Page 195 - Williams Hematology ( PDFDrive )

170 Part IV: Molecular and Cellular Hematology Chapter 12: Epigenetics 171

DOT1 (a histone H3 methyltransferase) inhibitors have proven useful the modification. Among many examples is EZH2, the main enzyme
in mouse models of MLL-fusion–induced leukemia (and in cell lines), for H3K27me addition, which is either greatly overexpressed or hyper-
45
and have entered phase I clinical trials (NCT01684150). Notably, the activated (via mutation) in various hematologic malignancies. For
55
involvement of BRD4 in this system, along with its known importance example, EZH2 is highly expressed in mantle cell lymphomas, whereas
in transcriptional activation in MYC-driven cancers, provides addi- activating mutations (conferred by amino acid substitutions in the
tional therapeutic possibilities. Here, inhibitors of BET-family bromo- catalytic domain) are common in diffuse large B-cell and follicular
56
domains (JQ1 and others) have proven very effective in cell lines from lymphomas. This hyperactivity has led to therapeutic strategies
patients, laying the foundation for clinical trials. 46–49 involving selective competitive inhibitors that mimic the cofactor
S-adenosyl-methionine, the methyl donor for the EZH2 enzyme, which
have proven effective in mouse xenografts. 56
Theme 2: Loss-of-Function Mutations in Chromatin Modifiers
Loss-of-function mutations in chromatin modifiers are now very com-
mon in many cancers. The key concept in this theme is that the loss of MISREGULATION OF DNA
epigenetic control confers both gene-specific and genome-wide epige- METHYLATION/DEMETHYLATION IN
netic variation, eliciting transcriptome variation and plasticity. As a
result, individual cells with transcriptomes that promote growth, sur- HEMATOLOGIC MALIGNANCIES
vival, and/or metastasis can be selected from a diverse population. For
example, this epigenetic variation can allow cells to sample a transcrip- Over the past several years, several studies have made striking links
tome that promotes invasion and later convert to a transcriptome that between hematologic malignancies and the misregulation of DNAme.
favors colonization. One mode involves the aberrant epigenetic silencing High-throughput sequencing of leukemias and lymphomas have
of tumor-suppressor proteins, either by the acquisition of “silencing” revealed three different types of mutations: (1) loss-of-function muta-
histone modifications, DNAme, or both. Epigenetic variation and selec- tions in the de novo DNMT3a enzyme; (2) loss-of-function mutations
tion are themselves powerful tools; however, they can also combine in particular TET proteins; and (3) gain-of-function mutations in the
with genetic mutations to provide a further fitness benefit and reinforce metabolic enzymes isocitrate dehydrogenase (IDH)1 and IDH2, which
oncogenic properties. create small molecule inhibitors of TET proteins (expanded below in
Examples of mutations in epigenetic factors in hematologic malig- this section). To begin, hypomorphic mutations in DNMT3a are com-
57
nancies are numerous; even a partial list of factors and their impact is mon in acute myeloid leukemias (AMLs), although precisely how a
beyond the scope of this chapter. However, mutations in certain fac- reduction in DNMT3a activity promotes leukemia is not yet under-
1
tors are found in many hematologic malignancies and help to illustrate stood. Of the three TET-family genes, particular TET genes are mutated
the concepts above; consequently, they are treated further here. One at high frequency in hematologic malignancies. Strikingly, mutations in
example that builds on an earlier section “Epigenetics and Hemato- TET2 are found in almost half of chronic myelomonocytic leukemias
logic Malignancies” involves mutations in MLL genes. MLL is actually a (CMML), 58,59 and are also common in certain T-cell lymphomas. The
family of five similar genes; however, whereas MLL1 is most commonly key emerging concept is that defects in DNA demethylation by TET2
involved in leukemogenic fusion proteins (discussed earlier), mutations mutations may lead to increases in DNAme at certain CpG island
60
in MLL2 are very common in lymphomas, with mutation rates as high regions (including those bearing H3K27me), which may confer gene
as 89 percent for follicular lymphoma. Other chromatin factors are also silencing of developmental and/or tumor-suppressor type genes; how-
50
mutated at high frequency in BCLs, including the HAT complex mem- ever, clear cause-and-effect links have not yet been made. Consistent
bers EP300 and CREBBP. 50 with this mechanism, TET2 mutations are often found with mutations
Mutations that affect the addition or removal of the repressive in EZH2, which catalyzes H3K27me addition. 51,59
histone modification H3K27me3, are increasingly common in hema- Of particular interest are recent links between metabolic dysreg-
tologic malignancies. For example, mutations in the polycomb repres- ulation, hematologic malignancies, and gliomas. Remarkably, gain-of-
sive complex 2 (PRC2) complex, which adds H3K27me3, are associated function mutations in the TCA cycle enzymes IDH1 and IDH2 are
with myeloproliferative diseases, myelodysplastic syndromes, and oncogenic. 61–66 Normally, IDH1/2 convert isocitrate to 2OG (also
T-cell ALL. 51–53 In addition, mutations in the main enzyme that known as α-ketoglutarate), a cofactor for both TET enzymes and
removes H3K27me3, known as X-chromosome encoded ubiquitously JmjC-class lysine demethylases. Notably, oncogenic IDH1/2 mutations
transcribed tetratricopeptide repeat (UTX), are common in multiple create proteins that additionally convert 2OG to (R)-2-hydroxygluta-
myeloma. Furthermore, mutations in these enzymes are known to rate (R-2HG). 66,67 This both depletes the normal cofactor for TETs and
54
synergize with mutations in other epigenetic enzymes, such as TET pro- JmjC demethylases and further creates a potent “oncometabolite” inhib-
teins, in myeloid disorders. Notably, many genes that become improp- itor of TET enzymes and prolyl hydroxylases (which regulate hypoxia-
51
erly methylated in cancer cells were marked by H3K27me earlier in inducible transcription factor [HIF] proteins). Additionally, mutations
their development and were DNA demethylated. Thus, proper regula- in other Krebs cycle enzymes (e.g., succinate dehydrogenase [SDH])
tion of H3K27me3—a modification present at many silent but “poised” accumulate succinate, which can likewise inhibit TET, JmjC, and pro-
68
developmental genes—appears critical for tumor prevention. lyl hydroxylases ; notably, SDH mutations and HIF mutations are both
common in neuroendocrine tumors. These observations have inspired
68
multiple therapeutic approaches, including: (1) reversing the impact
Theme 3: Gain-of-Function Mutations in Chromatin Modifiers of these oncometabolites on the epigenome (e.g., via DNMT, HDAC
Gain-of-function of epigenetic enzymes typically occurs either through or HMT inhibitors); (2) implementing selective inhibitors of these
upregulation of expression (through copy number variation or pro- gain-of-function IDH mutant proteins to prevent (R)-2HG production;
moter fusions) or via mutations that upregulate the activity of the (3) using selective inhibitors of prolyl-hydroxylases; and (4) the use
enzyme. The main concept in this theme is that high levels and/or the of ascorbic acid (vitamin C), which can enhance TET protein activity
inability to turn off an epigenetic enzyme can lead to sustained acti- by affecting the reduction-oxidation (redox) state of iron, an essential
vation or silencing of target loci, depending on the main function of cofactor for TET enzymes.

Kaushansky_chapter 12_p0165-0172.indd 170 17/09/15 6:30 pm