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Phenotypic, molecular, functional, and structural aspects of novel DCX and LIS1 mutations causing the subcortical band heterotopia/lissencephaly (SBH/LIS) spectrum (P2.227)

OBJECTIVE: To analyse novel DCX and LIS1 mutations employing phenotypic, molecular and functional/structural techniques. BACKGROUND: Most patients with (SBH/LIS) spectrum have either DCX or LIS1 mutations, associated with predominantly anterior or posterior distribution of the malformation, respectively. DESIGN/METHODS: Detailed review of the phenotype of two patients with SBH/LIS spectrum caused by novel mutations in DCX and LIS1 ; functional analysis of DCX mutation using in vitro fluorescence-based assays with dynamic microtubules; bioinformatic analysis of the LIS1 mutation and mapping on to a structural model of the mutated LIS 1 protein. RESULTS: Patient 1 (Pt1) is a 46-year-old woman of French-Canadian ancestry; Patient 2 (Pt2) is a 28-year-old man of British ancestry. Both patients presented with developmental delay and refractory epilepsy, at 3 and 6 months respectively. Brain MRI in Pt1: double cortex predominating in the frontal regions; in Pt2: predominantly posterior lissencephaly associated with partial callosal agenesis, cavum septum pellucidum, and diffuse cerebellar atrophy. Parents declined genetic testing.
 DCX sequencing in Pt1 showed a c.578delA variant. LIS1 sequencing in Pt2 revealed duplication of five nucleotides in exon 8 (c.728_732dupATCAA).
The recombinant mutated DCX protein was found to be defective in promoting microtubule nucleation and polymerization, and showed impaired cooperative binding to microtubules. The change in the mutated LIS1 protein introduces a five residue stretch of altered sequence followed by a premature stop codon at residue 250, early in the 4th 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. Our structural interpretation of the LIS1 variant suggests that the LIS1 protein does not fold properly, is unable to bind dynein, and is likely targeted for degradation in cells. Disclosure: Dr. Amrom has nothing to disclose. Dr. Brouhard has nothing to disclose. Dr. Bechstedt has nothing to disclose. Dr. Toropova has nothing to disclose. Dr. Dubeau has nothing to disclose. Dr. Andermann has nothing to disclose. Dr. Melancon has nothing to disclose. Dr. Tampieri has nothing to disclose. Dr. Reck-Peterson has nothing to disclose. Dr. Andermann has nothing to disclose.