Chromothriptic Translocation t(1;18): A Paradigm of Genomic Complexity in a Child with Normal Intellectual Development and Pyridoxine-Dependent Epilepsy.

BACKGROUND: Pyridoxine-dependent epilepsy (PDE) is a rare disorder characterized by seizures resistant to conventional treatments but responsive to pyridoxine therapy. Typically caused by biallelic variants in ALDH7A1, PNPO, or PLPBP, a few patients present a similar clinical phenotype but without confirmed molecular diagnoses. We report a child with a 13-year PDE diagnosis and normal intellectual development, whose seizures recurred after pyridoxine withdrawal but resolved with reintroduction, despite unremarkable whole-exome sequencing results. METHODS: Following negative results from WES, optical genome mapping (OGM) and whole-genome sequencing (WGS) were performed to highlight any potential structural variants involving known PDE-associated genes. RESULTS: OGM and WGS revealed a recurrent 16p11.2 BP4-5 duplication, inherited from his healthy father, along with a de novo chromothripsis-type unbalanced t(1;18)(p22.3;q12.3), affecting several genes not currently associated with epilepsy (RIT2, PIK3C3, COL24A1, LRRC8D, DIPK1A, and DPYD), with RIT2 being a plausible candidate for the neurological phenotype due to its neuron-specific expression along with a likely reshuffling of topologically associating domains (TADs) involving SYT4, an epilepsy-candidate gene. DISCUSSION: While the molecular data do not pinpoint a single gene or locus as the cause of seizures in this case, a key aspect of our patient's phenotype is true pyridoxine dependence, rather than just pyridoxine responsiveness. We propose that the genomic complexity associated with the chromothriptic t(1;18) and the 16p11.2 BP4-5 duplication may create a unique metabolic environment in which pyridoxine-dependent pathways are disrupted through unconventional mechanisms. The preservation of cognitive function in our case has been observed in small groups of PDE patients, especially those diagnosed and treated early. This may indicate a distinct phenotypic subgroup that warrants further genetic investigation.