Biomedical Engineering Reference
In-Depth Information
cells. These approaches seem logical and have worked in one or
more animal tests, but still remain unproven in humans, despite
some small human studies.
The specific genetic changes responsible for the uncontrolled
growth of cancer cells are found in oncogenes , genes everyone has
that promote the growth of tumors if the sequence changes, and
tumor suppressor genes that normally suppress the growth of tumors.
Some of the tumor suppressed genes that are changed in cancer cells
are those that normally prevent the death of cells by suicide or
trigger cell death if the cell DNA is damaged. This programmed cell
suicide is called apoptosis (Figure 7.2). Several human tumors have
mutations in the tumor suppressor genes that prevent those genes
from functioning. Delivery of normal, unchanged tumor suppressor
genes to trigger apoptosis has been promising as a cancer treatment
both in animal tests and in limited human trials. For example, an
adenovirus vector carrying the p53 tumor suppressor gene reduced
the size of the tumors when injected directly into human tumors.
Three months after treatment with the p53-carrying adenovirus,
combined with radiation, 63% of patients with a form of lung cancer
had no detectable tumors. Another gene therapy approach being
explored for cancer is delivery to tumor cells of a gene for an enzyme
protein that changes an inactive form of a drug to the active drug
that kills the tumor cell. The tumor cell is thus made into the engine
of its own death, which is why this type of trial is sometimes called
a “suicide gene” trial. The enzyme genes are derived from viruses or
non-mammals, such as a gene for an enzyme that converts a drug
used to fight virus infections into a drug that is toxic to cells.
Tumors are masses of cancer cells and can only survive if nutri-
ents and oxygen are provided by blood. Researchers are developing
promising drugs, including recombinant proteins, that deprive
tumors of this sustenance by preventing the development of
blood vessels at the tumor site. The growth of blood vessels is called
angiogenesis . Gene therapy strategies and vectors are being developed
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