Cancer is an abnormal growth of cells caused by multiple changes in gene expression leading to
dysregulated balance of cell proliferation and cell death and ultimately evolving into a population
of cells that can invade tissues and metastasize to distant sites, causing significant
morbidity and, if untreated, death of the host.
Cancer is a group of diseases of higher multicellular organisms. It is characterized by alterations
in the expression of multiple genes, leading to dysregulation of the normal cellular program for
cell division and cell differentiation. This results in an imbalance of cell replication and cell death
that favors growth of a tumor cell population. The characteristics that delineate a malignant
cancer from a benign tumor are the abilities to invade locally, to spread to regional lymph nodes,
and to metastasize to distant organs in the body. Clinically, cancer appears to be many different
diseases with different phenotypic characteristics. As a cancerous growth progresses, genetic
drift in the cell population produces cell heterogeneity in such characteristics as cell antigenicity,
invasiveness, metastatic potential, rate of cell proliferation, differentiation state, and
response to chemotherapeutic agents. At the molecular level, all cancers have several things
in common, which suggests that the ultimate biochemical lesions leading to malignant
transformation and progression can be produced by a common but not identical pattern of
alterations of gene readout. In general, malignant cancers cause significant morbidity and will be
lethal to the host if not treated. Exceptions to this appear to be latent, indolent cancers that may
remain clinically undetectable (or in situ), allowing the host to have a standard life expectancy.
Some points in the description may not seem intuitively obvious. For example, cancer doesn’t
just occur in humans, or just mammals for that matter. Cancer (or at least tumorous growths—
these may or may not have been observed to metastasize) has been observed in phyla as old as
Cnidaria, which appeared almost 600 million years before the present, and in other ancient
phylasuchasEchinodermata(>500millionyears old), Cephalopoda (500 million years old), Amphibia
(300 million years old), and Aves (150 million years old). Curiously, cancer has never
been seen (or at least reported) in a number of phyla such as Nematoda, Tradigrada, and Rotifera.
It is intriguing to consider that these organisms may have some protective mechanisms
that prevent them from getting tumors. If so, it would be important to find out what these
mechanisms are. One thing is clear, though, which is that
cancer is a disease of multicellular organisms.
