Mitochondrial Disease

Background: Mitochondrial disease is a rare disorder. It can cause poor growth, developmental delays, muscle weakness, and other symptoms. The disease is usually inherited. It can be present at birth or develop later in life. Infection is a major cause of disease and death in people with this disease. Researchers want to learn more about these infections and the declining health of people who have this disease. To do this, researchers will study the DNA of people who become ill. Their DNA will be compared to the DNA of their household/family members. Objective: To learn more about how genes affect people with mitochondrial disease. Eligibility: People age 2 months and older with mitochondrial disease and their household/family members. .\<TAB\> Design: Participants will complete a questionnaire about their health history. Their medical records may be reviewed. They will give a blood sample. If the participant becomes ill, they may have a videoconference with a doctor or nurse at the NIH to perform a physical exam. They may be contacted after their illness to give updates on their health. They may be asked to give extra blood samples or complete extra questionnaires. Participants genetic data will be put into a database. The data will be labeled with a code and not their name. The data will be shared with other researchers. Participation lasts about 1 year. This may be extended if the participant is very ill.

Mitochondrial diseases (MDs) are the most common metabolic disorders. Due to their great clinical and genetic heterogeneity, their diagnosis relies exclusively on the identification of pathogenic variants in nuclear genes or in mitochondrial DNA (mtDNA). However, to date, 50% of affected patients remain without a definitive diagnosis. The advent of next-generation sequencing (NGS) has improved diagnostic yield, but many identified variants remain of uncertain significance (VUS), preventing a definitive diagnosis. The clinical interpretation of these newly identified rare variants therefore represents a major challenge. In the context of MDs, one of the major criteria for mtDNA variant pathogenicity is a good correlation between heteroplasmy level and tissue or cellular involvement. Heteroplasmy refers to the coexistence, within the same cell or tissue, of mutated and non-mutated mtDNA molecules. Pathogenic mtDNA variants are most often heteroplasmic, with the most affected tissues harboring a higher proportion of mutated mtDNA. In muscle biopsies from patients with MDs, muscle fibers may show a cytochrome c oxidase (COX) enzymatic deficiency (COX-negative fibers), reflecting dysfunction of the mitochondrial respiratory chain (MRC). Single-fiber analysis makes it possible to isolate muscle fibers by LASER microdissection and to quantify the heteroplasmy level of a variant within them. The presence of a high heteroplasmy level in COX-negative fibers, in contrast to fibers without deficiency (COX-positive fibers), is strong evidence supporting the pathogenicity of the variant. In previous projects, we tested two variant quantification techniques (PCR-RFLP and NGS), but these methods remain too labor-intensive or costly and are therefore difficult to maintain in routine practice. This pilot study aims to develop a new method for quantifying heteroplasmy levels using digital PCR. Faster and less expensive, this approach could simplify technical implementation and reduce analysis costs, thereby facilitating its integration into clinical practice. Initially, a feasibility study will begin with validation of digital PCR on DNA extracted from blood samples of two patients carrying pathogenic variants identified in a previous study (AOI 2017 IDRCB: 2017-A00688-45). This validation will compare digital PCR with the PCR-RFLP method for heteroplasmy quantification and assess the reliability and reproducibility of the technique. Once validation is achieved, digital PCR will be tested on DNA extracted from microdissected muscle fibers from the same patients to evaluate its feasibility at the single-fiber level, with comparison to PCR-RFLP results. If feasibility at the single-fiber level is confirmed, the method will then be tested in four new patients from the Mitochondrial Diseases Reference Center, in whom variants of uncertain significance have been identified and for whom muscle biopsies with COX-negative fibers are available. Additional samples (blood, urine, and buccal swabs) will be collected to assess heteroplasmy levels across different tissues. If validated, this technique could be applied to a larger number of patients and integrated into the diagnostic strategy for mitochondrial diseases. Moreover, digital PCR could also be used to quantify mtDNA copy number, an essential biomarker for monitoring patients with MDs. To this end, mtDNA is partitioned into thousands of nanowells, and absolute quantification is obtained by counting fluorescent signals emitted by positive partitions, with a nuclear DNA probe serving as an internal control. This validation will be performed using the same blood samples and muscle fibers, by comparing digital PCR results with those obtained using the reference method, quantitative PCR (qPCR). The primary objective of this study is to reduce diagnostic odysseys by simplifying existing methods. In addition, the application of digital PCR to quantify mtDNA copy number could offer new perspectives, particularly as a biomarker for patient monitoring and the development of clinical trials.

Past mitochondrial disease treatment studies have been unsuccessful in determining treatment efficacy, and a major factor has been the lack of validated biomarkers in mitochondrial myopathy (MM). There is currently a growing number of potential new treatments to be tested through MM clinical intervention trials, which has created a pressing need for quantitative biomarkers that reliably reflect MM disease severity, progression, and therapeutic response. The purpose of the study is to measure the efficacy of an electrochemical oxygen nanosensor to measure in vivo mitochondrial function in human muscle tissue, and its ability to discriminate MM patients from healthy volunteers. The data and results from this nanosensor study may contribute to current and future research, including improved diagnostic and therapeutic approaches for patients with mitochondrial disease.

N-Acetylcysteine (NAC), an anti-oxidant, will be studied to investigate the effects on brain glutathione levels, cognitive skills, motor skills, and quality of life. A group of 18 participants will take either 1800, 3600 or 5400 mg per day of N-acetylcysteine (NAC) for 3 months in this dose escalation study. The investigators want to determine first if the 3600 mg dose per day is safe and might provide some efficacy. If the 3600 mg dose is safe, then additional participants will be treated with 5400 mg per day of NAC, for up to a total of 18 participants. If the 3600 mg per day dose is unsafe, then participants will be treated with the 1800 mg per day dose. Data from this pilot study will be used to determine the most safe and effective dose of NAC for a future clinical trial.

Leber Hereditary Optic Neuropathy (LHON) is a rare genetic disease that causes sudden and severe vision loss, usually in young adults. It is linked to mutations in mitochondrial DNA that impair energy production in retinal ganglion cells, leading to degeneration of the optic nerve. Currently, treatment options are very limited and often ineffective. Recent research has shown that patients with LHON have lower levels of nicotinamide (vitamin B3), a key molecule for mitochondrial energy metabolism. Nicotinamide is a precursor of NAD, an essential cofactor for cellular energy production. Experimental studies and clinical trials in related optic nerve diseases suggest that nicotinamide may protect retinal ganglion cells. Our hypothesis is that supplementation with high-dose nicotinamide could restore NAD levels, support mitochondrial activity, and help preserve or improve vision in LHON. This pilot study will evaluate the effectiveness and safety of oral nicotinamide (2 grams per day for 12 months) in patients who developed LHON within the past 18 months and carry one of the two most severe mutations (m.11778G\>A or m.3460G\>A). The main goal is to measure changes in visual acuity over time using standardized eye charts. Secondary objectives include assessing visual fields, retinal structure by optical coherence tomography (OCT), blood nicotinamide levels, and quality of life. Liver function will be monitored to ensure safety. If this study shows promising results, it could pave the way for a larger randomized trial and ultimately offer a new therapeutic option.

The purpose of this expanded access program is to enable access to R007 (probucol) tablets for the compassionate treatment of adults with mitochondrial disease who also have chronic kidney disease.