A functional identification of protein tyrosine phosphatase receptor - type kappa (PTPRK) gene that restores replicative senescence in breast tumor cells
Authors: Vikramjeet Zhawar, Jinglan Liu, Gursurinder Kaur, Gurpreet Kaur, Jagmohan Singh, Megha Patel, Ming Jin, Raghbir Athwal
Journal: Cancer Research (2008)

1838  Applying a functional strategy, we identified PTPRK gene that plays a role in cell senscence. Ectopic expression of PTPRK gene restores normal growth pattern and senescence in breast and ovarian tumor cells. In numerous studies, three regions on chromosome 6q, 6q16-21, 6q22.3-23.1 and 6q25-27, have been described to carry tumor suppressor, genes for breast and ovarian cancers. Microcell transfer of a normal chromosome 6 or 6q inhibits tumorigenicity, metastasis, cell growth or restores normal cell growth pattern and senescence in a variety tumor cell lines (Sandhu et al. 1994, PNAS 91: 5498-5502; Sandhu et al. 1996, Oncogene 12: 247-252). These studies provided functional evidence for the presence of cell senescence/tumor suppressor genes on 6q. Applying a deletion strategy, following microcell mediated transfer of a normal human chromosome 6 or 6q into immortal breast tumor and SV40 immortalized cells, we mapped a candidate cell senescence locus, within 1cM genetic interval, at 6q22.3. Human genome database search, for cDNAs mapped within this locus, identified PTPRK (Protein-Tyrosine Phosphatase Receptor Type K) as candidate cell senescence/tumor suppressor gene.  Following initial identification, a 5.6 Kb full length cDNA for PTPRK gene was recovered by RT-PCR, and characterized for genomic structure and regulatory region. These studies revealed the expression of two different transcript variants for PTPRK gene in tumor v/s normal cells. Open reading frame for PTPRK gene was cloned in a retroviral expression vector, in frame with EGFP tag, where EGFP is added after the signal peptide. In this vector, PTPRK-EGFP gene is expressed by a CMV promoter along with puromycin gene for selection, as a single transcript. Ectopic expression of PTPRK induced complete growth arrest and senescence in breast and ovarian tumor cells. Gene transfer clones that multiplied indefinitely contained a truncated form of introduced PTPRK gene, confirming that intact gene indeed induces senescence in tumor cells. These results suggest that PTPRK may be a cell senescence/tumor suppressor gene that plays a role in the etiology of breast cancer. PTPRK is a member of a large family of genes for Protein Tyrosine Phosphates (PTPs). The interplay of Protein Tyrosine Kinases (PTKs) and PTPs in phosphorylation and dephosphorylation of proteins, involved in the cascade of signaling circuitry, governs physiological functions involved in cell growth and differentiation. We hypothesize that PTPRK gene is either mutated or abnormally expressed in tumor cells and loss or abnormal expression of PTPRK gene contributes to the neoplastic growth of cells in breast cancer. Future studies are designed to delineate the signaling pathways responsive to the expression of PTPRK gene. PTPRK gene, cloned in an Adeno viral vector, will be tested for effect on tumors formation in an animal model.