mitochondrial DNA] sequence of 93 CFS [chronic fatigue syndrome] patients from the UK and RSA, without finding evidence of clinically proven mtDNA mutations."
So said the results reported by Elizna Schoeman and colleagues  (open-access available here) who drew on other research previously covered on this blog (see here) suggesting that "symptoms of mitochondrial diseases and CFS frequently overlap and can easily be mistaken" to look-see whether issues with mitochondrial DNA might be important to at least some cases of CFS. Their investigation found no evidence that within this cohort at least, undiagnosed mtDNA disease was apparent. They do caution however that their findings do not "exclude a role for mtDNA population variation in the susceptibility to CFS", neither also did they provide any functional biological analysis of the presentation of mitochondrial function/disease in cases on this research occasion. I might add that the lack of an association between mitochondrial genomes and CFS is not a new finding .
Mitochondria represent the so-called 'powerhouse' of cells as a result of their link with compounds such as ATP (adenosine triphosphate) among other things. Functional issues with mitochondria are known to manifest as 'fatigue' on some occasions and so it stands to reason that they should be explored in conditions where fatigue is a primary symptom such as in CFS. Indeed, other authors have looked at 'targeting' mitochondria onward to tackling some of those fatigue-related symptoms present in cases of CFS (see here).
The Schoeman results are a bit of a blow to the idea that genetically speaking, issues with mitochondria are a core part of CFS such that "CFS does not fall within the spectrum of inherited mtDNA disorders." But I would perhaps draw your attention to some of the 'interventions' that have been talked about in relation to CFS that have a 'mitochondrial' edge to them (see here) and have 'helped' under experimental conditions. Further, how just because CFS is not an inherited genetic condition as a result of underlying [genetic] mitochondrial disease does not necessarily rule out the targeting of mitochondrial functions in at least some cases of CFS. This bearing in mind the typical onset age of something like CFS (see here) and that various 'trigger' factors have been implicated with onset in mind (see here).
To close, I also note that the Cochrane guidance on the use of exercise therapy in relation to CFS has been updated (again) recently . Personally, I'm minded to be a little cautious about quite sweeping statements like: "We think the evidence suggests that exercise therapy might be an effective and safe intervention for patients able to attend clinics as outpatients" made by the authors, particularly when accompanying statements like: "Serious side effects were rare in all groups, but limited information makes it difficult to draw firm conclusions about the safety of exercise therapy" are made. 'First do no harm' and all that and then perhaps focus a little more on important concepts like PEM in light of other meta-analysed findings ...
 Schoeman EM. et al. Clinically proven mtDNA mutations are not common in those with chronic fatigue syndrome. BMC Med Genet. 2017 Mar 16;18(1):29.
 Billing-Ross P. et al. Mitochondrial DNA variants correlate with symptoms in myalgic encephalomyelitis/chronic fatigue syndrome. J Transl Med. 2016 Jan 20;14:19.
 Larun L. et al. Exercise therapy for chronic fatigue syndrome. Cochrane Database Syst Rev. 2017 Apr 25;4:CD003200.
 Loy BD. et al. Effect of Acute Exercise on Fatigue in People with ME/CFS/SEID: A Meta-analysis. Med Sci Sports Exerc. 2016 Oct;48(10):2003-12.
Schoeman EM, Van Der Westhuizen FH, Erasmus E, van Dyk E, Knowles CV, Al-Ali S, Ng WF, Taylor RW, Newton JL, & Elson JL (2017). Clinically proven mtDNA mutations are not common in those with chronic fatigue syndrome. BMC medical genetics, 18 (1) PMID: 28302057