An international team of scientists has found that a source of mounting genomic chaos, or instability, common to chronic myeloid leukemia (CML) may lie in a pool of leukemia stem cells that are immune to treatment with potent targeted anticancer drugs. The team was led by researchers from Temple University School of Medicine.
They have shown in mice with cancer that even after treatment with the highly effective imatinib (Gleevec), stem cells that become resistant to these drugs - tyrosine kinase inhibitors (TKIs) - may continue to foster DNA damage, potentially leading to disease relapse and a downward spiral to a much more deadly "blast" stage of leukemia.
Identifying a source of such genomic instability may help investigators eventually devise new treatment strategies against drug-resistant CML and other difficult-to-treat diseases.
"We showed that mice that mimic human CML are continuing to accumulate DNA damage - despite treatment with imatinib," said senior investigator Tomasz Skorski, MD, PhD, Professor of Microbiology and Immunology at Temple University School of Medicine. "This implies that whether a patient with CML is being treated or not, DNA damage can continue to accumulate and can confer resistance and possibly lead to relapse in some patients." Dr. Skorski, first author Elisabeth Bolton Gillespie, a graduate PhD student in the Department of Microbiology and Immunology, and their co-workers presented their findings December 11 at the 54th American Society of Hematology Annual Meeting and Exposition in Atlanta.
In CML, an enzyme called ABL1 goes into overdrive because of a chromosomal mix-up that occurs during blood cell development. The genes ABL1 and BCR become fused and produce a hybrid BCR-ABL1 enzyme that is always turned on. This overactive BCR-ABL1 protein drives the excessive production of white blood cells that is the hallmark of CML.
While treatment with imatinib is usually effective in controlling CML growth, it does not eradicate the disease, and small pockets of both active and dormant leukemia stem cells remain. In earlier research, Dr. Skorski and his group showed that both active and quiescent stem cells have more oxidative DNA damage than normal cells.
Tracing Sources of Instability
According to Dr. Skorski, CML in its chronic phase is marked by genomic instability, and results in mutations that cause the cancer to become resistant to TKIs, potentially leading to disease relapse. Understanding the source of such mutations and DNA damage - and instability - could be important in developing new therapies.
"We wanted to understand the mechanism of this genomic instability," Dr. Skorski said. "If stem cells keep accumulating these aberrations, they are like ticking time bombs. Eventually, if they are activated by the accumulation of a right combination of mutations, there is a relapse and/or a blast crisis, which can be fatal. Imatinib keeps the cancer at bay, but if the DNA damage repair process is overwhelmed, resistance occurs, and there may be disease progression."
To determine if TKI-resistant human leukemia stem cells are the source of genomic instability, the researchers turned to a mouse model of chronic phase CML.
"We looked at oxidative DNA damage in stem cells, quiescent stem cells and subpopulations of stem cells and found there is elevated oxidative DNA damage in not only the stem cell pool in CML-like mice but also within a sub-population of leukemia stem cells such as long- and short-term stem cells, and quiescent stem cells," said Ms. Bolton Gillespie.
The researchers subsequently examined the genomic instability within leukemic bone marrow cells, and looked for five common point mutations - alterations in single amino acids - that conferred resistance to imatinib. They also examined the cells for other types of alterations and major changes in genes, such as the loss of copies of genes. They found three of the most common point mutations in mouse cells that conferred drug resistance in animals treated with imatinib as well as in those animals that did not receive the drug.
Link to Leukemia in Patients
"We found numerous genetic aberrations that have already been associated with advanced disease in humans," Ms. Bolton Gillespie said. "We were able to mimic the same kind of oxidative DNA damage and genomic instability in this mouse model that is seen in patients."
"It was very important that we found aberrations in the mice that underwent imatinib therapy," Dr. Skorski said. "The assumption now is that even patients undergoing imatinib treatment will continue to accumulate genetic aberrations. These patients may not be safe. As a result, we feel that patients treated with imatinib may benefit from adding a drug that will stop such genetic aberrations. Ongoing research in our lab is aimed at the development of this approach."