細胞遺伝学

Cytogenetics is the study of chromosome structure and behavior during cell division and heredity. It enables direct observation of a cell's karyotype—its chromosome number and structure—to detect abnormalities associated with diseases. For example, the Philadelphia chromosome causes chronic myelogenous leukemia, while Trisomy 21 results in Down syndrome. These chromosomal defects are often detectable through cytogenetic analysis, making it essential for medical genetics research history and modern techniques.

Chromosome staining uses various dyes to highlight specific features. G-banding marks gene-poor regions rich in A-T bases, while R-banding highlights G-C-rich regions. C-banding reveals the densest regions around the centromere, and T-banding highlights telomeres at chromosome ends. These distinctive banding patterns allow researchers to identify structural abnormalities and arrange chromosomes for karyotype analysis.

Fluorescence in situ hybridization (FISH) uses fluorescently labeled oligonucleotide probes to detect specific chromosomes, DNA regions, or RNA transcripts. Unlike classical karyotyping, FISH can hybridize to uncondensed chromosomes in non-dividing cells, making it more broadly applicable. This flexibility allows researchers to screen for chromosomal abnormalities in diverse cell types and clinical samples more efficiently.

FISH preparation involves fixing tissues with paraformaldehyde to preserve chromosome morphology, creating a chromosome spread through mechanical disruption, and dehydrating in ethanol solutions. Chromosomes are then permeabilized with pepsin, denatured with formamide to separate DNA strands, and dehydrated again. Finally, fluorescently labeled probes are applied, unbound probe is washed away, and samples are mounted with a counterstain like DAPI for microscopic observation.

Comparative genomic hybridization isolates and fragments genomic DNA from test and control subjects, labeling each with different colored fluorophores. These preparations are mixed and competitively hybridized to a normal chromosome spread. Colors indicate whether a region is duplicated in the test sample, generating more binding fragments, or deleted, resulting in preferential control fragment binding and revealing chromosomal imbalances.

FISH enables preimplantation genetic diagnosis by analyzing a single blastomere biopsied from an embryo three days after fertilization. The cell is lysed and fixed on a microscope slide, then hybridized with probes targeting potential chromosomal disorders. Deviations from expected hybridization signal patterns indicate disruptions in chromosome number or structure, allowing selection of chromosomally normal embryos for implantation.

Spectral karyotyping (SKY) labels all chromosomes with multiple chromosome-specific probes bearing different fluorescent labels, giving each chromosome a unique spectral color. This concurrent observation of all labeled chromosomes in a single mixture is particularly useful for identifying karyotype defects, including aneuploidy, deletions, and translocations. SKY provides comprehensive chromosomal analysis from a single hybridization event.