Molecular diagnostics in cancer screening Flashcards
Biomarker used in cancer
Definition: Biomarker are measured as an indicator of normal biological processes, pathogenic processes or responses to an exposure or intervention, including therapeutic interventions
Human papilloma virus (HPV): Cervical cancer
BRCA1 and BRCA2 gene mutations: Breast, ovarian, cervical cancer
CA125 protein: Ovarian cancer
alpha-fetoprotein: Hepatocellular carcinoma
Prostate Specific Antigen (PSA): Prostate cancer
HER2/neu gene mutations: Breast cancer
Calprotectin: Colorectal inflammations and cancer
KIT gene: Acute myelogenous leukemia and gastrointestinal Stromatumors
EGFR, ALK, and KRAS gene mutations: Lung cancer
Biomarker
Questions that can be answered by cancer biomarkers
Prognostic: Is it likely to develop this cancer?
Diagnostic: What type of cancer is it?
Reduktive: Is this the optimal drug for my cancer?
Pharmacodynamics: What’s the optimal dose for my body?
Recurrence: Will the cancer return?
How reliable are cancer biomarkers?
Technical (assay) validation/Biological and clinical validation/Cost effectiveness
Imaging biomarker evaluated in vitro, in animals and in humans
-> Transnational gap 1
Imaging biomarker is a reliable measure used to test hypotheses in clinical cancer research
-> Translational gap 2
Imaging biomarker routinely used in the management of patients with cancer within the healthcare system
- Cross translational gap 1 to become robust medical research tools
- Translational gap 2 to be integrated into routine patient care
Prognostic marker: Prostate specific antigen
- Prostate-specific antigen (PSA) was approved by the United States Food and Drug Administration in 1986 to monitor men with prostate cancer
- In 1994 approved for cancer detection
- Member of kallikrein-related peptidase family
- Secreted by the epithelial cells of the prostate gland
- Liquefies semen and allows sperm to swim freely
- Present as inactive forms:
- preproPSA: formed in epithelial cells -> co-translational cleavage
- proPSA: proenzyme (zymogens), activated to PSA in the seminal lumen through hK2 (Kallikrein-2)
- In serum active PSA can be bound to protease inhibitors (zinc ions) -> inactive
- In cancer: Truncated forms of proPSA are formed, which are all enzymatically inactive
PSA detection in serum
- proPSA is secreted into the seminal lumen and activated by hK2
- In healthy men a small fraction of active PSA can diffuse into the serum; bound to protease inhibitors; Active PSA can be inactivated by proteolysis and diffuse into serum as free inactive PSA
- Prostate cancer: disruption of the basal cell layer and basement membrane leads to decrease in luminal processing resulting in increasing levels of complexed PSA and proPSA in serum, and decreasing levels of serum free PSA
Validity, specificity and sensitivity
- Analytical validity is a measure of how well the test measures what it purports to measure (a test designed to detect a mutation associated with melanoma should not give a positive result for an unrelated mutation associated with diabetes)
Specificity:
* ability to correctly identify patients without cancer
* a specific test gives a positive result only when the cancer is present
Sensitivity:
* ability to correctly identify patients with cancer
* a sensitive test correctly identifies everyone who has the cancer
Importance of cancer prevention
- A higher cutoff setting results in higher specificity and lower sensitivity and vice versa
- Cut-off 2: Positive result would include all clinical positives, but also a lot of negatives
- Cut-off 4: Positive result would include most of clinical positive and some negatives
- Cut-off 6: Positive result would exclude all clinical negatives, but also half of clinical positives
PSA in prostate cancer patients
- High levels of PSA are not specific for prostate cancer
- Associated with inflammation of the prostate and benign prostatic hypertrophy (enlargement of the prostate) -> relatively common health conditions in older men
- Approximately 70-80 % of men who have a positive PSA test do not have prostate cancer
- PSA test as a screening tool for prostate cancer lacks specificity
- New approaches: In men with prostate cancer, the ratio of free (unbound) PSA to total PSA is decreased
Faecal calprotectin
- In the presence of calcium, calprotectin is capable of sequestering the transition metals iron, manganese and zinc via chelation
- Comprises as much as 60% of the soluble protein content of the cytosol of a neutrophil
- Secreted by an unknown mechanism during inflammation
- Faecal calprotectin correlates significantly with disease activity in people with confirmed inflammatory bowel disease and colon cancer
- Can be false-positive if the laboratory uses low calprotectin cut-off levels
Receiver operating characteristics
- Graphical plot that illustrates the diagnostic ability of a binary classifier system as its discrimination threshold is varied
- To draw an ROC curve, only the true positive rate (TP) and false positive rate (FP) are needed
- Best possible prediction method would yield a point in the upper left corner or coordinate (0,1) of the ROC space, representing 100% sensitivity (no false negatives) and 100% specificity (no false positives)
- Points above the line of no discrimination represent good classification results (better than random); points below the line represent bad results (worse than random)
