Page 1066 - Hematology_ Basic Principles and Practice ( PDFDrive )

Chapter 60 Myelodysplastic Syndromes 949

many patients are either compound heterozygotes or have uniparental
U2AF1 disomy (UPD) at chromosome 4q, leading to effective abrogation of
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TET2 function. Indeed, patients with TET2 mutations have been
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U2AF1 encodes an auxiliary factor in the U2 spliceosome that is shown to have globally altered methylation profiles. TET2 muta-
responsible for recognizing the AG splice acceptor dinucleotide at the tions often occur early in MDS pathogenesis and are thought to alter
3′ end of introns. U2AF1 mutations occur in about 12% of patients HSC homeostasis, a theory supported by the fact that HSCs in TET2
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with MDS. Similar to other commonly mutated splicing factor knockout mice display enhanced self-renewal and repopulation
genes, the two most common mutations are both heterozygous mis- capacity. In humans, however, TET2 mutations do not appear to
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sense substitutions, again implying a gain of function, and both themselves confer a specific phenotype; rather, they may create an
appear to alter sequence specificity of pre-mRNA binding and splic- epigenomic environment permissive to the acquisition of other muta-
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ing. The two mutations are in separate zinc finger DNA binding tions that are responsible for determining these factors. Recent evi-
domains, one at codon 34 toward the N-terminal of the protein, and dence suggests that TET2 mutations appear to predict a favorable
one at codon 157 toward the C terminal. Some studies have shown response to hypomethylating agents, particularly in the setting of
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an increased tendency of MDS with U2AF1 mutations to evolve into wildtype ASXL1. TET2 mutations were not previously felt to
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secondary AML, but given the small numbers of patients in these confer prognostic information, but newer data suggest that, at least
studies, the strength of this effect is unclear. in patients undergoing stem cell transplant, they predict an inferior
outcome. 104
Epigenetic Modifier Mutations
DNMT3A
Epigenetic changes are biochemical modifications that affect chroma-
tin structure, and thereby gene expression, without actually altering DNMT3A is a member of the family of DNMTs, which catalyze the
the DNA sequence itself. The two types of epigenetic changes most addition of methyl groups to cytosine residues of CpG dinucleotides.
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relevant to MDS are DNA methylation and histone modification. These dinucleotides tend to cluster in 5′ promoter regions upstream
DNA methylation involves the addition of a methyl group to the of genes, and increased methylation of these CpG islands is associated
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cytosine residues of cytosine-guanine pairs by members of the DNA with decreased expression of the associated downstream gene. The
methyltransferase (DNMT) family of enzymes. These cytosine- observation that many cancers often display aberrant methylation
guanine pairs frequently cluster together in “CpG islands,” which are relative to healthy tissue has led to the hypothesis that hypermethyl-
typically located just upstream of promoter regions. CpG islands tend ation, particularly in the promoters of tumor suppressor genes, plays
to be unmethylated at baseline, but their progressive methylation a role in cancer pathogenesis. This hypothesis has been somewhat
leads to transcriptional silencing of the downstream genes. Studies supported by the efficacy, albeit imperfect, of so-called hypomethylat-
have shown that many MDS patients display aberrant methylation ing agents like decitabine and 5-azacitidine in MDS and AML (see
patterns compared to healthy controls, with isolated hypermethyl- section on Therapy, later).
ation in the promoters of critical tumor suppressors despite global The DNMT3A gene consists of 29 exons and encodes a 908-
hypomethylation elsewhere. This phenomenon has been hypothesized amino acid protein that, along with DNMT3B, is one of the two
to play a role in the pathogenesis of MDS, but attempts to glean enzymes responsible for de novo CpG methylation independent of
prognostic or predictive information from specific methylation pat- replication, whereas a third methyltransferase, DNMT1, is respon-
terns in MDS patients have been largely unsuccessful, and the success sible for maintenance of baseline hemimethylation during active
of so-called hypomethylating agents such as 5-azacitidine and replication. Only DNMT3A mutations, however, have been found to
decitabine (which may in fact not truly act by reducing global occur recurrently in myeloid malignancies, perhaps suggesting dif-
methylation) has been variable. ferential expression in hematopoietic cells. Analysis of DNMT3A
A second type of epigenetic regulation involves the biochemical mutations in patients with MDS has shown a preponderance of
modification of histones, the structural protein complexes that form missense single nucleotide variations predicted to alter protein func-
scaffolding for chromatin packaging. The interaction between his- tion, although nonsense mutations, insertions, and deletions (indels)
tones and chromatin represents an additional level of transcriptional have been observed as well. The mutations occur throughout the
control, in which unwinding of chromatin is required for the tran- gene, although a mutational hotspot at R882H, in the methyltrans-
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scription machinery to physically access DNA. The dynamics of ferase domain, has been described in a minority of MDS patients.
chromatin-histone interactions are largely mediated by complex In AML, the R882H mutant protein has been shown to inhibit
biochemical modifications of specific histone amino acids. In MDS wildtype DNTM3A, suggesting a dominant negative mechanism. 107
and other myeloid disorders, these modifications can either be In earlier studies, DNMT3A-null HSCs transplanted into mice
affected directly by mutations in genes coding for histone-modifying displayed aberrant global methylation patterns, increased self-renewal,
enzymes or indirectly by the permutation of biochemical pathways and impaired differentiation capacity compared to wildtype HSCs,
that regulate the balance between open and closed chromatin. but the mice themselves did not develop dysplasia or other hemato-
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logic malignancies. In more recent studies, however, mice trans-
planted with DNMT3A-null HSCs had shortened overall survival
TET2 and developed a spectrum of hematopoietic malignancies similar to
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that seen in humans, including leukemia and MDS. The different
TET2, which encodes a member of the Ten-Eleven Translocation outcomes in these two studies is thought to be caused in large part
gene family, is the most commonly mutated epigenetic regulator in by the fact that mice in the older experiments were serially trans-
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MDS. TET2 mutations occur in 20% to 30% of all MDS and are planted at 18 weeks, before they had a chance to develop overt
particularly enriched in CMML, where they can be found in 40% hematologic disease, whereas mice in the later experiments were
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to 50% of cases. The TET2 protein is a methylcytosine oxy- observed for 6 months. The long latency to development of malig-
genase responsible for converting 5-methylcytosine (5mC) into 5- nancy in these experiments may speak to the subtlety of the epig-
hydroxymethylcytosine (5hmC) using iron and α-ketoglutarate enomic abnormalities initially conferred by acquisition of DNMT3A
(α-KG, produced by IDH1 and IDH2), and for further oxidizing mutations.
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5hmC to 5-formyl- and 5-carboxycytosine. These reactions are In humans, the significance of DNMT3A mutations in the
thought to contribute to active demethylation through base excision pathogenesis of MDS is not clear. They occur with somewhat less
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repair back to unmodified cytosine. Mutations in TET2 tend to be frequency than the most common recurrent mutations (between 8
inactivating frameshift or nonsense mutations or specific missense and 13% in most studies 80,110 although some studies have quoted
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substitutions predicted to lead to abrogation of protein function, and much lower frequencies below 5%). On the other hand, clonal