Cellular
and Molecular Basis of Cancer Development
Any
hyper- or hypo-proliferation of cells is potentially a cell cycle
disease, and the most obvious of such uncontrolled cell proliferation
is cancer. Cancer or malignant tumor differs from benign
tumor as the former is not encapsulated, and it often invades
neighboring tissues or metastases to distant parts of the body
from its site of origins. However, the most important difference
is that cancer is potentially lethal.
It is now known that cancer is a genetic disease. The genes that
have been implicated in cancer development (also termed as carcinogenesis)
are divided into two broad categories: oncogenes and tumor-suppressor
genes.
There
are some genes in a normal cell that encode proteins for various
functions, and these genes are called proto-oncogenes. Mutations
of these genes will produce proteins that promote the loss of
growth control and the conversion of a cell to a malignant state.
These mutated genes are termed as oncogenes, which act as accelerators
of cell proliferation and carcinogenesis. Tumor-suppressor genes,
on the other hand, act as a cell's brakes as they encode proteins
that restrain cell growth, and prevent cells from becoming malignant.
Most of the oncogenes encode uncontrollable mediators or messengers
of an otherwise normal signal transduction pathway, which allows
the cell to divide properly in response to the external conditions,
such as the presence of growth factors. On the contrary, tumor-suppressor
proteins may inhibit progression of cell cycle, induce the enzymes
required for repairing the damaged DNA, and induce cell suicides
when their damages are not recoverable (also called programmed
cell death or apoptosis).
Very often since the above cancer-causing genetic alterations
arise in the DNA of a somatic cell during the lifetime of an affected
individual, cancers in most cases are not inheritable.
