Objective:To summarize the clinical characteristics of children with mitochondrial short-chain enoyl-CoA hydratase-1 deficiency (ECHS1D) caused by c.489G>A (p.Pro163=) compound heterozygous variants in the ECHS1 gene, and to explore genotype-phenotype correlations.Methods:A case series study was performed to analyze clinical, biochemical, metabolic, imaging, genetic, treatment and follow-up outcomes of 24 children with ECHS1 gene c.489G>A(p.Pro163=) variant, who were diagnosed in the Department of Neurology, Beijing Children′s Hospital from July 2010 to June 2024. Disease severity was assessed using the Newcastle Paediatric Mitochondrial Disease Scale, and Fisher exact test was applied to compare the improvement rate between valine-restricted and non-restricted groups.Results:These 24 children were all diagnosed after 2022, with a disease duration of 3.35 (1.25, 6.52) years at diagnosis. A total of 8 children initially had negative genetic results, and were finally confirmed by abnormal splicing of ECHS1 gene via skin fibroblast RNA sequencing, with the longest diagnostic time of 14 years. All 24 children presented with Leigh syndrome, including 11 boys and 13 girls, with an onset age of 1.46 (0.96, 2.79) years; 16 children (67%) were mild cases. Common initial symptoms included developmental delay (9 cases) and paroxysmal dystonia (9 cases), followed by developmental regression (3 cases), nystagmus (2 cases), and epilepsy (1 case). Main manifestations were dystonia (18 cases), developmental regression (14 cases), nystagmus (12 cases), developmental delay (11 cases), ataxia (10 cases), vision loss (9 cases), seizures (2 cases), and hearing impairment (1 case). Among 22 children who underwent blood and urine metabolic screening, 21 children (95%) had elevated urinary 2, 3-dihydroxy-2-methylbutyric acid and 19 children (86%) had elevated urinary S-(2-hydroxypropyl) cysteamine. All 24 children had symmetric abnormal signals in bilateral globus pallidus on cranial magnetic resonance imaging, 10 children had isolated globus pallidus involvement, and other common involved sites included caudate nucleus and brainstem (9 cases each), putamen (7 cases), and cerebral white matter (5 cases). At last follow-up, all 24 children survived, with a follow-up duration of 5.40 (2.75, 8.02) years and a maximum age of 17.8 years; 17 children (71%) had varying degrees of clinical improvement. There was no statistical difference in the improvement rate between children with or without valine-restricted diet (12/14 vs. 5/8, P=0.309). A total of 18 pathogenic variants in the ECHS1 gene were identified among 24 children, 13 of which were distributed in exons 7 and 8; those carrying c.308T>C, c.523G>A, c.796A>G, and c.832G>A variants were mostly severe cases. Conclusions:Children carrying ECHS1 gene c.489G>A(p.Pro163=) compound heterozygous variants face significant diagnostic delay. Clinical awareness of this synonymous variant needs further improvement for timely diagnosis. All these cases present as Leigh syndrome, mostly mild, with no clear genotype-phenotype correlation identified.

Objective:To summarize the clinical characteristics of children with mitochondrial short-chain enoyl-CoA hydratase-1 deficiency (ECHS1D) caused by c.489G>A (p.Pro163=) compound heterozygous variants in the ECHS1 gene, and to explore genotype-phenotype correlations.Methods:A case series study was performed to analyze clinical, biochemical, metabolic, imaging, genetic, treatment and follow-up outcomes of 24 children with ECHS1 gene c.489G>A(p.Pro163=) variant, who were diagnosed in the Department of Neurology, Beijing Children′s Hospital from July 2010 to June 2024. Disease severity was assessed using the Newcastle Paediatric Mitochondrial Disease Scale, and Fisher exact test was applied to compare the improvement rate between valine-restricted and non-restricted groups.Results:These 24 children were all diagnosed after 2022, with a disease duration of 3.35 (1.25, 6.52) years at diagnosis. A total of 8 children initially had negative genetic results, and were finally confirmed by abnormal splicing of ECHS1 gene via skin fibroblast RNA sequencing, with the longest diagnostic time of 14 years. All 24 children presented with Leigh syndrome, including 11 boys and 13 girls, with an onset age of 1.46 (0.96, 2.79) years; 16 children (67%) were mild cases. Common initial symptoms included developmental delay (9 cases) and paroxysmal dystonia (9 cases), followed by developmental regression (3 cases), nystagmus (2 cases), and epilepsy (1 case). Main manifestations were dystonia (18 cases), developmental regression (14 cases), nystagmus (12 cases), developmental delay (11 cases), ataxia (10 cases), vision loss (9 cases), seizures (2 cases), and hearing impairment (1 case). Among 22 children who underwent blood and urine metabolic screening, 21 children (95%) had elevated urinary 2, 3-dihydroxy-2-methylbutyric acid and 19 children (86%) had elevated urinary S-(2-hydroxypropyl) cysteamine. All 24 children had symmetric abnormal signals in bilateral globus pallidus on cranial magnetic resonance imaging, 10 children had isolated globus pallidus involvement, and other common involved sites included caudate nucleus and brainstem (9 cases each), putamen (7 cases), and cerebral white matter (5 cases). At last follow-up, all 24 children survived, with a follow-up duration of 5.40 (2.75, 8.02) years and a maximum age of 17.8 years; 17 children (71%) had varying degrees of clinical improvement. There was no statistical difference in the improvement rate between children with or without valine-restricted diet (12/14 vs. 5/8, P=0.309). A total of 18 pathogenic variants in the ECHS1 gene were identified among 24 children, 13 of which were distributed in exons 7 and 8; those carrying c.308T>C, c.523G>A, c.796A>G, and c.832G>A variants were mostly severe cases. Conclusions:Children carrying ECHS1 gene c.489G>A(p.Pro163=) compound heterozygous variants face significant diagnostic delay. Clinical awareness of this synonymous variant needs further improvement for timely diagnosis. All these cases present as Leigh syndrome, mostly mild, with no clear genotype-phenotype correlation identified.

Objective:To summarize the clinical characteristics of children carrying the m.8344A>G variant of MT-TK gene.Methods:A case series study was conducted to retrospectively collect data of 22 children with mitochondrial disease caused by MT-TK gene m.8344A>G variation who were treated at the Department of Neurology of Beijing Children′s Hospital of Capital Medical University from January 2012 to January 2024. Their clinical data, laboratory tests, muscle pathology, genetic testing, and the follow-up results were analyzed. Pearson correlation analysis was used for correlation analysis.Results:Among the 22 children, there were 13 boys and 9 girls. The age of onset was 5.00 (2.75, 9.00) years. Fifteen children had myoclonic epilepsy with ragged-red fibers (MERRF), 3 had Leigh syndrome (LS), and 4 had LS-MERRF overlap syndrome (LS-MERRF). Myoclonus presented and worsened progressively in all 15 MERRF children, with 10 as the initial symptom and 5 developing progressively during the disease course. Myoclonus was predominantly focal, worsening with fine motor tasks or stress. Electroencephalogram monitoring in the 15 MERRF children revealed myoclonic seizures in 10 children, with 6 classified as myoclonic epilepsy, and 4 as subcortical myoclonus. Two children had generalized myoclonic seizures, and 1 each had absence seizures and generalized seizures. Twelve children had cerebellar ataxia, 10 children exhibited exercise intolerance, and 8 children had muscle weakness. Magnetic resonance imaging (MRI) revealed periventricular white matter involvement in 1 child and bilateral hippocampal involvement in 1 child, likely due to frequent seizures. All 3 children with LS exhibited developmental regressions, accompanied with 2 symptoms include cerebellar ataxia, muscle weakness, and dysphagia. The clinical manifestations of 4 LS-MERRF overlap children presented with combined features of MERRF and LS. Cranial MRI in the 7 LS and LS-MERRF children showed brainstem involvement (all affecting the midbrain) in 6 children and basal ganglia involvement in 4 children. Among the 22 children, 12 had m.8344A>G variant levels >90%, 3 had >80%-90%, 4 had >70%-80%, and 3 had >60%-70%. Higher variant level correlated with the LS phenotype and earlier onset age ( r=0.47, -0.50; P=0.018 and 0.029, respectively). Sanger sequencing in 19 mothers revealed m.8344A>G variations in 18, with 4 showing exercise intolerance. Follow-up of 13 children on antimyoclonic treatment showed>75% reduction in seizures with levetiracetam monotherapy in 2 children, with combination therapy required in others. Most achieved >50% seizures reduction within 2 years, but the effectiveness declined with disease progression. Conclusions:The m.8344A>G variant is rare, with MERRF being the most common phenotype, while LS and LS-MERRF are less common. Children with higher ratio of the m.8344A>G variant are more likely to present LS phenotype. Myoclonus, primarily focal, is a key feature, with levetiracetam as the first-line treatment and benzodiazepines recommended for refractory cases.