Genetic disorders
Cytogenetics is a branch of genetics that focuses on the study of chromosomes and their role in inheritance and disease. It involves various specialized fields, including oncology cytogenetics, which examines chromosomal alterations in cancer, and reproductive cytogenetics, which identifies genetic conditions such as Down syndrome through chromosome analysis. The field utilizes techniques such as karyotyping to visualize and classify chromosomes, allowing for the detection of abnormalities like translocations and deletions.
Cytogenetic analysis typically begins with cell collection, followed by culture and chemical treatment to visualize chromosomes, culminating in the creation of a karyotype for evaluation. The process can take several days to weeks, with results aiding in the diagnosis and treatment of various conditions, particularly cancers. As the field has evolved, molecular cytogenetics has emerged, integrating molecular biology techniques like fluorescence in situ hybridization (FISH) to identify smaller chromosomal abnormalities with greater precision. This ongoing advancement in cytogenetics is crucial for understanding genetic disorders and guiding personalized medical strategies.
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Subject Terms
Cytogenetics
Also known as: Chromosome analysis, karyotyping
Definition: Cytogenetics is the study of chromosomes as seen under the microscope. Observation of gain, loss, or breakage of chromosomes is used to diagnose cancers. Many cancers are defined by cytogenetic test results.
Subspecialties:Oncology cytogenetics, reproductive cytogenetics, molecular cytogenetics
Cancers treated: All
Training and certification: Technologists perform cytogenetic analysis under the direction of laboratory directors and technical supervisors. In the United States, laboratory directors and technical supervisors are medical doctors or scientists with doctoral degrees who are experienced in cytogenetics and certified in clinical cytogenetics by the American Board of Medical Genetics and Genomics (AMBGG). In addition, they may be required to pass state board examinations. Supervisors and technologists are certified by the American Society for Clinical Pathology (ASCP) Board of Certification. ASCP certification is not required to practice in the United States, although several states require professional licensing for cytogeneticists to practice. The licenses very closely follow the requirements of the ASCP certification, and California accepts all ASCP examinations for state licensing purposes, while New York accepts the ASCP exam for cytotechnologist (CT) as well as those for medical laboratory technician (MLT), medical laboratory scientist (MLS), and histotechnician (HT).
ASCP certification as a cytotechnologist (CT(ASCP)), technologist in cytogenetics (CG(ASCP)), or specialist in cytotechnology (SCT(ASCP)) is available to those who have appropriate educational backgrounds. An applicant for CT(ASCP) certification must have completed a bachelor's degree and a cytotechnology program accredited by the Commission on Accreditation of Allied Health Education Programs (CAAHEP), while an applicant for CG(ASCP) certification has three options: one, a bachelor's degree and completion of a cytogenetics education program accredited by the National Accrediting Agency for Clinical Laboratory Sciences (NAACLS); two, a bachelor's degree with either a major or thirty semester hours of course work in chemistry, medical sciences, or biological sciences, plus one year of qualifying full-time work experience; or three, a graduate degree in genetics or molecular biology, plus nine months of full-time work experience. To qualify for SCT(ASCP) certification, one must already have CT(ASCP) certification, plus a bachelor's degree and three years of full-time experience following prior certification.
Services and procedures performed: In cytogenetic analysis, cells are collected from a patient and then grown in cell culture. The cells are chemically treated so that the chromosomes are visible and grown in culture media that support the growth of that particular cell type. A chemical that stops cells during cell division is used to arrest the dividing cells at the metaphase stage, when the chromosomes are most condensed.
Depending on the specimen type, the chromosomes may be either grown on or later dropped onto microscope slides. The resulting slides are then stained using various special techniques. The staining results in a unique banding pattern for each of the chromosomes. This allows the identification of the chromosomes and enables cytogeneticists to identify abnormal chromosomes. Multiple cells are analyzed by the technicians.
The length of the process, from collecting the sample at the physician’s office or hospital to processing, analysis, and reporting the results to the physician, varies depending on the tissue type. Cell growth can take from just a few days to more than a week.
The results are given in the form of a photograph of the chromosomes (a karyotype) and a written report. The karyotype shows the chromosomes aligned and paired by their numerical classification. After the metaphase cells are karyotyped, it is much easier to compare the bands and to identify abnormalities. Typical abnormal results include the loss or gain of a whole chromosome, the loss of part of a chromosome, inversion (when a middle piece of one chromosome is turned upside down), and translocation (when two or more chromosomes break and exchange parts). One or many changes may happen in any one tumor cell, and the cancer can be complicated by the presence of many different but related cancer-cell lines that are mutating and changing continuously. These changes can include loss or gain of different abnormal chromosomes and structural abnormalities. Doctors can determine the type and severity of the cancer based on the results of cytogenetic tests. After successful treatment, subsequent cytogenetic testing should show normal chromosomes.
Related specialties and subspecialties: In addition to studying cancer, cytogeneticists can identify birth defects such as Down syndrome, which is characterized by the presence of an extra chromosome 21. This use of chromosome analysis is referred to as reproductive cytogenetics. Amniocentesis specimens are most commonly used in reproductive cytogenetics.
Beginning in the 1980s, a new branch of cytogenetics began to develop. Molecular cytogenetics combines cytogenetics with molecular biology and applies deoxyribonucleic acid (DNA)–based tests to the cell samples. A major technique of molecular cytogenetics is called fluorescence in situ hybridization, or FISH, which uses the complementary nature of DNA to answer specific questions about particular regions of chromosomes. FISH can be used to identify abnormalities that may be too small to be seen using conventional cytogenetic methods. The chromosomes being analyzed are “probed” with tiny pieces of fluorescently labeled DNA, which attaches to other DNA of the same kind (complementary sequences). For example, a fluorescent probe for the TP53oncogene, which produces a substance called tumor protein p53, may be used to identify the presence or absence of the gene in a patient sample. If the TP53 gene is present, the probe will attach, and a fluorescent microscope will reveal a visible signal in the location of the gene. An abnormal specimen would lack this signal, indicating a deletion of the TP53 gene.
Bibliography
ASCP Board of Certification. US (Only) Procedures for Examination & Certification. Chicago: ASCP Board of Certification, 2014. ASCP. Web. 19 Sept. 2014.
Campbell, Lynda J., ed. Cancer Cytogenetics: Methods and Protocols. 2nd ed. New York: Humana, 2011. Print.
Das, Kakoli, and Patrick Tan. "Molecular Cytogenetics: Recent Developments and Applications in Cancer." Clinical Genetics 84.4 (2013): 315–25. Print.
Dolan, Michelle. "Conventional and Molecular Cytogenetics in Cancer." Molecular Testing in Cancer. Ed. George M. Yousef and Serge Jothy. New York: Springer, 2014. 15–25. Print.
Gersen, Steven L., and Martha B. Keagle, eds. The Principles of Clinical Cytogenetics. 3rd ed. New York: Springer, 2013. Print.
Harper, Peter S. First Years of Human Chromosomes: The Beginnings of Human Cytogenetics. Bloxham: Scion, 2006. Print.
Heim, Sverre, and Felix Mitelman, eds. Cancer Cytogenetics: Chromosomal and Molecular Genetic Aberrations of Tumor Cells. 3rd ed. Hoboken: Wiley, 2009. Print.
Mark, Hon Fong L., ed. Medical Cytogenetics. New York: Dekker, 2000. Print.
Riegel, Mariluce. "Human Molecular Cytogenetics: From Cells to Nucleotides." Supp. to Genetics and Molecular Biology 37.1 (2014): 194–209. Print.
Zneimer, Susan Mahler. Cytogenetic Abnormalities: Chromosomal, FISH and Microarray-Based Clinical Reporting. Hoboken: Wiley, 2014. Print.
Organizations and Professional Societies
National Credentialing Agency.
Association of Genetic Technologists.