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.