Dendritic Spine in Autism Genetics: Whole-Exome Sequencing Identifying De Novo Variant of CTTNBP2 in a Quad Family Affected by Autism Spectrum Disorder.

Autism spectrum disorder (ASD) affects around 1% of children with no effective blood test or cure. Recent studies have suggested that these are neurological disorders with a strong genetic basis and that they are associated with the abnormal formation of dendritic spines. Chromosome microarray (CMA) together with high-throughput sequencing technology has been used as a powerful tool to identify new candidate genes for ASD. In the present study, CMA was first used to scan for genome-wide copy number variants in a proband, and no clinically significant copy number variants were found. Whole-exome sequencing (WES) was used further for genetic testing of the whole quad family affected by ASD, including the proband, his non-autistic sister, and his parents. Sanger sequencing and MassARRAY-based validation were used to identify and confirm variants associated with ASD. WES yielded a 151-fold coverage depth for each sample. A total of 98.65% of the targeted whole-exome region was covered at >20-fold depth. A de novo variant in CTTNBP2, p.M115T, was identified. The CTTNBP2 gene belongs to a family of ankyrin repeat domain-containing proteins associated with dendritic spine formation. Although CTTNBP2 has been associated with ASD, limited studies have been developed to identify clinically relevant de novo mutations of CTTNBP2 in children with ASD; family-based WES successfully identified a clinically relevant mutation in the CTTNBP2 gene in a quad family affected by ASD. Considering the neuron-specific expression of CTTNBP2 and its role in dendritic spine formation, our results suggest a correlation between the CTTNBP2 mutation and ASD, providing genetic evidence for ASD spine pathology. Although the present study is currently insufficient to support the assertion that the de novo mutation M115T in CTTNBP2 directly causes the autism phenotype, our study provides support for the assertion that this mutation is a candidate clinically relevant variant in autism.

A de novo interstitial deletion of 7q31.2q31.31 identified in a girl with developmental delay and hearing loss.

We report on a 4-year-old female who presented with unilateral sensorineural hearing loss and a concern for developmental delay. A genome-wide SNP array analysis was performed and revealed a de novo 3.2 Mb interstitial deletion of chromosome 7q31.2q31.31. This region contains thirteen protein-encoding genes. It is unknown whether haploinsufficiency of any of these genes is responsible for the clinical features of our patient. We reviewed, the clinical phenotype of a previously published 7q31.3 deletion patient and 18 additional patients with overlapping 7q31 deletions listed in the DECIPHER database. The most consistent feature in these patients and our proband is delayed speech and language development. Hearing loss is presented both in our proband and the published 7q31.3 patient. Our study suggests that a small region on chromosome 7q31.3 encompassing four genes, CFTR, CTTNBP2, NAA38, and ANKRD7, may represent a new locus for congenital hearing loss and/or speech development. © 2016 Wiley Periodicals, Inc.

Cortactin-binding protein 2 increases microtubule stability and regulates dendritic arborization.

Neurons are characterized by subcellular compartments, such as axons, dendrites and synapses, that have highly specialized morphologies and biochemical specificities. Cortactin-binding protein 2 (CTTNBP2), a neuron-specific F-actin regulator, has been shown to play a role in the regulation of dendritic spine formation and their maintenance. Here, we show that, in addition to F-actin, CTTNBP2 also associates with microtubules before mature dendritic spines form. This association of CTTNBP2 and microtubules induced the formation of microtubule bundles. Although the middle (Mid) region of CTTNBP2 was sufficient for its association with microtubules, for microtubule bundling, the N-terminal region containing the coiled-coil motifs (NCC), which mediates the dimerization or oligomerization of CTTNBP2, was also required. Our study indicates that CTTNBP2 proteins form a dimer or oligomer and brings multiple microtubule filaments together to form bundles. In cultured hippocampal neurons, knockdown of CTTNBP2 or expression of the Mid or NCC domain alone reduced the acetylation levels of microtubules and impaired dendritic arborization. This study suggests that CTTNBP2 influences both the F-actin and microtubule cytoskeletons and regulates dendritic spine formation and dendritic arborization.

STRIPAK complexes: structure, biological function, and involvement in human diseases.

The mammalian striatin family consists of three proteins, striatin, S/G2 nuclear autoantigen, and zinedin. Striatin family members have no intrinsic catalytic activity, but rather function as scaffolding proteins. Remarkably, they organize multiple diverse, large signaling complexes that participate in a variety of cellular processes. Moreover, they appear to be regulatory/targeting subunits for the major eukaryotic serine/threonine protein phosphatase 2A. In addition, striatin family members associate with germinal center kinase III kinases as well as other novel components, earning these assemblies the name striatin-interacting phosphatase and kinase (STRIPAK) complexes. Recently, there has been a great increase in functional and mechanistic studies aimed at identifying and understanding the roles of STRIPAK and STRIPAK-like complexes in cellular processes of multiple organisms. These studies have identified novel STRIPAK and STRIPAK-like complexes and have explored their roles in specific signaling pathways. Together, the results of these studies have sparked increased interest in striatin family complexes because they have revealed roles in signaling, cell cycle control, apoptosis, vesicular trafficking, Golgi assembly, cell polarity, cell migration, neural and vascular development, and cardiac function. Moreover, STRIPAK complexes have been connected to clinical conditions, including cardiac disease, diabetes, autism, and cerebral cavernous malformation. In this review, we discuss the expression, localization, and protein domain structure of striatin family members. Then we consider the diverse complexes these proteins and their homologs form in various organisms, emphasizing what is known regarding function and regulation. Finally, we explore possible roles of striatin family complexes in disease, especially cerebral cavernous malformation.

Neuron-specific regulation on F-actin cytoskeletons: The role of CTTNBP2 in dendritic spinogenesis and maintenance.

Dendritic spines are neuron-specific actin-rich subcellular structures and are the location of excitatory synapses. Neurotransmitters released from presynaptic terminals activate the signals modifying the F-actin dynamics and stability and thus control dendritic spine morphology. Many ubiquitously expressed actin-associated proteins, including cortactin, have been shown to regulate dendritic spine morphology and density. Since dendritic spines are neuron-specific structures, neuron-specific proteins are expected to control F-actin cytoskeletons and dendritic spinogenesis. Recently, we demonstrated that cortactin-binding protein 2 (CTTNBP2), a neuron-specific protein, regulates the mobility and distribution of cortactin and controls the density of dendritic spines. This is the first example of a neuron-specific protein that controls the mobility of an F-actin associated protein and influences the dendritic spines. It provides a platform to explore the specific pathway triggering dendritic spinogenesis.