Page 133 - Hematology_ Basic Principles and Practice ( PDFDrive )

104 Part II Cellular Basis of Hematology

In normal HSPCs, the expression of HOX genes is regulated by of Dnmt3a, further pronounced by ablation of Dnmt3b, leads to an
320
MLL1, as part of a multiprotein complex that regulates the chro- accumulation of self-renewing HSCs in the BM which lose their
matin structure at HOX clusters. 321,322 MLL1 fusion proteins in differentiation capacity upon serial transplantation. 343,344 Next-
MLL1-rearranged leukemias further induce the transcription of generation sequencing studies have identified DNMT3A mutations
specific HOX genes including HOXA5, HOXA9 and HOXA10. 323,324 in most hematologic malignancies, in particular in patients with
345
Indeed, HOX gene overexpression is essential for MLL1-fusion cytogenetically normal AML (over 30%), where they can be
347
induced leukemogenesis as demonstrated by the dependence of detected as preleukemic lesions, 286,346 and T-ALL (16% ; for a
348
transplanted AMLs induced by both MLL1-ENL and MLL1-AF4 complete list see ). In contrast, DNMT1 mutations have been rarely
rearrangements on HOXA9. 325,326 found in AML. 334
Recently it was discovered that DNA methylation is in fact revers-
ible, 349,350 owing to the activity of ten-eleven translocation (TET)
Transcription Factor Networks proteins (TET1, TET2, and TET3), which iteratively oxidize
351
5-methylcytosine present in methylated CpG dinucleotides. Mice
Transcription factors act within larger multiprotein complexes. In the deficient for Tet2, the only TET expressed in the BM, show enhanced
setting of hematopoiesis, transcription factors often act positively to self-renewal of HSCs and develop a CMML-like disease. 352–354 TET2
355
sustain their own expression, while simultaneously acting to cross- is mutated in 49% of CMML and up to 23% of AML 356–358
regulate other transcription factors, thereby establishing complex patients. Interestingly, gain-of-function mutations in isocitrate dehy-
transcriptional networks. With the advent of genome-wide molecular drogenase 1 and 2 (IDH1/2) that indirectly impair TET2 function
studies, networks can be constructed computationally. Examples are mutually exclusive with TET2 mutations in AML. IDH1 and 2
include a core heptad regulatory network consisting of SCL, LYL1, are found mutated in 13–33% of AML cases and lead to similar
LMO2, ERG, FLI1, GATA2 and RUNX1 bound to over 1,000 genes methylation profiles as TET2 mutations. 359,360–363 IDH1/2 enzymes
in HSPCs 327,328 or a regulatory module composed of GATA1, GFI1 catalyze the conversion of isocitrate to α-ketoglutarate (α-KG), but
and GFI1B with a potentially important role in specifying early mutated IDH1/2 further convert α-KG to 2-hydroxyglutarate, which
329
lymphoid cells. Also, profiling of transcription factors in single inhibits the α-KG dependent TET2 enzyme. 364,365
HSPCs combined with computational lineage progression analysis The effects of histone modifications are combinatorial in nature,
suggests a role for GATA2 in driving a network that specifies mega- and there is crosstalk between DNA methylation and histone modi-
330
366
karyocytic and erythroid from lymphomyeloid lineage cells. As fications. Histones have protruding flexible and charged NH 2 -
these constructed networks become more mature, in silico methods termini (“tails”) that can be posttranslationally modified in various
will predict developmental outcomes that can be tested experimen- ways, by methylation, acetylation, phosphorylation, ubiquitination
367
tally by modulation of one or multiple transcription factors in a direct and sumoylation, to name but a few. Many histone modifications
331
manner. A comprehensive understanding of how gene regulatory have been implicated in HSC self-renewal by knockout or overexpres-
networks are perturbed in hematologic malignancies may lead to sion of chromatin regulators that function as their “writers”, “readers”
368
new therapeutic approaches based on restoring normal regulatory or “erasers” (for a recent list, see ). Recurrent mutations of CREBBP
patterns. and/or and EP300, members of the histone acetyltransferase family
that activate transcription, have been detected in several lymphoma
369
types, and 18% of relapsed pediatric ALL patients exhibit muta-
Epigenetic Regulation of HSC Self-Renewal tions in the enzymatic domain of CREBBP, which are thought to
contribute to drug resistance. 370
Epigenetic regulation leads to a “stably heritable phenotype resulting Unlike histone acetylation, methylation of histones can have
from changes in a chromosome without alteration in the DNA activating or repressive effects, dependent on the context and the
332
sequence”. Epigenetic modifications include DNA methylation, targeted histone residue. With respect to active marks, most epigen-
covalent histone modification, chromatin remodeling and mecha- etic studies have focused on lysine methylation carried out by lysine
nisms involving noncoding RNAs which will be discussed further methyltransferases (KMTs) and removed by lysine demethylases
later. By modifying chromatin structure and accessibility, epigenetic (KDMs). The large multidomain KMT MLL1 is essential for the
371
mechanisms regulate the expression of genes involved in determining generation of definitive HSCs in the mouse, likely through its
320
the balance between self-renewal and lineage commitment of HSCs, regulation of HOX cluster genes. The MLL1 locus has been found
ultimately leading to all hematopoietic cell types. Epigenetic dysregu- to be frequently involved in chromosomal translocations in up to
lation has been implicated in the pathogenesis of virtually all hema- 70% of infant and childhood AML and ALL and in 5–10% of adult
372
373
tologic malignancies. 333,334 In contrast to genetic mutations, epigenetic leukemia, generally associated with a poor prognosis. 85% of
alterations are in principle reversible, making epigenetic modifiers MLL1 translocations involve six proteins (AF4, AF9, ENL, AF10,
attractive targets for the innovative treatment of hematologic malig- ELL, and AF6), of which MLL1-AF4 in ALL and MLL1-AF9 in
374
nancies. Fig. 9.2 depicts key epigenetic factors within the hematopoi- AML are most common. Many MLL1 fusion partners belong to
etic hierarchy, and Table 9.1 summarizes main roles of critical the super elongation complex (SEC); when fused to MLL1 the aber-
epigenetic factors in HSPCs and hematologic malignancies. rantly recruited SEC bypasses the normal transcription initiation-to-
DNA methylation plays a critical role in HSC self-renewal and elongation checkpoints leading to high expression of MLL1-regulated
375
commitment. 335–338 Through recruitment of multiprotein complexes, genes such as HOX, WNT and leukemic stem cell target genes.
methylated DNA results in transcriptional repression of nearby genes. Furthermore, almost all fusion proteins aberrantly recruit another
In mammalian cells, DNA methylation occurs at cytidines, mostly in KMT, DOT1L, which by methylating H3K79 induces the expres-
the context of CpG dinucleotides, dispersed throughout the genome sion of MLL1 target genes sufficient for leukemogenesis such as
311
and clustered (as CpG islands) within gene promoters. In a number HOXA9 and MEIS1. In addition, MLL1 fusion proteins provoke
376
of hematologic cancers, for example, the progression from MDS to a global loss of DNA methylation in AMLs. In fact, Dnmt1
AML, promoter CpG islands become hypermethylated and the haploinsufficiency, while not perturbing normal HSC function, sig-
339
affected genes, including tumor suppressor genes silenced. Further- nificantly delays leukemia progression in the MLL-AF9 AML mouse
more, subgroups of AML can be defined by unique DNA methylation model. 377
profiles, which can be used to stratify AML patients with respect to At many lineage-specifying promoters in HSCs, epigenetic activa-
340
their overall survival. The family of DNA methyltransferases tors are balanced with repressive complexes, namely the polycomb
includes the de novo DNA methyltransferases DNMT3A and repressive complex 1 (PRC1) and PRC2. The latter typically com-
DNMT3B that establish methylation whereas another family prises the catalytically active EZH2, as well as EED and SUZ12, and
member, DNMT1, maintains DNA methylation. 341,342 DNA meth- is responsible for di- and trimethylation of H3K27, a repressive
ylation is required for HSCs to differentiate. Conditional knockout histone mark that is then “read” and maintained by PRC1 through