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Page 318 Meyer et al. Cancer Drug Resist 2019;2:313-25 I http://dx.doi.org/10.20517/cdr.2019.11

were retained and often duplicated at the time of disease relapse, suggesting that they are selected for during
treatment and may play a causal role in chemoresistance. Interestingly, these mutations were rarely present
in diagnostic samples from patients who did not relapse. Upon functional analysis, the authors demonstrated
that CREBBP mutations were associated with impaired histone acetylation and corresponding gene
dysregulation. Importantly, some of these dysregulated genes were found to be important glucocorticoid
[24]
receptor target genes . Glucocorticoids are a key component of chemotherapy for ALL, and sensitivity to
[25]
glucocorticoids is an important prognostic factor . These data therefore provide a mechanistic link between
the presence of CREBBP mutations in relapsed ALL and the associated acquisition of chemoresistance at
disease relapse. Similarly, in a cohort of pediatric patients with high hyperdiploid ALL, which is typically
associated with a good prognosis, HAT domain mutations in CREBBP were found in 63% of relapsed
samples. Again, many of these mutations were present in the corresponding diagnostic samples, but CREBBP
[26]
mutations were absent in diagnostic samples from children who were cured . Consistent with these
findings, another more recent study demonstrated that low CREBBP expression at diagnosis is associated
with a poor clinical response to glucocorticoids and with increased MRD following the first month of
[27]
therapy , providing clinical evidence that loss of CREBBP activity may contribute to chemoresistance.

In addition to HATs, aberrant regulation of HDAC function is also a common feature of ALL. In an analysis
of the transcript expression of HDACs across 94 pediatric ALL samples relative to healthy control cells,
ALL cells were found to have overexpression of HDACs 2, 3, 6, 7, and 8, with overexpression of several of
these HDACs significantly associated with decreased five-year EFS . Consistent with these expression
[28]
[29]
data, Sonnemann et al. performed functional studies to assess HDAC activity in ALL cells relative to
healthy controls. This analysis revealed a significant increase in HDAC activity in ALL, suggesting that these
[21]
changes may underlie the global histone hypoacetylation that is commonly observed in cancer . As with
CREBBP mutations, aberrant HDAC expression has been causally implicated in resistance to glucocorticoids.
In a correlative analysis of HDAC expression with clinicopathologic parameters, elevated HDAC4 expression
[30]
was found to be associated with poor responses to the glucocorticoid prednisone . Accordingly, Bachmann et al.
[31]
determined that epigenetic silencing of the gene encoding the pro-apoptotic protein BIM is associated
with poor in vivo responses to glucocorticoids in a patient-derived xenograft model of ALL. Across patient
samples, this glucocorticoid resistance phenotype correlated with a reduction in histone acetylation. Upon
pharmacologic restoration of histone acetylation with the HDAC inhibitor vorinostat, BIM expression and
glucocorticoid sensitivity were restored.

Histone methylation
ALLs harboring rearrangements involving the MLL (KMT2A) gene comprise approximately five percent
of pediatric ALLs. MLL rearrangements are particularly common in infant ALL and are associated with
intrinsic drug resistance and with an unfavorable prognosis . MLL encodes a histone H3 lysine 4 (H3K4)
[1]
[32]
methyltransferase that functions to regulate Hox gene expression during development . Interestingly, in
ALL, the most common MLL rearrangements result in the deletion of the H3K4 methyltransferase domain.
However, these rearrangements result in fusions to partners that similarly function to covalently modify
histones. For example, in ALLs harboring MLL-AF4 or MLL-AF10 fusions, AF4 and AF10 interact with the
DOT1L methyltransferase, which mediates H3 lysine 79 methylation (H3K79). This aberrant methylation
leads to increased Hox gene expression [33,34] . Furthermore, it has been shown that aberrant DOT1L activity in
the context of leukemias with MLL-AF4 fusions results in aberrant H3K79 methylation of the gene encoding
the anti-apoptotic protein BCL-2. In a xenograft model of MLL-AF4 ALL, the BCL-2 inhibitor ABT-199
[35]
synergized with conventional induction-type chemotherapy , suggesting that altered histone methylation
may directly contribute to poor chemosensitivity in this subtype of ALL.

In addition to a role for aberrant histone methylation as a driver of leukemogenesis, targeted sequencing of
epigenetic regulators in matched pairs of diagnostic and relapsed samples from pediatric patients with B-ALL