Impaired transcriptional compensation for oxidative damage in a porcine model of ischaemic cardiomyopathy
Authors: Baz-Lopez D , Bruno V D , Johnson T W , Ascione R , Sammut E 
Published: October 2021

Abstract

Background
The prognosis of heart failure remains poor. Energetic imbalances related to mitochondrial dysfunction, impaired oxidative phosphorylation and oxidative damage have been implicated in the pathogenesis or worsening of heart failure (1,2). Improved understanding of the metabolic alterations in heart failure may provide new biomarkers or therapeutic targets.

Purpose
We explored mutation rate, mitochondrial copy number and regional mitochondrial gene transcription in a porcine model of ischaemic cardiomyopathy. Specifically, we investigated the Dloop region – a promotor for mitochondrial DNA- in which mutations have been implicated in many cancers. We hypothesised that there may be differences in mutation rate and oxidative damage within the mitochondrial genome.

Methods
15 female Yorkshire pigs were studied. 7 animals underwent percutaneous balloon catheter myocardial infarction followed by termination at 4 weeks. A group of 8 healthy animals served as controls. Reverse transcription quantitative PCR (RT-qPCR) assays were performed to determine the gene expression levels of mitochondrial DNA codified genes (ND1, ND2, ND4, ATP6 and ND6). Quantitative PCR (qPCR) methodology was modified to obtain the relative mitochondrial copy number, mutation rate, and oxidative damage according to established methods (3–5).

Results
Significant mitochondrial transcriptional activity of the genes studied was seen in both groups (see Table 1 and 2). When examining the ND2-ND6 region (excluding the Dloop) in control animals, we note an inverse correlation with increased oxidative damage corresponding to a significantly lower mutation rate (p=0.017). There was no correlation between the mutation rate and oxidative damage in the ND6-ND2- including Dloop- region. However, when examining the Dloop specifically, there was a marked inverse correlation between oxidative damage and mutation rate (p=0.007). This suggests that in controls, there is regional variation in the susceptibility to damage within the mitochondrial genome which may trigger repair mechanisms. Indeed, the relative mitochondrial copy number was inversely associated to the mutation rate (p=0.08) in controls.
In contrast, in the chronic animals, we noted no correlation with the level of oxidative damage in the ND2-ND6, D-loop, or ND6–2 regions compared to mutation rate (p=0.52, p=0.53 and p=0.17 respectively). This indicates that there is a loss in the ability to instigate repair mechanisms in the setting of ischaemic cardiomyopathy.

Conclusions
This study demonstrates that in control animals, there appears to be regional variation in the ability to mitigate against mutations in response to oxidative damage within the mitochondrial genome. In contrast, this protection and/or the effectiveness of repair mechanisms appear to be impaired in the setting of ischaemic cardiomyopathy. This may be a driver, and in turn a therapeutic target, for adverse remodelling in this setting.

Funding Acknowledgement
Type of funding sources: Other. Main funding source(s): This research work was supported by grants awarded to Professor Ascione: the British Heart Foundation (BHF) (BHF IG/14/2/30991, BHF RM/13/2/30158), and the Medical Research Council (MRC) (MRC MR/L012723/1).