Chromosome Mutations and Karyotyping Techniques

Chromosome Mutations

Part 1: Techniques for Studying Chromosome Aberrations

  • Chromosome aberrations can affect gene dosage, gene interaction, chromosomal architecture, and evolution.
  • Types of chromosome mutations include:
    • Loss/Deletion: loss of chromosome segments.
    • Gain/Duplication: gain of chromosome segments.
    • Translocation: segments moving from one chromosome to another.
    • Inversion: segments flipped in orientation.

What is a Chromosome?

  • DNA is highly packaged in chromosomes, with a packaging ratio of >1:10,000.
  • Human Chromosome 17 is an example of chromosome structure.
  • Chromosomes are highly condensed during the M phase of the cell cycle, making them visible under light microscopy.
  • Definitions for human cells:
    • n (number of different chromosomes) = 23
    • C (DNA content) = 3.5 pg

Karyotyping: Method 1 - G-banding

How to Visualize a Chromosome Set

  1. Sample preparation involves culturing lymphocytes in a mitotic medium, incubating at 37°C in 5% CO₂ for 72 hours.
  2. Use a mitotic inhibitor (e.g., Colcemid) to obtain metaphase chromosomes.
  3. Hypotonic shock (0.075 M KCl) is applied for chromosome dispersion.
  4. Chromosomes are stained using Giemsa, providing banding that reveals structural details.
  5. Nomenclature example: 46,XY represents a total of 46 chromosomes, including sex chromosomes (X, Y).

G-banding Details

  • A total of 400 bands visible when properly stained.
  • Dark bands are AT-rich and relatively gene-poor while light bands are GC-rich and relatively gene-rich.

Karyotyping: Method 2 - FISH

  • Fluorescence In Situ Hybridization (FISH): Chromosome hybridization with fluorescent probes to visualize specific DNA sequences.
    • (A) Standard FISH uses a single type of DNA fragment for chromosome preparation.
    • (B) Chromosome painting employs a complex mixture of DNA for multiple colors to identify different chromosomes.
  • Example: BCR-ABL1 fusion detected by a reciprocal translocation (t(9;22)), significant in chronic myeloid leukemia (CML).

Karyotyping: Method 3 - CGH & Array CGH

  • Comparative Genomic Hybridization (CGH) compares chromosomal regions between proband and reference DNA by hybridization.
  • Array CGH allows for the detection of microdeletions and gains using a mixture of labeled patient and reference DNA.

Chromosome Aberrations and Numerical Aberrations

  • Euploidy: normal set of chromosomes (e.g., diploid 2n = 46,XX).
  • Aneuploidy: abnormal number of chromosomes (e.g., monosomic 2n-1, trisomic 2n+1).

Non-Disjunction and Numerical Aberrations

  • Non-disjunction can lead to aneuploidies during meiosis I or II.
    • Examples of numerical aberrations:
    • Turner Syndrome (45,X)
    • Down Syndrome (Trisomy 21)
    • Edwards Syndrome (Trisomy 18)
    • Patau Syndrome (Trisomy 13)

Chromosome Aberrations: Frequency and Types

  • Frequency of various chromosomal aberrations per live births:
    • Pericentric inversion: 1:100
    • Balanced translocation: 1:500
    • Trisomy 21: 1:700
    • Unbalanced translocation: 1:2000

Structural Aberrations of Chromosomes

  • Types of structural changes include:
    • Translocations
    • Deletions
    • Inversions
    • Insertions
    • Duplications
  • These rearrangements can lead to gene imbalance and be caused by double-stranded DNA breaks.

Nomenclature for Chromosome Banding and Abnormalities

  • Karyotype examples include descriptions using numerical and structural information:
    • p: short arm
    • q: long arm
    • Example: 46,XY,del(5)(q13q33) indicates a deletion on chromosome 5 between bands q13 and q33.

Conclusion of Week's Learning

  • Chromosome studies utilize various staining techniques (G-banding, FISH, CGH) to identify karyotypes and aberrations.
  • Important aspects include the impact of altered chromosome numbers or structures on diseases.