Leukocyte cytokine responses in adult patients with mitochondrial DNA defects

Patients with oxidative phosphorylation (OxPhos) defects causing mitochondrial diseases appear particularly vulnerable to infections. Although OxPhos defects modulate cytokine production in vitro and in animal models, little is known about how circulating leukocytes of patients with inherited mitochondrial DNA (mtDNA) defects respond to acute immune challenges. In a small cohort of healthy controls (n=21) and patients (n=12) with either the m.3243A>G mutation or single, large-scale mtDNA deletions, we examined: i) cytokine responses (IL-6, TNF-α, IL-1β) in response to acute lipopolysaccharide (LPS) exposure, and ii) sensitivity to the immunosuppressive effects of glucocorticoid signaling (dexamethasone) on cytokine production. In dose-response experiments to determine the half-maximal effective LPS concentration (EC50), relative to controls, leukocytes from patients with mtDNA deletions showed 174 -179% lower responses for IL-6 and IL-1β (pIL-6=0.031, pIL-1β=0.009). Moreover, IL-6 response to LPS in presence of GC was also blunted in cells from patients with mtDNA deletions (pIL-6=0.006), but not in leukocytes from patients with the m.3243A>G mutation. Overall, these ex vivo data provide preliminary evidence that some systemic OxPhos defects may compromise immune cytokine responses and glucocorticoid sensitivity. Further work in larger cohorts is needed to define the nature of immune dysregulation in patients with mitochondrial disease, and their potential implications for disease phenotypes.

Mitochondrial DNA Instability Is Common in HIV-Exposed Uninfected Newborns

Worldwide, one million HIV-exposed uninfected (HEU) children are born yearly, and chronic health impairments have been reported in these children. Mitochondrial DNA (mtDNA) instability and altered mtDNA content have been evidenced in these children, but an exhaustive characterization of altered mitochondrial genomes has never been reported. We applied deep mtDNA sequencing coupled to the deletion identification algorithm eKLIPse to the blood of HEU neonates (n = 32), which was compared with healthy controls (n = 15). Dried blood spots (DBS) from African HEU children were collected seven days after birth between November 2009 and May 2012. DBS from French healthy controls were collected at birth (or <3 days of life) in 2012 and in 2019. In contrast to the absence of mtDNA instability observed at the nucleotide level, we identified significant amounts of heteroplasmic mtDNA deletions in 75% of HEU children and in none of controls. The heteroplasmy rate of the 62 mtDNA deletions identified varied from 0.01% to up to 50%, the highest rates being broadly compatible with bioenergetic defect and clinical expression. mtDNA integrity is commonly affected in HEU neonates. The nature of the deletions suggests a mechanism related to aging or tumor-associated mtDNA instability. This child population may be at risk of additional mtDNA genetic alterations considering that they will be exposed to other mitotoxic drugs including antiretroviral or anti-tuberculosis treatment.

Ethoxyquin is neuroprotective and partially prevents somatic and autonomic neuropathy in db/db mouse model of type 2 diabetes

Ethoxyquin (EQ), a quinolone-based antioxidant, has demonstrated neuroprotective properties against several neurotoxic drugs in a phenotypic screening and is shown to protect axons in animal models of chemotherapy-induced peripheral neuropathy. We assessed the effects of EQ on peripheral nerve function in the db/db mouse model of type II diabetes. After a 7 week treatment period, 12-week-old db/db-vehicle, db/+ -vehicle and db/db-EQ treated animals were evaluated by nerve conduction, paw withdrawal against a hotplate, and fiber density in hindlimb footpads. We found that the EQ group had shorter paw withdrawal latency compared to vehicle db/db group. The EQ group scored higher in nerve conduction studies, compared to vehicle-treated db/db group. Morphology studies yielded similar results. To investigate the potential role of mitochondrial DNA (mtDNA) deletions in the observed effects of EQ, we measured total mtDNA deletion burden in the distal sciatic nerve. We observed an increase in total mtDNA deletion burden in vehicle-treated db/db mice compared to db/+ mice that was partially prevented in db/db-EQ treated animals. These results suggest that EQ treatment may exert a neuroprotective effect in diabetic neuropathy. The prevention of diabetes-induced mtDNA deletions may be a potential mechanism of the neuroprotective effects of EQ in diabetic neuropathy.

POLRMT mutations impair mitochondrial transcription causing neurological disease

While >300 disease-causing variants have been identified in the mitochondrial DNA (mtDNA) polymerase γ, no mitochondrial phenotypes have been associated with POLRMT, the RNA polymerase responsible for transcription of the mitochondrial genome. Here, we characterise the clinical and molecular nature of POLRMT variants in eight individuals from seven unrelated families. Patients present with global developmental delay, hypotonia, short stature, and speech/intellectual disability in childhood; one subject displayed an indolent progressive external ophthalmoplegia phenotype. Massive parallel sequencing of all subjects identifies recessive and dominant variants in the POLRMT gene. Patient fibroblasts have a defect in mitochondrial mRNA synthesis, but no mtDNA deletions or copy number abnormalities. The in vitro characterisation of the recombinant POLRMT mutants reveals variable, but deleterious effects on mitochondrial transcription. Together, our in vivo and in vitro functional studies of POLRMT variants establish defective mitochondrial transcription as an important disease mechanism.

De Novo Development of mtDNA Deletion Due to Decreased POLG and SSBP1 Expression in Humans

Defects in the mitochondrial genome (mitochondrial DNA (mtDNA)) are associated with both congenital and acquired disorders in humans. Nuclear-encoded DNA polymerase subunit gamma (POLG) plays an important role in mtDNA replication, and proofreading and mutations in POLG have been linked with increased mtDNA deletions. SSBP1 is also a crucial gene for mtDNA replication. Here, we describe a patient diagnosed with Pearson syndrome with large mtDNA deletions that were not detected in the somatic cells of the mother. Exome sequencing was used to evaluate the nuclear factors associated with the patient and his family, which revealed a paternal POLG mutation (c.868C > T) and a maternal SSBP1 mutation (c.320G > A). The patient showed lower POLG and SSBP1 expression than his healthy brothers and the general population of a similar age. Notably, c.868C in the wild-type allele was highly methylated in the patient compared to the same site in both his healthy brothers. These results suggest that the co- deficient expression of POLG and SSBP1 genes could contribute to the development of mtDNA deletion.

DNA methylation of PGC-1α is associated with elevated mtDNA copy number and altered urinary metabolites in Autism Spectrum Disorder

BackgroundAutism Spectrum Disorder (ASD) is a complex disorder that is underpinned by numerous dysregulated biological pathways, including canonical mitochondrial pathways. Epigenetic mechanisms contribute to this dysregulation and DNA methylation is an important factor in the aetiology of ASD. We examined the relationship between DNA methylation of peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α), an essential transcriptional regulator of mitochondrial homeostasis, and mitochondrial dysfunction in an ASD cohort of South African children.ResultsUsing targeted Next Generation bisulfite sequencing, we found 12 highly variable CpG sites in PGC-1α that were significantly differentially methylated (p<0.05) between ASD (n = 55) and controls (n = 44). In ASD, eight CpG sites were hypermethylated in the PGC-1α promotor with a putative binding site for CAMP response binding element 1 (CREB1) spanning one of these CpG sites (p = 1 × 10−6). Mitochondrial DNA (mtDNA) copy number, a marker of mitochondrial function, was elevated (p = 0.002) in ASD compared to controls and correlated significantly with DNA methylation at the PGC-1α promoter. There was a positive correlation between methylation at PGC-1α at CpG#1 and mtDNA copy number (Spearman’s r = 0.2, n = 49, p = 0.04) in ASD, but a negative correlation between methylation at PGC-1α at CpG#4 promoter and mtDNA copy number in controls (Spearman’s r = −0.4, n = 42, p = 0.045). While there was no relationship between mtDNA deletions and PGC-1α methylation in ASD, mtDNA deletions correlated negatively with methylation at PGC-1α at CpG#4 (Spearman’s r = −0.4, n = 42, p = 0.032) in controls. Furthermore, levels of urinary organic acids associated with mitochondrial dysfunction correlated significantly (p<0.05) with DNA methylation at PGC-1α CpG#1 and mtDNA copy number in ASD (n= 20) and controls (n= 13) with many of these metabolites involved in altered redox homeostasis and neuroendocrinology.ConclusionsThese data show an association between PGC-1α promoter methylation, elevated mtDNA copy number and metabolomic evidence of mitochondrial dysfunction in ASD. This highlights an unexplored link between DNA methylation and mitochondrial dysfunction in ASD.