Page 1037 - Hematology_ Basic Principles and Practice ( PDFDrive )

920 Part VII Hematologic Malignancies

with a poor response to therapy. This fusion protein binds regulatory Epigenetic Alterations in Acute Myeloid Leukemia
elements of the polycomb complex and results in HOX gene
activation. AML is frequently characterized by epigenetic alterations, including
covalent modifications to DNA and chromatin factors, dysregulated
expression of small and large noncoding RNAs, and changes in long-
Spliceosome Complex range DNA interactions resulting in altered gene expression. In many
cases (discussed earlier), these epigenetic regulators are targets of
The spliceosome complex consists of a number of proteins around a somatic mutation or are aberrantly recruited by fusion proteins in
small nuclear ribonucleic acid (snRNA) core, which identifies splicing AML.
motifs in pre-mRNA, removing introns and religating exons, to gener- Methylation of cytosine on DNA at CpG sites, which are enriched
ate a diversity of mRNA isoforms from each coding gene. The splicing at gene promoter regions, is a key regulator of gene expression.
machinery is highly conserved, and in addition to five snRNAs, there The AML genome has increased methylation when compared
are numerous associated proteins. Recurrent mutations have been with normal tissues, and certain genetic subtypes of AML, such
identified in hematologic malignancies in a number of the core splicing as AML with RUNX1-RUNX1T1 or PML-RARA rearrangements,
components, most often affecting SF3B1, U2AF1, SRSF2, or ZRSR2. have distinct gene methylation patterns. Recurrent missense muta-
These mutations are typically heterozygous and tend not to cooccur tions in the de novo DNA methyltransferase, DNMT3A, likely
within patients, suggesting that a second mutation in the pathway function in a dominant negative fashion and lead to focal regions
confers no additional selective advantage or is not tolerated by hema- of reduced DNA methylation. Oxidation of 5-methylcytosine to
topoietic cells. Alternative splicing was noted in AML samples prior to 5-hydroxymethylcytosine by the TET enzymes is an intermediate
the discovery of spliceosome mutations, but the cause and biological step toward subsequent demethylation. TET2 inhibition, either
consequences were not understood. Collectively, splicing factor muta- through an acquired loss-of-function mutation, or via 2-HG pro-
tions are detectable in approximately 15% of patients with AML. These duced preferentially in the setting of IDH1/2 mutations, results in
mutations are highly associated with specific subtypes of MDS and aberrant hydroxymethylation, particularly at key HOX sites, which
MPNs; consequently, AML with splicing factor mutations frequently leads to myeloid expansion and a dysplastic phenotype in mouse
have a history of these antecedent disorders. SF3B1 is the most common models. Causal links between mutations in these regulatory enzymes,
mutation in the splicing complex; mutations in this gene are tightly cytosine modifications, altered gene expression, and leukemogenesis
associated with the refractory anemia with ringed sideroblasts MDS remain elusive, but it is unlikely that global changes in methylation
subtype. SRSF2 mutations are common in CMML and are retained are responsible for the leukemic phenotype.
when these patients progress to sAML. DNA methylation and histone modification are often coordinately
regulated. For example, EVI1 has DNA methylation activity, but also
associates with histone deacetylases and methyltransferases. The
Cohesin Complex polycomb repressive complex 2 (PRC2) catalyzes the methylation of
histone H3K27, resulting in gene silencing. PRC2 is comprised of a
The cohesin complex is involved in the alignment of sister chromatids noncatalytic subunit, as well as a SET domain with methyltransferase
throughout replication, from the initial DNA synthesis during S-phase activity via EZH1 or EZH2. Mutations in EZH2 found in myeloid
and on through mitosis and segregation during M-phase. The cohesin malignancies are predicted to be loss of function. In mouse models,
core complex is comprised of the structural maintenance of chromo- EZH2 appears to have either tumor suppressor or oncogene charac-
somes (SMC) proteins SMC1, SMC3, RAD21 (SCC1), and stromalin teristics, depending on the timing of the mutation in relation to the
antigens STAG1 or STAG2. These core proteins form a ring structure, stage of disease, and has a role in inhibiting hematopoietic cell dif-
with a hinge formed by SMC1 and SMC3, and a closed loop formed ferentiation. Similarly, ASXL1 interacts with PRC2, resulting in tri-
through the binding of RAD21 with STAG1 and STAG2, which methylation at H3K27; loss-of-function mutations may promote
regulates chromosome segregation. Recurrent mutations in the genes leukemia due to relative activation of PRC2-specific genes. Finally,
for this core complex have been identified in approximately 15% of MLL rearrangements bring a diversity of fusion partners to targets
AML cases. The cohesin complex preferentially localizes at “super normally regulated by MLL. Many of these fusion partners interact
enhancers,” which regulate gene expression related to cell lineage and with DOT1L, a histone H3K79 methyltransferase, which then
self-renewal. Perturbation of this mechanism may be more relevant to methylates critical MLL-dependent regulatory genes, such as HOXA9.
the pathogenesis of AML, since cohesin mutations have not been
associated with increased risk of aneuploidy in this disease.
BIOLOGY OF ACUTE MYELOID LEUKEMIA

Nucleophosmin Role of the Bone Marrow Microenvironment
Nucleophosmin (NPM1) is a molecular chaperone with multiple AML arises in hematopoietic cells residing in a bone marrow micro-
functions, including ribosomal protein assembly, the prevention of environment known as the niche, where the stromal infrastructure
nucleolar protein aggregation, and regulation of the tumor suppres- helps to promote leukemic cell survival. AML cells in culture have
sors TP53 and alternative reading frame (ARF). NPM1 mutations are improved survival when in the presence of bone marrow fibroblasts,
seen in approximately one third of patients with AML, and are which increase expression of the antiapoptotic proteins Bcl-2 and
enriched in patients with normal karyotypes. They occur as insertions Bcl-XL. Moreover, the bone marrow stroma contributes to chemore-
in exon 12, most frequently of the four base pairs TCTG (type A), sistance via the binding of fibronectin on stromal cells to VLA-4
causing a frameshift mutation with an added nuclear export signal expressed on AML blasts. Bone marrow stromal cells also produce
motif at the carboxy-terminus, and resulting in cytoplasmic localiza- the chemokine SDF-1, or chemokine CXC motif ligand 12
tion and loss of function. Less common mutations with similar effect (CXCL12), which binds to CXC-chemokine receptor 4 (CXCR4), a
include alternate insertions of CATG (type B) or CCTG (type D), chemokine receptor expressed on hematopoietic progenitors as well
among others; there does not appear to be a clinical impact according as leukemic cells. This signal maintains the normal hematopoietic
to the type of mutation. Murine models incorporating the NPM1 progenitor niche; it also facilitates proliferation and survival.
type A mutation result in increased megakaryocytes, but do not A variety of strategies that interfere with the interactions between
develop AML. Consistent with this, mutations in NPM1 are thought leukemic and stromal cells have been tested as a means to enhance
to represent early events in the development of AML. Rarely, NPM1 the efficacy of cytotoxic agents. In addition, there is evidence that
is involved in chromosomal translocations, including t(2;5)(p23;q35) AML blasts sustain an immunosuppressive microenvironment via an
with ALK, and t(5;17)(q35;q21) with RARA, the latter resulting in arginase-dependent mechanism that may be amenable to pharmaco-
an uncommon variant of APL. logic inhibition.