2012 EDUCATIONAL BOOK - American Society of Clinical Oncology

ASXL1 and L3MBTL1
ASXL1 belongs to the Enhancer of trithorax and Polycomb
gene family. Its biologic roles are not well understood;
however, one epigenetic function ascribed to the Drosophilia
homolog is modification of chromatin complexes, including
ubiquitination of histone H2A lysine 119. 48 Similar to other
epigenetic modifiers, mutation of ASXL1 (primarily exon 12)
is found in a wide spectrum of myeloid malignancies, but
among the MPNs, seems to be preferentially associated with
MF at a rate up to 13% to 23%. 45
Another polycomb family member, L3MBTL1, is the human
homolog of the Drosophila lethal, 3 malignant brain
tumor (L3MBT) that functions as a tumor suppressor. It is
located on the long arm of chromosome 20 (q12), within a
region commonly deleted in several myeloid malignancies
and is a candidate tumor suppressor gene. Depletion of
L3MBTL1 from human cells causes replicative stress, DNA
breaks, activation of the DNA damage response, and
genomic instability. 49 In addition to being involved in histone
H4 methylation, 50 it has been found that haploinsufficiency
for L3MBTL1 promotes erythroid differentiation,
suggesting a role in PV development. 51
Extra-TK Functions of JAK2 V617F
Recently, the unexpected finding was made that normal
JAK2 can translocate to the nucleus. Among its nuclear
roles, JAK2 can phosphorylate histone H3Y41, releasing the
transcriptional repressor HP1alfa from chromatin. 52 Exclusion
of HP1alfa resulted in increased gene transcription and
elevated expression of the oncogene LMO2 in leukemia
cells, which was reversed with inhibition of JAK2. It still
remains to be determined how JAK2’s effects on chromatin
structure translate into expression of specific genes and
promotion of oncogenesis in certain cellular or disease contexts.
A nucleus-associated gain-of-function of JAK2 V617F,
separate from its role in activated JAK-STAT signaling, is
its capacity to phosphorylate the protein arginine methyltransferase
5 (PRMT5). 53 This modification reduces PRMT5’s
ability to methylate histones H2A and H4. These chromatin
changes were modeled with knockdown of PRMT5 by a short
hairpin RNA in CD34-positive cells, resulting in erythroid
differentiation and increased colony formation. These data
suggest that the inhibitory effects of JAK2 V617F on PRMT5
activity may be relevant to MPN pathobiology.
Other Genetic Mutations and Pathway
Alterations in MPNs
Hemizygous deletions encompassing all or part of the gene
encoding the Ikaros (IKZF1) transcription factor on chromosome
7 are associated with MPN transformation. 54 Modeling
of IKZF1 haploinsufficiency in mice progenitors with SiRNA
knockdown of IKZF1 led to an increase in cytokinedependent
growth and STAT5 activation. In another study
of post-MPN AML, P53 mutations were found at a frequency
of 27%, including independent bi-allelic abnormalities and
homozygous mutations caused by acquired uniparental disomy
of chromosome 17p. 55 Other genes mutated in the
leukemic phase of MPNs include NRAS and RUNX1. The
role of all of these genes in disease initiation compared to
MPN transformation remains to be clarified.
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NGUYEN AND GOTLIB
Genetic Instability and Apoptosis Resistance
Increased DNA damage and evasion of apoptosis may
also play a role in MPN pathogenesis. Expression of the
antiapoptotic protein Bcl-xL is increased in PV erythroid
progenitors, suggesting that this is one possible mechanism
of resistance to apoptosis that normally occurs on withdrawal
of Epo. 56 In cell line experiments, overexpression of
V617F induced an increase in homologous recombination
and genetic instability. 57 Furthermore, a marked increase in
homologous recombination was found in CD34 positive progenitors
from patients with PV and PMF when compared
with controls. Nonenzymatic deamidation of Bcl-xL is a key
step in the induction of apoptosis in response to DNA
damage. This pathway is inhibited in both CML and PV. 58
JAK2 inhibitors restored the Bcl-xL deamidation pathway
in primary samples from PV patients, suggesting that activated
signaling mediated by V617F may lead to resistance to
apoptosis.
Hypermethylation and Phosphorylation of SOCS Family Members
The SOCS proteins are SH2-domain containing proteins
that function as key negative regulators of JAK-STAT signaling.
SOCS1 and SOCS3 can bind to the catalytic domain
of JAK2 and inhibit its kinase activity, as well as target
JAK2 for ubiquitination and subsequent degradation. Recent
studies have suggested that hypermethylation of SOCS
genes may also play a role in MPN pathogenesis. Hypermethylation
of SOCS3, and to a lesser extent, SOCS1, has
been observed in either V617F or wild type JAK2 chronic
and blast phase MPN patient samples. 59
Phosphorylation of SOCS proteins leads to enhanced
SOCS protein degradation, and this process may be altered
in MPNs. In addition to its previously highlighted gain-offunctions,
activated JAK2 V617F has also been shown to
overcome normal SOCS regulation by hyper-phosphorylating
SOCS3, thus effectively blocking its inhibitory
activity and perhaps even potentiating the proliferative
capacity of the mutant V617F allele. 60 To date, somatic
mutations in SOCS genes in MPNs have not been demonstrated.
Inherited Susceptibility to MPNs
A five- to seven-fold increased risk of MPN development is
found in first-degree relatives of patients with MPN. Three
studies demonstrated that a germline haplotype (GGCC,
referred to as ‘46/1’) encompassing the 3� region of JAK2
gene is associated with a three- to four-fold risk of developing
a V617F-positive (as well as MPL-mutated) MPN. 61-63
Patients who were heterozygous for this haplotype preferentially
acquired the V617F mutation in cis with the predisposition
allele, suggesting that the haplotype may lead to
hypermutability at the JAK2 locus. However, the haplotype
was also weakly associated with JAK2 V617F-negative
MPNs, suggesting that it may confer a more generalized
propensity for MPN development independent of JAK2
V617F. One possibility is that the germline haplotype may
result in a functional difference such that cells with the risk
haplotype gain a selective advantage. However, evidence to
support this notion has been lacking. No differences in JAK2
expression level or nonsynonymous JAK2 coding polymorphisms
associated with this haplotype have been identified.
Given the low penetrance of the haplotype, and a lack of
correlation with specific-disease related features, testing for