Multifaceted functions of RNA-binding protein vigilin in gene silencing, genome stability, and autism-related disorders.

RNA-binding proteins (RBPs) are emerging as important players in regulating eukaryotic gene expression and genome stability. Specific RBPs have been shown to mediate various chromatin-associated processes ranging from transcription to gene silencing and DNA repair. One of the prominent classes of RBPs is the KH domain-containing proteins. Vigilin, an evolutionarily conserved KH domain-containing RBP has been shown to be associated with diverse biological processes like RNA transport and metabolism, sterol metabolism, chromosome segregation, and carcinogenesis. We have previously reported that vigilin is essential for heterochromatin-mediated gene silencing in fission yeast. More recently, we have identified that vigilin in humans plays a critical role in efficient repair of DNA double-stranded breaks and functions in homology-directed DNA repair. In this review, we highlight the multifaceted functions of vigilin and discuss the findings in the context of gene expression, genome organization, cancer, and autism-related disorders.

Genome wide nucleosome landscape shapes 3D chromatin organization.

The hierarchical chromatin organization begins with formation of nucleosomes, which fold into chromatin domains punctuated by boundaries and ultimately chromosomes. In a hierarchal organization, lower levels shape higher levels. However, the dependence of higher-order 3D chromatin organization on the nucleosome-level organization has not been studied in cells. We investigated the relationship between nucleosome-level organization and higher-order chromatin organization by perturbing nucleosomes across the genome by deleting Imitation SWItch (ISWI) and Chromodomain Helicase DNA-binding (CHD1) chromatin remodeling factors in budding yeast. We find that changes in nucleosome-level properties are accompanied by changes in 3D chromatin organization. Short-range chromatin contacts up to a few kilo-base pairs decrease, chromatin domains weaken, and boundary strength decreases. Boundary strength scales with accessibility and moderately with width of nucleosome-depleted region. Change in nucleosome positioning seems to alter the stiffness of chromatin, which can affect formation of chromatin contacts. Our results suggest a biomechanical "bottom-up" mechanism by which nucleosome distribution across genome shapes 3D chromatin organization.

Role of histone acetyltransferases MOF and Tip60 in genome stability.

The accurate repair of DNA damage specifically the chromosomal double-strand breaks (DSBs) arising from exposure to physical or chemical agents, such as ionizing radiation (IR) and radiomimetic drugs is critical in maintaining genomic integrity. The DNA DSB response and repair is facilitated by hierarchical signaling networks that orchestrate chromatin structural changes specifically histone modifications which impact cell-cycle checkpoints through enzymatic activities to repair the broken DNA ends. Various histone posttranslational modifications such as phosphorylation, acetylation, methylation and ubiquitylation have been shown to play a role in DNA damage repair. Recent studies have provided important insights into the role of histone-specific modifications in sensing DNA damage and facilitating the DNA repair. Histone modifications have been shown to determine the pathway choice for repair of DNA DSBs. This review will summarize the role of important histone acetyltransferases MOF and Tip60 mediated acetylation in repair of DNA DSBs in eukaryotic cells.

Role of Histone Methylation in Maintenance of Genome Integrity.

Packaging of the eukaryotic genome with histone and other proteins forms a chromatin structure that regulates the outcome of all DNA mediated processes. The cellular pathways that ensure genomic stability detect and repair DNA damage through mechanisms that are critically dependent upon chromatin structures established by histones and, particularly upon transient histone post-translational modifications. Though subjected to a range of modifications, histone methylation is especially crucial for DNA damage repair, as the methylated histones often form platforms for subsequent repair protein binding at damaged sites. In this review, we highlight and discuss how histone methylation impacts the maintenance of genome integrity through effects related to DNA repair and repair pathway choice.

Autism-Associated Vigilin Depletion Impairs DNA Damage Repair.

Vigilin (Vgl1) is essential for heterochromatin formation, chromosome segregation, and mRNA stability and is associated with autism spectrum disorders and cancer: vigilin, for example, can suppress proto-oncogene c-fms expression in breast cancer. Conserved from yeast to humans, vigilin is an RNA-binding protein with 14 tandemly arranged nonidentical hnRNP K-type homology (KH) domains. Here, we report that vigilin depletion increased cell sensitivity to cisplatin- or ionizing radiation (IR)-induced cell death and genomic instability due to defective DNA repair. Vigilin depletion delayed dephosphorylation of IR-induced γ-H2AX and elevated levels of residual 53BP1 and RIF1 foci, while reducing Rad51 and BRCA1 focus formation, DNA end resection, and double-strand break (DSB) repair. We show that vigilin interacts with the DNA damage response (DDR) proteins RAD51 and BRCA1, and vigilin depletion impairs their recruitment to DSB sites. Transient hydroxyurea (HU)-induced replicative stress in vigilin-depleted cells increased replication fork stalling and blocked restart of DNA synthesis. Furthermore, histone acetylation promoted vigilin recruitment to DSBs preferentially in the transcriptionally active genome. These findings uncover a novel vigilin role in DNA damage repair with implications for autism and cancer-related disorders.