2.2.2 Histone acetyltransferase (HAT)
Genes encoding histone acetyltransferases (HATs) CREB binding protein (CREBBP) and EP300 are recurrently mutated in activated B-cell-like and germinal center (GC) B-cell-like subtypes of diffuse large B-cell lymphoma (DLBCL). Hind Hashwah et al 35 used CRISPR Cas9 to edit CREBBP and EP300 genes in the GC B-cell compartment of mice and determined the tumor suppressive impact of HAT in human diffuse large B-cell lymphoma (DLBCL) cell lines and mice. CREBBP and EP300 are tumor suppressors whose gene products contribute to histone H3 acetylation and promote active transcription in DLBCL cells, and affect many gene expression, most notably the MHCII gene, which controls MHCII expression and promotes tumor immune control. CREBBP deficiency also decreases MHCII expression in the B-cell compartment of the germinal center, impairs immunotherapy of tumors, and leads to B-cell overproliferation and predisposition to the development of MYC-driven lymphomas. Loss of CREBBP leads to MYC-driven lymphoma MHCII defects, ensuing in immune escape. CREBBP used to be proven to be a regulator of the enhancer/super-enhancer network, inhibiting enhancer activity via the BCL6/SMRT/HDAC3 complex and regulating GC and plasma cell development as well as antigenic rapture to result in formed lymphomas 36,37.
2.2.3 Locus control region (LCR)
Chi-Shuen Chu et al38 observed that OCA-B acted particularly on germinal center B cells, promotes GC cell growth, and impacted the BCL6 promoter thru the interaction of OCA-B with MED1. In addition, OCA-B forms a ternary complication with the lymphoid-rich OCT2 and the GC-specific transcription factor MEF2B that occupies and activates the locus control region (LCR) via the octameric form. The LCR is positioned 150 kb upstream of BCL6 on chromosome 3q26 and acts appreciably on adjoining and distal genes consisting of the BCL6 proto-oncogene and its function is GC-specific39. CRISPR interference with gRNA targeting OCT2 and OCA-B both resulted in a significant reduction in BCL6 mRNA levels. Knockdown of each gene left the expression level of the other unaffected however MEF2B expression used to be decreased after OCT2 and OCA-B knockdown, and for that reason OCA-B and OCT2 are required for MEF2B-mediated activation of the constitutive enhancer CE1. Furthermore, Bcl6 and its adjacent LCR can function in vivo only if they are located on the same chromosome, reflecting the direct cis-regulatory role of the LCR in the induction of Bcl6 and GC formation. The order of assembly of these key factors on DNA is OCT2→OCA-B→MEF2B. In the early stage of GC formation, these element sequences are activated and inhibit the transcription of BCL6 promoters 40,41. Thus, a hierarchical model of BCL6 regulation by progressive activation of the LCR can be proposed.