Constitutional cytogenetics involves the detection of heritable genetic abnormalities in children and adults, whereas cancer cytogenetics is associated with the detection of acquired genetic abnormalities for the diagnosis and therapy of various types of cancer.
Constitutional cytogenetic analysis is done by studying amniotic fluid, fetal tissues, blood, bone marrow, or skin biopsy and may be recommended in the following cases:
Growth and developmental delay and short stature in children
Intellectual and learning disability in children
In the case of disorders like autism, Obsessive Compulsive Disorder (OCD), seizures, etc.
History of infertility or recurrent abortions
Birth of a child with a chromosomal anomaly
Birth of a child with malformations, dysmorphisms or other defects
If the mother is above 35 years of age
Abnormalities in the ultrasound of the fetus
History of chromosomal or genetic anomalies in the family
Conventional cytogenetics and molecular cytogenetics
Conventional cytogenetic methods to detect genetic changes include karyotyping on cells obtained from cell cultures using banding techniques like G-banding, R-banding, etc. Molecular cytogenetic methods include fluorescence in situ hybridization (FISH), multicolor FISH, reverse transcriptase PCR (RT-PCR), array CGH (comparative genomic hybridization), etc.
Conventional cytogenetics is the most widely used method to analyze chromosomal anomalies like the ones due to a change in the number or structure of chromosomes. However several issues like the need for fresh samples, the limited resolution of classic banding techniques lead to an increase in the use of molecular cytogenetic methods like FISH.
Techniques to view chromosomes
G-banding, Q-banding, R-banding and C-banding
In G-banding, chromosomes can be individually identified by first treating them with an enzyme called trypsin and then with Giemsa stain.
Q-banding is useful for detecting variations in chromosomal structures. This is performed by staining the chromosomes with quinacrine mustard and then examined by fluorescence microscopy.
In the R-banding method, chromosomes are first treated with heat, which enables the patterns to be easily analyzed.
In C-banding, only regions of chromosomes containing heterochromatin are stained. Fluorescent in situ Hybridization (FISH)
In this method, DNA or RNA probes, which are labelled with a distinct fluorescent dye, are hybridized with their complementary target DNA sequences on chromosomes. It allows the detection of specific nucleic acid sequences in chromosomes and the number or structure of chromosomes, cells, and tissues, which is used by cytogeneticists to detect chromosomal abnormalities.
Spectral Karyotyping (SKY) or Multicolor FISH (M-FISH)
FISH led to a more powerful method, known as M-FISH. M-FISH allowed each of the 24 pairs of human chromosomes to be painted in a different color. This allowed the simultaneous presentation of all 24 different human chromosomes. M-FISH makes use of chromosome-specific paints (fluorochromes).
Comparative Genomic Hybridization (CGH)
Seeing that FISH was time-intensive, CGH was developed. In this process, instead of hybridizing a single labeled probe to human chromosomes on a slide (as was the case in FISH), thousands of distinct probes could be printed on a glass slide. Array CGH equals conducting thousands of FISH experiments, in one go.
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