What Causes Cancer: Genetics, Environment, and the Biology of Malignant Disease

What Is Cancer?

Cancer is not a single disease but a broad collection of more than 100 distinct conditions, all characterized by the uncontrolled growth and division of abnormal cells. Under normal circumstances, cell division is tightly regulated by genetic instructions that govern when a cell grows, divides, and dies. Cancer develops when these regulatory systems break down — due to accumulated mutations in a cell's DNA — allowing cells to divide unchecked, invade surrounding tissue, and potentially spread to other parts of the body through a process called metastasis.

According to the World Health Organization (WHO), cancer is the second leading cause of death globally, responsible for approximately 10 million deaths per year. The most common cancers worldwide are breast, lung, colon and rectum, prostate, skin, and stomach cancers. This article is for general educational purposes. Consult a qualified healthcare provider for personal medical guidance.

The Molecular Basis of Cancer

At the cellular level, cancer begins with mutations — changes in the DNA sequence of a cell's genome. A single mutation is rarely sufficient to cause cancer; typically, cancer develops after a cell accumulates multiple mutations across key genes over a period of years to decades.

Key Genes Involved in Cancer

Three major categories of genes, when mutated, contribute to cancer development:

• Proto-oncogenes: Genes that normally promote cell growth and division. When mutated into oncogenes, they become permanently or excessively active, driving uncontrolled proliferation. A well-known example is the RAS gene family, mutated in approximately 25% of all human cancers.
• Tumor suppressor genes: Genes that normally restrain cell division, repair DNA damage, or initiate programmed cell death (apoptosis) when damage is irreparable. Cancer arises when both copies of a tumor suppressor gene are inactivated. The most studied examples are TP53 (mutated in more than 50% of all cancers) and RB1 (the first tumor suppressor gene identified).
• DNA repair genes: Genes responsible for detecting and correcting errors that arise during DNA replication. Mutations in these genes allow errors to accumulate throughout the genome. Mutations in BRCA1 and BRCA2, for example, impair repair of double-strand DNA breaks and significantly increase the risk of breast and ovarian cancers.

Hallmarks of Cancer

In their landmark 2000 paper in the journal Cell, biologists Douglas Hanahan and Robert Weinberg described the core capabilities that a cell must acquire to become malignant — now known as the "Hallmarks of Cancer." These include:

• Sustaining chronic proliferative signaling
• Evading growth suppressors
• Resisting cell death (apoptosis)
• Enabling replicative immortality (via telomerase activation)
• Inducing formation of new blood vessels (angiogenesis)
• Activating invasion and metastasis
• Reprogramming energy metabolism
• Evading immune destruction

Major Risk Factors

Cancer risk arises from an interplay of genetic susceptibility, lifestyle, and environmental exposures. The majority of cancers are driven by acquired (somatic) mutations accumulated over a lifetime, rather than inherited mutations.

Tobacco

Tobacco smoking is the single largest preventable cause of cancer globally. It causes approximately 22% of all cancer deaths and is causally linked to cancers of the lung, mouth, throat, esophagus, stomach, pancreas, kidney, bladder, cervix, and more. Cigarette smoke contains more than 70 known carcinogens, including benzene, formaldehyde, polycyclic aromatic hydrocarbons, and nitrosamines. Lung cancer, the deadliest cancer worldwide, is caused by smoking in approximately 85% of cases.

UV Radiation and Ionizing Radiation

Ultraviolet (UV) radiation from the sun and tanning beds is the primary cause of skin cancers, including melanoma, basal cell carcinoma, and squamous cell carcinoma. UV radiation damages DNA by causing pyrimidine dimers — abnormal bonds between adjacent bases in the DNA strand. Ionizing radiation (X-rays, gamma rays, radon gas) can cause double-strand DNA breaks and is an established risk factor for leukemia, thyroid cancer, and other malignancies.

Infectious Agents

Approximately 13% of cancers globally are attributable to infectious agents:

Alcohol

Alcohol consumption is a Group 1 carcinogen (causally linked to cancer in humans), associated with cancers of the mouth, pharynx, larynx, esophagus, liver, colon, rectum, and breast. Risk increases with the amount consumed; there is no established safe threshold for cancer risk.

Obesity and Physical Inactivity

Excess body fat is a risk factor for at least 13 types of cancer, including endometrial, esophageal, kidney, liver, pancreatic, colorectal, postmenopausal breast, and others. Adipose (fat) tissue produces hormones including estrogen and insulin-like growth factor-1 (IGF-1) that can promote cell proliferation. According to the International Agency for Research on Cancer (IARC), overweight and obesity are the second largest preventable cause of cancer after tobacco.

Genetic Predisposition

An estimated 5–10% of cancers are primarily attributable to inherited germline mutations. Well-characterized hereditary cancer syndromes include:

• BRCA1/BRCA2 mutations: Increase lifetime risk of breast cancer to 45–72% and ovarian cancer to 17–44%
• Lynch syndrome (HNPCC): Mutations in mismatch repair genes; 50–80% lifetime risk of colorectal cancer
• Familial adenomatous polyposis (FAP): Nearly 100% lifetime risk of colorectal cancer if untreated
• Li-Fraumeni syndrome: TP53 germline mutations; broad cancer susceptibility across multiple organ systems

How Cancer Spreads: Metastasis

Metastasis — the spread of cancer from its site of origin to distant organs — is responsible for the majority of cancer deaths. The process involves several steps:

1. Local invasion through the basement membrane into surrounding connective tissue
2. Entry into blood vessels or lymphatic vessels (intravasation)
3. Survival in circulation
4. Exit from blood vessels at a distant site (extravasation)
5. Formation of a micrometastasis, then a clinically detectable secondary tumor

Different cancers have characteristic patterns of metastatic spread. Prostate cancer frequently metastasizes to bone; lung cancer to the brain, adrenal glands, and liver; breast cancer to bone, brain, lung, and liver.

Cancer Incidence by Type (Global, 2022)

Source: IARC GLOBOCAN 2022.

Cancer Treatment Approaches

Treatment depends on cancer type, stage, molecular characteristics, and patient health status:

• Surgery: Physical removal of the tumor; most effective for localized disease.
• Radiation therapy: High-energy rays kill cancer cells by damaging their DNA, preventing replication.
• Chemotherapy: Drugs that target rapidly dividing cells; effective but often toxic to healthy proliferating cells.
• Targeted therapy: Drugs designed to block specific molecular drivers of a cancer (e.g., imatinib targeting the BCR-ABL fusion protein in chronic myelogenous leukemia; trastuzumab targeting HER2-positive breast cancer).
• Immunotherapy: Strategies that harness or enhance the immune system's ability to fight cancer. Immune checkpoint inhibitors (e.g., pembrolizumab, nivolumab) block proteins like PD-1 and CTLA-4 that cancer cells exploit to evade immune destruction. This approach has transformed outcomes for melanoma, lung cancer, and others.
• CAR-T cell therapy: A patient's own T cells are genetically engineered to express chimeric antigen receptors (CARs) targeting cancer cells. Currently approved for several blood cancers, with clinical trials ongoing for solid tumors.

Advances in genomic sequencing, liquid biopsy (detecting tumor DNA in blood), and artificial intelligence-assisted diagnostics are continuing to improve early detection rates and treatment precision — steadily improving survival outcomes for many cancer types.