Page 1119 - Hematology_ Basic Principles and Practice ( PDFDrive )

Chapter 62 Acute Myeloid Leukemia in Children 985

when exposed to retinoic acid. Based on this observation, Zhen-Yi PML to be localized to the nuclear matrix where it becomes
Wang treated a 5-year-old girl with refractory APL who was in a sumoylated and subsequently degraded by proteasomes. When global
critical condition with all-trans-retinoic acid (ATRA). With single- transcription and protein patterns affected by arsenic were compared
agent ATRA, she achieved a complete remission (CR) and remains to those altered by ATRA, it was found that while arsenic regulated
disease free to this day. Wang and colleagues subsequently treated 24 a significant number of genes also regulated by ATRA, the total
patients with APL and were able to improve the CR rate to 96% with number of affected genes was much less. Arsenic altered a more sig-
ATRA alone. In addition to high CR rates, ATRA has improved nificant change in protein patterns than ATRA, suggesting that its
long-term outcomes, with 5-year EFS rates as high as 85% in adults main mechanism was through protein alterations as opposed to gene
and 91% in children. This remarkable response rate to ATRA was expression modulation.
specific for APL, leading researchers to explore the retinoic acid
receptor (RAR) in this population. In the early 1990s, several groups
simultaneously demonstrated that the translocation that characterizes Acute Megakaryocytic Leukemia
this malignancy encodes a chimeric protein fusing the PML gene with
the RAR (RARA) gene. In contrast to the expected paradigm, whereby AMKL is a subtype of AML characterized by abnormal megakaryo-
molecular biology leads to the development of a targeted agent, blasts that express platelet-specific surface glycoprotein. Bone marrow
ATRA was found to be effective prior to the understanding of the biopsy frequently demonstrates extensive myelofibrosis, often making
mechanism. In studying the response of APL to this agent, much of aspiration in these patients difficult. AMKL is rare in adults, occur-
the biology of APL has come to light, demonstrating the constant ring in only 1% of AML patients, but comprises between 4% and
interplay that exists between science and medicine. 15% of childhood AML cases. In pediatrics, the disease is divided
Greater than 98% of patients with APL carry t(15;17)(q22q1221), into two major subgroups: AMKL in patients with Down syndrome
which fuses the PML gene with the RARA gene to create the chimeric (DS-AMKL) and AMKL in patients without Down syndrome (non-
oncogene PML-RARA. Retinoid signaling is relayed by two families DS-AMKL). AMKL is the most frequent type of AML in children
of nuclear receptors, the RARs and the retinoid X receptor, that with Down syndrome, and the incidence in these patients is 500-fold
together form heterodimers. In the absence of retinoic acid, these higher than in the general population. Somatic mutations in GATA1
heterodimers bind to target gene promoters and repress transcription are found in almost all cases of DS-AMKL and precede the develop-
through the recruitment of NCoR, SMRT), and HDAC corepres- ment of leukemia, as indicated by their presence in patients with
sors. When retinoic acid binds, a conformational change allows the transient myeloproliferative disease (TMD) in the neonatal period.
recruitment of coactivators and histone acetyltransferases, resulting Pediatric non-DS-AMKL is a heterogeneous group of patients, a
in activation of transcription. RARA is highly expressed in myeloid significant proportion of which carry chimeric oncogenes including
cells, and activation of its transcriptional targets promotes granulo- RBM15-MKL1, CBFA2T3-GLIS2, NUP98-KDM5A, and MLL gene
poiesis. PML-RARA expression impairs normal responses to retinoic rearrangements.
acid as the fusion protein binds to transcriptional corepressors and DS-AMKL is associated with a hematologic disorder in infancy,
histone deacetylases with a higher affinity than wild-type RARA. termed TMD. In this disorder, a clonal population of megakaryo-
In addition to the effects on transcription, the fusion gene alters blasts accumulates in the peripheral blood. These blasts are pheno-
localization of PML. In wild-type cells, PML is localized in discrete typically indistinguishable from AMKL leukemic blasts, and in the
nuclear subdomains, called nuclear bodies, which likely play a role in majority of cases remission is spontaneous within 3 months in the
senescence, growth control, telomere lengthening, and DNA repair. absence of treatment. In approximately 20% of TMD cases patients
These nuclear bodies are disrupted by PML-RARA in a manner will develop MDS or AMKL. TMD is felt to originate in utero, as
that is reversible by treatment, suggesting that they are important mutations in GATA1, the genetic lesion associated with TMD, have
in apoptosis and growth control. PML is in fact a p53 target gene, been found to be present at birth in patients that suffered from TMD.
and regulates p53 stability by sequestering Mdm2, a negative Exome sequencing of TMD has revealed that nonsilent mutations in
regulator of p53, to the nucleolus, thus providing a mechanism for these blasts are primarily limited to the GATA1 gene. In contrast,
this phenotype. PML contains a sumoylation site that is present AMKL blasts carry a higher burden of mutations, with additional
on the PML-RARA chimeric protein and is required for leukemic lesions in epigenetic and kinase-signaling genes leading to progression
transformation. of the disease. Collectively, these findings support a model whereby
The mechanism whereby retinoic acid induces differentiation of TMD blasts arise secondary to GATA1 mutations, acquiring this
APL cells carrying PML-RARA was delineated following the demon- so-called first hit and persist in the bone marrow. Additional lesions
stration of its clinical efficacy. Retinoic acid binds to the hormone- can then occur, providing the cooperating events that are necessary
binding site of PML-RARA, inducing a conformational change that for full blown leukemia to develop.
triggers co-repressor release and co-activator recruitment, opening up The GATA proteins are transcription factors, three of which are
the chromatin structure and relieving the transcriptional repression. expressed principally in hematopoietic cells (GATA1, GATA2, and
Global transcription and protein alterations following treatment with GATA3). GATA1 is required for the development of erythrocytes,
retinoic acid are significant for a large number of genes involved megakaryocytes, eosinophils, and mast cells. Mutations detected in
in granulocyte differentiation, such as CEBPs, cytokines, cytokine DS patients with AMKL consist of short deletions, insertions, and
receptors, and molecules downstream of cytokine signaling. Another point mutations within exon 2 that introduce a premature stop
effect of ATRA is to induce proteasome degradation of the chimeric codon. This shorter mutant protein retains the ability to bind DNA
oncogene. and interact with its cofactor, but lacks the transcriptional activation
ATRA is not the only targeted agent for APL. Ai-ling 1, a Chinese domain and hence has reduced transactivation potential. GATA1 is
remedy historically used to treat a variety of illnesses, was tested by able to activate lineage specific genes and repress progenitor mainte-
a group from Harbin Medical University in the 1970s in more than nance genes depending on the cofactors present. Deregulation of
1000 patients with different cancers and was found to induce remis- these targets contributes to the differentiation arrest seen with the
sions in approximately two-thirds of patients with APL. Zhu Chen, truncated GATA1 that is no longer able to transactivate transcription
a scientist in Shanghai, collaborated with the Harbin team in 1994 of lineage-specific genes. Given that only 20% of TMD progresses to
to demonstrate that the effective component of this remedy was leukemia, what then are the subsequent events or alterations that
arsenic (III) trioxide. Importantly, arsenic was able to induce remis- promote the preleukemic state to that of a fully transformed malig-
sions in patients who had failed ATRA and conventional chemo- nancy? Exome and targeted sequencing of 46 genes has provided
therapy. Subsequently, many groups confirmed these findings and the insight to this question, identifying recurrently mutated genes in
mechanism of action has been under study ever since. At high con- three major categories: cohesin, epigenetic regulators, and signaling
centrations, arsenic induces apoptosis of APL, while low doses molecules. These include the cohesin complex genes STAG2, RAD21,
promote maturation and differentiation. Arsenic treatment causes SMC3, SMC1A, NIPBL and CTCF; PRC2 complex genes EZH2 and