Abstracts

Rationale: Subcortical band heterotopia (SBH), also called double cortex syndrome, and lissencephaly (LIS) are part of a spectrum of malformations of cortical development due to deficient neuronal migration, referred to as the SBH/LIS spectrum. Most patients have either DCX or LIS1 gene mutations, associated with predominant anterior or posterior distribution of SBH/LIS respectively. Pachygyria and lissencephaly have also been less commonly associated with pathogenic variants in other genes. Methods: We report the phenotypic, molecular and functional/structural analysis of novel DCX and LIS1 mutations causing SBH/LIS and epilepsy. Results: Patient 1 is a 46-year-old woman of French-Canadian ancestry who had onset of epilepsy at 3 months of age. She had multiple seizure types, most of them generalized, including drop attacks, staring episodes, and tonic-clonic seizures, and a diagnosis of Lennox-Gastaut syndrome that was refractory to antiepileptic medication. She had severe developmental delay. Brain MRI showed double cortex predominating in the frontal regions. DCX sequencing showed a c.578delA variant. The parents were not available for genetic testing. Using assays with dynamic microtubules, we measured in vitro the ability of recombinant mutated DCX protein to interact with microtubules. The 578delA variant was found to be defective in its ability to promote microtubule nucleation and polymerization, and showed impaired cooperative binding to microtubules. Patient 2 is a 28-year-old man of United Kingdom ancestry. At 6 months of age, he presented a mild developmental delay and his first seizures. He had multiple seizure types, most of them generalized, including infantile spasms, drop attacks, staring episodes, and tonic-clonic seizures. After an intermittent response to antiepileptic drugs, the seizures became refractory to medication. On clinical examination, he showed bilateral facial diplegia, pseudobulbar signs and severe developmental delay. Brain MRI showed a predominantly posterior lissencephaly associated with partial callosal agenesis and a cavum septum pellucidum, as well as diffuse cerebellar atrophy. LIS1 sequencing showed duplication of five nucleotides in exon 8 (c.728_732dupATCAA). The parents declined genetic testing. Bioinformatic analysis of the mutant sequence and mapping onto the LIS1 structure showed that the change introduces a five residue stretch of altered sequence followed by a premature stop codon at residue 250, early in the 4^th WD repeat of the Lis1 beta propeller. Conclusions: We report two novel pathogenic variants causing severe phenotypes of the SBH/LIS spectrum. Our functional analyses show that the DCX variant disrupts microtubule binding as well as the cooperative interaction between DCX molecules. Based on our structural interpretation of the LIS1 variant, it is very likely that the LIS1 protein does not fold properly, is unable to bind dynein, and is likely targeted for degradation in cells.