Learning outcomes
- To understand the importance of plasma proteins in the human body
- To know and understand the clinical relevance of plasma protein test for diagnosis of diseases
- To understand the uses of tumor marker in cancer diagnosis
- To describe patient management and the use of tumor markers in colon and breast cancers
Plasma Proteins
Two major groups: albumin & globulins
Individual plasma protein conc affected by:
- nutritional status
- physiologic changes
- synthesis rate
- extracellular distribution
- clearance rate
Most synthesized in the liver, except immunoglobulins & protein hormones
Prealbumin (Transthyretin)
- ^^small transporter protein^^ that migrates before albumin in classic serum protein electrophoresis
- ^^transports thyroid hormones^^
- also binds to retinol-binding protein for the ^^transportation of vitamin A (retinol)^^
- synthesised in the liver & choroid plexus of the central nervous system
- stimulated by glyuocorticoid hormones, androgens & NSAIDs
- [[Laboratory Investigations[[
- immunonephlometry
- immunoturbidimetry
- Clinical revelance
- indicates protein nutrition
- higher than normal: severe renal failure, corticosteroid use, oral contraceptive use
- lower than normal: malnutrition, liver disease, serious infection, trauma, inflammation, serious or long term illness, hyperthyroidism
Albumin
- small protein found in blood, CSF, interstitial fluid, urine & amniotic fluid
- synthesised in liver:
- stimulated by hormones, e.g. insulin, cortisol & growth hormone
- inhibited by proinflammatory substances
- regulated by colloidal osmotic pressure and protein intake
- catabolised mainly in the muscle, liver & kidneys
- Primary function:
- maintain the colloidal osmotic pressure in the intravascular & extravascular compartments
- serves as a transport protein for fatty acids, phospholipids, cholesterol, amino acids, hormones, bilirubin, drugs, toxins, metallic ions & gas
- Laboratory investigations
- measured colorimetrically by using bromocresol green dye
- Clinical relevance
- related to liver or kidney disease, or underlying nutritional deficits
- hyperalbuminemia: dehydration, increased insulin level, blood transfusion, exogenous albumin administration, anabolic steroid use, androgen / growth hormone administration
- hypoalbuminemia: overhydration, hepatic disease, nephrotic syndrome, protein losing states, inflammation, poor nutrition
α2-Macroglobulin
- serine protease inhibitor
- inhibits different types of proteinases
- inhibits enzymes in the kinin, complement, coagulation & fibrinolytic pathways
- carrier protein for cytokines, growth factors & cations
- modulates immunologic & inflammatory reactions
- very large glycoprotein synthesised mainly in the liver
- Laboratory investigations
- serum protein electrophoresis
- immunologic assay
- Clinical Relevance
- increased level: nephrotic syndrome, related to estrogen & age
- decreased level: acute phase response, pancreatitis, prostate cancer
Immunoglobulins
Tumour Markers
Production:
- directly by the tumours; or
- as an effect of the tumours on healthy tissue; or
- other cells of the body in response to cancer
Traditional concept on tumour markers:
- proteins or other substances that are present at a higher amount in cancers
- found in blood, urine, stool, tumours, or other tissues & body fluids of cancer patients
Carcinoembryonic Antigen (CEA)
- ]]expressed in embryonic tissue of gut, pancreas & liver]]
- elevated level in many different types of cancer
- 60-90% of colorectal carcinoma
- 50-80% of pancreatic carcinoma
- 25-50% of gastric and breast carcinoma
- also elevated in several benign disorders, i.e., alcoholic cirrhosis, hepatitis & ulcerative colitis
- lacking sensitivity & specificity for early detection of cancers
- Tumour burden
- CEA is measured before and after surgical resection to confirm successful removal of tumour burden
- Monitor the efficacy of treatment
- CEA should drop within the reference interval in 1-4 months if the treatment is successful
- Detect recurrence - serially monitored every 2-3 months
α1-fetoprotein (AFP)
- major glycoprotein in fetal plasma
- serum level declines rapidly after birth; undetectable level within several months after birth
- synthesised by embryonic yolk sac and fetal liver
- produced and secreted by certain malignant tumours
- utilised as tumour marker
- Elevated level in adult males and non-pregnant females
- hepatocellular carcinoma, germ cell tumours, ovarian tumours
- Diagnosis
- clearly, marked elevation of α1- fetoprotein level + presence of liver mass
- Prognostication
- elevated level indicates aggressive tumour & poor prognosis in hepatocellular carcinoma
- Normally detected in maternal serum, and peaks at 12-15 weeks of pregnancy
- prenatal surveillance - elevated in neural tube defects
- part of triple or quadruple screening test for fetal abnormalities
- Non-cancerous conditions, e.g., hepatitis & liver cirrhosis, also reports an elevation in α1- fetoprotein level
Tumour Markers (pt 2)
Important characteristics:
- tumour-specific
- absent in healthy individuals
- readily detectable in body fluids
5 broad uses:
- screening
- diagnosis
- prognosis
- treatment outcome prediction/monitoring treatment
- relapse
Screening
Current widely-used screening methods
- mammography, cytology, fetal occult blood test
Serum tumour markers
- lack of sensitivity or specificity
- high proportions of false negatives/positives
- may be elevated in chronic inflammation or those without cancers
- usually elevated when the cancer is already well-establish
Diagnosis
Ideally, the sensitivity & specificity should approach 100%
To date, none of the serum tumour markers reach this level of diagnostic performance
- poor diagnostic indicator
Gold standard - histology
Prognosis
Concentration level of tumour markers increases with tumour progression
May reflect the aggressiveness of tumour and predict outcome
May reflect the size of tumour and predict survival
Determination & monitoring of treatment
Presence or absence of tumour marker indicates the responsiveness towards a particular treatment
- determine the most suitable treatment approach
Monitor the efficacy of therapy
- after surgical resection, radiation, chemotherapy or targeted therapy
- direct quantitative relationship between tumour marker concentration and tumour load
Criteria used to assess effectiveness of treatment:
- no change: tumour marker >50% of value at t=0
- improvement: tumour marker <50% of value at t=0
- response: tumour marker <10% of value at t=0
- complete response: tumour marker undetectable
Relapse
Monitor relapse in patients whom the initial treatment is effective/tumour is eliminated successfully after treatment
An increased level of tumour marker level/appearance of detectable tumour marker
- indicates re-emergence and/or spread of the tumour to new sites
Allow earlier identification and treatment, e.g., prostate specific antigen (PSA) in prostate cancer
Laboratory Measurement
- Immunoassays
- most commonly used
- automated testing & relative ease of use
- High-performance liquid chromatography
- used to detect small molecules, e.g., catecholamine metabolites
- more labour intensive & requires more experience and skill
- Immunohistochemistry & immunofluorescence
- solid tumour tissue markers from fine-needle aspirate or biopsy samples
- determine particular cell type & subcellular location
Two main considerations:
- lack of standardisation between different assays
- use of same methodology or same kit for monitoring
- vary in concentration by orders of magnitude
- take note of dilution protocols and risk of antigen excess
DNA Markers
Oncogenes
- usually encode for proteins function in cell growth and division
- activated by point mutations, insertions, deletions, translocations or inversions, gene amplification
- responsible in the transformation of tumour cells and abmormal cell proliferation
- i.e., RAS, HER2, Myc, cyclin D
Tumour suppressor genes
- inhibit cell proliferation, limit the growth and development of tumours
- repair DNA damage and initiate apoptosis f abnormal cells
- lost or inactivated by mutations in cancers
- examples: TP53, BRCA1, BRCA2
Utility in Cancer Patient Management
Breast Cancer
Serum-based tumour marker: cancer antigen 15-3 (CA15-3)
- the product of MUC1 gene
- higher level is found in serum of breast cancer patients
- used as prognostic indicator, in determining the suitable therapy and treatment monitoring in metastatic breast cancer
Tissue-based tumour markers
- biopsy
- expression of genes associated with breast cancer, e.g.,
- estrogen receptor
- progesterone receptor
- HER2 protein receptor
- ER/PR-positive patients have a better prognosis with hormone therapy
- HER2-amplified tumours is likely to be successfully treated with trastuzumab
DNA markers
- mutations in breast cancer susceptibility genes, e.g.,
- related to hereditary breast cancers (>90%)
- BRCA1 & BRCA2 augment the risk of breast cancer by 65% & 45%, respectively
- BRCA1 tumours show more aggressive clinicopathological features than BRCA2
Colorectal Cancer
Serum-based tumour marker: CEA
- neither sensitive nor specific to be used in screening
- widely used in prognostication, monitoring patient’s response to treatment and recurrence detection
- also elevated in inflammatory bowel diseases, pancreatitis, alcoholic cirrhosis, during pregnancy and in smokers
DNA markers
- mutations in KRAS, BRAF and NRAS genes via real-time PCR
- microsatellite instability status (MSI) via DNA sequencing
- protein expression of DNA mismatch repair genes, i.e., MLH1, MSH2, MSH6, PMS2, via immunohistochemistry
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