loss at the genomic level (P ¼ 0.001). In addition, the effect of small interfering RNA (siRNA) PTEN knockdown on KIT signaling was examined in GIST-T1 and GIST430 cell lines, in the absence or presence of a dual PI3K/mTOR inhibitor NVP-BEZ235, alone or in combination with imatinib. In both cell lines, siRNA silencing of PTEN resulted in the substantial upregulation of PI3K-AKT and MAPK pathways. The MAPK hyperactivation was further potentiated by NVP-BEZ235 in the imatinib-sensitive GIST-T1 cells; yet, this effect was counteracted efficiently by combined treatment. In the imatinib-resistant GIST430 cells, neither NVP-BEZ235 alone or in combination with imatinib yielded sufficient inhibition of hyper-phosphorylated MAPK and downstream intermediate S6 protein. In conclusion, depleted PTEN expression associated with mono-allelic PTEN loss occurs frequently in imatinib-resistant GIST and might serve as a biomarker for stratifying patients for optimal treatment. In vitro, the PTEN insufficiency leads to hyperactivation of AKT and MAPK pathways in tumor cells. Novel therapies targeting multiple components of the integrated KIT receptor signaling pathways in imatinibresistant GIST warrant further studies.

Phosphoinositide 3-Kinase Inhibitors Combined with Imatinib in Patient-Derived Xenograft Models of Gastrointestinal Stromal Tumors: Rationale and Efficacy

“…Indeed, PTEN acts as a negative regulator of the PI3K-AKT signaling pathway (27). This hypothesis and our results are consistent with a study conducted by Quattrone and colleagues (28), in which the effect of PTEN silencing was evaluated in the imatinib-sensitive GIST-T1 (KIT exon11 p.V560_Y579del) and imatinib-resistant GIST430 (KIT exon11þ13 p.V560_L576delþV654A) cell lines. Importantly, silencing of PTEN resulted in over-activation of AKT in both models in vitro.…”

“…Indeed, the absence of AKT inactivation under singleagent PI3Ki in UZLX-GIST4 was counteracted by combining IMAþPI3Ki, which led to improved efficacy over either single treatment. The same evidence comes from Quattrone and colleagues (28) who have shown that PTEN silencing reduced the inhibitory effect of BEZ in GIST cell lines, which were reverted substantially by IMAþBEZ combination. Floris and colleagues (15) showed that GDC-0941 had a more pronounced inhibitory effect on AKT in KIT exon11 mutants without PTEN loss as compared with PTEN-deficient KIT exon11 mutants.…”

Combination of Imatinib Mesylate and AKT Inhibitor Provides Synergistic Effects in Preclinical Study of Gastrointestinal Stromal Tumor

“…Although these inhibitors have revolutionized the treatment of GIST and other malignancies, management of intrinsic and acquired resistance mechanisms remain a clinical challenge. Activation of the PI3K/AKT pathway, downstream of activated RTKs, has been shown to both predict and promote resistance to RTK inhibitors in GIST and in other malignancies (7,9,10,39-41). Clinical trials are currently underway to investigate the use of PI3K/AKT inhibitors in combination with RTK inhibitors in CLL, melanoma, and NSCLC.…”

“…Recently, PI3K inhibitors were combined with IM in preclinical studies using GIST xenografts, and this combination demonstrated superior and more durable responses as compared to either single agent (13,14). Reduced or absent expression of the phosphatase and tensin homolog (PTEN) was significantly associated with IM treatment in GIST patient samples, and PTEN deficiency in vitro results in hyperactivation of AKT (9). Two recent studies in IM-sensitive and resistant GIST cellular models established links between KIT activity (modified by specific KIT-targeting miRNAs), AKT expression and phosphorylation, and cell viability and apoptosis (42,43).…”

Frequency and clinicopathologic profile of PIK3CA mutant GISTs: molecular genetic study of 529 cases

“…3,5,6 Relatively little is known about the genetic alteration of PI3K/AKT/mTOR pathway in GISTs, although one study reported frequent mono-allelic loss of PTEN, a negative regulator of PI3K. 11,12 In addition, activation of the PI3K/AKT pathway was proposed to be involved in tumor cell survival in imatinib-sensitive and -resistant GISTs. 13 PIK3CA, a gene that encodes the p110α catalytic subunit of PI3K, has been shown to be mutated in common carcinomas, malignant melanoma, and some sarcomas.…”