Methylation and silencing of the gene for the receptor-type protein tyrosine phosphatase PTPRO in primary human tumors

Proc Amer Assoc Cancer Res, Volume 46, 2005 923 Promoter methylation is a major mechanism for the transcriptional silencing of many tumor suppressor genes. Using Restriction Landmark Genomic Scanning technique to identify methylated genes, we found that the gene for the receptor-type protein tyrosine phosphatase receptor-type O (PTPRO) is suppressed in primary rat hepatocellular carcinoma induced by folate/methyl deficient diet, which probably occurs as a result of regional/localized hypermethylation (Motiwala, T. et.al., Oncogene, 22, 6319-6332, 2003). We further demonstrated methylation of this gene in many primary human solid tumors including hepatocellular carcinoma and lung cancer by COBRA (combined bisulfite restriction analysis) and bisulfite genomic sequencing. Real-time PCR showed that the methylated PTPRO in many tumor samples is silenced. The suppressed gene in some lung cancer cell lines could also be re-activated upon treatment with 5-aza-deoxycytidine (decitabine), a potent inhibitor of DNA methyltransferase I. Ectopic expression of the full-length isoform in a lung cancer cell line resulted in reduced proliferation, delayed re-entry into cell-cycle following release from cell-cycle arrest, increased susceptibility to apoptosis (Motiwala, T. et. al., PNAS, 101, 13844-9, 2004) and reduced tumor forming potential in nude mice. To demonstrate that the methylation-mediated suppression of PTPRO is a common characteristic of different types of cancers we analyzed a large number of primary chronic lymphocytic leukemia (CLL) samples. Normal B-lymphocytes express the truncated isoform of PTPRO (PTPROt). While the B-lymphocytes from normal individuals were free of methylation, approximately 75% (n=92) of primary CLL samples exhibited methylation at the PTPRO CpG island. Further, ectopic expression of PTPROt in leukemia cells resulted in slower growth rate and loss of anchorage-independent growth. These observations suggest that this specific receptor-type tyrosine phosphatase plays an important role in regulating growth and proliferation of mammalian cells. In addition to exhibiting these characteristics of tumor suppressor genes, the PTPRO gene is localized to chromosomal region 12p12.3 that is characterized by loss of heterozygosity in several tumor types, another hallmark of classical tumor suppressors. Protein tyrosine phosphatases are gaining recognition as important cellular molecules due to their ability to function as positive and negative regulators of signal transduction pathways involved in different cellular processes. Identification of PTPRO as a candidate tumor suppressor, therefore, provides a novel molecular target in cancer therapy. Further, its expression could also be used as a prognostic marker in cancer therapy. Supported, in part, by grants CA 81024, CA86978, and ES 10874 (STJ) from the National Institutes of Health.