Page 1120 - Hematology_ Basic Principles and Practice ( PDFDrive )

986 Part VII Hematologic Malignancies

SUZ12; as well as kinases such as JAK1, JAK2, JAK3, MPL, KRAS, there are likely additional factors contributing to the development of
and NRAS. megakaryoblastic disease. Cooperating mutations, the target cell, and
t(1;22), which is seen exclusively in infants with AMKL, fuses the microenvironment all have the potential to direct lineage during
RBM15 and MKL1. MKL1 is a transcriptional coactivator for serum the process of transformation.
response factor (SRF), a transcription factor that regulates the expres-
sion of genes involved in cell growth, proliferation, and differentia-
tion, as well as genes that control the actin cytoskeleton. In Cytogenetically Normal AML
unstimulated cells MKL1 associates with G-actin monomers and is
retained in the cytoplasm. Following stimulation and Rho-mediated Approximately 20%–25% of pediatric AML cases lack chromosomal
actin polymerization, G-actin pools are depleted and MKL1 translo- aberrations and are prognostically defined as intermediate risk. To
cates to the nucleus, associating with SRF to activate gene transcrip- understand the underlying lesions driving this form of AML and to
tion. RBM15 encodes a protein containing three N-terminal RNA use this information to further refine risk stratification of these
recognition motifs that bind to nucleic acids and a Spen paralogue patients, efforts have been made by a number of groups to identifying
and orthologue C-terminal (SPOC) domain that is thought to the genetic lesions within this AML subtype. Identified genetic
interact with the SMRT and NCoR corepressor complexes, as well lesions have included mutations within the genes FLT3, NPM1,
as RBPJ, a transcription factor downstream of Notch signaling. The IDH, RAS, and CEBPA. More recent whole-genome sequencing
fusion of MKL1 to RBM15 deregulates the normal intracellular analysis of cytogenetically normal adult AML has identified these
localization of MKL1 such that it is becomes constitutively localized previously described mutations, as well as the mutation of a number
to the nucleus, resulting in SRF activation even in the absence of of other genes including DNMT3A, which are thought to contribute
stimuli. In addition to the SRF transcriptional program, the fusion to tumorigenesis. Below we discuss the most frequent somatic muta-
also aberrantly activates RBPJ transcriptional targets. While both tions identified within pediatric AMLs with a normal karyotype.
transcription programs have been shown to be deregulated by the
fusion gene, the degree to which they contribute to transformation Nucleophosmin
is still unclear. Nucleophosmin (NPM) is often mutated in cytogenetically normal
Until recently, with the exception of the RBM15-MKL1 fusion, AML and is unique in that it has both oncogenic and tumor sup-
the genetic etiology of non-DS-AMKL had remained elusive. Tran- pressor functions. The protein shuttles between the nucleus and
scriptome sequencing of a small cohort identified a cryptic inversion cytoplasm, taking part in many cellular processes including regulation
on chromosome 16 [inv(16)(p13.3q24.3)] in half of the patients that of ribosomal RNA transcription/processing, transport of preribo-
resulted in the joining of CBFA2T3, a member of the ETO family somal particles to the cytoplasm, DNA-histone and nucleosome
of nuclear corepressors, to GLIS2, a member of the GLI family of assembly, as well as regulating the activity and stability of tumor
transcription factors. The gene expression profile of CBFA2T3-GLIS2 suppressors such as p53 and ARF. Alterations of NPM in cancer
AMKL was distinct from that of AMKL cells lacking this chimeric include its overexpression in a variety of epithelial cancers; involve-
transcript, and from other genetic subtypes of pediatric AML. Fur- ment in chromosomal translocation in several hematological malig-
thermore, the CBFA2T3-GLIS2 fusion gene conferred a poor prog- nancies including the t(2;5)[NPM-ALK] in anaplastic large-cell
nosis, a finding that has since been confirmed. Expression of lymphoma (ALCL), t(3;5)[NPM-MLF1] in myelodysplastic syn-
CBFA2T3-GLIS2 in Drosophila and murine hematopoietic cells drome and AML and t(5;17)[NPM-RARA] in variant APL; and
induces bone morphogenic protein (BMP) signaling, a pathway not point mutations that alter its C-terminus resulting in the creation of
previously implicated in AML, and results in a marked increase in a new nuclear export signal. Each of these genetic alterations result
the self-renewal capacity of hematopoietic progenitors. CBFA2T3- in alteration of the normal shuttling of NPM between the cytoplasm
GLIS2-expressing cells remained growth factor dependent in vitro and nucleus, resulting in constitutive cytoplasmic localization. Muta-
and fail to induce leukemia in mice, consistent with a requirement tions in NPM are found in 35% of cytogenetically normal adult
for cooperative mutations. Overall, the total burden of somatic muta- AMLs but only between 2% and 12% of pediatric AMLs with normal
tions in CBFA2T3-GLIS2-expressing cases is low; however, several cytogenetics.
have been found to carry lesions in either a Janus kinase(JAK) gene The tumor suppressor function of NPM in hematologic malig-
and/or a somatic amplification of the Down syndrome critical region nancies is attributed to its role in maintenance of genomic stability
on chromosome 21. and in the regulation of the ARF-p53 tumor suppressor pathway.
In addition to CBFA2T3-GLIS2, approximately 8% of pediatric Wild-type NPM forms a complex with ARF and HDM2, which
non–DS-AMKL cases carry the NUP98-KDM5A fusion. NUP98, stabilizes ARF and prevents HDM2-mediated p53 degradation.
a nucleoporin family member with transactivation activity, fused to More recently, it has also been shown that in the absence of NPM or
KDM5A, an H3K4me3-binding PHD finger, was initially described in the presence of an NPM mutant, cells express increased protein
in adult AML. When introduced into murine bone marrow, this levels of the MYC proto-oncogene due to a loss of degradation by
fusion oncogene induces a myeloid differentiation arrest and mice the NPM-stabilized ubiquitin ligase, FBW7G. Given that mutations
develop AML with an average latency of 69 days. Wang and colleagues in NPM are able to suppress the ARF-p53 tumor suppressor pathway
demonstrated this fusion to be bound to H3K4me3 mononucleo- and enhance the oncogenic MYC pathway, it is perhaps not surpris-
somes, showing the PHD finger plays a role in targeting the fusion ing that the mutation occurs in the context of cytogenetically normal
to the genome. Interestingly, microarray analysis identified several AML with a minimal number of secondary lesions.
polycomb proteins carrying H3K4me3 marks to be transcriptionally
upregulated in response to the fusion, while housekeeping genes with Isocitrate Dehydrogenase and TET2
constitutive H3K4me3 marks remained unchanged. Affected poly- Isocitrate dehydrogenase 1 (IDH1) was initially identified as a target
comb targets confirmed by chromatin immunoprecipitation include of cancer-associated mutations in a study that performed whole-
genes upregulated in MLL-rearranged leukemia such as HOXA5, exome sequencing on glioblastoma multiforme (GBM). Subsequent
HOXA7, HOXA9, HOXA10, MEIS1, and PBX1. Furthermore, the analyses revealed IDH1 or IDH2 mutations in up to 16% of adult
authors demonstrate a block in PRC2 binding, the complex that and about 7% of pediatric AMLs with normal cytogenetics. The
antagonizes polycomb proteins through transcriptional repression mutations in both GBM and AML have been heterozygous and
of target genes. Therefore, the NUP98-KDM5A fusion is able to restricted to arginine 132 in exon 4 of IDH1, or to either the homolo-
prevent silencing of critical transcription factors that play a role in gous residue in IDH2, R172, or to a second arginine, R140, also
maintaining hematopoietic progenitor status, similar to MLL gene located in its substrate binding pocket. Although the distribution of
rearrangements. It is perhaps not surprising then, that MLL-AF9 specific IDH1/IDH2 mutations varies between GBM and AML, each
and MLL-AF10 fusion events have also been detected in non-DS- results in a loss of the enzyme’s ability to catalyze the oxidative car-
AMKL. As these lesions are also found in other subtypes of AML, boxylation of isocitrate to α-ketoglutarate (α-KG), coupled with a