2012 EDUCATIONAL BOOK - American Society of Clinical Oncology

GENETICS OF MYELOPROLIFERATIVE NEOPLASMS
missense mutation (E208Q) in the PH domain. BaF3-MPL
cells transduced with these LNK mutants displayed augmented
and sustained thrombopoietin-dependent growth
and signaling and primary samples from these patients
exhibited aberrant JAK-STAT activation. Follow-up studies
indicate a low frequency of LNK mutations in the chronic
phase of MPN (�5%), increasing to approximately 10% with
transformation to AML. 30 Mutations have also been found
in a few patients with idiopathic erythrocytosis/PV. 31 Most
mutations cluster in an exon 2 ‘hot spot,’ which are expected
to disrupt the PH domain, but frameshift, missense, and
nonsense mutations in exons 5, 7, and 8 have now been
reported, and may coexist with other MPN mutations in the
same patient. Taken together, these findings indicate that
JAK-STAT activation caused by loss of LNK negative feedback
regulation can phenocopy MPN disease.
Mutations in Components of the Epigenetic Machinery
TET2
SNP and CGH arrays identified acquired LOH on chromosome
4q in patients with myeloid neoplasms, with further
mapping defining the TET oncogene family member 2
(TET2) gene as a mutated locus. Somatic mutations in TET2
are a mixture of deletions, frameshifts, stop codons, or
conserved amino acid substitutions and occur at a patient
frequency of 7% to 16% in PV, 4% to 11% in ET, and 8% to
17% in PMF. 32,33 The biologic and biochemical roles of TET
family proteins and the functional consequences of TET2
mutations are becoming better clarified. TET proteins are
�-ketoglutarate-dependent enzymes that catalyze the conversion
of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine
(5hmC). 34 As DNA methylation is known to be an
important modulator of gene expression, it has been speculated
that the conversion of 5mC to 5hmC may alter chromatin
structure and restrict access to DNA methyltransferases,
which mediate repression of gene transcription. It follows
that loss of TET enzyme activity results in increased methylation,
and in a sample of patients with TET2 mutations,
a reduction in levels of 5hmc levels is demonstrated. 35 In
various TET2 knockout murine models, loss of TET2 results
in a myeloproliferative phenotype (e.g., splenomegaly, extramedullary
hematopoiesis) with expansion of the hematopoietic
stem cell compartment. 36 Expression of mutant
TET2 in early hematopoietic precursors skews hematopoiesis
in favor of myeloid over lymphoid progenitors. 37
DNMT3A
Whole-genome sequencing of patients with AML revealed
recurrent mutations in 22% of AML patients in the gene for
DNA methyltransferase 3A (DNMT3A), another epigenetic
regulator. 38 In this study, DNMT3A mutations were associated
with poor outcome. In one analysis of patients with
MPN, 12 DNMT3A variants were found in 115 patients
(10%). 39 Mutations were most frequently detected in patients
during blastic phase transformation (6/35, 17%) and
MF (3/20, 15%), followed by PV (2/30, 7%) and ET (1/30, 3%).
In another study of 94 patients (46 PMF, 22 post-PV/ET MF,
11 blast-phase MPN, and 15 CMML), only three DNMT3A
mutations were found among 46 PMF patients (7%). 40 In
both studies, mutations were always heterozygous, and in
some cases, coexisting JAK2 V617F, TET2, ASXL1, and IDH
1/2 mutations were found. Although DNMT3A mutations
are ubiquitous among myeloid neoplasms, their mechanistic
consequences are less clear. Using a conditional ablation
model in mice, it was found that loss of DNMT3A results in
impairment of hematopoietic stem cell (HSC) differentiation
and expansion of HSC with serial transplantation. 41 However,
analysis of HSC from DNMT3A-null mice showed both
increased and decreased methylation at specific loci. In the
study of AML patients, global methylation patterns and
5-methylcytosine content in genomes were not significantly
altered in patients with DNMT3A mutations. The potential
cooperative effects of DNMT3A with other epigenetic modifiers,
and the gene’s role in disease transformation requires
further study.
IDH 1/2
The genes IDH1 and IDH2 encode enzymes that catalyze
oxidative decarboxylation of isocitrate to �-ketoglutarate.
Mutant IDH has decreased affinity to isocitrate, but displays
neomorphic catalytic activity toward �-ketoglutarate,
resulting in accumulation of 2-hydroxyglutarate. IDH1 and
IDH2 mutations were first reported in gliomas/glioblastomas
and AML. In a multi-institutional cohort of 1,473
MPN patients, 38 IDH mutations were found, and ranged in
frequency from 0.8% to 4% among chronic phase PV, ET, and
PMF patients (highest in the latter), and significantly increased
to 21% in blast phase MPN. 42 This study corroborates
the results of smaller studies, which documented a
higher incidence of IDH 1/2 mutations in the leukemic phase
of MPNs. In a multivariate analysis, the presence of IDH
mutations predicted a worse survival in a subgroup of 43
blastic phase MPN patients from the Mayo Clinic. 42
It is noteworthy that TET2 activity is impaired in cells
with mutated IDH 1/2 because its activity depends on
a-ketoglutarate. 43 Mutual exclusivity of IDH 1/2 and TET2
mutations has been observed in AML and was speculated
to extend to MPNs as well. However, in the aforementioned
study, IDH mutational frequencies were similar among
patients with JAK2 (3.6%), MPL (4.3%) and TET2 (3.2%)
mutations. 42 Given the coexistence of these molecular derangements,
the specific contribution of IDH 1/2 mutations
to epigenetic dysregulation, and MPN biology in general,
becomes more difficult to dissect.
Mutations of the Polycomb Repressive Complex (PRC2)
The polycomb repressive complex (PRC2) is the major
methyltransferase for histone H3K27 methylation, one type
of post-translational histone modification involved in regulation
of gene transcription. A low frequency of mutations
in the PRC2 enzymatic component EZH2, noncatalytic components
EED and SUZ12, and PRC2-associated protein
JARID2 have been described in either MDS, MPNs, or
MDS/MPNs overlap disorders. 44-46 Acquired uniparental
disomy on chromosome 7q led to identification of EZH2
mutations, which are either heterozygous or homozygous
and result in loss-of-function. 44 To date, a well-defined
picture of the effect of PRC2 complex-related mutations on
myeloid pathobiology has not yet emerged. The prognostic
relevance of EZH2 mutations was assessed in a survey of
370 patients with PMF and 148 patients with post PV/ET-
MF. Twenty-five different EZH2 mutations were found in
5.9% of PMF, 1.2% of post PV-MF, and 9.4% of post-ET MF
patients. Leukemia-free and overall survival was significantly
reduced in patients with EZH2-mutated PMF. 47
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