The first mouse model for the most
common form of infant leukemia has been created by researchers from
the University of Chicago Medicine and the Cincinnati Children's
Hospital Medical Center. Their discovery, published in the Cancer Cell, could hasten development and testing of new drug therapies.
Pro-B acute lymphoblastic leukemia (ALL) with the (4;11)
translocation is responsible for about 70% of infant and 10% of both childhood and adult acute lymphoblastic leukemias. The
new mouse model replicates the human genetic flaw that causes this
disease, making it much easier to study.
‘The newly developed mouse model of infant leukemia replicates the human genetic flaw that causes this disease, making it much easier to study.’
This subtype of leukemia results from a genetic fusion t(4;11),
known as a translocation. This combines parts of two separate genes. One
of those genes, MLL (short for mixed-lineage leukemia), comes from
chromosome 11. The other fragment, AF4 (short for ALL fused gene) from
chromosome 4. The hybrid MLL-AF4 gene results in leukemia.
Children and adults with this disease produce vast numbers of
dysfunctional blood cells, which eventually crowd out functional cells.
MLL-AF4 leukemia has a dismal prognosis, among the worst of any subset
of acute leukemia.
"For 20 years, scientists have repeatedly tried and consistently
failed to make a model of MLL-AF4 Pro-B acute lymphoblastic leukemia,"
said Michael Thirman, Associate Professor of Medicine at the
University of Chicago. "Even though we understood the basic genetic
flaw, no one had been able create a mouse model that mimicked the human
disease, which is crucial for evaluating potential therapies."
That frustrated many researchers, who shifted their focus to test
alternative hypotheses on the causes of this leukemia or refocused their
laboratories to study different aspects of this disease.
Thirman's team, including longtime colleague Roger Luo began
working on this problem "years ago," he said, and stayed with it. They
quickly identified two hurdles.
The first was a problem with the retrovirus that scientists used to
insert the leukemia-causing gene into mouse cells. That gene, acquired
from leukemia patients, consisted of a human gene fragment from MLL
linked to the human fragment from AF4.
"We soon discovered that the virus wasn't working," Thirman
explained. "We knew that certain parts of human DNA can decrease viral
titers. So we switched from the human version of AF4 to the mouse
version, Af4, which is slightly different. This increased viral titers
That worked, but it led to hurdle two. The mice injected with virus
transporting MLL-Af4 developed leukemia, but it was the wrong kind. They
developed acute myeloid instead of acute lymphoblastic leukemia.
"Despite the use of lymphoid conditions," the study authors wrote, "no
lymphoid leukemia was observed."
Next, they collaborated with James Mulloy at Cincinnati
Children's Hospital Medical Center, whose graduate student Shan Lin
inserted the fused MLL-Af4 gene into human CD34 cells, derived from cord
or peripheral blood from volunteer donors. They transferred those cells
to mice with immune systems that permit the growth of human cells. This
time, the mice developed Pro-B ALL, identical to the leukemia found in
"The model worked perfectly," Thirman said. Within 22 weeks, all of
the mice developed exactly the same type of leukemia as observed in
Expression of MLL-Af4 in human cells "recapitulates the pro-B ALL
observed in patient with t(4:11) as shown by immunophenotype, chromatin
targeting of the fusion, nuclear complex formation, and gene expression
signatures," the authors wrote. "It mimics the disease found in humans
both phenotypically and molecularly."
"The differences in the type of leukemia that developed using mouse
versus human cells were striking," said Mulloy. "Researchers need to
consider these differences carefully when choosing which model to use to
mimic human disease. The available evidence now indicates that the
approaches are not equivalent."
They conclude that "our MLL-Af4 model will be a valuable tool to
study this most prevalent MLL-fusion leukemia with such a poor
However, there is more work to be done. "MLL fusion disease is not a
single genetic entity," the authors note. "Each has its own genetic and
biological features associated with particular fusion partners." This
highlights the need for "more models specific to each fusion. Our
MLL-Af4 model will be a valuable tool."