Early infantile epileptic encephalopathy with suppression-burst (EIEE), also known as Ohtahara syndrome, is one of the most severe and earliest forms of epilepsy. Using array-based comparative genomic hybridization, we found a de novo 2.0-Mb microdeletion at 9q33.3-q34.11 in a girl with EIEE. Mutation analysis of candidate genes mapped to the deletion revealed that four unrelated individuals with EIEE had heterozygous missense mutations in the gene encoding syntaxin binding protein 1 (STXBP1). STXBP1 (also known as MUNC18-1) is an evolutionally conserved neuronal Sec1/Munc-18 (SM) protein that is essential in synaptic vesicle release in several species. Circular dichroism melting experiments revealed that a mutant form of the protein was significantly thermolabile compared to wild type. Furthermore, binding of the mutant protein to syntaxin was impaired. These findings suggest that haploinsufficiency of STXBP1 causes EIEE.

Radixin is a member of the ezrin/radixin/moesin (ERM) family of proteins, which play a role in the formation of the membrane-associated cytoskeleton by linking actin ®laments and adhesion proteins. This cross-linking activity is regulated by phosphoinositides such as phosphatidylinositol 4,5-bisphosphate (PIP2) in the downstream of the small G protein Rho. The X-ray crystal structures of the radixin FERM domain, which is responsible for membrane binding, and its complex with inositol-(1,4,5)-trisphosphate (IP3) have been determined. The domain consists of three subdomains featuring a ubiquitin-like fold, a four-helix bundle and a phosphotyrosine-binding-like domain, respectively. These subdomains are organized by intimate interdomain interactions to form characteristic grooves and clefts. One such groove is negatively charged and so is thought to interact with basic juxtamembrane regions of adhesion proteins. IP3 binds a basic cleft that is distinct from those of pleckstrin homology domains and is located on a positively charged¯at molecular surface, suggesting an electrostatic mechanism of plasma membrane targeting. Based on the structural changes associated with IP3 binding, a possible unmasking mechanism of ERM proteins by PIP2 is proposed.

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Flap endonuclease-1 (FEN1) is a key enzyme for maintaining genomic stability and replication. Proliferating cell nuclear antigen (PCNA) binds FEN1 and stimulates its endonuclease activity. The structural basis of the FEN1-PCNA interaction was revealed by the crystal structure of the complex between human FEN1 and PCNA. The main interface involves the C-terminal tail of FEN1, which forms two b-strands connected by a short helix, the bA-aA-bB motif, participating in b-b and hydrophobic interactions with PCNA. These interactions are similar to those previously observed for the p21 CIP1/WAF1 peptide. However, this structure involving the full-length enzyme has revealed additional interfaces that are involved in the core domain. The interactions at the interfaces maintain the enzyme in an inactive 'locked-down' orientation and might be utilized in rapid DNA-tracking by preserving the central hole of PCNA for sliding along the DNA. A hinge region present between the core domain and the C-terminal tail of FEN1 would play a role in switching the FEN1 orientation from an inactive to an active orientation.

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ERM (ezrin/radixin/moesin) proteins recognize the cytoplasmic domains of adhesion molecules in the formation of the membrane-associated cytoskeleton. Here we report the crystal structure of the radixin FERM (4.1 and ERM) domain complexed with the ICAM-2 cytoplasmic peptide. The non-polar region of the ICAM-2 peptide contains the RxxTYxVxxA sequence motif to form a b-strand followed by a short 3 10 -helix. It binds the groove of the phosphotyrosinebinding (PTB)-like subdomain C mediated by a b±b association and several side-chain interactions. The binding mode of the ICAM-2 peptide to the FERM domain is distinct from that of the NPxY motif-containing peptide binding to the canonical PTB domain. Mutation analyses based on the crystal structure reveal the determinant elements of recognition and provide the ®rst insights into the physical link between adhesion molecules and ERM proteins.

Congenital hypomyelinating disorders are a heterogeneous group of inherited leukoencephalopathies characterized by abnormal myelin formation. We have recently reported a hypomyelinating syndrome characterized by diffuse cerebral hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCAHC). We performed whole-exome sequencing of three unrelated individuals with HCAHC and identified compound heterozygous mutations in POLR3B in two individuals. The mutations include a nonsense mutation, a splice-site mutation, and two missense mutations at evolutionally conserved amino acids. Using reverse transcription-PCR and sequencing, we demonstrated that the splice-site mutation caused deletion of exon 18 from POLR3B mRNA and that the transcript harboring the nonsense mutation underwent nonsense-mediated mRNA decay. We also identified compound heterozygous missense mutations in POLR3A in the remaining individual. POLR3A and POLR3B encode the largest and second largest subunits of RNA Polymerase III (Pol III), RPC1 and RPC2, respectively. RPC1 and RPC2 together form the active center of the polymerase and contribute to the catalytic activity of the polymerase. Pol III is involved in the transcription of small noncoding RNAs, such as 5S ribosomal RNA and all transfer RNAs (tRNA). We hypothesize that perturbation of Pol III target transcription, especially of tRNAs, could be a common pathological mechanism underlying POLR3A and POLR3B mutations.

A de novo 9q33.3-q34.11 microdeletion involving STXBP1 has been found in one of four individuals (group A) with early-onset West syndrome, severe hypomyelination, poor visual attention, and developmental delay. Although haploinsufficiency of STXBP1 was involved in early infantile epileptic encephalopathy in a previous different cohort study (group B), no mutations of STXBP1 were found in two of the remaining three subjects of group A (one was unavailable). We assumed that another gene within the deletion might contribute to the phenotype of group A. SPTAN1 encoding alpha-II spectrin, which is essential for proper myelination in zebrafish, turned out to be deleted. In two subjects, an in-frame 3 bp deletion and a 6 bp duplication in SPTAN1 were found at the initial nucleation site of the alpha/beta spectrin heterodimer. SPTAN1 was further screened in six unrelated individuals with WS and hypomyelination, but no mutations were found. Recombinant mutant (mut) and wild-type (WT) alpha-II spectrin could assemble heterodimers with beta-II spectrin, but alpha-II (mut)/beta-II spectrin heterodimers were thermolabile compared with the alpha-II (WT)/beta-II heterodimers. Transient expression in mouse cortical neurons revealed aggregation of alpha-II (mut)/beta-II and alpha-II (mut)/beta-III spectrin heterodimers, which was also observed in lymphoblastoid cells from two subjects with in-frame mutations. Clustering of ankyrinG and voltage-gated sodium channels at axon initial segment (AIS) was disturbed in relation to the aggregates, together with an elevated action potential threshold. These findings suggest that pathological aggregation of alpha/beta spectrin heterodimers and abnormal AIS integrity resulting from SPTAN1 mutations were involved in pathogenesis of infantile epilepsy.