An atypically mild case of ethylmalonic encephalopathy with pathogenic ETHE1 variant

Ethylmalonic encephalopathy (EE) is a rare, severe, autosomal recessive condition caused by pathogenic variants in ETHE1 leading to progressive encephalopathy, hypotonia evolving to dystonia, petechiae, orthostatic acrocyanosis, diarrhea, and elevated ethylmalonic acid in urine. In this case report, we describe a patient with only mild speech and gross motor delays, subtle biochemical abnormalities, and normal brain imaging found to be homozygous for a pathogenic ETHE1 variant (c.586G>A) via whole exome sequencing. This case highlights the clinical heterogeneity of ETHE1 mutations and the utility of whole‐exome sequencing in diagnosing mild cases of EE.

The clinical manifestations of EE have been linked to pathogenic variants in the ETHE1 gene leading to dysregulation of sulfur metabolism (Tiranti & Zeviani, 2013). ETHE1 encodes the Ethe1 protein (Ethe1p), a 30 kDa polypeptide found in mitochondria involved in the catabolism of hydrogen sulfide (H 2 S) and thiosulfate (Tiranti & Zeviani, 2013). Deficiency of Ethe1p activity leads to local accumulation of H 2 S in brain, muscle, colonic mucosa, and liver as well as elevated thiosulfate levels detectable in blood and urine. In tissues of accumulation, H 2 S acts as a signaling molecule leading to the clinical symptoms characteristic of EE. High local concentrations of H 2 S also inhibits short-chain acyl-CoA dehydrogenase and cytochrome c oxidase activities leading to elevated lactate and C4/C5 acylcarnitines (Tiranti et al., 2009), lab abnormalities also seen in EE patients. Symptom onset is typically within the first several months of life and patients usually die within the first decade (Tiranti & Zeviani, 2013;Tiranti et al., 2006).
Certain pathogenic ETHE1 mutations have been associated with findings such as joint hypermobility (di Rocco et al., 2006), genitourinary abnormalities , or stroke-like episodes (Lim et al., 2021) in addition to the typical findings described above. This case report describes at patient exhibiting a mild case of EE despite being homozygous for ETHE1 c.586G>A, a pathogenic variant previously identified as causal in a more severe case.

| CASE
A 19-month-old toddler boy presented for initial genetic evaluation for developmental delay and mild hyperammonemia (ammonia 40-63, ref 13-37 μmol/L). This patient was the product of a consanguineous marriage (paternal great-grandfather is brother to maternal great-grandfather) and was born at 37 weeks and 4 days gestation via vaginal delivery. Pregnancy was uncomplicated with a normal 20-week anatomic ultrasound. Newborn screening was normal. During infancy, there were initial concerns for feeding difficulty requiring thickened feeds which improved over time. At the time of initial evaluation, our patient had a 5-word vocabulary with strong receptive language and a wide-based gait. There were no dysmorphic features. Lab work obtained notable for the following: plasma amino acids (within normal limits), plasma acylcarnitine profile (Iso-/Butyrylcarnitine, C4-1.41 nmol/mL [Ref <1 .06]), and urine organic acids (trace methylmalonic acid; small ethylmalonic acid). The small amount of urine ethylmalonic acid was interpreted to be within the range of normal.
At 3 years and 5 months of age, the patient was evaluated by his primary care provider who noted difficulty with articulation and had concerns for his gait. History obtained during a pediatric neurology evaluation at 3 years and 8 months of age was notable for articulation difficulty with <50% speech intelligibility and inability to jump with both feet. Neurologic exam was significant for paresis of lateral tongue movements and when asked to run, had right arm flexion and right leg out-swinging. Despite this, he had normal axial and appendicular tone, 5/5 and symmetric strength in all extremities with confrontational testing, no dysmetria, normal upper-and lower-extremity deep tendon reflexes, and down-going toes with plantar stimulation bilaterally. He was otherwise noted to have good social development without concern for seizures or regression. The right arm flexion and leg extension with running was most suspicious for a dystonia (potentially localizing to the left basal ganglia or its connections with the cerebellum), or less likely mild spasticity not detected during formal tone assessment (localizing to the corticospinal tract originating from the left motor cortex). The paresis of bilateral lateral tongue movements without atrophy or fasciculations could localize to the bilateral corticobulbar tracts as part of a pseudobulbar palsy. While a single localization in the absence of other brainstem or extremity symptoms seemed unlikely, given the potential multifocal localization at the level of the medulla or above, an MRI brain with and without contrast was pursued, which was normal (Figure 1). Chromosomal microarray was obtained revealing approximately 7.1% of single nucleotide polymorphism homozygosity consistent with known parental consanguinity.
Subsequent neurology evaluation at 4 years and 2 months of age was notable for persistent expressive language deficit, difficulty with lateral tongue movement, and behavioral concerns for which audiology evaluation and re-evaluation by the genetics team was recommended.
His hearing test was normal and whole-exome sequencing with mitochondrial genome sequencing was pursued by the genetics team. This Cytochrome c oxidase activity has not been measured as the patient has not undergone muscle or skin biopsy. He was subsequently initiated on N-acetylcysteine and intermittent metronidazole to reduce sulfur burden. He remains without concern for seizures, developmental regression, or acrocyanosis. He did have a brief period of diarrhea with documented bacterial illness suggesting an infectious etiology. In support of an infectious etiology, symptoms have since completely resolved and not recurred.

| DISCUSSION
EE is a severe mitochondrial disease of infancy caused by pathogenic variants in the ETHE1 gene leading to dysregulation of sulfur metabolism (di Meo et al., 2017;Tiranti & Zeviani, 2013). Resultant local tissue accumulation of H 2 S leads to progressive encephalopathy, hypotonia evolving to dystonia, petechiae, orthostatic acrocyanosis, diarrhea, and ethylmalonic acid in urine (Tiranti & Zeviani, 2013). We describe a patient born from consanguineous parents homozygous for the ETHE1 c.586G>A variant. Clinically, this patient had only mild findings which included expressive speech delay, gross motor delay, initial abnormal right-sided extremity movements with running, initial paresis of lateral tongue movements, low-normal muscle tone, and small elevation in urine ethylmalonic acid. Asymmetric extremity and restricted lateral tongue movements spontaneously improved. His brain MRI did not show abnormal findings. He remains without seizures, worsening weakness, petechiae, acrocyanosis, or diarrhea. The diagnosis of EE was made from whole exome sequencing and led to initiation of N-acetylcysteine and metronidazole to buffer serum sulfur burden and intestinal production of absorbable sulfur compounds, respectively (Tiranti & Zeviani, 2013). For more severe cases, orthotopic liver transplant has also been trialed to as an additional means to filter H 2 S from the blood (Tam et al., 2019).
Homozygosity for the ETHE1 c.586G>A variant has been previously described (Mineri et al., 2008). In contrast to our patient, the previously reported patient was born from non-consanguineous parents and developed classic/more severe findings of EE (Mineri et al., 2008). Specifically, they had frequent vomiting and diarrhea within the first year of life followed by proximal muscle weakness, axial hypotonia, and poor trunk control (Mineri et al., 2008 (Chen et al., 2020). MRI brain for the third patient showed evidence of F I G U R E 1 Axial MRI images of patient's brain. Scan performed when patient was 3 years and 9 months of age. (a) T2-weighted image at the level of the midbrain which includes amygdala. (b) T2-weighted image at the level of the thalamus, caudate, putamen, globus pallidus, and external capsule. Small foci of T2 hyperintensity in basal ganglia not evident on corresponding FLAIR sequences likely represent Virchow-Robin spaces (c, d).
demyelination and during follow-up, developmental delay was