Biochemical and clinical response to a sulfur-restricted diet in ethylmalonic encephalopathy

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.