November 22, 2002
Researchers Identify Cause of Aggressive Childhood Cancer
Researchers have generated a mouse model of a new type of tumor
suppressor gene that triggers a rapidly advancing cancer that affects
children. The discovery of the fast-onset cancers that result from
inactivation of the gene and the technique used to generate the model
will likely prove useful in studying genes involved in other forms of
cancer.
The research team, which was led by Howard Hughes Medical Institute
investigator Stuart H.
Orkin, and former HHMI physician postdoctoral fellow Charles
Roberts, reported its findings in the November 2002 issue of the
journal Cancer Cell. Orkin and Roberts are at the Dana-Farber
Cancer Institute, Children’s Hospital, Boston, and Harvard Medical
School.
“It will be especially important to link this tumor suppressor with a known pathway of tumorigenesis.”
Stuart H. Orkin
The tumor suppressor gene, called SNF5, codes for a protein
that is a component of a large complex called SWI/SNF that attaches to
chromatin to regulate the expression of genes. Chromatin is the complex
of DNA and proteins in the nucleus of the cell.
“There has been indirect evidence that some types of chromatin
remodeling complexes might play a role in cancer,” said Roberts.
In a key finding reported in 1998, French researchers showed that
mutations that inactivated SNF5 were present in tissue samples
from children with malignant rhabdoid tumors. “That’s what
first caught our interest, that we might be dealing with a new type of
tumor suppressor,” said Roberts. Malignant rhabdoid tumors are
rare but highly aggressive cancers that usually appear in infancy.
These tumors are resistant to treatment and usually cause death within
a year of diagnosis.
With the initial evidence that SNF5 was involved in such
tumors, Roberts, Orkin and their colleagues set out to establish in
mice that loss of SNF5 did indeed produce cancers. The problem,
said Roberts, was that the usual methods for knocking out the gene did
not produce a useful model of rhabdoid tumors in the mice.
“Mice that are deficient in SNF5 die very early in
embryonic development, and therefore cannot be used to analyze for
cancer,” he said. “And mice that lack only one of the two
genes show a relatively low prevalence of tumors, with a median onset
of about twelve months.” Thus, said Roberts, while these mouse
models did demonstrate that SNF5 was necessary for development,
and that its loss caused cancer, such mice could not be used to analyze
how SNF5 loss affected the development of this form of
cancer.
To construct a more useful model, the scientists turned to a
“conditional targeting approach” that enabled them to knock
out SNF5 in some mouse cells but not others. This approach
involved engineering the mice so that the SNF5 gene would
function normally throughout development, but could later be knocked
out in adult mice by the introduction of a triggering chemical. This
trigger chemical activates an enzyme that excises the gene under
study.
Deletion of SNF5 in the mice revealed that SNF5 was required for the
survival of adult mice and, in fact, for survival of virtually all
normal cells. In order to circumvent the lethality and generate a
working cancer model, Roberts and Orkin took the conditional targeting
a step further. They engineered the knockout system so that instead of
being snipped out, the SNF5 gene would randomly invert in the
process of being knocked out. In some cells, the gene would assume a
normal orientation after triggering, and in others it would be
inverted, and thus nonfunctional.
“This was an adaptation of a technique that researchers
Kong-Peng Lam and Klaus Rajewski had used to study lymphoid cells, but
it had not been applied to cancer modeling,” said Orkin.
“The trick was to make the gene we wanted to delete, instead of
being excised, to flip back and forth and then randomly settle in
either the active or inactive orientation.”
By employing this technique, Orkin and Roberts created mice whose
tissues had a delicate balance of cells with normal and inactivated
SNF5 genes. There were enough cells with normal SNF5 to
allow the mice to live longer, but enough with inactivated SNF5
genes to give rise to cancers. According to Orkin, the mice engineered
to have the “reversible, inverting conditional” knockout
genes showed immediate onset of cancers. Most of the mice developed
malignant lymphomas, or cancers of the blood cells, while many also
developed rhabdoid tumors.
“The fact that the mice showed consistent oncogenesis in a
very short time means that we can crossbreed the animals with other
genetically altered mice to sort out the cellular pathways that are
affected,” said Orkin. “It will be especially important to
link this tumor suppressor with a known pathway of tumorigenesis.
Ultimately, if we know what pathway is affected, we can target
therapies to that pathway.” Orkin and his colleagues believe that
the reversible knockout technique could be applied generally to aid the
study of other tumor suppressor genes in which complete deletion of the
gene proves lethal.
According to Roberts, understanding the mechanism of
SNF5-related cancers could have significant clinical impact.
“There have been many papers showing the role of SNF5 loss
in human cancers,” he said. “It’s clear that the gene
is involved in malignant rhabdoid tumors and that it may be involved in
certain other aggressive cancers in early childhood. This work has led
us to realize the existence of an entirely novel tumor suppressor
pathway, the SWI/SNF complex of which SNF5 is a core member. And, we
believe that understanding the basic genetics, biochemistry and
molecular biology of SWI/SNF, is likely to generate significant new
understanding, and potentially therapies, for many types of human
cancer.”
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