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Cytogenetics: Fluorescence In Situ Hybridization (FISH), Chromosome Analysis, and Karyotyping


Definition


  • FISH: Molecular hybridization of a fluorescently labeled, cloned sequence of interest to a mitotic chromosome or interphase nucleus.
  • Chromosome analysis: Microscopic visual inspection of banded mitotic chromosomes that assess the entire genome with the ability to detect chromosome aberrations larger than approximately 5 " “10 megabases.
  • Karyotyping: An ordered pairing of chromosomes that aids in detecting chromosome anomalies.

Use


  • FISH
    • Assessment of a specific region of the genome; allows detection of abnormalities that are too small to be visualized by conventional cytogenetics (e.g., microdeletions, microduplications)
    • Also may be performed on interphase (nondividing) cells, eliminating the necessity of cell culture and thereby allowing rapid turnaround times, and assessment of specimens that contain few or no dividing cells
  • Chromosome analysis: used to identify abnormalities of chromosome number and structure that may be causal for mental retardation, congenital anomalies, pregnancy loss, infertility, and cancer
  • Karyotyping
    • A tool in chromosome analysis.
    • Sometimes used (incorrectly) to mean chromosome analysis; a karyotype is not a stand-alone test.

Interpretation


  • FISH
    • Normal (two intact copies of sequence in a diploid cell)
    • Abnormal: examples include deletion of the genomic region, additional copies of region, and positional rearrangement of region
  • Chromosome analysis
    • Normal: 46,XY (male) or 46,XX (female)
    • Abnormal:
      • Numeric: incorrect chromosome number (e.g., +21 in Down syndrome)
      • Structural: abnormal chromosome structure (e.g., deletion of the chromosome 5 short arm (5p ¢ ˆ ’) in Wolf-Hirschhorn syndrome, translocation such as t(9;22) in CML)

Limitations


  • FISH: A targeted test, it cannot provide the total genomic assessment provided by conventional chromosome analysis.
  • Chromosome analysis: requires dividing cells; therefore, all submitted specimens must contain viable cells that can be cultured in the laboratory.
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