Cytoplasmic localization of p27 (CDKN1B / KIP1) in colorectal cancer: inverse correlations with nuclear p27 loss, microsatellite instability and CpG island methylator phenotype (CIMP)

Cytoplasmic mislocalization of p27 (cyclin-dependent kinase inhibitor-1B, CDKN1B/KIP1) is caused by activated AKT1, and has been associated with poor prognosis in various cancers. CpG island methylator phenotype (CIMP) in colorectal cancer is characterized by extensive promoter methylation, and is associated with microsatellite instability-high (MSI-H) and BRAF mutations. We have recently shown a positive correlation between MSI/CIMP and loss of nuclear p27. However, no study has examined cytoplasmic p27 mislocalization in relation to CIMP and MSI in colorectal cancer. Using MethyLight assays, we quantified DNA methylation in eight CIMP-specific gene promoters {CACNA1G, CDKN2A (p16), CRABP1, IGF2, MLH1, NEUROG1, RUNX3 and SOCS1} in 853 colorectal cancer samples obtained from two large prospective cohorts. We assessed expressions of nuclear and cytoplasmic p27 and nuclear p53 by immunohistochemistry. Cytoplasmic p27 expression was inversely associated with loss of nuclear p27 (p<0.0001), CIMP-high (p<0.0001), MSI-H (p<0.0001) and BRAF mutations (p<0.0001). The inverse association of cytoplasmic p27 with CIMP-high (or MSI-H) was independent of MSI (or CIMP) status. In addition, the inverse association of cytoplasmic p27 with CIMP-high was independent of KRAS/BRAF status. BRAF and CDKN2A (p16) methylation were not correlated with cytoplasmic p27 after stratification by CIMP status. The inverse associations of cytoplasmic p27 with MSI-H and CIMP-high were much more pronounced in p53-negative tumors than p53-positive tumors. In conclusion, cytoplasmic p27 expression is inversely associated with MSI-H and CIMP-high, particularly in p53-negative tumors, suggesting interplay of functional losses of p27 and p53 in the development of various molecular subtypes of colorectal cancer.

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