Investigating the effect of age-related mitochondrial dysfunction on intestinal tumour formation and progression

Abstract

The clonal expansion of mitochondrial DNA (mtDNA) point mutations and associated defects of oxidative phosphorylation (OXPHOS) are observed within the ageing colorectal epithelium. Clonally expanded mtDNA point mutations are also observed within colorectal tumours, however whether these mutations are passive bystanders or if they functionally contribute to tumour progression is unknown. In order to address this, a mouse model was developed in which PolγA mediated mtDNA mutation accumulation preceded the intestinal Lgr5+ stem cell specific deletion of Apc. In comparison with controls, mice harbouring OXPHOS defects demonstrated an accelerated clinical deterioration characterised by anaemia, impaired thermoregulation and weight loss. The reduced survival of these animals was attributed to a significantly greater intestinal adenoma burden in which a prominent complex I defect was revealed following immunofluorescent quantification of OXPHOS protein levels. A double thymidine analogue labelling technique demonstrated accelerated proliferation within complex I deficient intestinal adenomas while apoptotic cells were significantly fewer. Total RNA sequencing of OXPHOS deficient intestinal crypts prior to Apc deletion identified the upregulation of transcripts specific to the de novo serine synthesis pathway and mitochondrial one-carbon cycle (Whitehall, 2019). These pathways provide essential nucleotides, antioxidants and anabolic precursors vital to maximal tumour growth. RNA sequencing results were validated at the protein level within OXPHOS deficient intestinal crypts and adenomas and were further demonstrated to be upregulated in response to an age-related accumulation of OXPHOS defects in the murine small intestine and colon. Finally, these data are directly translatable to the normal human colonic epithelium in which the upregulation of serine synthesis and mitochondrial one-carbon cycle enzymes was identified within OXPHOS deficient crypts. These data show that age-related OXPHOS deficiency creates a permissive metabolic environment which in the event of malignant transformation, can actively contribute to accelerated intestinal cancer growth