What
other genes interact with DNMT3A that complicate the prognosis of AML?
FLT3 is a receptor tyrosine kinase that spans the
plasma membrane and has an important role in proliferation, survival and differentiation
of hematopoietic progenitor cells. The incidence of this mutation in AML
patients is about 25%, although this varies depending on age, clinical risk,
and is more common in adult AML instead of pediatric AML and myelodysplastic syndrome (MDS), which is a
related malignancy that can give rise to AML. It was noted in this paper that
several FLT3 inhibitors are currently in various stages of development.
It turns out that nucleophosmic (NPM1) is another gene
most commonly mutated in AML, most frequently found near CpG islands that
regulate gene promoters. You might recall from one of our earlier class
discussions that the addition of methyl groups at a cytosine base in a 5’-CG-3’
dinucleotide pair (CpG) around promoter regions of our DNA can either decrease
or eliminate transcription in particular genes and is a type of epigenetic genomic
instability, an important hallmark of cancer. Indeed, it turns out that AML
cancer cells exhibit areas of hypomethylation and hypermethylation of CpG
islands near these promoters, that serves to inactivate transcription and has
gene-silencing effects in tumor-suppressor genes.
DNMT is a whole family of methyl transferases that
include DNMT1, DNMT3A, and DNMT3B, which catalyzes the addition of methyl
groups to cytosine residues of CpG nucleotides mentioned earlier. An important
piece of information that researchers have found with regards to DNMT3A
specifically is that the majority of
these mutations are of the missense type that occur at residue R882 at the
carboxyl end of the DNMT3A protein. However there are also less common
nonsense, frameshift and splice site mutations throughout the DNMT coding
sequence (see Figure 1). Researchers currently do not understand why DNMT3A
mutation at R882 is so prevalent. Yan et al. reports that the mutated DNMT3A
enzymatic activity is substantially reduced at R882 in vitro as compared to
wild-type DNMT3A.
The involvement of DNMT3A though is nowhere even
close to being deciphered as the paper presents conflicting pieces of evidence
from various other sources that only fuel the controversy. For example, Ley et
al. have reported that there are no differences in the total expression levels
of DNMT3A protein of cells harboring mutant DNMT3A versus wild-type DNMT3A.
This study also reported that although AML patients with mutated DNMT3A
contained genomic regions with significantly different levels of methylation,
there was no correlation between any of the differentially methylated regions
and altered expression of nearby genes. Moreover, there were no clearly defined
gene expression patterns that were associated with DNMT3A mutation status. Clearly,
there is a still a lot of information that researchers don’t understand regarding this important gene in AML.
One other gene I would like to address is the IDH1/2
genes that play an important role in metabolism. The mutation in this gene is
heterozygous, resulting in proteins with newly acquired function
(gain-of-function) that catalyzes nicotinamide adenine dinucleotide phosphate
hydrogen-dependent reduction of α-KG to 2-hydroxyglutarate (2HG). This results
in a decrease in α-KG and an increase in 2HG, with 2HG subsequently acting as a
competitive inhibitor of α-KG-dependent reactions in the citric acid or Krebs
cycle. In AML, increased cellular 2HG levels contribute to epigenetic
mechanisms of pathogenesis by inhibiting α-KG-dependent enzymes that are
important for normal DNA methylation. The frequency of IDH mutations in AML is
6–16% for IDH1 and 8–19% for IDH2, both which exhibit similar effects on DNA
methylation (see also Figure 1).
 |
| Figure 1. Location and frequency of DNMT3A, IDH1, and IDH2 mutations as determined by the Cancer Genome Atlas. Mutation data was obtained from the cBio portal (Im A.P. 2014) |
What does seem to be clear at this point in the
majority of studies thus far is that older age, higher white blood cell and
platelet counts, normal cytogenetics and the presence of NPM1, FLT3-ITD and
IDH1 mutations have been found to be more common in patients with DNMT3A mutations
versus wild-type DNMT3A. Therefore it seems that the complex interplay between
these genes is what leads to a worse prognosis and on the whole, a negative
clinical outcome.
So
is there any hope for treatment??
The current goal of
AML chemotherapy is complete remission (CR) with return to normal
hematopoiesis, and this goal has been successful to some extent over the past
four decades, with the combined use of anthracycline (that is, daunorubicin or idarubicin) with cytarabine (60-70% success
rate in newly diagnosed AML patients). However, as with many anti-cancer drugs,
the disease manages to relapse in these patients, and so new and novel
therapies need to be created.
Age seems to play a significant role in whether the
disease can be cured or never return. It seems that for patients under the age
of 60, the combination of cytarabine and allogeneic hematopoietic cell transplantation (HCT) improves cure
rates to 50%. However, for individuals over the age of 60, the cure rate
remains quite low.
The
promise of hypomethylating agents in AML
Hypomethylating agents, such as decitabine and
azacitidine, are DNA methyltransferase inhibitors, and FDA approved agents for treatment
of high-risk MDS. Decitabine
is a deoxycytidine analog that is incorporated into DNA during S-phase of the
cell cycle and binds to DNA methyltransferase, rendering it inactive. Azacitidine is a cytidine
analog that primarily is incorporated into RNA, inhibiting RNA processing and
function. To a lesser extent, it is incorporated into DNA, similarly to
decitabine.
In a recent Experimental
Hematology and Oncology 2014 paper by Smith, a retrospective comparative
analysis study was conducted to study outcomes for AML patients treated with either
decitabine (DEC) or azacitidine (AZA) between January 2006 and June 2012. 487
patients were eligible for this study and over 70% of patients in each cohort
were at least 65 years old (mean age
was AZA 70.3 ± 11.8 years, DEC 69.4 ± 11.6 years). According to the study, most patient
characteristics were similar between the cohorts except that the decitabine
cohort had significantly more hospitalizations than the azacitdine cohort (62%
AZA, 71% DEC; p = 0.0323).
Below is a Kaplan Meier graph, analyzing the overall survival
(OS) of the two cohorts.
 |
| Figure 2. Overall Survival (OS) for AML patients treated with decitabine or azacitidine between January 2006 and June 2012 (B Douglas Smith 2014). |
Overall survival was significantly better in the
AZA-treated cohort compared with patients in the DEC-treated cohort (10.1
months vs. 6.9 months respectively; p = 0.007, Figure 2) and treatment with
azacitidine resulted in a significantly longer time to death when compared with
decitabine treatment.
The most crucial aspect of the entire study,
however, was that patients were not randomly assigned to treatment and administration
schedules (i.e. dosing and adherence) for each therapy were not controlled. Since
this was a retrospective, non-randomized control study, there is a possibility
that patients who were at the more advanced stage of the disease or who showed
worse prognosis were prescribed decitabine by their doctors, and hence had to
be hospitalized more often than the other cohort. The reasons for
hospitalization were stated to be primarily infections, bleeding events, or
both. Now whether these hospitalizations were largely due to the cancer itself
or side-effects of the treatment is not supported by thorough discussion in the
paper. Neither does the paper state what stage of disease these patients were
in prior to receiving either treatment as that could have a major impact on the
clinical outcome and results of the study. A study analyzing the effects of
both azacitidine and decitabine on the patient should also be conducted by
these researchers in the future.
References:
B
Douglas Smith, Charles L Beach, Dalia Mahmoud, Laura Weber, Henry J Henk
Exp
Hematol Oncol. 2014; 3: 10. Published online 2014 March
25. doi: 10.1186/2162-3619-3-10
PMCID: PMC3994315
Im, A. P., A. R. Seghal, M. P. Carroll, B. D. Smith,
D. E. Johnson, and M. Boyiadzis. "DNMT3A and IDH Mutations in Acute
Myeloid Leukemia and Other Myeloid Malignancies: Associations with Prognosis
and Potential Treatment Strategies." Leukemia 28.4
(2014): 727-980. Nature. Web. 12 May 2014.
<http://www.nature.com/leu/journal/vaop/ncurrent/full/leu2014124a.html>.
