What Are Germline (Inherited) and Somatic Mutations? What Is Their Role in Cancer Development?

• The genetic basis of cancer is highly important for developing treatment strategies and understanding individuals' cancer risks.
• Cancer usually develops as a result of genetic mutations that accumulate in cells. These mutations, or genetic changes, cause cells to escape their normal control mechanisms and divide uncontrollably.
• Genetic mutations are considered in two main groups: germline (inherited) and somatic mutations. Each contributes to cancer development through different mechanisms.
• Germline mutations are inherited and present throughout all cells, whereas somatic mutations arise during a person's lifetime and affect only specific groups of cells.

This article comprehensively examines what germline and somatic mutations are, how they arise, and how they contribute to cancer development.

What Are Germline Mutations?

Germline mutations are mutations that occur in an individual's reproductive cells (sperm or egg cells) and can therefore pass directly to the next generation. They have the following characteristics:

1. They Are Inherited: A parent carrying a germline mutation can pass it on to their children. This may result in multiple family members carrying the same mutation.
2. They Are Present in All Cells: These mutations are present in all cells from the formation of the zygote. Thus, all body cells in individuals carrying them contain the same mutation.
3. They Increase Cancer Risk: Germline mutations may predispose individuals to certain cancers. For example, germline mutations in genes such as BRCA1 and BRCA2 significantly increase the risk of breast and ovarian cancer.

How Germline Mutations Contribute to Cancer Development

Inherited mutations can cause “cancer predisposition syndromes” that increase susceptibility to cancer. Examples include:

• Li-Fraumeni Syndrome: An inherited mutation in the TP53 gene is responsible for this syndrome and creates a predisposition to various cancers.
• Hereditary Breast and Ovarian Cancer Syndrome: Mutations in the BRCA1 and BRCA2 genes are among the main causes of hereditary breast and ovarian cancers.

These mutations may cause cancer to develop at an earlier age and show different biological characteristics. When germline mutations contribute to cancer development, patients may have more than one primary cancer type.

What Are Somatic Mutations?

Somatic mutations can occur in any cell in the body during an individual's lifetime. Unlike germline mutations, they are not inherited and affect only a specific group of cells. Their main characteristics are:

1. They Are Not Inherited: Somatic mutations affect only the individual and are not passed on to their children.
2. Their Effects Are Local: These mutations are present only in the cell in which the mutation occurs and the cells produced by its division. For example, mutations caused by UV light in skin cells persist only in those cells.
3. They Can Result from Various Environmental Factors: Environmental factors such as UV light, radiation, and chemicals can trigger somatic mutations. The carcinogens in cigarette smoke can cause somatic mutations in lung cells, leading to lung cancer.

How Somatic Mutations Contribute to Cancer Development

Somatic mutations are a major driving force in cancer. Most cancer cases arise through the accumulation of somatic mutations. By causing loss-of-function or gain-of-function mutations in genes controlling the cell cycle, somatic mutations lead to uncontrolled cell division. These mutations may include:

• Gain-of-Function Mutations in Proto-Oncogenes: Mutations in proto-oncogenes make proteins that promote cell growth more active, leading to uncontrolled cell proliferation. For example, mutations in the RAS gene can initiate cancer development by continuously promoting cell growth.
• Loss-of-Function Mutations in Tumor Suppressor Genes: Mutations in tumor suppressor genes cause cells to lose their normal growth control mechanisms. For example, mutations in the TP53 tumor suppressor gene prevent cells from undergoing apoptosis, setting the stage for cancer to spread.

The image above explains how germline and somatic mutations arise and affect different cell types in the human body. On the left, germline mutations are shown passing to the next generation through reproductive cells (egg and sperm). Mutations in reproductive cells become present in the zygote following fertilization and spread to all body cells. Somatic mutations are shown on the right. They arise in body cells during a person's lifetime and affect only the group of cells in which the mutation occurs. These cells multiply through mitosis, but the mutations are not passed to the next generation.

Comparison of Germline and Somatic Mutations

Interaction Between the Two Types of Mutation in Cancer Development

Germline and somatic mutations can work together to contribute to cancer development. The Two-Hit Hypothesis describes a mechanism in which the first “hit” is present in the individual as an inherited mutation, while the second “hit” is a somatic mutation that occurs during life. For example, an inherited mutation in the Rb gene in a child who develops retinoblastoma (an eye tumor) is the first hit. However, for cancer to begin, a somatic mutation must develop in the second copy of the same gene. Together, these two mutations disrupt cell-cycle control and initiate tumor formation.

The Effect of Environmental Factors on Somatic Mutations

Environmental factors such as cigarette smoke, radiation, and UV light increase the somatic mutation rate, raising cancer risk. Particularly in cancers linked to environmental factors, such as lung cancer, damage to DNA repair mechanisms also accelerates the accumulation of somatic mutations in cells. This accumulation triggers the driver mutations needed for cancer development.

The Importance of Germline and Somatic Mutations in Cancer Treatment

Distinguishing between germline and somatic mutations is crucial in cancer treatment. Detecting germline mutations is critical for determining individuals' cancer risk and encouraging those with familial cancer risk to take preventive measures. Somatic mutation analysis, on the other hand, is used to identify treatment options such as targeted therapies (“smart drugs”) and immunotherapies. For example, patients with lung cancer carrying a somatic mutation in the EGFR gene may benefit from medicines targeting that mutation.

How Are Germline and Somatic Mutations Tested?

Genetic tests are now highly important for identifying germline and somatic mutations in cancer treatment.

• Germline mutations are generally detected using cancer predisposition panels , which assess individuals' risk of hereditary cancer syndromes. Blood or saliva samples are usually collected for these tests. Because germline mutations are present in all cells, DNA does not have to be isolated solely from tumor tissue. For example, BRCA1/2 gene tests are widely used to determine breast and ovarian cancer risk.
• Somatic mutations are preferably identified using methods such as tumor biopsy and liquid biopsy . By identifying the mutations present in cancer cells, these tests enable the selection of suitable treatment strategies for targeted therapies (such as EGFR inhibitors and ALK inhibitors ) and immunotherapies. Treatment can thus be personalized, making it more effective and specific.
• The most commonly used techniques to detect both germline and somatic mutations are based on Next-Generation Sequencing (NGS) and PCR (Polymerase Chain Reaction) . NGS enables the rapid and comprehensive simultaneous sequencing of a large genetic region or many genes, allowing inherited mutations associated with cancer risk to be detected. PCR-based methods are used to examine mutations in particular genes or variants in a more targeted manner.
• In addition, Sanger sequencing is often used to detect germline mutations for confirmation or for analyzing smaller genetic regions.

Conclusion

Germline and somatic mutations play an important role in the genetic basis of cancer, each contributing to its development in different ways. Germline mutations cause hereditary cancer syndromes that carry cancer risk across generations, while somatic mutations accumulate during life under the influence of environmental factors and form the basis of cancer development. Understanding cancer genetics is indispensable for assessing individuals' cancer risk, developing early diagnostic methods, and providing more effective treatment strategies. The interaction between these mutations and their relationship with environmental factors are important for understanding the multifaceted nature of cancer.