Loss of Function of AFG3L2 Leading to Developmental and Epileptic Encephalopathy

To delineate the clinical features of AFG3L2-related developmental and epileptic encephalopathy (DEE) and explore its pathogenic mechanisms.

Whole-genome and blood transcriptome sequencing were performed in undiagnosed DEE patients. Patient-derived skin fibroblasts were established for the analysis of RNA and protein expression as well as for mitochondrial functional assays, including OPA1 processing, mtDNA copy number, membrane potential, ATP production, mitochondrial morphology analysis, and mitochondrial stress testing. Additionally, published AFG3L2-related epilepsy cases were systematically reviewed.

We identified four novel AFG3L2 variants in four DEE patients from two unrelated families, including splice-site/intronic variants in one family and exon-deletion/intronic variants in the other, fitting a recessive model of disease. In these patients, plus six additional previously reported DEE patients, symptoms included severe developmental delay, intractable seizures, microcephaly, generalized spasticity, and progressive cerebral atrophy. Transcriptome and fibroblast functional analyses revealed aberrant splicing, reduced AFG3L2 expression, defective OPA1 processing, decreased mtDNA content, impaired membrane potential and ATP production, fragmented mitochondrial networks, and diminished respiratory capacity, supporting a loss-of-function mechanism. Compared with spastic ataxia 5-usually involving null-missense or missense-missense genotypes-DEE predominantly features null-null combinations.

We implicate AFG3L2 as a novel causative gene for DEE, likely through mitochondrial proteostasis failure and bioenergetic compromise, expanding the phenotypic and genotypic spectrum of AFG3L2-related disorders.