Liquid Biopsies

Question 1

Describe liquid biopsies and list the main types of biomarkers that can be detected

Answer 1

A liquid biopsy is a minimally invasive technology that detects molecular biomarkers without needing invasive procedures unlike surgical biopsies, enabling medical doctors to discover a range of information about a disease or a tumour through a simple blood sample

Several circulating biomarkers can be interrogated from this blood sample, including:

1) Circulating Tumour Cells (CTCs):

• These are cells that have shed into the vasculature or lymphatics from a primary tumour and are carried around the body in the bloodstream
• The presence of CTCs is a potential indicator of metastatic disease

2) Circulating Tumour DNA (ctDNA):

• Tumours shed DNA into the bloodstream, which can be detected using high-throughput sequencing technologies
• The analysis of ctDNA can provide information about the genetic makeup of a tumour, including the presence of specific mutations that might guide treatment decisions

3) Circulating RNA or microRNA:

• Similar to ctDNA, tumours also shed RNA into the bloodstream
• Certain patterns of microRNA expression have been associated with the presence of specific types of cancer

4) Exosomes:

• These are small vesicles released by cells that contain a variety of molecular constituents of their cell of origin, including proteins and RNA
• Tumours can produce a large number of exosomes, which can provide information about the protein expression patterns of the tumour

5) Tumour-educated platelets (TEPs):

• These are blood platelets that contain RNA transcripts from the tumour
• The RNA profile of TEPs can provide a snapshot of the gene expression pattern of the tumour

6) Circulating Proteins:

• Certain proteins are shed by tumours or produced by the body in response to cancer
• E.g. prostate-specific antigen (PSA) is often found at elevated levels in the blood of men with prostate cancer

7) Circulating Antibodies:

• Sometimes the immune system responds to a tumour by producing antibodies against tumour-specific antigens
• These antibodies can be detected in the blood and could provide clues about the presence and type of cancer

Question 2

discuss the main characteristics of CTCs and ctDNA

Answer 2

Circulating Tumour Cell (CTCs):

Circulating tumour cells (CTCs) are cells that have detached from a primary tumour and circulate in the bloodstream

1. Origin: CTCs are derived from primary or metastatic tumours
2. Presence in Blood: Although they circulate in the bloodstream, they are present in very low concentrations
3. Identification: CTCs can be identified in the blood by their expression of specific markers, such as the epithelial cell adhesion molecule (EpCAM) and cytokeratins. However, not all CTCs express these markers
4. Heterogeneity: Like tumour cells, CTCs are heterogeneous. They can vary in size, shape, and marker expression, and this heterogeneity can be a reflection of the heterogeneity within the tumour of origin
5. Clinical Relevance: The number of CTCs in a patient’s blood is often associated with disease prognosis. Higher CTC counts generally correlate with worse clinical outcomes

Circulating Tumour DNA (ctDNA):

Circulating tumour DNA (ctDNA) consists of small fragments of DNA that are released into the bloodstream by dying tumour cells

1. Origin: ctDNA is derived from tumour cells and carries the same genetic alterations as the tumour, such as mutations, copy number variations, and structural rearrangements
2. Size: ctDNA is typically fragmented into pieces that are about 160 to 180 base pairs in length
3. Presence in Blood: ctDNA is present in the cell-free fraction of blood (plasma or serum), along with DNA from healthy cells. The fraction of ctDNA among all cell-free DNA can vary widely among patients, ranging from less than 0.1% to more than 90%
4. Detection: requires highly sensitive methods due to its low concentration. Techniques such as digital PCR and next-generation sequencing are commonly used
5. Clinical Relevance: Changes in ctDNA levels can reflect tumour burden and response to treatment. In addition, the genetic alterations in ctDNA can provide information about the molecular characteristics of the tumour, resistance mutations, and potential therapeutic targets

Question 3

enumerate the advantages and disadvantages of liquid biopsies compared to standard methods

Answer 3

Advantages of Liquid Biopsies:

1. Non-invasive: Liquid biopsies require only a simple blood draw, which is less invasive and generally safer than surgical biopsies
2. Real-time Monitoring: Because they are relatively easy to perform, liquid biopsies can be done more frequently than traditional biopsies. This allows for real-time monitoring of disease progression and treatment response
3. Comprehensive View: Liquid biopsies can capture the heterogeneity of the tumour, as they detect ctDNA or CTCs shed from all tumour sites, not just one specific area
4. Detection of Minimal Residual Disease and Relapse: Liquid biopsies can detect small amounts of ctDNA even when imaging scans are clear, which can indicate minimal residual disease or early relapse
5. Early detection: Liquid biopsies have the potential to detect cancer in early stages by identifying cancer-specific genetic mutations or other biomarkers in the bloodstream. This could allow for early intervention and potentially improve patient outcomes

Disadvantages of Liquid Biopsies:

1. Sensitivity and Specificity: Current methods of detecting ctDNA or CTCs may not be sensitive or specific enough to detect all cases of cancer, particularly in early-stage disease. False positives can also occur
2. Quantitative Challenges: The amount of ctDNA or CTCs in the blood may not always correlate with tumour size or aggressiveness, making it challenging to interpret results
3. Lack of Spatial Information: Unlike tissue biopsies, liquid biopsies can’t provide information about the location of the tumour or its physical characteristics