Novel Chromatin Insulating Activities Uncovered upon Eliminating Known Insulators in vivo .

CCCTC-binding factor (CTCF) and MAZ are recognized insulators required for shielding repressed posterior genes from active anterior genes within the Hox clusters during motor neuron (MN) differentiation. CTCF and MAZ interact independently with cohesin and regulate three-dimensional genome organization. Here, we followed cohesin re-location upon CTCF and MAZ depletion in mouse embryonic stem cells (mESCs) to identify novel insulators. Cohesin relocated to DNA motifs for various transcription factors, including PATZ1 and other zinc finger proteins (ZNFs). Moreover, PATZ1 and ZNFs co-localized with CTCF, MAZ, and cohesin with apparent overlapping specificity as dictated by the site to be insulated. Similar to CTCF and MAZ, PATZ1 interacted with RAD21. Patz1 KO mESCs exhibited altered global gene expression. While the absence of MAZ impacts anterior CTCF-boundaries as shown previously 1 , Patz1 KO led to derepression of posterior Hox genes, resulting in cervicothoracic transformation of motor neuron (MN) fate during differentiation. These findings point to a varied, combinatorial binding of known and newly defined accessory factors as being critical for positional identity and cellular fate determination during differentiation.