MyD88 deficiency modestly attenuates disease in a Leigh syndrome mouse model while enrofloxacin accelerates disease.

Primary genetic mitochondrial diseases (GMDs) are a clinically and genetically diverse group of diseases estimated to impact over 1 in 4,000 individuals. Leigh syndrome (LS) is the most common pediatric presentation of GMD. LS typically presents within the first years of life and is a severe progressive multi-system disorder. Symmetric progressive inflammatory brain lesions are a defining feature of the disease. Patients can also present with seizures, metabolic dysfunction, muscle weakness, and other symptoms. No effective clinical treatments currently exist. Recent data from the Ndufs4(-/-) mouse model shows that peripheral macrophages contribute to brain lesions in LS, that disease is causally driven by innate immune populations, and that depletion of innate immune cells prevents LS disease. However, the precise mechanisms underlying immune activation remain unknown. Certain mitochondrial macromolecules retain bacterial signatures and can act as potent agonists for innate immune pathways. For example, cytoplasmic mitochondrial RNA and mitochondrial DNA are detected by Toll-like receptors (TLRs) 7 and 9, respectively, at the endosome. Accordingly, these are considered strong candidates for mediating innate immune activation in LS. Here, we generated TLR signaling deficient Ndufs4(-/-)/MyD88(-/-) animals to assess whether TLR signaling plays a role in disease onset or progression in LS. Loss of MyD88 in Ndufs4(-/-) animals statistically significantly increased survival and delayed the onset of some symptoms, but the benefits were modest compared to CSF1R inhibition from prior work. We conclude that Myd88-mediated immune signaling is not a primary driver of LS. Notably, prophylactic enrofloxacin treatment, which was necessary for production of innate immune deficient MyD88(-/-) animals, modestly decreased survival and accelerated disease. The impact of enrofloxacin and similar drugs in the context of mitochondrial disease warrants further investigation.