Colorectal Cancer (CRC) is the fourth leading cancer world-wide with approximately 1,360,602 new cases annually. The earliest known model of sporadic CRC includes the adenoma-carcinoma sequence characterized by chromosomal instability and alterations in specific oncogenes (e.g. KRAS) or tumour suppressor genes (e.g. TP53, APC etc). More recently, alternative molecular pathways characterised by multiple CpG Island Methylation (termed as CIMP) associated with distinct genetic alterations in BRAF and KRAS oncogenes are also identified. The interaction of HPV with frequently perturbed molecular events in colorectal cancer development has not been investigated in details.
The present studied the association of HPV with various genetic (MSI, KRAS, TP53 and BRAF mutations) and epigenetic (CpG island methylation) events colorectal cancer. HPV DNA was detected by PCR using My09/My11 and Gp5+/Gp6+ consensus primers and typed using HPV16 and HPV18 specific primers. MSI was detected using BAT 25 and BAT 26 markers and mutation of KRAS, P53 and BRAF V600E was detected by direct sequencing. Methyl specific polymerase chain reaction was used to determine promoter methylation of the classical CIMP panel markers (P16, hMLH1, MINT1, MINT2 and MINT31) and other tumour related genes (DAPK, RASSF1, BRCA1 and GSTP1).
HPV DNA was detected in 34/93 (36.5%) colorectal tumour tissues, HPV 18 being the predominant high-risk type. HPV presence was not associated with age, stage or grade of tumours, MSI or mutations in KRAS, P53 or BRAF genes; however a non-significant trend for better survival was observed in HPV positive cases. HPV was not only found to be associated with CIMP-high in both univariate and multivariate analysis, but also a synergistic association was observed between HPV and KRAS codon12, 13 mutations (OR= 8.01 [2.05-31.27], P=0.003) towards promoting aberrant CpG island methylation in colorectal cancer.
The present findings indicate that, KRAS oncogenic mutations might synergistically co-operate HPV to promote CpG island methylation in high-risk HPV associated colorectal cancer progression. HPV therefore, might be a potent risk factor of colorectal malignancies in developing countries and epigenetic deregulation of host genome might be an alternate pathway for HPV pathogenesis in CRC.