Exploration of Mutation and DNA Methylation of Polo-Like Kinase 1 (PLK1) in Colorectal Cancer

Polo-like kinase 1 (PLK1) is a serine/threonine kinase that plays critical roles in cell cycling and DNA damage response. Overexpression of PLK1 is associated with the poorer prognosis of cancers, including colorectal cancer (CRC). Although the downstream pathways of the overexpression that lead to oncogenesis have been extensively studied, little is known about the factors that cause the overexpression of PLK1 in CRC. DNA methylation was reported to be affecting the expression of PLK1 in some cancers. The study aims to investigate the contribution of genetic mutation and DNA methylation of the PLK1 gene to the overexpression of PLK1 in CRC. The study involves data mining from Catalogue of Somatic Mutations in Cancer (COSMIC) and UniProtKB, Sanger sequencing on DNA from the cell lines HCT116, SW48, Colo320DM and T84 to analyse the possible mutation of PLK1. Other than that methylation status of the PLK1 promoter in CRC are also analysed by using mass spectrometry (MS) and pyrosequencing. Data from the COSMIC show the low incidences of PLK1 mutation for CRC (3.02%) with 46 mutations identified. One of the mutation p.R337Q (c.1010G > A) is located in the D-box which is an important motif for protein ubiquitination and eventually proteasomal degradation. Hence this mutation may potentially result in stabilisation of the PLK1 protein. Mutations are detected at the upstream silencer region, the promoter region and Exon1 in HCT116 but are not located at the protein binding or functioning site. Similarly, the same mutation at promoter region is detected in SW48. Differential trends of changes in methylation status of PLK1 in the IR treated CRC cell lines detected by MS reveal the possible association between the methylation and the radiosensitivity. Furthermore, pyrosequencing shows that PLK1 methylation status in tumour tissues with high expression of PLK1 is not significantly different from those with no PLK1 expression. In conclusion, mutation of PLK1 gene is infrequent in CRC and the expression of PLK1 is unlikely to be dependent on DNA methylation in the promoter region of PLK1 in the CRC.

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