Causes of Cancer

The origin for many neoplasms is obscure. However, there are several theories of origin:
Environmental causes:
Chemicals: including those that are man-made (such as aniline dyes and bladder cancer), drugs
(cigarette smoke and lung cancer), and natural compounds (aflatoxins and liver cancer) which are
carcinogenic.
Oncogenic viruses: such as human papillomavirus (HPV) implicated in most squamous cell
carcinomas of cervix and anogenital squamous papillomas, Epstein-Barr virus (EBV) implicated in
African Burkitt's lymphoma, and hepatitis B virus (HBV) implicated in development of hepatocellular
carcinomas.
Radiation: including ultraviolet light that induces pyrimidine dimers in DNA and promotes skin
cancers. Ionizing radiation (such as gamma radiation) induces mutations in DNA and promotes
malignancies such as leukemia, thyroid, lung, colon, and breast cancers.
Chemical carcinogenesis
There are two steps: initiation and promotion
An initiating carcinogenic agent irreversibly damages cell DNA (it is mutagenic) to start the process.
Examples of carcinogenic initiators include: alkylating agents like cyclophosphamide, polycyclic
aromatic hydrocarbons like epoxides found in smoked foods, aromatic amines or azo dyes used in
food coloring, aflatoxins in moldy peanuts, nitrosamines in pickled foods.
A promoting agent (which may be the same as the carcinogen) then acts (reversibly) to cause
proliferation of a neoplastic cell clone, but there appears to be a "dose-threshold" concentration of
promoter below which neoplasia will not occur. Examples of promoters include: hormones such as
estrogen, drugs such as diethylstilbesterol, and chemicals such as cyclamates used as sweeteners.
Hereditary causes:
Chromosomes which have absent or defective anti-oncogenes that control growth (retinoblastoma
results from defective chromosome 13)
Obscure defects: racial predilections (American women have breast cancer more often than Japanese
women; Japanese men have stomach cancer far more often than American men).
Age: older persons have a greater propensity to develop neoplasms from lack of effective control
mechanisms.
Altered DNA:
All of the above are probably mediated by the cause, whatever it is, producing a mutation in, or
damage to, cell DNA
There can be mutations involving tumor suppressor genes (such as p53), which then fail to exert a
controlling influence upon growth activation. The majority of human neoplasms probably arise via this
mechanism.
In some cases these mutations are probably mediated by proto-oncogenes (genes which control
cellular growth) that undergo mutation to oncogenes which give rise to neoplasia. Proto-oncogenes
can be activated by point mutations, translocations, and by gene amplification.
An example of this is chronic myelogenous leukemia (CML) which is a neoplastic proliferation of
white blood cells. All cases of CML have the "Philadelphia chromosome" which is a translocation
between chromosomes 9 and 22. This translocation juxtaposes the proto-oncogene ABL with the
breakpoint cluster region (BCR) on chromosome 22. The chimeric ABL-BCR gene leads to
production of a mutant protein with enhanced tyrosine kinase activity. This protein may play a role
in regulation of cell growth in CML.
About 15 to 20% of human cancers have been linked to oncogenic activity. The ras oncogene is the
transforming gene found most frequently in human cancers.
Oncogenic viruses may bring oncogenes with them, so-called viral oncogenes (typical of RNA
containing "retroviruses" such as human T-lymphotropic viruses (HTLV's).
DNA repair mechanisms may be affected. There are DNA excision repair genes that can be mutated,
introducing genomic instability and a greater likelihood that mutations in other genes will occur to
drive oncogenesis. Examples include:
DNA mismatch repair genes: defective nucleotide "spell checker" introducing "microsatellite
instability" of tandem repeat sequences in DNA. Seen in hereditary non-polyposis colon cancer
(HNPCC)
Nucleotide excision repair genes: defective function in xeroderma pigmentosa, allowing DNA
damage from pyrimidine dimer formation induced by ultraviolet light
Growth factors such as epidermal growth factor (EGF), platelet-derived growth factor (PDGF) and
colony-stimulating factor-1 (CSF-1) assist oncogene activity. Transforming growth factor (TGF-
alpha) also promotes tumor growth.