A Secondary Syndrome: Acute Myeloid Leukemia
"Secondary" acute myeloid leukemia (sAML) arises in approximately one third of patients with myelodysplastic syndrome (MDS). sAML is generally resistant to traditional chemotherapy and overall survival is universally poor.
As the average age of our population increases, the incidence of sAML in Maine will continue to grow. In our local community, Cancer Care of Maine estimates a 3-year survival rate of MDS patients at only 17%, while the national survival rate is 35%. This underscores the immediate need in Maine for research to identify the mechanisms driving MDS-tosAML progression, for the development of better diagnostics and therapies. Furthermore, developing improved, non-invasive diagnostics will allow better management and care of MDS patients in Maine that may reside in more rural locations without easy access to specialists.
Two significant unmet needs in clinical management of MDS patients are:
reliably identifying patients at high risk of progressing to sAML, and
- having effective therapeutic strategies in-hand to prevent or delay progression to sAML.
A lack of knowledge of the mechanisms underlying MDS-to-sAML progression has hindered efforts to develop more effective therapies. The goal of this pilot project is to identify specific mechanisms driving MDS-to-sAML progression, which will allow development of biomarkers to stratify patients at high risk and new therapies to prevent this progression. The incidence of MDS rises steeply with age. In the State of Maine, MDS occurs in ~30 per 100,000 people at age 70 years and beyond.
MDS-to-sAML progression is characterized by a conversion from cytopenia (lack of circulating peripheral blood cells), to leukemia (uncontrolled proliferation of circulating peripheral blood cells) (Figure 1). These cellular changes are accompanied by three major alterations in bone marrow hematopoietic stem cells (HSCs): (1) marked changes in DNA methylation, (2) genomic instability, and (3) acquisition of genomic DNA mutations followed by clonal selection. The roles of these molecular alterations in MDS-to-sAML progression are poorly understood. A key question in the field is which of these alterations are responsible for driving MDS-tosAML progression ("driver" mechanisms), and which may accompany MDS-to-sAML progression but are not directly responsible for causing it ("passenger" mechanisms). This project proposes to determine the contribution of DNA methylation, genomic instability, and recurrent genomic mutations to driving the progression of MDS to sAML (Figure 1). Aim 1 will test the hypothesis that alterations in DNA methylation and chromatin structure occur in DNA double-strand breakpoint (DSB) regions in the MDS genome that drive sAML progression. Aim 2 will test the hypothesis that induction of specific mutations defined as recurrent in human MDS-to-sAML progression are drivers of this progression.
Alterations in DNA methylation and genomic instability in MDS have largely been reported as global trends identified by accumulation of data from numerous patient samples. One report examining DNA methylation and chromosomal abnormalities in the same set of MDS patient samples found cooperation between aberrant DNA methylation and genomic instability at particular genomic loci3. Furthermore, clinical data suggests that DNA methylation is one of the driving forces in MDS-to-sAML progression. Between 20-40% of MDS patients respond to the DNA methylation inhibitor 5-azacitidine, resulting in a 2-fold decrease in the rate of the progression to sAML4. As the direct relationship between DNA methylation and chromosome instability in driving MDS-to-sAML progression is unknown, I propose to elucidate this mechanistic link in Aim 1.
Whole-genome DNA sequencing of matched sAML patient samples with their antecedent MDS samples has provided a wealth of information about mutation acquisition and clonal selection in MDSto- sAML progression5. These data have identified four specific mutations in WT1, RUNX1, PTPN11 and CDH23 that are recurrent in sAML but not found in MDS samples from the same patient(s), suggesting that they may drive disease progression. I propose to directly determine the role of these mutations in driving MDS-to-sAML progression in Aim 2.
Identifying and distinguishing the driver and passenger mechanisms in MDS-to-sAML progression requires use of an experimental model system to induce specific changes and follow their functional consequences, which is not possible to do directly in patients. Accomplishing this also requires the availability of validated in vitro and in vivo model systems to assay MDS-to-sAML progression. While several genetically engineered mouse models of MDS have been developed, none accurately recapitulate human MDS-to-sAML progression6. Therefore, in this project I will collaboratively develop a novel xenograft mouse model and culture system for in vivo and in vitro examination of human MDS-tosAML progression utilizing de-identified human MDS bone marrow samples from Cancer Care of Maine.