A full-fledged cancer cell generally develops from more than one somatic mutation, and this may explain why cancer occurs mostly on older people. If cancer develops from the aggregation of mutations and if mutations occur all over life, then the longer we live, the higher the chance of developing a cancer.
The model of multi-step path to cancer is very well supported by studies of one of the most understood types of cancer, colorectal cancer. This type of cancer affects the colon and the rectum. Each year, there are more than 100,000 new cases of colorectal cancer in the United States, and the disease causes 50,000 deaths per year. Colorectal cancer develops gradually like most other cancers. A small benign growth in the colon lining called polyp is often the first sign. Although polyp cells divide unusually often, they look normal. Eventually, the tumor becomes malignant and invades the other tissues. Gradual aggregation of mutations that turns normal genes into oncogenes (genes that potentially causes cancer) is paralleled by the development of a malignant tumor. The genes often involved in the cancer development are the ras oncogene and a mutated p53 tumor-suppressor gene.
At DNA level, about half a dozen changes must occur before a cancer cell develops. These changes usually include the activation of at least one oncogene and the mutation or loss of some tumor-suppressor genes. Additionally, mutated tumor-suppressor alleles are usually recessive, and most of the time, mutations must knock out both alleles in the genome of a cell to block tumor suppression. On the other hand, most oncogenes behave as dominant alleles.
Routine screenings such as colonoscopy are recommended to recognize and remove any suspected polyps. For the past 20 years, colorectal cancer mortality rate has been declining due to increased screening and better treatments. There are also improvements in treating other types of cancer. Medical researchers compare the genes expressed by various types of tumors and by the same type in various people with the advances in DNA and mRNA sequencing. The comparisons have resulted in the idea that every cancer is unique and requires personalized treatments based on the molecular characteristics of the tumor.
The second most common form of cancer in the United States is the breast cancer. Each year, breast cancer strikes more than 200,000 women and some men in the United States and kills 40,000 people. The worldwide mortality for this cancer is more than 400,000. Understanding breast cancer is difficult due to its diverse characteristics. To improve treatments and decrease mortality rate, it is important to identify the differences between the types of breast cancer. In 2012, four major types of breast cancer were identified based on their molecular signatures. It is now routine to screen for the presence of specific signaling receptors in any breast cancer tumors. Breast cancer patients and their doctors can now make more informed decisions about their treatments.
Urry, Lisa A.. Campbell Biology (p. 389). Pearson Education. Kindle Edition.