Mainz: In leukemia cells it is often the case that genes are reactivated that, in physiological terms, mediate the self-renewal of blood stem cells. In a common subtype of acute myeloid leukemia, this abnormal activation of such self-renewing genes is apparently caused by structural modifications of the DNA packaging. In turn, these modifications are caused by two specific proteins of the so-called chromatin regulator group, on which leukemia cells are dependent. These discoveries were made by oncologist Dr. Michael Kühn from the Department of Internal Medicine III, which is a part of the University Center for Tumor Diseases (UCT) at the Mainz University Medical Center, in a collaborative effort with researchers from the Memorial Sloan-Kettering Cancer Center in New York and Harvard University in Boston.
researchers were able to demonstrate that a targeted drug-based
inactivation of the two chromatin regulators will interrupt the
self-renewing program, thereby causing leukemia cells to revert to
normal blood cells. The results have been published in the October issue
of Cancer Discovery.
Acute myeloid leukemia (AML) refers to a group
of disorders that are also known as blood cancer. AML is an aggressive
disease of malignant immature blood cells which, if left untreated,
almost always causes the death of the affected patient. The established
method of treatment is the use of a combination of various
chemotherapeutic agents. However, dependent on the genetic subtype and
the age of the patient, only about half of those with AML respond to
this kind of treatment.
The goal of current research is thus to develop
more efficient and less toxic forms of treatment. To achieve this, Dr.
Michael Kühn of the University Medical Center of Johannes Gutenberg
University Mainz (JGU) has been collaborating with the work groups of
Professor Scott Armstrong in New York and Boston. They built on the
relatively recent scientific discovery that changes to the "packaging
structure" of DNA can contribute to the development of cancers. These
chemical modifications particularly occur in the so-called histone
proteins. These proteins are responsible for the coiling of DNA in
mammalian cells. Various chemical modifications of these histone
proteins will result in an increase or decrease in the relevant gene
activity. DNA wrapped around histones is also called chromatin.
Accordingly, the proteins writing, reading, or removing the chemical
modifications of histones are called chromatin regulators.
These modifications represent a layer of
information that can be passed from a parent cell to a daughter cell but
is not encoded in the DNA sequence. This field of research is therefore
known as "epigenetics". Medical research focusing on epigenetics is
currently trying to block the enzymes that regulate these changes
thereby silencing cancer promoting genes. One example of such research
is the study undertaken by Dr. Michael Kühn and his colleagues. Its
subject is the NPM1-mutated (NPM1mut) AML subtype, which is one of the
most common leukemias in adults under the age of 60 years.
It has been known for quite some time that
NPM1mut AMLs are associated with the activation of the so-called
homeobox (HOX) stem cell genes. The HOX genes play a fundamental role in
the developmental processes of organisms. They are particularly
responsible for the self-renewal of blood stem cells. It has been
assumed that activation of HOX genes turns normal blood cells into
leukemia cells by initiating stem cell-like self-renewal. However, it
has been unclear to date how this activation occurs. In an attempt to
answer this question, the researchers undertook a targeted manipulation
of leukemia cell DNA in the lab. Using a relatively new technology
called CRISPR/Cas9, they managed to accurately cut out specific DNA
sequences from leukemia cells. This enabled them to analyze the
functioning of two proteins, namely, the mixed lineage leukemia (MLL)
protein and the disruptor of telomeric silencing 1-like (DOT1L) protein.
Based on these experiments, the researchers were
able to demonstrate that the survival of NPM1mut leukemia cells depends
on these two proteins. Both proteins belong to a group of regulators
that control chromatin and thus an important structural component of the
cell nucleus. The researchers then used two highly specific chemical
agents to block the specific functions of those proteins. While they
were able to block DOT1L directly using an inhibitor substance currently
being tested in a clinical trial for a different type of leukemia, a
direct drug-based inhibition of MLL proved impossible. The researchers
therefore inhibited chromatin binding of MLL using drugs that target
this protein indirectly.
Both drugs reduced the activity of the homeobox
stem cell genes in NPM1mut leukemia cells, while the combination of the
two compounds resulted in nearly complete inactivation of these genes.
Following combined exposure to the two substances, the leukemia cells
underwent substantial changes and, to the surprise of the researchers,
started to turn back into normal blood cells.
The described approach represents the first
molecularly targeted treatment of NPM1mut leukemias by reversing a key
mechanism of leukemogenesis and builds a basis for future clinical
trials assessing these drugs in patients with NPM1mut leukemia.