Detectie van Inter-chromosomale Stable aberraties door Multiple Fluorescentie<em> In Situ</em> Hybridisatie (mFISH) en Spectral Karyotypering (SKY) in bestraalde muizen

The two most reliable methods of identifying radiation-induced inter-chromosomal stable aberrations are multiple fluorescence in situ hybridization (mFISH), which allows the painting of two or more chromosomes simultaneously, and spectral karyotyping (SKY), which imparts a distinct color to each homologous chromosome pair in the genome. Unlike unstable aberrations, stable aberrations are persistent in nature and may be propagated for several generations in irradiated populations¹, and are regarded as critical molecular “signatures” of radiation-induced cytogenetic lesions². Studies by various groups have shown that stable aberrations are associated with the pathogenesis and development of a number of diseases, including cancer³. Before the era of chromosome painting (also referred as molecular cytogenetics), the conventional G-banding technique was the only method for detecting stable chromosomal aberrations. However, chromosome banding is a challenge to cytogeneticists because the resolution is limited, reproducibility is uncertain, it is a labor-intensive procedure, and it requires highly skilled and experienced cytogeneticists for reliable data interpretation⁴. Moreover, the classic banding technique does not allow detection of complex chromosomal rearrangements, which involve the interaction of three or more breaks distributed among two or more chromosomes, a common outcome of radiation damage. Complex aberrations may persist in individuals many years after radiation exposure, making them useful for retrospective biodosimetry⁵. Therefore, an alternate approach was required to overcome the limitations of conventional banding techniques to detect stable chromosomal rearrangements.

In the late 1960s, the pioneering work of Gall and Pardue (1969) on molecular hybridization using nucleic acid probes labeled with radioactive material commenced a new era in the field of cytogenetics, which allowed detection of a specific DNA sequence on chromosomes⁶. However, the use of radioactive probes for molecular hybridization had several drawbacks: radioactive probes are relatively unstable, probe activity depends on radioactive decay of the isotope used, hybridization takes a longer time, the resolution is limited, the probes are relatively costly, and the radioactive materials are a health hazardous. Thus, it became necessary to develop and design non-radioactive probes. The introduction of fluorescent tagged nucleic acid probes in the 1980s and 1990s overcame the limitations of radioactive probes and greatly enhanced the safety, sensitivity, and specificity of the hybridization technique^7-10. Fluorescent probes give rise to extremely bright signals when observed under fluorescence microscopes equipped with the appropriate excitation and emission filters. Any loss, gain, or rearrangement of fluorescent labeled chromosome/s or a part of the chromosome is easily identifiable with this FISH technique.

Analysis of chromosomal aberrations by FISH painting has led to marked progress in cytogenetic research over the years. Designing fluorescent labeled probes for specific applications ranging from locus-specific probes to whole-chromosome painting probes has advanced the field significantly; this has also facilitated the detection of submicroscopic (“cryptic”) rearrangement, which was not possible by conventional chromosome banding. Chromosome painting by mFISH and SKY have proven to be valuable tools for the identification of simple and complex inter-chromosomal rearrangements. The basic principles for both techniques are similar, but the method of detection and discrimination of fluorescent signal after in situ hybridization and the process of image acquisition are different. In mFISH, separate images of each of the four fluorochromes are captured by using narrow bandpass microscope filters; dedicated software is then used to combine the images. While in SKY, image acquirement is based on a combination of epifluorescence microscopy, charge-coupled device imaging, and Fourier spectroscopy, which allows the measurement of the entire emission spectrum with a single exposure at all image points. In both mFISH and SKY, monochrome images are captured independently, then merged, and finally, unique pseudo-colors are assigned to the chromosomes in monochromatic images based on the specific dye attached to each fluorochrome probe.

The contribution of mFISH and SKY analysis in the radiation biology field is remarkable, particularly for retrospective dose estimation of human exposure to IR (radiation biodosimetry)^11-14, radiation carcinogenesis risk assessment¹⁵, as well as detection and risk estimation of radiotherapy-related secondary cancer¹⁶. A recent study on mice has shown that a FISH-based chromosome painting technique is also an important tool for evaluating the efficacy of radiation countermeasure¹⁷. In the present study, the effect of total body radiation exposure on the induction of stable chromosomal aberrations in the bone marrow cells of mice has been demonstrated using mFISH and SKY techniques.