Investigation of Disease lectures 4-6 Flashcards
What is a biomarker?
A biological molecule that changes in blood level in response to a specific disease
How is activity defined?
Defined as amount of product formed in a given time by a set amount of enzyme
How can activity be monitored?
Coloured products
In what 2 ways can enzyme activity be measured?
- Kinetic fixed time (or end point e.g after 5 minutes) assay
- Kinetic continuous monitoring (or reaction rate - e.g every 30 seconds)
What is an isoenzyme?
An enzyme that can exist in a number of different molecular forms
Describe the molecular leakage from healthy, reversibly damaged and permanently damaged tissue
Healthy - Intracellular proteins, second messangers and other biomarkers retained
Reversibly damaged - transient leakage of biomarker e.g. soluble, cytosolic enzymes
Permanent damage - many component proteins released e.g sarcomeric proteins from muscle cells
Define the terms ischemia, myocardial infarction and angina
Ischemia - blockage of blood vessels supplying oxygen to the heart muscle leading to oxygen deficiency
Myocardial infarction - Cell death and breakdown of muscle cells
Angina - heart pain
How can muscle damaged be detected?
Assays of enzymes in blood: - aspartate aminotransferase - lactate dehydrogenase - creatine kinase Immunoassays of cardiac isoforms of troponin: - troponin T and I
When are biomarker tests not very accurate when dealing with a myocardial infarction (MI) patient?
When MI is associated with road traffic accident or other trauma
- Damage to skeletal muscle complicates anaylsis for LDH, CK etc
What usually occurs on an ECG in MI patients?
S/T elevation (STE)
If no alteration to ECG occurs (nSTEMI) what should be done?
Blood tests for other biomarkers
What is the difference between analytic specificity and analytical sensitivity of an assay?
Analytical specificity of an assay is its ability to measure only the analyte in question
Analytical sensitivity of an assay reflects the smallest amount of an analyte it can detect
What is the difference between clinical specificity and clinical sensitivity of an assay?
Clinical specificity of an assay describes its ability to detect only patients with a particular disease (few false positives)
Clinical sensitivity of an assay reflects its ability to only detect that disease
Name the most abundant plasma protein and also some others
Most abundant = Albumin Others: Haptoglobin Transferrin Complement C3 and C4
Where are most plasma proteins (other than Ig’s) synthesised?
Liver
What is the name of the response that causes a release of cytokines as a consequence of infection, trauma or inflammatory diseases that alters the synthesis of plasma proteins?
Acute phase response
The synthesis of which 2 proteins drops as a result of the acute phase response? (All others rise)
Albumin and transferrin
What is the structure and 2 main functions of albumin?
- Single large (584 residues) polypeptide of Mr ~ 67,000
2 main functions:
1) Contribute a large proportion of the oncotic pressure of blood
2) Carrier for sparingly soluble substances including: - fatty acids
- bilirubin
- acid drugs
- divalent cations ( Ca, Cu, Zn)
- hormones (cortisol, aldosterone)
Name some things that can cause hypoalbuminaemia
- an acute phase reaction
- Liver disease
- Malnutrition (not enough AA to synthesise)
- Malabsorption (less AA)
- Nephrotic syndrome (albumin lost in urine)
- protein losing enteropathy (loss from intestine)
What is oedema?
A condition in which excess fluid accumulates in the tissue
- results from low albumin in blood - oncotic pressure not enough to prevent leaking of fluid from capillaries to interstitial fluids
What 4 main factors are used to classify cancer severity?
1) Tumour size
2) Histology
3) Regional lymph node involvement
4) The presence of metastasis (spreading of cancer to other organs)
Explain the 5 main stages of cancer classification (0-4)
0: Carcinoma in situ I: Localized II: Early locally advanced III: Late locally advanced ( stage I - III related to tumour size and spread) IV: Metastasized
What do the letters in the TNM system stand for and what are the variations of each?
T - extent of tumour
N - extent of spread to lymph nodes
M - presense of metastasis
TX - tumour cannot be evaluated
T0 - No evidence of primary tumour
Tis - Carcinoma in situ
T1 - T4 - size of primary tumour
NX - regional lymph nodes cannot be evaluated
N0 - No regional lymph node involvment
N1-N3 - number of nodes/extent of spread
MX - cant be evaluated
M0 - No distant metastasis
M1 - distant metastasis present
What would T3 N2 M0 indicate about a tumour?
Large tumour
Spread to nearby lymph nodes
Not spread to other parts of the body
What are tumour markers?
Substances produced by tumours or by the bodys response to cancer
- usually proteins
- can be specific to one cancer, or produced by many
- tested by blood tests or tumour samples
Name some criteria for the ‘ideal’ tumour marker
1) Tumour specific
2) Absent in health or benign disease (high specificity)
3) High sensitivity
4) Detectable at early stage (useful for screening)
5) Concentration reflects prognosis (prognosticator)
6) Useful for monitoring
7) Easily measured
Name some molecules that can be tumour markers
Serum proteins Oncofetal antigens Hormones Metabolites Receptors Enzymes
What are themain 7 tumour markers?
AFP (alpha-fetoprotein) CA125 CA15-3 (MUC1) CA19-9 CEA (carcinoembryonic antigen) HCG (human chorionic gonadotrophin) PSA (prostate spectific antigen)
What molecule is AFP and what is it encoded by?
- Glycoprotein with carbohydrate moiety on Asn 222
- Most abundant plasma protein in human foetus (fetal form of albumin)
- Encoded by AFP gene
What conditions cause raised AFP?
1) Heptocellular carcinoma (most common liver cancer): high in 80% of patients, over 1000µg/L in 40%
2) Non-seminomatous germ cell tumours
3) Gastrointestinal cancers: Gastric cancer, bililary tract cancer, pancreatic cancer
4) Some non malignant conditions e.g alcoholic liver disease, bililary tract obstruction
What is the hook effect?
An artifact of tumour marker immunoassay kits that causes the reported quantity of tumour marker to be incorrectly low when quantity is high
- can cause AFP to be reported lower than actual amount
What does ‘CA’ stand for in tumour marker abbreviations?
Cancer antigen
What is CA125, what gene encodes it and how is it tested for?
- Large glycoprotein ~22,000 AA
- Ovarian marker
- MUC16 gene
- Defined by OC-125 antibody
- Most frequent used biomarker for ovarian cancer detection
Why should CA125 not be used to diagnose ovarian cancer?
- Only elevated in 50% of early ovarian cancers
- Raised in many non cancerous conditions e.g endometriosis, acute pancreatitis, cirrhosis
What is the percentage elevation of CA15-3 in women with early stage and advanced breast cancer?
10-30% elevation in early stage
60-70% elevation in advanced
What is the relation between CA15-3 and CA27.29?
Different epitopes on the same protein antigen product of the breast cancer associated MUC1 gene
- CA27.29 in more sensitive and specific than CA15-3
What levels of CA15.3 and CA27.29 suggest malignancy?
CA15-3 - over 25 U/mL
CA27.29 - over 100 U/mL
Why should CA15-3 not be used in screening or diagnosis of breast cancer?
May be elevated in breast and other adenocarcinomas (a malignant tumour formed from glandular structures in epithelial tissue)
The tumour marker CA19.9 is used in the management of which cancer?
Pancreatic cancer
CA19.9 is not expressed even in those with large tumours if they lack what antigen?
Lewis antigen
Why should CA19.9 not be used for screening or diagnosis of colorectal cancer
- Elevated in most pancreatic cancers
- elevated in 50% of gastric cancers
- elevated in 30% of colorectal cancers
- may be increased in pancreatitis, heptocellular jaundice, cirrhosis, obstruction of bile ducts
Describe the structure and function of CEA (carcinoembryonic antigen)
- Cell surface anchored glycoprotein
- carbohydrate composition ranging from 50-85% of molecular mass
- Involved in cell adhesion
- Produced during fetal development, not produced after birth
When is CEA expressed and what levels suggest colon cancer?
Expressed only in cancer cells, especially adenocarcinomas e.g colon, lung, breast, stomach, pancreas
- 5-10 times upper limit of normal suggests colon cancer
Describe the structure of hCG (human chorionic gonadotrophin) and name the part of the molecule that acts as the tumour marker
- Glycoprotein composed of 237 AA, Mr 25.7 kDa
- Is heterodimeric:
- alpha subunits - identical to LH, FSH and TSH
- beta subuit unique to hCG
Beta subunit used as marker (antibody binds to it)
What is the primary purpose of hCG testing?
To test for pregnancy
The Beta subunit of hCG is secreted by which cancers?
1) Seminoma (germ cell tumour of testicle or, rarely, other exta-gonadal locations)
2) Choriocarcinoma (malignant tumour of uterus)
3) germ cell tumours
4) Hydatidiform mole formation (cell mass in uterus that will not develop into a baby)
5) Islet cell tumour
What is the normal range of hCG in men?
Between 0-5 mIU/mL
beta-hCG can be measured in combination with which other tumour marker to monitor germ cell tumours?
AFP
What type of molecule is PSA (prostate specific antigen) and what gene encodes it?
A glycoprotein enzyme
encoded by KLK3 gene
60% of PSA forms a complex with what type of molecule in serum?
With protease inhibitors
40% free
What is the major PSA-protease inhibitor complex found in the serum?
PSA-ACT (antichymotripsin)
What free PSA to total PSA value is linked with prostate cancer?
What other factors can cause an increase in PSA?
Levels rise after ejaculation but fall within 24-48 hours
Raised in prostatitis (-titis means ‘inflammation of’)`
What levels of PSA are normal for a man aged 60 or under, 60-69 and 70 and over?
60 or under: 3ng/ml or less
60-69: 4ng/ml or less
70 + : 5ng/ml or less
Who should have PSA screening?
Males aged 50 and over
If family history - start at 40-45
How many men out of 3 with raised PSA actually have prostate cancer?
1/3
Name some drawbacks of tumour markers
- Lack of reliability
- Proteins vary between individuals, cell type etc
- Normal cells as well as cancer cells produce most markers
- Markers not always present in early stage cancers
- Can be present in non cancerous conditions
- People with cancer may not have elevated marker
Comment on use of tumour markers in screening, diagnosis, prognosis (treatment), monitoring post treatment
- In some cases can be used in screening, mainly PSA, but highly criticized; too many variables
- NEVER use tumour markers for diagnosis
- Generally tumour markers adopted in prognosis and monitoring post treatment
- Can be used to narrow down possibilities
