Mitochondrial Mutagenesis and Inflammation in the Colorectal Adenoma-Carcinoma Sequence
Aoife Maguire, Kieran Sheahan, Edward Fox, Petra Martin, David Hughes, Robert Geraghty, Niall Swan, Hugh Mulcahy, John Hyland, Diarmuid O'Donoghue, Jacintha O'Sullivan. St. Vincent's University Hospital, Dublin, Ireland; University College Dublin, Dublin, Ireland; University of Washington, Seattle; Trinity Centre for Health Sciences, Dublin, Ireland
Background: Inflammation is known to play a role in colorectal carcinogenesis. We previously found that mitochondrial lipid peroxidation increases as the colorectal adenoma-carcinoma sequence advances suggesting that mitochondrial damage may contribute to neoplastic progression. This led us to investigate mitochondrial dysfunction & its relationship to inflammation in this sequence.
Design: The random mutation capture assay was used to detect mitochondrial DNA(mtDNA) point mutation frequency & mitochondrial:nuclear DNA ratio in fresh frozen tissue from 4 normal colonic biopsies, 6 adenomas with low-grade dysplasia(LGD), 4 adenomas with high-grade dysplasia(HGD) & 8 colorectal cancers(CRC) with matched normal tissue. T-lymphocytes(CD3) & macrophages(CD68) were assessed on formalin fixed paraffin embedded tissue sections from these cases, with IHC. MtDNA encoded cytochrome c oxidase subunit II(COII, part of electron transport chain complex IV), was also assessed with IHC. Data were analysed with Wilcoxon sign rank test, Mann-Whitney test & Spearman correlation.
Results: MtDNA mutation rate was lower in adenomas with LGD compared to HGD & CRC (all p values ≤ 0.02). A similar pattern was seen for mitochondrial:nuclear DNA ratio with the lowest ratio found in adenomas with LGD & higher ratios in adenomas with HGD & CRCs (all p values ≤ 0.05). There was significant positive correlation between mtDNA mutation frequency & COII loss in CRCs (r=0.8, p=0.03). Increased mutation frequency correlated with increased numbers of CD3-positive cells (r=0.9, p=0.05) in adenomas.
Conclusions: T cell infiltration increases in adenomas with higher mutation frequencies suggesting a link between inflammation & mitochondrial mutagenesis. As the adenoma-carcinoma sequence progresses to HGD & CRC, mutational frequency & mitochondrial:nuclear DNA ratio increase. Increasing COII deficiency correlates with increasing mtDNA mutation frequency, demonstrating an association between random mutations & inefficient oxidative phosphorylation in CRC. Adenomas with LGD had the lowest mutation frequency and lowest amount of mtDNA relative to nuclear DNA. We postulate that mutated mtDNA may be preferentially degraded to avoid accumulation of mutations which may impair cellular metabolism at this early stage in neoplasia. Degrading damaged mtDNA may assume a lower level of priority in HGD & CRC.
Monday, March 19, 2012 1:00 PM
Poster Session II # 115, Monday Afternoon