Table of Contents  
REVIEW ARTICLE
Year : 2012  |  Volume : 4  |  Issue : 2  |  Page : 87-95

Tumor markers: An overview


Department of Oral Pathology, Vishnu Dental College, Bhimavaram, Andhra Pradesh, India

Date of Web Publication17-Jan-2013

Correspondence Address:
A Naga Supriya
Department of Oral Pathology, Vishnu Dental College, Bhimavaram - 534 202, Andhra Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0975-8844.106192

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  Abstract 

Tumor markers are substances, usually proteins that are produced by the body in response to cancer growth or by the cancer tissue itself. Measurement or identification of these markers is useful in patient diagnosis and clinical management. An ideal tumor marker should be highly sensitive, specific, accurate, reliable and easily assayable. But none of the tumor markers have all these characteristics. Apart from their limitations, tumor markers are precious tools for screening a healthy and a high risk population for the presence of cancer, making a diagnosis of a specific type of cancer, along with determining the prognosis and monitoring the course of the disease in the patient, at the time of remission or during the course of treatment. This overview discusses and emphasizes currently available tumor markers, their role in cancer and their recent development in cancer diagnosis and prognosis.

Keywords: Diagnostic marker, prognostic marker, tumor marker


How to cite this article:
Babu G S, Supriya A N, Raj Kumar N G, Swetha P. Tumor markers: An overview. J Orofac Sci 2012;4:87-95

How to cite this URL:
Babu G S, Supriya A N, Raj Kumar N G, Swetha P. Tumor markers: An overview. J Orofac Sci [serial online] 2012 [cited 2017 Mar 25];4:87-95. Available from: http://www.jofs.in/text.asp?2012/4/2/87/106192


  Introduction Top


A tumor marker can be defined as substances present in, or produced by, a tumor itself or produced by host in response to a tumor that can be used to differentiate a tumor from normal tissue or to determine the presence of a tumor based on measurements in blood or secretions. [1]

Mostly tumor markers are proteins. These markers may be detected within exfoliated or distributed cells or as circulating agents within peripheral blood or plasma. Other surrogate biological specimens, typically bodily fluids (e.g.: Urine, saliva, sputum, cerebrospinal fluid, or effusions) may also contain tumor markers. Tumor markers include a variety of substances like cell surface antigens, cytoplasmic proteins, enzymes, hormones, oncofetal antigens, receptors, oncogenes and their products. Quantitative as well as qualitative evaluation of these markers is possible through modern techniques of sensitive immunoassays like Radio Immuno Assay (RIA) and Enzyme Linked Immuno Sorbent Assay (ELISA) using monoclonal or polyclonal antibodies immunoassays in majority of cases and biochemical and molecular biological techniques in other cases. Each tumor marker has a variable profile of usefulness for screening, determining diagnosis and prognosis, assessing response to therapy, and monitoring for cancer recurrence.

An ideal tumor marker theoretically should have the following criteria: [2]

  • It should be highly sensitive and should have low false negatives
  • It should be highly specific and should have low false positive
  • It should have high positive and negative predictive value
  • 100% accuracy in differentiating between healthy individuals and tumor patients
  • It should be able to differentiate between neoplastic and non-neoplastic disease and show positive correlation with tumor volume and extent
  • It should predict early recurrence and have prognostic value
  • It should be clinically sensitive i.e., detectable at early stage of tumor
  • Its levels should be preceding the neoplastic process, so that it should be useful for screening early cancer
  • It should be either a universal marker for all types of malignancies or specific to one type of malignancy
  • It should be easily assayable and be able to indicate all changes in cancer patients receiving treatment.
History

The first known attempt to find markers for malignancy was made 2000 years ago and is described in an Egyptian Papyrus where breast cancer was distinguished from mastitis. [3] Incidentally the first tumor marker in modern medicine was identified by Bence-Jones, who in 1846 detected a heat precipitate in samples of acidified urine from patients suffering from "Mollities ossium". [1]

Since then numerous potential tumor markers have been reported in the literature. Tissue polypeptide antigen (TPA) is a chemically well defined substance identified by Bjorklund in 1957. [3] In 1965, GOLD et al. isolated a glycoprotein molecule from specimens of human colonic cancer and thus discovered the first "tumor antigen", later identified as carcinoembryonic antigen (CEA). [1]

The first modern tumor marker used to detect cancer was Human Chorionic Gonadotropin (HCG) the substance doctors look for in pregnancy tests. Women who have ended a pregnancy but still have an enlarged uterus are tested for HCG. A high level of HCG in the blood may be a sign of a cancer of the placenta called Gestational Trophoblastic Disease (GTD). This cancer continues to produce HCG. [4] Cancer Antigen-125 was introduced in 1983 by Bast for ovarian cancer. [3]

At present there are hundreds of tumor markers available and currently studies are going on to determine the accuracy of tumor marker in cancer detection, measurement of tumor treatment response and determination of recurrence.

Classification [2]

Epithelial markers

  • Cytokeratins (CK)
  • Epithelial membrane antigen (EMA)
  • Oncofetal antigens
    1. Alpha-fetoprotein (AFP)
    2. Carcinoembryonic antigen (CEA)
  • Desmoplakin
Mesenchymal markers

Muscle antigens

  • Desmin
  • Actin
  • Myoglobin
  • Myosin
Vascular antigen

  • CD 34
  • CD 31
Neural antigens

  • S 100
  • Neuron specific enolase (NSE)
  • Glial fibrillary acidic protein (GFAP)
  • Synaptophysin
  • Nerve growth factor receptor
Prognostic markers

Cell adhesion molecules

  • Cadherins
  • Integrins
  • Selectins
Proliferation markers

  • PCNA
  • Ki67
  • AgNORs
Biochemical markers

Enzymes and isoenzymes

  • Prostatic acid phosphatase (PAP)
  • Prostate Specific Antigen (PSA)
  • Placental Alkaline Phosphatase (PALP)
  • Lysozyme
Protein

  • Ferritin
  • Glycoprotein
  • Beta protein
  • Immuno globulins
Hormone receptors

  • Estrogen receptor (ER)
  • Progesterone receptor (PR)

  Epithelial Markers Top


Cytokeratin

Cytokeratins are also known as keratins and they are classified as intermediate filament proteins and are found with in the cytoplasm of all epithelial cells. Cytokeratin filaments are made up of a highly complex multigene family of proteins.

Cytokeratins are a family of 20 members which are present in normal epithelial cells and their tumors. They are divided in to two broad groups according to their isoelectric point.

  1. Acidic group includes keratins 10-19. They have a molecular weight ranging from 40-56.5 kD.
  2. Basic group contains keratins from 1-8. They have a molecular weight ranging from 52-67 kD.
Cytokeratin expression in normal oral mucosa

Keratins are usually found in pairs, with one type I (nos 9-20) co-existing with one type II (nos 1-8), each encoded by its own gene. Keratins help maintain cell shape and appear to be attached to the cell membrane. In the oral mucosa all basal cells express K5 and K14 and in non-'cornifying' sites K19. Expression of K19 in basal cells from cornifying sites is sporadic. Suprabasal cells in the non-'cornifying' regions, e.g., buccal mucosa, ventral tongue, express K4 and K13. In cornifying sites e.g., hard palate, dorsal tongue, K1 and K10 are largely found. In regions of increased proliferation K6 and K16 are found. The so-called 'simple' epithelial keratins K8 and K18 are not normally found in stratified squamous epithelium, but rather in single cells, such as glandular tissue (e.g., salivary glands). Occasionally K8 positive cells may be observed in the oral mucosa, but these are Merkel cells (of neuroectodermal origin) rather than epithelial cells.

Cytokeratin expression in pathology

During embryogenesis, the low molecular weight keratins appear early in development, the higher molecular weight keratins appear later with the formation of more complex stratified epithelia. Low molecular weight keratins are seen in more simple non-stratified epithelia and tumors derived from them (i.e., breast carcinomas or gastrointestinal carcinomas derived from cuboidal or simple columnar epithelia). The higher molecular weight keratins are seen in more complex stratified squamous epithelia and their corresponding tumors (i.e., squamous cell carcinomas). Cytokeratins 8, 18, and 19 are the most abundant in simple epithelial cells. [5]

Keratins are remarkably diverse, highly resistant and the most conserved cytoskeletal proteins present in all types of epithelial cells. In epithelial tumors some keratin polypeptides are either not expressed or are over expressed. After their release from proliferating or apoptotic cells, cytokeratins act as useful markers for epithelial malignancies. Therefore, keratins have gained importance as marker proteins useful in diagnosis of tumors of epithelial origin. [4]

Keratins have been used effectively as markers for epithelial carcinomas, especially those of stratified and squamous-cell origin, e.g. Squamous cell carcinoma, lung carcinomas, breast carcinomas, urinary bladder carcinomas, thymomas, and cervical carcinomas. It should be remembered that cytokeratin expression is not strictly restricted to epithelia and their tumors. Some non-epithelial derived tumors, such as synovial sarcoma, epitheliod sarcoma, myosarcoma, ewing's tumor, malignant melanoma, schwannoma, glial tumors, large cell anaplastic lymphoma and malignant mesothelioma may also express immunoreactivity for cytokeratins. [5]

Cytokeratin tumor markers can accurately predict disease status before conventional methods. They offer a simple, noninvasive, cheap, and reliable tool for more efficient management. [4] The three most applied cytokeratin markers which can be used clinically are tissue polypeptide antigen (TPA), tissue polypeptide specific antigen (TPS), and CYFRA 21-1. [4] TPA is a broad spectrum test, and it measures cytokeratins 8, 18, and 19. TPS and CYFRA 21-1 assays are more specific and they measure cytokeratin 18 and cytokeratin 19.

Cytokeratins are particularly useful tools in oncology diagnostics. [4] Normal fetal and adult Merkel cells also express CK 20. CK 20 is currently the best immunohistochemical marker for Merkel-cell carcinomas. CK 13 and less frequently CK 20 act as markers for poorly differentiated squamous cell carcinoma. CK 20 also acts as a marker for transitional cell carcinomas of the urinary tract. [6] Their main use is to monitor treatment and evaluate response to therapy. CK 14 and 19 acts as a marker for odontogenic epithelial origin. [7] CKs are also used as prognostic markers particularly on tumor progression and metastasis formation. [4]

Desmoplakin

Desmoplakins are highly conserved proteins present within the desmosomal plaques of epithelial cells. Utilizing antibodies directed against desmoplakin I and II, immunoreactivity can be localized to peripheral punctate regions in a wide variety of epithelial cells, meningeal cells, and mesothelium.

Desmoplakins can also be identified within the glandular component of synovial sarcomas but not in other sarcomas. Thus desmoplakins represent an additional marker of epithelial differentiation independent of keratin. Mutations in this gene are the cause of several cardiomyopathies and keratodermas as well as the autoimmune disease paraneoplastic pemphigus. [8]

Alpha fetoprotein

AFP is a very popular and extensively studied carcinoembryonic glycoprotein/Oncofetal antigen. Alpha-fetoprotein (AFP) is a 70kD a glycoprotein. It is increased in the majority of patients with Hepatocellular Carcinoma (HCC) and other gastrointestinal tumors. [8] It is a major fetal serum globulin with a molecular weight of approximately 65,000. The single chain glycoprotein has carbohydrate content of 3% and amino acid sequence similar to that of albumin.

AFP is normally produced during fetal development by the liver and yolk sac. After birth, the levels drop off rapidly, and by the second year only trace amounts are detectable in serum. AFP may be useful in the diagnosis and follow-up of cases of HCC. [8] Although it is abundant in fetal blood, its concentration in normal adults is below 15 ng/ml. The appearance of excess amount of serum AFP beyond 500 ng/ml indicates underlying malignancy except in cases of pregnancies.

The role of alpha fetoprotein in malignancy

Serum AFP measurement is of valuable clinical aid in diagnosis, prognosis and monitoring primary hepatocellular carcinoma, hepatoblastoma, non-seminomatous testicular germ cell tumors the embryonal carcinoma, teratoma, choriocarcinoma and yolk sac carcinoma etc., germ cell tumors of ovary and extragonadal germ cell tumors. [2] Alpha fetoprotein is a more sensitive and more specific marker for hepatoma than CA 19-9. Increased Alpha fetoprotein concentrations are found in 80% of patients with liver cell cancer and substantially increased values are rarely found in patients with benign liver disease or other gastrointestinal malignancies. But in CA 19-9 the concentrations are mostly found in benign conditions than in hepatoma or other malignant conditions. [9] Measurement of serum AFP has been helpful aid in the diagnosis and prognosis of AFP secreting malignancies. It has also been useful in monitoring efficacy of chemotherapy, surgery and radiotherapy in primary hepatocellular carcinoma, hepatoblastoma, non-seminomatous testicular and other germ cell tumors.

Carcinoembryonic antigen

Carcinoembryonic antigen (CEA), is an oncofetal glycoprotein. It is expressed in normal mucosal cells and it is over expressed in adenocarcinoma, especially in colorectal Cancer. Elevated levels of CEA is also seen in other malignancies such as salivary gland tumors i.e. carcinomas with glandular differentiation and can also be observed in squamous cell carcinomas. Non-neoplastic conditions associated with elevated CEA levels include cigarette smoking, peptic ulcer disease, inflammatory bowel disease, pancreatitis, hypothyroidism, biliary obstruction, and cirrhosis. [10]

CEA is a normal cell product and it is overexpressed in adenocarcinomas, primarily of the colon, rectum, breast, pancreas, and lung. Elevated levels of CEA is also found in patients with primary gastric cancer. [10] It is normally found in small amounts in the blood of most healthy people. It is also elevated in some benign conditions like liver dysfunction, hydroneph rosis, peptic ulcer disease, pancreatitis, biliary obstruction, bowel obstruction, inflammatory bowel disease, and post-5-fluroouracil (5-FU)/-levamisole chemotherapy.

According to the available literature, the primary use of CEA is in monitoring colorectal cancer (CRC), especially when the disease has metastasized. CEA has a low sensitivity and specificity; therefore it has not been recommended for the screening, diagnosis, staging, or follow-up in patients with lung or breast cancer after primary therapy. [10] But it is very helpful in monitoring the treatment response. [11] This marker is used to follow-up the colorectal cancers rather than for the purpose of diagnosis. [12]

CA 19-9 (Cancer antigen 19-9)

CA 19-9 is a tumor marker of first choice for cancer pancreas and cancer gall bladder. The marker is 210 kD tumor associated glycoprotein antigen. It is present as carbohydrate determinant on glycolipid and glycoprotein. Appreciable concentration of CA 19-9 is also present in mucin rich saliva, seminal fluid, gastric juice, amniotic fluid, urine, ovarian cyst fluid, pancreatic, gall bladder and duodenal secretions. [2]

CA 19-9 is an intracellular adhesion molecule. It is elevated primarily in patients with pancreatic and biliary tract malignancies. CA 19-9 can be found in healthy individuals. It is also increased in sera obtained from patients with gastrointestinal adenocarciomas and squamous cell carcinomas. CA 19-9 levels are also elevated in benign conditions such as cirrhosis, cholestasis, cholangitis, and pancreatitis. [11]

In 99.6% of healthy adults, serum CA 19-9 levels are lower than 37 u/ml. In malignant tumors the values are elevated to more than 100,000 U/L. CA-19-9 is neither tumor specific nor organ specific. The diagnostic sensitivity (85 %) and specificity (95 %) of CA-19-9 are highest for the adenocarcinoma pancreas. This tumor marker can be used for diagnosis and monitoring of the treatment response. [11]


  Mesenchymal Markers Top


Desmin

Desmin belongs to the family of intermediate filaments. It has a molecular weight of 53kD. It is a cytoplasmic protein. It is found in muscle cells. It serves as an integral part of the cytoskeleton of cardiac, skeletal and smooth muscle fibers. Desmin is a muscle cell marker. Antibody to desmin reacts with striated muscle (skeletal and cardiac) as well as smooth muscle cells. In skeletal and cardiac muscles, it is confined to the Z zone between the myofibrils, and it serves as a binding material for fibrils. In smooth muscle it is associated with cytoplasmic dense bodies and subplasmalemmal dense plaques.

Desmin can be identified immunohistochemically within skeletal, cardiac, and smooth muscle. Desmin is encountered in some subsets of myofibroblasts, reticulum cells of lymph node, endometrial stromal cells, fetal mesothelium, stromal cells of fetal kidney, and chorionic villi. Desmin is considered to be highly specific for myogenic tumors, including leiomyomas derived from smooth muscle and rhabdomyosarcomas derived from skeletal muscle. [5] Desmin levels are elevated in cases such as Rhabdomyosarcomas and Leiomyosarcomas. Desmin can also differentiate rhabdomyosarcoma from other types of round cell tumors. Desmin immunoreactivity is identified focally in various spindle cell lesions which are traditionally not considered smooth muscle, but are of focal myoblastic differentiation which includes fibromatosis, malignant fibrous histiocytoma and myofibroblastoma of breast. Many sarcomas which have no relationship to smooth muscle such as malignant peripheral nerve sheath tumor, epithelioid sarcoma, liposarcoma and angiomatoid fibrous histiocytoma also express desmin positivity. Desmin is mainly used as a diagnostic marker. [13]

Actin

Actins are a family of contractile proteins. They are composed of molecular weight of about 42 kD. They are seen distributed in mammalian cells. It can be divided in to alpha, beta, and gamma subtypes depending on electrophoretic mobility. The most widely used actin antibody (HHF-35) reacts with alpha and gamma smooth muscle actins and therefore will be seen in cardiac, skeletal, and smooth muscle cells from all sites. It also recognizes pericytes, myoepithelial cells, and myofibroblasts. This can be used for immunostaining of myofibroblastic cells within granulation tissue, scar tissue, nodular fasciitis, and fibromatosis. In context of round cell sarcoma, immunoreactivity for actin would be highly suggestive of rhabdomyosarcoma, especially since other round cell sarcomas such as neuroblastoma and Ewing's sarcoma are usually negative. Elevated levels of actin can be seen in Rhabdomyosarcoma. Thus it acts as a marker for Rhabdomyosarcoma. It is mainly used as a diagnostic marker. [14]

Myoglobin

Myoglobin is the oxygen binding heme protein. It is a protein present in humans. It is encoded by the MB gene. It has a molecular weight of 17,800 daltons. It is found exclusively in skeletal and cardiac muscle. It is more specific and less sensitive antigen. It is primary oxygen-carrying pigment of the muscle tissues. It is suggested that myoglobin may be transferred from skeletal muscle to the infiltrating tumor cells. Reactive histiocytes and even carcinomas; lymphomas and malignant melanomas; when infiltrating skeletal muscle may show a positive reaction for myoglobin. It acts as a marker for Rhabdomyosarcomas. Desmin is found in a higher percentage of rhabdomyosarcomas than myoglobin. Myoglobin is used as a diagnostic marker. [15]

Vascular antigens

CD 34 (Human progenitor cell antigen)

The CD-34 antigen is a protein. It has a molecular weight of 105-120 kD. It is encoded by a gene which is located on the chromosome 1 q. It is expressed on the surface of hematopoietic progenitor cells of lymphoid and myeloid lineage in the bone marrow and in some acute leukemias. It has been identified in vascular endothelial cells and fibroblasts.

It acts as a specific marker of endothelial differentiation. It can be detected in nearly all benign vascular tumors. It is present in malignant vascular tumors such as Kaposi's sarcoma. It is seen in a variety of non-vascular tumors such as dermatofibrosarcoma protuberans, solitary fibrous tumors of pleura and peritoneum, benign nerve sheath tumors of gastrointestinal and soft tissue origin. [16]

CD-31 (Platelet-endothelial cell adhesion molecule; PECAM-1)

PECAM-1 is a 130 kD integral membrane glycoprotein. Previously it was thought to be a member of the immunoglobulin super family. Now it is considered as a marker of endothelial differentiation. It is present on the surface of endothelial cells as well as various hematopoietic cells, including megakaryocytes, platelets, and plasma cells. This antigen is expressed by all benign vascular tumors. In malignancies it is expressed in angiosarcomas. It is expressed by non-vascular tumors such as sporadic case of malignant mesothelioma and leiomyosarcoma. [17]

Neural antigens

S-100 protein

S-100 protein is so named because of its 100% solubility in ammonium sulphate. It is an acidic protein. It is widely distributed in the central and peripheral nervous systems. Its function is unknown, but its relation to calcium and potassium has lead to the hypothesis that it plays a role in ionic regulation in the brain. [18] S-100 is a low molecular weight protein. It is a member of a family of calcium-binding proteins isolated from bovine brain. It is expressed in glia, schwann cells, melanocytes, Langerhans cells of the epidermis, histiocytes, chondrocytes, lipocytes, skeletal and cardiac muscle, myoepithelial cells and some epithelial cells of the breast, salivary and sweat gland epithelium. [5]

It is used in the diagnosis of soft tissue lesions such as benign nerve sheath tumors and melanoma. It is present in virtually all neurilemmomas and neurifibromas. S-100 protein can be identified within granular cell tumors which are of neural origin. It is helpful in separating malignant peripheral nerve sheath tumors from other similar appearing sarcomas (e.g., fibrosarcoma) [18] S-100 protein can be found in most melanoma cells. Tissue samples may be tested for this marker to help in diagnosing melanoma metastasis. [10] S-100 protein is present within occasional non-neural tissues and tumors. It has been identified in normal adipocytes, lipomas, and occasional liposarcomas. It is also identified in chondrocytes, chondromas, chondrosarcomas and synovial sarcomas. [19]

Serum S-100 was first described to be elevated in metastatic malignant melanoma by Fagnart et al. 1988. Serum S-100 in the sera of melanoma patients has been thought to be derived from tumor tissue. Expression of S-100 is found in malignant melanoma. Elevated serum levels of S-100 have been described in metastatic malignant melanoma. [20] Previously, increased serum S-100 has been identified as a marker of disease progression in metastatic malignant melanoma. Therefore, serum S-100 might be a useful adjunct in the clinical staging and monitoring of metastatic malignant melanoma. S-100 acts as a good prognostic marker for overall survival in metastatic malignant melanoma. [20]

Neuron-specific enolase

Neuron-specific enolase (NSE) is a member of the family of enolase dimeric isoenzymes. These are formed of three types of subunits: Alpha, beta and gamma. [5] NSE is the gamma subunit of enolase enzyme. It is present predominantly in neurons and neuroendocrine cells. [2] NSE has been detected in patients with neuroblastoma, small cell lung cancer (SCLC), Wilms' tumor, melanoma, and cancers of the thyroid, kidney, testicle, and pancreas. [11] It is a marker of first choice for SCLC. Monitoring of NSE concentrations is also utilized in assessing prognosis and monitoring therapy in 85% of neuroblastomas and SCLC. [2] This marker can be used for patient's prognosis, and the patient's response to treatment. This marker is not used as a screening test for cancer. [11]

Glial acidic fibrillary protein

Glial acidic fibrillary protein (GFAP) is a 51 kD intermediate filament protein. It is expressed by glial cells, such as astrocytes, oligodendrocytes and ependymal cells of the central nervous system. Apart from the central nervous system, it has also been reported in myoepithelial cells of the salivary gland and in breast epithelial cells. It is a useful antigen in the diagnosis of glial versus non-glial tumors of the central nervous system, but its use is restricted to soft tissue. Chondroid syringomas, schwannomas, neurofibromas and pleomorphic adenomas of the salivary glands also express GAFP. [5]

Synaptophysin

It is a membrane protein found in the presynaptic vesicles of nerve cells. It can be identified within the nerve cells of peripheral, central nervous system and neuroendocrine cells. Neuroblastic tumors (neuroblastoma, ganglioneuroblastoma, ganglioneuroma) and paragangliomas also contain this membrane protein. [21]


  Proliferation Markers Top


AgNOR

AgNOR-Silver stained (Ag) nucleolar organizer regions (NORs). Nucleolar organizing regions (NORs) are loops of rDNA placed in the nucleolus. They play an important role in the synthesis of ribosomes and gene proteins. In human karyotype, NORs are located on each of the short arms of acrocentric chromosomes 13, 14, 15, 21, and 22. NORs can be detected by staining with silver nitrate and the structures thus demonstrated are termed AgNORs. Simple silver staining technique can recognize these argyrophil associated proteins. They appear as black dots after silver staining in nucleolar and extranucleolar regions. In a normal cell 20 black dots of AgNORs can be seen (2 per arm of chromosome i.e. 2 × 10 = 20) but only one or two dots are seen as the dots are tightly packed. In the dysplastic cells and malignant cells, as the amount of DNA increases, the number of AgNOR dots (AgNOR count) also increases.

The number and size of NOR dots in malignant cells is significantly different from that in normal or benign cells. Marked cellular atypia is present in repair and regeneration of squamous and columnar epithelia. Cells from epithelial repair have enlarged nuclei which vary in size and shape. Parts of nuclei are hyperchromatic due to unevenly distributed chromatin, nuclear membrane is irregular, and nucleoli are prominent and irregular in size and shape. The AgNOR technique provides an index of cell proliferation. The number, shape and distribution of AgNOR dots counted in the cell gives information not only about the morphology but also about the behaviour of the cell. There is progressive increase in the mean AgNOR count in cells with squamous metaplasia and in cells undergoing repair up to the various grades of Cervical Intraepithelial Neoplasia demonstrating ongoing proliferation.

AgNOR count is a reproducible simple efficient and inexpensive method which can be used as an adjunct to routine cytology and histopathology for diagnosis of cervical intraepithelial neoplasia especially in doubtful cases. Studies have reported that dysplasia with low AgNOR counts are more likely to regress as compared to those with high AgNOR counts. High risk counts are likely to progress to invasive cancer showing its prognostic significance. AgNOR is the best diagnostic marker for cervical intraepithelial neoplasia. It acts as a proliferation marker also. [22] AgNOR is a better marker of malignant changes in pancreatic duct cell adenocarcinoma. It is also useful in the diagnosis of lymphoma, breast cancer, prostatic tumors, oral cavity tumors, and other tumors. [23]

Malignant tumors were found to contain more AgNORs than their benign counterparts. [24] AgNOR can be used to differentiate malignant and benign neoplasms and grading of malignant neoplasms. AgNOR method can be applied to a wide range of diagnostic problem cases such as breast, salivary glands, and lung malignant neoplasms. [25] It is well known that definitive histological diagnosis of NHL is often difficult. However, treatment, prognosis, and 5-year survival rates are closely related to the histological grading and staging of tumor. Due to their shortcomings, present NHL classifications fail to effectively predict biologic behavior and treatment plans for this diseases. For these reasons, alternative methods like AgNOR staining could be employed to determine cell proliferation rate and differentiation of benign from malignant neoplasms like NHL, bone tumor, skin tumor, and liver tumor.

Enzymes and isoenzymes

Prostatic acid phosphatase

Prostatic acid phosphatase (PAP) is biochemically a glycoprotein of 100,000 molecular weight, consisting of two dimmers and approximately 10% carbohydrate and 90% peptide with microheterogeneity (i.e., multiple isoelectric points). N-terminal amino-acid is lysine and partial sequence is also known. Acid phosphatase activity is 200 times more abundant in prostate tissue than in any other tissue. PAP is useful in identifying metastatic prostate cancer and in monitoring disease status and therapeutic response of prostate cancer. Its activity in serum can be estimated by several synthetic substrates, but now specific antibodies are available for immunoassays. [2] It is rarely used now, because the PSA test is much more sensitive.

Prostate specific antigen

Prostate specific antigen (PSA) was termed earlier as gammaseminoprotein due to its presence in seminal plasma. PSA is 34 kD single chain glycoprotein consisting of 93% amino acids and 7% carbohydrate. It is a monomer made up of 240 amino acid residues. PSA is a neutral serine protease, having chymotrypsin and trypsin like activities and it belongs to glandular kallikrein family. [2] Prostate-specific antigen is a glycoprotein produced by prostatic epithelium. The PSA level can be elevated in prostate cancer, prostatitis, benign prostatic hypertrophy (BPH), and prostatic trauma, as well as after ejaculation. [11] Elevated levels of PSA may be found in the blood of men with benign prostate conditions, such as prostatitis and BPH, or with a malignant growth in the prostate. [10] PSA is synthesized in very low quantity by normal healthy prostate, in moderate quantity by inflamed or hyperplastic prostate and in excess amount by the malignant prostate. PSA has emerged as the most useful and clinically relevant tumor marker for carcinoma of prostate. This tumor marker is useful for screening, diagnosis, follow-up after primary treatment and very much helpful in monitoring the treatment response. [11]

Lysozyme

Lysozyme is an enzyme and is found primarily in monocytes and neutrophils. An elevated level of lysozyme is commonly associated with granulocyte destruction. Elevated serum lysozyme levels are seen in acute granulocytic leukemia, acute myelomonocytic leukemia and acute myeloid leukemia. It is rarely elevated in acute lymphoid leukemia. This is shown to be of value in following the disease course and chemotherapy. [26]

Protein

Ferritin

It is the major protein of human tissues and is also found in small quantities in serum. [27] Ferritin is a well known protein related to iron storage and metabolism. It is present abundantly in the liver, spleen and bone marrow. Biochemically it exists in isomeric forms and is generally termed as isoferritins. Serum Ferritin is one of the acute phase reactant. It is an intracellular protein and plays a role in storage of iron. Increased level of serum ferritin is reported in cancer patients. Ferritin levels are also increased in advanced cancers of breast, ovaries, lungs, colon and esophagus. Elevated levels of ferritin are also seen in acute myelocytic leukemia and teratoblastoma, [2] Squamous Cell Carcinoma of head and neck. [2],[27] Serum ferritin can be used in evaluating prognosis and for monitoring treatment response of the disease. [27]

Monoclonal immunoglobulin/paraprotein

Abnormal immunoglobulins are well known tumor markers. In fact, Bence Jones protein is perhaps the first tumor marker used effectively in clinical diagnosis of cancer patients. [1] Monoclonal gammapathies are most commonly observed in cancer patients involving plasma cells where a single malignant clone of precursor cell outgrow other cells and resulting in the production of a homogenous monoclonal immunoglobulin. Biclonal gammapathies are rare but do occur. Monoclonal immunoglobulin content is of great value in diagnosis as well as for monitoring efficacy of therapeutic management of plasma cell neoplasms namely multiple Myeloma, Waldenstrom's macroglobulinemia, plasmacytoma, B cell leukaemias and lymphomas. [2] It can be used for diagnosis and prognosis. [1]




  Hormone Receptor Top


Estrogen receptor, Progesterone receptor

Estrogen receptor (ER) is a 70 kD protein. It is present in nuclei of mammary and uterine tissues. ER and progesterone receptor (PR) belong to receptor super gene family. [2] These are intracellular receptors that are measured directly in tumor tissue. [10] They include receptors for thyroid hormone, vitamin D3 and retinoic acid. [4] In breast tumor patients the ER and PR measurements help in identifying whether the patients are likely to achieve benefit from endocrine therapy. [2]

PR is a more sensitive indicator than ER in predicting effective responsiveness to endocrine therapy. [2] Mostly the ER and the PR are not only used to predict the probability of response to hormonal therapy at the time of diagnosis, but also to predict the likelihood of recurrence. Patients with invasive breast cancer whose tumors lack ERs and PRs may not respond to or derive benefit from hormonal therapy. [10] Estrogen and PRs are usually used for diagnosis and not used for monitoring or follow-up, as their measurements are determined directly from tissue samples. [10]


  Conclusion Top


Tumor markers are precious tools for screening a healthy and a high risk population for the presence of cancer, making a diagnosis of a specific type of cancer, along with determining the prognosis and monitoring the course of the disease in the patient, at the time of remission or during the course of treatment.

To date, no specific marker has been established as a practical cancer screening tool. The reason for this is the relative lack of sensitivity and specificity of the available tests. Hence, more focus has to be directed towards identification and proper use of suitable tumor markers which may prove to be an invaluable tool to detect the deadly disease in mankind.

 
  References Top

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  In this article
Abstract
Introduction
Epithelial Markers
Mesenchymal Markers
Proliferation Ma...
Hormone Receptor
Conclusion
References

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