Mechanism Of Nucleosome Targeting By Pioneer Transcription Factors

Transcription factors (TFs) forage the genome to instruct cell plasticity, identity, and differentiation. These developmental processes are elicited through TF engagement with chromatin. Yet, how and which TFs can engage with chromatin and thus, nucleosomes, remains largely unexplored. Pioneer TFs are TF that display a high affinity for nucleosomes. Extensive genetic and biochemical studies on the pioneer TF FOXA, a driver of fibroblast to hepatocyte reprogramming, revealed its nucleosome binding ability and chromatin targeting lead to chromatin accessibility and subsequent cooperative binding of TFs. Similarly, a number of reprogramming TFs have been suggested to have pioneering activity due to their ability to target compact chromatin and increase accessibility and enhancer formation in vivo. But whether these factors directly interact with nucleosomes remains to be assessed. Here we test the nucleosome binding ability of the cell reprogramming TFs, Oct4, Sox2, Klf4 and cMyc, that are required for the generation of induced pluripotent stem cells. In addition, we also test neuronal and macrophage reprogramming TFs. Our study shows that reprogramming TFs bind nucleosomes with a range of nucleosome binding affinities, indicating that although specific cocktails of TFs are required for reprogramming, mechanistically these TFs show differential nucleosome interacting behaviors. These results allowed us to assess differential features between TFs nucleosome binding ability and to correlate their binding with reprogramming potential. To determine how general is nucleosome binding we extended our analysis to screen 593 of the 2,000 predicted human TFs in the genome for potential nucleosome binding and validated their binding in solution. Based on 3D structural analysis, we proposed that strong nucleosome binders anchor DNA through short -helixes and have a flexible and adaptable DNA binding domain while weak nucleosome binders use -sheets or unstructured regions and have a higher rigidity within their DNA binding domain. Through the experiments presented in this dissertation we present the first study revealing the shared structural features contributing to nucleosome binding potential of pioneer TFs and thus allow for predication of novel pioneer TFs with cell reprogramming potential

Pioneer Transcription Factors Target Partial DNA Motifs on Nucleosomes to Initiate Reprogramming

Pioneer transcription factors (TFs) access silent chromatin and initiate cell-fate changes, using diverse types of DNA binding domains (DBDs). FoxA, the paradigm pioneer TF, has a winged helix DBD that resembles linker histone and thereby binds its target sites on nucleosomes and in compacted chromatin. Herein, we compare the nucleosome and chromatin targeting activities of Oct4 (POU DBD), Sox2 (HMG box DBD), Klf4 (zinc finger DBD), and c-Myc (bHLH DBD), which together reprogram somatic cells to pluripotency. Purified Oct4, Sox2, and Klf4 proteins can bind nucleosomes in vitro, and in vivo they preferentially target silent sites enriched for nucleosomes. Pioneer activity relates simply to the ability of a given DBD to target partial motifs displayed on the nucleosome surface. Such partial motif recognition can occur by coordinate binding between factors. Our findings provide insight into how pioneer factors can target naive chromatin sites

Pioneer transcription factors target partial DNA motifs on nucleosomes to initiate reprogramming.

Pioneer transcription factors (TFs) access silent chromatin and initiate cell-fate changes, using diverse types of DNA binding domains (DBDs). FoxA, the paradigm pioneer TF, has a winged helix DBD that resembles linker histone and thereby binds its target sites on nucleosomes and in compacted chromatin. Herein, we compare the nucleosome and chromatin targeting activities of Oct4 (POU DBD), Sox2 (HMG box DBD), Klf4 (zinc finger DBD), and c-Myc (bHLH DBD), which together reprogram somatic cells to pluripotency. Purified Oct4, Sox2, and Klf4 proteins can bind nucleosomes in vitro, and in vivo they preferentially target silent sites enriched for nucleosomes. Pioneer activity relates simply to the ability of a given DBD to target partial motifs displayed on the nucleosome surface. Such partial motif recognition can occur by coordinate binding between factors. Our findings provide insight into how pioneer factors can target naive chromatin sites

Using the dCas9-KRAB system to repress gene expression in hiPSC-derived NGN2 neurons

Summary: We describe a CRISPR inhibition (CRISPRi) protocol to repress endogenous gene expression (e.g., ATP6V1A) in human induced pluripotent stem cell-derived NGN2-induced glutamatergic neurons. CRISPRi enables efficient and precise gene repression of one or multiple target genes via delivering gRNA(s) to direct a dCas9-KRAB fusion protein to the gene(s) of interest. This protocol can also be adapted for gene activation and high-throughput gene manipulation, allowing assessment of the transcriptomic and phenotypic impact of candidate gene(s) associated with neurodevelopment or brain disease.For complete details on the use and execution of this protocol, please refer to Ho et al. (2017) and Wang et al. (2021)

Haploinsufficiency of POU4F1 causes an ataxia syndrome with hypotonia and intention tremor

De novo, heterozygous, loss-of-function variants were identified in Pou domain, class 4, transcription factor 1 (POU4F1) via whole-exome sequencing in four independent probands presenting with ataxia, intention tremor, and hypotonia. POU4F1 is expressed in the developing nervous system, and mice homozygous for null alleles of Pou4f1 exhibit uncoordinated movements with newborns being unable to successfully right themselves to feed. Head magnetic resonance imaging of the four probands was reviewed and multiple abnormalities were noted, including significant cerebellar vermian atrophy and hypertrophic olivary degeneration in one proband. Transcriptional activation of the POU4F1 p.Gln306Arg protein was noted to be decreased when compared with wild type. These findings suggest that heterozygous, loss-of-function variants in POU4F1 are causative of a novel ataxia syndrome

The functional and evolutionary impacts of human-specific deletions in conserved elements.

Conserved genomic sequences disrupted in humans may underlie uniquely human phenotypic traits. We identified and characterized 10,032 human-specific conserved deletions (hCONDELs). These short (average 2.56 base pairs) deletions are enriched for human brain functions across genetic, epigenomic, and transcriptomic datasets. Using massively parallel reporter assays in six cell types, we discovered 800 hCONDELs conferring significant differences in regulatory activity, half of which enhance rather than disrupt regulatory function. We highlight several hCONDELs with putative human-specific effects on brain development, includin

The functional and evolutionary impacts of human-specific deletions in conserved elements

Conserved genomic sequences disrupted in humans may underlie uniquely human phenotypic traits. We identified and characterized 10,032 human-specific conserved deletions (hCONDELs). These short (average 2.56 base pairs) deletions are enriched for human brain functions across genetic, epigenomic, and transcriptomic datasets. Using massively parallel reporter assays in six cell types, we discovered 800 hCONDELs conferring significant differences in regulatory activity, half of which enhance rather than disrupt regulatory function. We highlight several hCONDELs with putative human-specific effects on brain development, including HDAC5, CPEB4, and PPP2CA. Reverting an hCONDEL to the ancestral sequence alters the expression of LOXL2 and developmental genes involved in myelination and synaptic function. Our data provide a rich resource to investigate the evolutionary mechanisms driving new traits in humans and other species