Introduction

Cancer cells exhibit a range of extensive genetic mutations that are not present in normal cells. Genetic mutations in cancer cells are diverse and vary across different types of cancer. However, certain mutations are more common (widespread) and play significant roles in cancer development and progression. These mutations can occur in several ways:

• Inherited mutations: some people are born with genetic alterations predisposing them to certain cancers
• Acquired mutations: they develop over time due to ageing, exposure to carcinogens like cigarette smoke, harmful chemicals or environmental factors such as ultraviolet(UV) radiation

Types of affected genes

There are some genetic mutations in cancer cells which are frequently observed. The genetic changes in cancer cells typically affect three main types of genes:

Tumour suppressor genes

These genes, which normally control cell division and induce apoptosis, are often inactivated in cancer cells. Tumour suppressor genes are crucial in preventing cancer by regulating cell growth, division, and survival. These genes act as safeguards against uncontrolled cell proliferation and malignant transformation.¹

The tumour suppressor gene TP53 is the most commonly mutated in cancer:

• It is mutated in approximately 36.6% of all cancers¹
• In some cancer types, such as squamous cell carcinoma, TP53 mutations are present in up to 63% of cases²
• TP53 mutations cause cells with damaged DNA to grow and divide uncontrollably²

Here are the key ways tumour suppressor genes are involved in preventing cancer: 

DNA damage response

Many tumour suppressors are involved in detecting and responding to DNA damage:

• The p53 protein, often called the "guardian of the genome," monitors for cellular stress, DNA damage, and inappropriate signalling³
• When damage is detected, p53 can trigger cell cycle arrest, allowing time for DNA repair³
• If the damage is irreparable, p53 can induce apoptosis (programmed cell death) to eliminate potentially cancerous cells³

Apoptosis induction

Tumour suppressors can promote apoptosis when necessary:

• p53 activates pro-apoptotic genes like BAX and inhibits anti-apoptotic genes like BCL2³
• This function helps eliminate cells with extensive DNA damage or other potentially cancerous characteristics³

Cell cycle regulation

Tumour suppressor genes control cell cycle progression, and therefore, when mutated in the case of cancer, the cell becomes resistant to external cues (signals) from the surrounding environment.⁴

• They inhibit passage through critical cell cycle checkpoints, particularly the G1 to S transition⁴
• For example, the Rb protein, encoded by the RB gene, represses transcription of genes involved in cell cycle progression and DNA synthesis⁴
• The p16 protein, encoded by the INK4 gene, inhibits cyclin-dependent kinase 4 (CDK4) activity, further regulating the cell cycle

DNA repair genes

When damaged, these genes fail to correct errors in DNA replication, leading to the accumulation of mutations.

In the genetic context of DNA repair genes, cancer cells differ from normal cells in several key ways:

DNA repair mechanisms

• Impaired DNA repair 

Cancer cells often have defects in DNA repair pathways crucial for maintaining genomic stability. This impairment can lead to the accumulation of mutations, contributing to cancer development and progression.^5,6

• Genomic instability

The inability to properly repair DNA damage results in genomic instability, a hallmark of cancer. This instability allows cancer cells to acquire additional mutations that enhance their growth and survival.^5,6

• Mutations in DNA repair genes

Cancer cells may have mutations in genes involved in DNA repair, such as BRCA1 and BRCA2, which are commonly found in breast and ovarian cancers. These mutations impair the cell's ability to repair DNA breaks, leading to increased genetic alterations.

Impact on cancer development

• Accumulation of mutations 

The inefficiency of DNA repair mechanisms in cancer cells leads to a higher mutation rate. This can result in the activation of oncogenes and the inactivation of tumour suppressor genes, driving cancer progression.⁶

• Resistance to therapy

Defects in DNA repair pathways can also confer resistance to certain cancer therapies, such as chemotherapy and radiation, which work by inducing DNA damage.⁵

• Targeted therapies 

Understanding these differences has led to the development of targeted therapies that exploit the impaired DNA repair capabilities of cancer cells. For example, PARP inhibitors effectively treat cancers with BRCA mutations by further impairing DNA repair.⁵

Genetic differences

• Mutational load 

Cancer cells typically have a higher mutational load than normal cells, partly due to defective DNA repair mechanisms.

• Genetic heterogeneity 

The genetic composition of cancer cells can differ markedly, even within the same tumour. Ongoing mutations and selection pressures influence this heterogeneity.

Cancer cells differ from normal cells in DNA repair capabilities, leading to genomic instability and a higher mutation rate. These genetic differences are crucial for cancer development and progression, offering opportunities for targeted therapeutic interventions.

Proto-oncogenes 

In cancer cells, proto-oncogenes (normal growth-regulating genes) become overactive or altered, becoming oncogenes and promoting uncontrolled cell growth.

Oncogene mutations

Several oncogenes are frequently mutated in various cancers:

• PIK3CA
  • Mutated in 12.4% of tumours across all cancer types¹
  • Often found in breast, colorectal, and endometrial cancers

• KRAS
  • Mutated in 11.1% of all tumours¹
  • Particularly common in pancreatic, colorectal, and lung adenocarcinomas

• BRAF
  • Mutated in 6.6% of tumours overall¹
  •  Most frequently mutated gene in melanoma⁴

Other common mutations 

• MUC16: mutated in 18.9% of tumours across all cancer types¹
• CSMD3: a tumour suppressor gene mutated in 13.7% of cancers¹
• LRP1B: another frequently mutated gene, present in 13.5% of cancers¹
• KMT2C and KMT2D: these genes, involved in epigenetic regulation, are among the ten most commonly mutated driver genes²
• ARID1A: also among the top ten most frequently mutated cancer driver genes²

Cancer-specific gene mutations

Certain genetic mutations occur more frequently in certain cancer types:

• EGFR mutations in lung cancer
• APC mutations in colorectal cancer
• BRAF mutations in melanoma and thyroid cancer
• BRCA1/2 mutations in breast and ovarian cancers

It is crucial to highlight that the incidence and severity of these mutations might range greatly between cancer types and individual patients. Understanding these genetic changes is critical for creating targeted treatments and personalised treatment plans in cancer care.

Aside from genetic mutations, other important characteristics of cancer cells include:

Chromosomal abnormalities

Cancer cells often exhibit significant chromosomal differences compared to normal cells:⁷

• Abnormal chromosome numbers: cancer cells may have too many or too few chromosomes
• Structural changes: chromosomes in cancer cells can have missing or additional segments
• Translocations: parts of chromosomes may be rearranged, such as in the Philadelphia chromosome in chronic myelogenous leukaemia

Clonal origin

While normal tissues consist of genetically diverse cells, tumours often originate from a single mutated cell.⁷ This clonal origin contributes to the unique genetic profile of cancer cells within a tumour.

Epigenetic changes

Although most cancer-causing changes are genetic, epigenetic alterations (changes in gene expression without DNA sequence changes) can also play a role in cancer development.⁷

Summary

Cancer cells differ considerably from normal cells in their genetic makeup, with a variety of changes that fundamentally alter their behaviour and properties. These differences are the result of a complex, non-stop interaction of genetic and epigenetic modifications that accumulate over time, causing normal cells to convert into malignant ones. 

These genetic abnormalities explain cancer cells' distinctive behaviours, such as uncontrolled proliferation and metastasis, and offer prospects for targeted therapeutics and personalised medical techniques. Understanding the distinctions is critical for developing more effective cancer treatments and improving patient outcomes. 

The dynamic and evolving nature of cancer genetics underscores the disease's complexity and highlights the need for continued research to unravel the intricate genetic landscape of cancer cells. As our knowledge expands, so does our ability to combat this formidable disease through innovative genetic-based strategies.