Epigenetic silencing of receptor-type protein tyrosine phosphatase K in Hodgkin lymphoma /cby Wong Hon Yu

Abstract

Tyrosine phosphorylation is an important mechanism of eukaryote physiology in many aspects, as well as human health and disease. Since phosphorylation of tyrosine by protein tyrosine kinases (PTKs) play a crucial role in the regulation of proliferation, differentiation, adhesion, metabolic homeostasis, and are directly related to cancer cell growth, the potential tumor suppressing activities of protein tyrosine phosphatase (PTPs) are of great interest in cancer research recently. The decreasing dephosphorylation activity of PTPs may lead to abnormal cell growth and proliferation, hence finally lead to cancer. DNA hypermethylation of the promoter is the most common epigenetic mechanism of gene silencing. It is believed that DNA hypermethylation of the promoter of tumor suppressor genes often results in the inactivation of gene expression permanently. We hypothesized that gene silencing of receptor-type tyrosine-protein phosphatase K (PTPRK) by promoter hypermethylation is associated with Hodgkin lymphoma. In this study, DNA methylation status of PTPRK promoter was studied in 32 Hodgkin lymphoma cases. DNA was extracted from Hodgkin lymphoma biopsy specimens and PTPRK methylation was detected by bisulfite conversion followed by Methylation Specific Polymerase Chain Reaction (MSP). Bisulfite conversion is regarded as the gold standard technology for DNA methylation detection because it provides an efficient approach to identify methylation of cytosine at single nucleotide resolution. In bisulfite conversion, unmethylated cytosines will be converted into uracil residues. As a result, unmethylated cytosines will be recognized as thymine after subsequent PCR amplification. However, methylated cytosines are resistant to bisulfite treatment and remain as cytosine residues. Hence, methylated cytosines can be distinguished from unmethylated cytosines by the presence of cytosine “C” and thymine “T” residues respectively during sequencing. The bisulfite treated products were then followed by MSP and gel electrophoresis. Results showed that 23 of 32 Hodgkin lymphoma cases (71.9%) were found to have methylated PTPRK promoter, which was positively correlated with decreased PTPRK expression levels observed by immunohistochemistry (IHC) (p<0.05). In conclusion, the results showed that gene silencing of PTPRK by promoter methylation is frequent in Hodgkin lymphoma. With the known association between EBV-positive Hodgkin lymphoma cells and down-regulation of PTPRK protein expression by the EBV–encoded EBNA1, the findings of this project further showed that PTPRK is also silenced by promoter hypermethylation in Hodgkin lymphoma.