The Mechanism Of P53-p21 Pathway Inhibits Expression Of The Histone Methyltransferase NSD2 And The Function Of NSD2 In DNA Damage Response

Nuclear Receptor-binding SET domain 2（NSD2） is a histone methyltransferase that is abnormally expressed in Wolf-Hirschhorn syndrome（WHS） and many kinds of carcinomas including melanoma. However, except for WHS and multiple myeloma（MM）, we know little about the mechanism that is responsible for its aberrant expression.p53 wild-type human melanoma cell line 92-1 was used as a cell model for this study. Firstly,92-1 cells were exposed to four different kinds of DNA damage inducer to prove that NSD2 is downregulated at protein level after serious DNA damage, and its downregulation is a general phenomenon which is related to upregulation of p53 and its downstream target gene p21. Then, the effects of etoposide on 92-1 cells was compared with the hydroxyurea’s at m RNA levels of p53、p21 and NSD2. RNA interference technology was also introduced to analyze the relationship between p53、p21 and NSD2. From the comparisons, it is easy to conclude that it’s p53, rather than p21 that directly represses the transcription of NSD2, and the repression depends on a certain posttranslational modification of p53 which is obviously different between the two different treatments. Then, it was still a puzzle that why the abundance of NSD2 protein in 92-1 cells gradually decreased in 48 hours after hydroxyurea treatment, since p53 did not repress the transcription of NSD2 in the context. It is speculated that p21 mediated the degradation of NSD2 by premature activation of the APC/C^cdh1 after DNA damage induced G2/M arrest. Si-p21, si-cdh1 and a proteasome inhibitor-MG132 were utilized to verify this speculation and positive results were obtained. Finally, after NSD2 knock down, the DNA damage response of 92-1 cells exposed to X-ray or hydroxyurea were detected to explore whether NSD2 functions differently in the DNA damage response to these two different treatments.Conclusions:1. p53 directly represses the transcription of NSD2.2. p21 mediates the degradation of NSD2 by premature activation of the APC/C^cdh1 during cell senescence.3. The functions of NSD2 in DNA damage response which is induced by X-ray or hydroxyurea are exactly different.