Background

There are several types of radiation including alpha-, beta-, gamma-, and neutron radiation. The way that radiation affects our cells is very complex and depends on many factors including the radiation type, the exposure time and magnitude, shielding and distance from the radiation source, and the type of cells that were exposed to the radiation. In accident conditions, methods including external dosimetry (‘badges’), internal dosimetry (detection of radionuclides in body fluid samples) and biodosimetry (the detection of changes in human cells or cell components due to radiation exposure) can be used to determine how much radiation a person has received.

Gamma radiation can penetrate objects easily (Brain, 1998-2004), but deposits little energy in one place. However, gamma can still cause interchromosomal translocations and (much more rarely) intrachromosomal rearrangements. To detect these translocations, methods such as the dicentric chromosome assay (DCA) (Thrower and Bloom, 2001), and whole chromosomal fluorescence in situ hybridization (whole chromosomal FISH) are used. DCA detects rearrangements that produce a chromosome with more than one centromere. Whole chromosomal FISH is used to ‘paint’ whole chromosomes a specific colour. In this way, interchromosomal rearrangements are quantified by counting unexpected colour junctions. A colour junction is when, for example, there are both green- and red-coloured segments on one chromosome (Figure 1). When such an observation is made, it is evident that a rearrangement has taken place since the green chromosome must have had an exchange (translocation) with a red chromosome.

Figure 1: Example of translocation

Note that both the DCA and whole chromosome FISH techniques test for alterations in the segments between two chromosomes. However these methods do not detect any intrachromosomal alterations (inversions). Radiation types that deposit their energy over a short distance such as alpha radiation can more frequently cause intrachromosomal alterations than gamma radiation because these types of radiation deposit their energy in one region, so several strand breaks may occur along one chromosome. Alpha particles can cause both interchromosomal and intrachromosomal rearrangements, (Anderson, Stevens, et al., 2002) but when considering alpha exposures, the use of only the DCA or whole chromosome FISH may underestimate the dose, since intrachromosomal mutations are undetected and thus not accounted for.

A New Method

M- banding (multi-coloured banding) is introduced primarily for the purpose of observing intrachromosomal alterations. M- banding, a method of colouring a single chromosome with different bands of colour, can be used to detect intrachromosomal mutations. Using this technique, a set pattern of colours is produced along the length of the chromosome. Any change in the order of the colours will indicate an intrachromosomal rearrangement.

The question then is why chromosome 8 is used rather than the other 22 chromosomes in the human karyotype (Figure 2). The first reason for using chromosome 8 is for its size. Chromosome 1 is the largest, chromosome 22 is the smallest, and the 23rd chromosomal pair is the sex-determining pair. The reason that chromosome 8, which consists of approximately 155 million base pairs, is used is because it is in the middle of the human karyotype; it is not too big to work with, but not too small (so a sufficient number of bands can be observed).

The Human Karyotype

Figure 2: The Human Karyotype (Genome News Network, 2003)

Mutations of the genes on chromosome 8 may also result in many of the diseases that humans may develop during their lifetime. Alterations in chromosome 8 may lead to diseases such as: liver cancer, metal retardation, epilepsy, monocytic leukemia, etc. The possible applications to cancer research make chromosome 8 more appealing to work on.

Once probes for all of chromosome 8 are prepared, and a barcode is achieved, the cells can be exposed, in vitro, to radiation and the frequency of intrachromosomal rearrangements can be quantified. This method of M-Banding can then be used in conjunction with other biodosimetric assays to increase the effectiveness and accuracy of radiation dose estimates.