Epigenetic silencing of the receptor tyrosine phosphatase, PTPRK, located at the frequently deleted 6q22.33-q23.2 region, leads to tumor growth via the constitutive activation of STAT3 in nasal-type NK/T-cell lymphoma

Abstract

Tumorigenesis is a multi-step process and involves the silencing of tumor suppressor genes (TSGs) by genetic and/or epigenetic mechanisms. Aberrant hypermethylation of gene promoters is a major epigenetic mechanism associated with TSG silencing in cancer. To identify putative TSGs that might be epigenetically silenced in extranodal NK/T-cell lymphoma, nasal type (ENKL), a genome-wide screening was performed in a commonly deleted region 6q22.33-q23.2. PTPRK (protein tyrosine phosphatase, receptor type, kappa) was identified as the only gene out of 77 genes mapped to the 6q22.33-q23.2 region that was upregulated in four of five ENKL cell lines after treatment with the demethylation agent 5-aza-2' deoxycytidine (5-aza-dC). Further analysis by methylation-specific PCR (MSP) and bisulfite genomic sequencing (BGS) confirmed that the CpG island surrounding the transcriptional start site of PTPRK was methylated in ENKL cell lines not expressing PTPRK. Similarly, a significant correlation between methylation of the PTPRK promoter and downregulation of PTPRK mRNA and protein expression (p=0.019 and p=0.048, respectively) was detected in 39 primary ENKLs. PTPRK gene allelic loss was detected in 80% of cell lines and 47% of primary ENKLs. Functional analyses by in vitro assays showed that the re-expression of PTPRK by retroviral transduction in the PTPRK non-expressing NKYS cell line suppressed the size and number of colonies formed, led to a remarkable increase in the apoptotic cell population and cell cycle arrest at G0/G1 phase. Moreover, PTPRK re-expression substantially reduced the migration and invasion of NKYS cells. Conversely, the inhibitory effect of PTPRK was significantly decreased by partial shRNA knockdown of PTPRK expression in the PTPRK-expressing SNK-6 cell line. The re-expression of PTPRK in another PTPRK non-expressing YT cell line suppressed tumor growth and metastasis in a nude mouse xenograft model. Consistent with these in vitro and in vivo findings, clinicopathological correlation analysis showed that PTPRK silencing was detected mostly in the ENKL patients with advanced and metastatic disease. Examination of the PTPRK protein sequence revealed that the cytoplasmic domain possesses a consensus STAT3 binding domain (YXXQ). Re-expression of PTPRK in NKYS cells resulted in a significant decrease in the level of phospho-STAT3 (Tyr705), whereas PTPRK knockdown in SNK-6 cells resulted in an increased level of phospho-STAT3 (Tyr705). These data suggested that PTPRK dephosphorylates and regulates the oncoprotein STAT3. Overall, this study shows that PTPRK is a putative TSG in the 6q22.33-q23.2 region that is frequently deleted and epigenetically silenced in ENKL, and the loss of PTPRK expression promotes tumor growth via the aberrant constitutive activation of STAT3 in ENKL. Specific therapies aimed at targeting STAT3 warrants further exploration.