Ethylmalonic encephalopathy is a rare autosomal recessive mitochondrial disorder caused by biallelic pathogenic variants in ETHE1, the gene encoding mitochondrial persulfide dioxygenase, an enzyme crucial for hydrogen sulfide (H2S) detoxification. Loss of this enzyme results in H2S accumulation, cytochrome c oxidase inhibition, oxidative stress, and disrupted energy metabolism. Clinically, ethylmalonic encephalopathy manifests during early infancy with developmental delay, hypotonia, progressive encephalopathy, seizures, chronic diarrhea, and microvascular abnormalities such as petechiae and acrocyanosis. Fewer than 100 cases have been reported globally, mostly among Mediterranean and Arab populations, with scarce data from Latin America. We report the first documented case of ethylmalonic encephalopathy in a Mexican patient. The affected male infant, born to healthy nonconsanguineous parents of indigenous Maya origin from Yucatán, presented at 2 weeks of age with persistent hemorrhagic diarrhea, followed by metabolic acidosis, hyperammonemia, hyperlactatemia, elevated C4-acylcarnitine, and increased urinary ethylmalonic acid. Neurological findings included developmental delay, hypotonia, and myoclonic epilepsy. Whole-exome sequencing revealed a homozygous frameshift pathogenic variant in ETHE1 (NM_014297.5):c.19_20dup (p.Val8Glyfs*7), predicted to introduce a premature stop codon and abolish protein function. Despite targeted interventions-antiepileptic therapy, ammonia-lowering treatment, and metabolic support-the patient's condition progressively worsened, culminating in death at 15 months after metabolic decompensation and brain death. This case broadens the known mutational spectrum of ETHE1 by identifying a previously unreported pathogenic variant and underscores the need to include ethylmalonic encephalopathy in the differential diagnosis of infants presenting with chronic diarrhea, vascular lesions, and neurological deterioration, even in regions where the condition is not typically observed.

Hydrogen sulfide (H2S) regulates multiple human physiological processes, its reactivity and range of action being tightly controlled through regulation of H2S-synthesizing and -detoxifying enzymes. H2S detoxification is mainly achieved by a mitochondrial sulfide detoxifying pathway including persulfide dioxygenase (PDO). Human PDO (known as ethylmalonic encephalopathy protein 1, ETHE1), a homodimeric enzyme with a mononuclear iron centre active site, catalyzes the conversion of glutathione persulfide (GSSH) and O2 to reduced glutathione (GSH) and sulfite. Here we report that ETHE1 is potently inhibited by authentic nitric oxide (NO) gas at physiological concentrations, as observed by high resolution respirometry. Inhibition is reversible, occurs via NO binding to the reduced mononuclear iron center and becomes more potent and persistent at lower O2 levels. Incubation with s-nitrosoglutathione (GSNO) also appears to partially and transiently inhibit ETHE1, this effect likely resulting from s-nitrosation of cysteine residues. While ETHE1 is devoid of NO reductase activity, in aerobic conditions it displays low NO degrading activity. These findings unravel a novel layer of cross-regulation between the H2S and NO gasotransmitters with possible implications on the regulation of numerous physiological and pathophysiological processes.

This study investigated the role of ferroptosis in acute depleted uranium (DU)-induced nephrotoxicity. Using Sprague-Dawley rats and HK-2 cells to establish models of acute DU exposure (rats: 10 mg/kg; cells: 500 μM for 24 h), we found that DU exposure caused mitochondrial dysfunction, lipid peroxidation, and iron accumulation, all hallmarks of ferroptosis, which were inhibited by ferrostatin-1 (Fer-1). We identified mitochondrial ethylmalonic encephalopathy 1 (ETHE1) as a key DU target. ETHE1 downregulation exacerbated DU-induced reactive oxygen species (ROS), ferrous ions (Fe2+) overload and ferroptosis, while exogenous ETHE1 protein alleviated them. Furthermore, DU-triggered ROS activated the p38 mitogen-activated protein kinase (P38-MAPK) pathway, an effect enhanced by ETHE1 knockdown. Inhibiting P38-MAPK with adezmapimod (SB203580) suppressed ferroptosis and autophagy, and reduced the expression of nuclear receptor coactivator 4 (NCOA4), a mediator of ferritinophagy. Knockdown of NCOA4 also attenuated ferroptosis. In conclusion, acute DU exposure downregulates ETHE1, promoting mitochondrial ROS that activates P38-MAPK signaling. This pathway induces NCOA4-mediated ferritinophagy, ultimately leading to renal cell ferroptosis. These findings elucidate a novel mechanism for DU-induced kidney injury.

Ethylmalonic encephalopathy (EE) is an often-severe inborn error of metabolism caused by biallelic variants in the ETHE1 gene leading to impaired detoxification of hydrogen sulfide (H2S). H2S is produced both exogenously by anerobic intestinal bacteria as well as by the endogenous catabolism of the sulfur-containing amino acids methionine and cysteine. Existing therapies including metronidazole, N-acetylcysteine (NAC), and orthotopic liver transplantation (OLT) have been pursued with the objective of reducing or detoxifying exogenously produced H2S. However, strategies to reduce endogenously produced H2S using a methionine and cysteine restricted diet are an understudied therapeutic avenue. We performed an open-label, single-arm study to evaluate the effects of dietary intervention with a methionine and cysteine restricted diet (20-30 mg/kg/day) on biochemical parameters and overall clinical trajectory in three patients with molecularly confirmed ethylmalonic encephalopathy (two with attenuated phenotypes, one classically affected). All three patients were receiving a combination of medical therapy with metronidazole and NAC and were status-post OLT at the time of diet initiation. Plasma butyrylcarnitine (C4) levels were measured at diagnosis, serially following initiation of medical therapy and OLT, and at regular follow-up visits in a metabolic clinic after diet initiation. Additionally, we obtained untargeted metabolomics studies and directly evaluated ethylmalonate, butyrylcarnitine, isobutyrylcarnitine, isovalerylcarnitine, 2-methylbutyrylcarnitine, glutarylcarnitine, and methylsuccinate levels in the pre- OLT/medical therapy, post- OLT/medical therapy, and post- sulfur-restricted diet states. We observed a 20-38 % reduction in plasma C4 levels in all three patients following OLT and combination medical therapy with NAC and metronidazole. An 8-10 % reduction in C4 was observed following the introduction of dietary therapy in the two patients with attenuated phenotypes and an 82 % increase in C4 was seen in the patient with the classical phenotype. The metabolic profile as assessed by untargeted metabolomics analysis was largely unchanged in the pre-OLT/medical therapy, post-OLT/medical therapy, and post-diet states. The modest biochemical response to a sulfur-restricted diet observed in our cohort likely reflects the relatively minor contribution of endogenous sulfur-containing amino acid catabolism to overall H2S production. Further work is needed to study the impact of dietary intervention on the natural history of EE including diet only trials in the animal model as well as in the pre-OLT period in human participants.

Elevated hydrogen sulfide (sulfide) levels are observed in tissues, including the brain, of patients with ethylmalonic encephalopathy. Clinical manifestations of this disorder involve severe neurological symptoms and abnormalities such as developmental delay, pyramidal and extrapyramidal signs, cortical atrophy and basal ganglia lesions. To elucidate the pathophysiology of basal ganglia alterations, we investigated the effects of sulfide on bioenergetics, redox status and mitochondrial quality control in the striatum of Wistar rats. After placing the rat in a stereotaxic apparatus, a single intrastriatal administration of sulfide (NaHS; 2 or 4 µmol) or PBS (control) was performed. Thirty minutes after the administration, the rats were euthanized, and the striatum was used for the determination of biochemical parameters. Sulfide administration, at both doses, altered the activities of antioxidant enzymes. At the lowest dose, sulfide showed a strong tendency toward increased activity of citrate synthase. Furthermore, the highest dose of sulfide also reduced respiratory chain complex IV activity and mitochondrial respiration with NADH- and FADH2-linked substrates. Levels of Nrf2, the main factor that regulates the expression of antioxidant defenses, were also reduced by 4 µmol of sulfide. The metabolite further increased the content of MFN1, suggesting mitochondrial fusion. Additionally, sulfide elevated Parkin and TBC1D15 and reduced LC3 levels, indicative of mitophagy dysregulation. The content of markers of mitochondrial mass and fission were not changed. Our study shows that high levels of sulfide in the striatum of rats affect bioenergetics, redox status and mitochondrial quality control. We suggest that these pathomechanisms are involved in the pathophysiology of basal ganglia alterations verified in ethylmalonic encephalopathy.