An Open Label, Long Term Safety Study of REN001 in Primary Mitochondrial Myopathy Patients (Stride Ahead)

Inclusion Criteria: * mtDNA-PMM subjects: Completed treatment in STRIDE or was participating in Study REN001-101 when the study stopped due to the COVID-19 pandemic, and in the opinion of the Investigator and Sponsor had been compliant with the study requirements OR nDNA-PMM subjects: Subjects aged 18 years or older with known nuclear (nDNA) pathogenic variants with a major muscle phenotype consisting of objective myopathy with poor exercise tolerance. Proof of pathogenicity must be provided. Must be able to walk at least 100m in the screening 12MWT and the limitations in walk test must be primarily due to the energy deficit and not due to ataxia or any other condition. For subjects under 25 years old only: confirmation of bone growth plate closure by wrist radiograph. * Have PMM which continues to be primarily characterized by exercise intolerance or active muscle pain. * Willing and able to swallow the REN001 gelatin capsules. * Concomitant medications (including supplements) intended for the treatment of PMM or other co-morbidities likely to remain stable throughout participation in the study where clinically possible. * Signed and dated informed consent document indicating that the subject has been informed of all pertinent aspects of the study. * Females should be either of non-child-bearing potential or must agree to use highly effective methods of contraception from baseline through to approximately 30 days after the last dose of study drug. Males with partners who are women of childbearing potential (WOCBP) must also use contraception from baseline through to 14 weeks after the last dose of study drug. Exclusion Criteria: * Anticipated to need a peroxisome proliferator-activated receptor (PPAR) agonist other than REN001 during the study. * Intent to donate blood, or blood components during the study or within one month after completion of the study. * Current drug dependency. Use of opiates/cannabis for medical reasons is acceptable with prescription evidence or at the Investigator's discretion. * Current alcohol dependency. * Any medical, psychiatric or laboratory condition that may increase the risk associated with study participation or interfere with the interpretation of study results and, in the judgment of the Investigator and Medical Monitor, would make the subject inappropriate for entry into this study. * Pregnant or nursing female Subjects with mtDNA mutations can enroll at STRIDE Week 24 visit, STRIDE-FU visit, after exiting from STRIDE or after exiting REN001-101 (UK only). Subjects enrolling after exiting from either of the 2 feeder mtDNA studies and all subjects with nDNA mutations will be required to fulfill additional exclusion criteria during their additional screening visit. This is required for the mtDNA-PMM subjects due to the gap in study drug treatment and period of time without study assessments. The additional exclusion criteria are: 1. Clinically significant kidney disease or impairment calculated as eGFR Grade 2 or above \<60ml/min/1.73m2 using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation at Screening. 2. Clinically significant liver disease or impairment of AST or ALT Grade 2 or above (\>2.5 x ULN), or Total bilirubin \> 1.6 x ULN or \>ULN with other signs and symptoms of hepatotoxicity at Screening. 3. Subjects with uncontrolled diabetes and/or a Screening HbA1c of ≥11%. 4. Evidence of significant concomitant clinical disease that may need a change in management during the study or could interfere with the conduct or safety of this study. (Stable well-controlled chronic conditions such hypercholesterolemia, gastroesophageal reflux, or depression under control with medication (other than tricyclic antidepressants), are acceptable provided the symptoms and medications would not be predicted to compromise safety or interfere with the tests and interpretations of this study.) 5. Subjects with a history of cancer. A history of in situ basal cell carcinoma in the skin is allowed. 6. Clinically significant cardiac disease and/or clinically significant ECG abnormalities such as 2nd degree heart block, symptomatic tachyarrhythmia or unstable arrhythmia (right bundle branch block, left fascicular block and long PR interval are not excluded) that in the opinion of the Investigator should exclude the subject from completing exercise tests.