Cytogenetics is a branch of genetics concerned with the study of chromosomal structure, location, and function of the cells. The study includes chromosome number, appearance or karyotyping, location of genes on chromosomes and chromosomal behavior in cell processes such as cell division.
There are various techniques used to study chromosomes which we will discuss later.
History and Development of Cytogenetics
Chromosomes were first observed in plant cells by Karl Wilhelm von Nägeli in 1842. However, the term was coined by von Waldeyer in 1888.
It was discovered and accepted in 1956 that humans have 46 chromosomes.
After chromosomes could be counted, diseases with aberrant chromosomes or chromosome number were discovered.
Few examples are
• Down syndrome
• Turner syndrome,
• Klinefelter syndrome
• Patau syndrome [trisomy 13]
• Edwards syndrome [trisomy 18]
The Philadephia chromosome was found in 1960 as a small chromosome in the white blood cells of patients with chronic myeloid leukemia. After 13 years, it was shown to be the result of a translocation of chromosomes 9 and 22. Identification of the Philadelphia chromosome by cytogenetics is diagnostic for chronic myeloid leukemia.
Cytogenetics is continuously evolving and expanding. Molecular cytogenetics and cancer cytogenetics have added greater dimension to this science.
Molecular cytogenetics involves the combination of molecular biology and cytogenetics. Using various reagents, it works to distinguish normal and abnormal cells.
Advances the molecular cytogenetics were made in the 1980s. The progress was made using a technique called fluorescence in situ hybridization or FISH.
Advances in micromanipulation and examination of chromosomes including the technique of chromosome microdissection whereby aberrations in chromosomal structure could be isolated, cloned and studied made the minuter study possible.
Molecular cytogenetics is a useful tool for the diagnosis and treatment of various malignancies such as brain tumors, hematological malignancies, etc.
The entire genome can be assessed for copy number changes using virtual karyotyping generated from arrays made of thousands to millions of probes.
Cancer cytogenetics is molecular genetics dedicated to the study of the chromosomal behavior – number, an aberration for the oncogenic effect and the effect of treatments.
Various Techniques Employed by Cytogenetics
Banding
When chromosomes are stained with a dye, there is a banded pattern of alternating light and dark regions along the length of a chromosome.
There are many methods of banding but G-banding or Giemsa banding is the benchmark for the routine analysis of human chromosomes.
The molecular mechanism and reason for these patterns are unknown.
Karyotyping
Karyotyping refers to the chromosome analysis of metaphase chromosomes [when the cell is in the metaphase stage of cell division].
For Karyotying, the chromosomes are banded by using Giemsa and analyzed under a microscope. Generally, 20 cells are analyzed for enough representation.
Fluorescent in situ hybridization or FISH
FISH is a method to visualize chromosomes using fluorescent-labeled probes. This technology permits the detection of specific nucleic acid sequences in morphologically preserved chromosomes, cells, and tissues.
This method is useful in investigating patients with various congenital and malignant neoplastic disorders [like leukemia and lymphoma]
Spectral Karyotyping (SKY) and Multicolour FISH (M-FISH)
Spectral karyotyping (SKY) or multicolor FISH (M-FISH) are even more advanced than FISH.
M-FISH allows all of the 24 human chromosomes to be painted in different colors. By using various combinations and concentrations of fluorescent dyes, it is even possible to give every single chromosome a different color. This can be particularly useful when dealing with complex aberrations as in solid tumors.
Comparative Genomic Hybridisation
Comparative genomic hybridization is a technique that allows the detection of chromosomal copy number changes without the need for cell culture. It provides a global overview of chromosomal gains and losses throughout the whole genome of a tumor.
The array-CGH is even more promising. Array-CGH is the equivalent of conducting thousands of FISH tests at once and is faster. Using array CGH, the laboratory is able to look at thousands of separate sections of DNA with a better resolution.
Future of cytogenetics
Advances now focus on molecular cytogenetics. This includes automated systems for counting the results of standard FISH and techniques for virtual karyotyping.