title: Investigating the molecular and cellular implications of polycyclic aromatic hydrocarbons in bowel cancers
Creators: name: Cheng, Tracie T
Publisher: name: Griffith University
Year: 2022
Subjects: subject: Polycyclic aromatic hydrocarbon
subject: bowel
subject: cancer
subject: carcinoma
subject: colon
subject: diet
subject: meat
subject: rectal
subject: CYP1A1
subject: genetic variant

description: Colorectal carcinoma (CRC) is a devastating disease whose incidence continues to rise in countries that have adopted a Western lifestyle. The consumption of red and processed meats confers an increased risk of CRC. In 2015, the International Agency for Research on Cancer put forward several classes of compounds as possible causative carcinogens. Of these compounds, polycyclic aromatic hydrocarbons (PAHs) are molecules formed through an incomplete pyrolysis reaction, thus meats cooked at high temperatures or exposed to smoke are likely to be large sources of PAHs. This thesis begins by systematically reviewing the literature to assess the current state of knowledge and identify gaps that this thesis should focus on (Chapter 2). After ingestion, PAHs can undergo metabolic activation via one of three pathways; diol epoxide pathway, radical cation pathway, or the aldo-keto-reductase pathway. The main metabolic pathway is the diol epoxide pathway, a three-step metabolic conversion that uses cytochrome P450 1A1 (CYP1A1) and epoxide hydrolase to create a carcinogenic diol epoxide. These diol epoxides are capable of binding DNA to create DNA-adducts that create genomic instability and lead to cancer. Genetic variants of PAH associated metabolic enzymes and DNA-adduct repair genes are associated with CRC risk. Moreover, population studies link PAH intake to increased CRC risk. It was also noted that current studies tended to link these aspects to the risk of CRC but do not investigate clinical parameters in patients with CRC. Following this, an experimental study sought to confirm the presence of PAHs and prove their genetic role in CRC (Chapter 3) prior to deep diving into genetic analyses with clinicopathological parameters. Samples of commonly consumed ready-to-eat meats were collected and PAHs were extracted using QuEChERS method of solid phase extraction. PAHs were successfully detected and quantified using gas chromatography-mass spectrometry, which showed that various PAHs were present across all meat samples and smoking ham significantly increased PAH concentrations. PAHs were then used as in vitro treatments to assess effects on CRC gene expressions. Benzo[a]pyrene and phenanthrene exerted gene expression changes that supported a carcinogenic role, whilst anthracene did the opposite. Given the presence of PAHs in meats and their effects on CRC gene expressions, the next portion of the thesis examined CYP1A1 exon 7 in CRC patients (Chapter 4). 85 CRC patients were recruited and DNA was extracted from their cancer and matched non-neoplastic mucosa tissues. This underwent high-resolution melt curve analysis and Sanger sequencing to detect any mutations at this locus. All tumour and matched tissues were identical, and three known single nucleotide polymorphisms (SNPs) were detected, rs1048943, rs1799814, and rs41279188. 33% of patients had one of these SNPs, and all patients harboured the heterozygous allele for their respective SNP. Patients who had a SNP were significantly more likely to be of an advanced age (&gt;65 years) and less likely to have perineural infiltration of their cancer. Finally, in addition to mRNA gene expression experiments, a similar methodology from Chapter 4 was applied to investigate the AHR gene (Chapter 5), that encodes for aryl hydrocarbon receptor that regulates CYP1A1 gene induction. Tumour tissues that exhibited less than -2-fold change in comparison to their matched non-neoplastic mucosa were significantly more likely to be above 50 years of age, have cancer perforation, and have mucinous colorectal adenocarcinoma. Further inquiry into exon 10 of AHR in patients with CRC revealed that 27% (21/79) patients harboured the SNP rs2066853. These individuals were significantly more likely to exhibit more favourable clinicopathological characteristics such as lack of synchronous cancer, lack of perineural infiltration, less than 10 polyps present, cancer stage 1 or 2, and lower depth of tumour invasion. Although not significant, rs2066853 polymorphism was also associated with stable microsatellite instability phenotype and more advanced age (&gt;65). These associations clearly implicate roles for both CYP1A1 and AHR genes in CRC. This provides an avenue for further research into the relationship between PAHs from red and processed meat consumption and CRC to ultimately better understand this link and improve CRC incidence and patient outcomes.