Comparative Genomic Hybridization (CGH) is a technique used to detect changes in the number of copies of DNA sequences (gains or losses) throughout the entire genome, typically in tumor cells, without needing to culture the cells first.
In simpler terms, CGH testing allows scientists and doctors to:
- Identify missing or extra pieces of chromosomes within a cell's DNA.
- Get a "big picture" overview of chromosomal abnormalities.
- Do all this directly from a sample, without having to grow the cells in a lab.
How CGH Works:
- DNA Extraction: DNA is extracted from both a patient sample (e.g., tumor cells) and a normal, healthy control sample.
- Labeling: The patient and control DNA are labeled with different fluorescent dyes (e.g., green for patient, red for control).
- Hybridization: The labeled DNA samples are mixed and hybridized (combined) onto a normal chromosome spread or a DNA microarray.
- Scanning and Analysis: A special scanner reads the fluorescence intensity along the chromosomes.
- Data Interpretation:
- A region that shows more of the patient's color (e.g., green) indicates a gain (amplification) of that particular DNA sequence in the patient's sample.
- A region that shows more of the control's color (e.g., red) indicates a loss (deletion) of that DNA sequence in the patient's sample.
- An equal balance of both colors indicates that the copy number is normal in that region.
Key Advantages of CGH:
- Genome-Wide Screening: It allows for the simultaneous analysis of the entire genome for copy number changes.
- No Cell Culturing Required: This is particularly useful for samples where cells are difficult to grow in the lab, such as many tumor samples.
- Detection of Novel Aberrations: CGH can identify previously unknown chromosomal abnormalities.
Limitations of CGH:
- Cannot Detect Balanced Translocations or Inversions: CGH only detects copy number changes; it cannot identify rearrangements where the total amount of DNA remains the same.
- Resolution Limitations: The resolution of traditional CGH is limited by the size of the chromosome bands. Array CGH (aCGH) offers higher resolution.
Types of CGH:
- Traditional CGH: Uses metaphase chromosome spreads for hybridization.
- Array CGH (aCGH): Uses DNA microarrays containing thousands of DNA probes to provide a higher resolution and more detailed analysis.
Applications of CGH:
- Cancer Research and Diagnostics: Identifying chromosomal abnormalities associated with different types of cancer.
- Prenatal Diagnosis: Detecting chromosomal abnormalities in fetuses.
- Genetic Research: Studying the role of copy number variations in various diseases and conditions.
