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Bennett. J Transl Genet Genom 2020;4:36-49 I https://doi.org/10.20517/jtgg.2020.17 Page 45

themselves, is not readily reoxidized. In liver from rat models of this MD, then, both low ATP production
and ROS production are likely and both sequelae would need to be addressed in a patient.

EPR of muscle paints a different picture. The human disease being modeled is characterized by weak muscle
performance [125] . In rats, expression of this phenotype is far milder, possibly due to underlying metabolic
[79]
corrections . The reduced iron-sulfur cluster signals from each of Complex I, II, and III in MD rat muscle
were diminished by 50% to 75%, with the Complex I signals depleted most. In addition, the intensity
+
of the Complex II S3 oxidized [3Fe4S] cluster signal increased by a factor of three in MD rats. These
data, in contrast to liver, suggest a global increase in redox potential (i.e., decrease in ATP-synthesizing
thermodynamic driving force) in the muscle of DGUOK-depleted rats. A number of scenarios that might
explain the phenotype and mitochondrial pathology were considered and rejected and, ultimately, a depletion
of succinate combined with the inability of Complex I to release electrons from the mitochondrially-encoded
ND1 subunit was considered the most likely explanation.

One perhaps surprising result was that the EPR of heart muscle in both MD and w/t rats was identical and
indicated a fully reduced MRC and an absence of markers for oxidative stress. A reason for the observation
may simply be that in any other case the rat would not be alive for study. The cause may be that the heart has
large redundancy in the number of mitochondria in order to fully benefit from reducing equivalents from
primary metabolism even when under stress.

CONCLUSION
EPR of biopsy tissue of a subject with suspected MD can provide some limited information, largely through
the ratio of intensities of the Complex I iron-sulfur centers reporting on the ability to maintain a low redox
potential, and through the observation of biomarkers for historical and ongoing ROS-mediated oxidative
stress. When the EPR of such tissues can be compared with good controls, or with a yet-to-be established
database of control spectra, quantitative information on the intensities of multiple MRC components can
be obtained that can inform on disease mechanism. However, it is when EPR is combined with multiple
complementary techniques that it becomes most useful. The primary advantages of EPR are that the sample
requires no processing or fixing, and that it is relatively straightforward to determine whether the main
outcome of mitochondrial dysfunction is likely to be diminished ATP synthesis, elevated ROS production,
or both. One lesson from the study discussed above is that important information on an MD that is
characterized in humans by poor muscle performance may be obtained from study of other tissue, e.g., liver.

DECLARATIONS
Authors’ contributions
The author contributed solely to the article.

Availability of data and materials
Not applicable.

Financial support and sponsorship
None.

Conflicts of interest
The author declared that there are no conflicts of interest.

Ethical approval and consent to participate
Not applicable.