Traditional techniques
Cytogenetics - the study of genetics of cells/chromosomes, began in the 1950s when the exact number of human chromosomes was first established as 46. In the following years, several syndromes were identified to be caused by numerical chromosome aberrations, such as Down‘s syndrome (three copies of chromosome 21), Edward’s syndrome (three copies of chromosome 18), Patau’s syndrome (three copies of chromosome 13), or Ullrich-Turner’s syndrome (one copy of chromosome X). Despite the very low resolution level of microscopic evaluation of chromosomes at this time (about 20-30 Mb), some structural anomalies in the form of terminal deletions could be detected, for example deletion of the short arm of chromosome 5 - 5p (cri du chat syndrome) and deletion of 4p (Wolf-Hirschhorn‘s syndrome).
The introduction of chromosome staining techniques (banding) in the early 1970s allowed the analysis of the genome in greater detail (5-10 Mb) and led to the discovery of further deletion syndromes such as WAGR syndrome and Jacobsen’s syndrome. High-resolution banding could improve the resolution level to 3-5 Mb and led to the identification of the first microdeletion syndromes – Prader-Willi, DiGeorge and Smith-Magenis syndrome.
In the mid 1980s fluorescence in situ hybridization (FISH) became a standard method in the routine cytogenetics laboratory. FISH uses fluorescently labelled DNA probes, to specifically bind to individual regions of chromosomes - different coloured fluorescent labels can be used simultaneously to identify breaks and rearrangements of chromosomes in a genome. The use of FISH rapidly expanded the spectrum of microdeletion syndromes (Williams- Beuren syndrome, microdeletion 1p36) and revealed the first microduplication syndrome (microduplication 22q11.2). The great disadvantage of FISH is the requirement of a tentative pre-diagnosis, as FISH is a targeted technique and not a screening tool. FISH is however ideal to use as a confirmatory technique.
In the early 1990's a molecular cytogenetic method enabled the analysis of gains and losses of genetic material across all chromosomes. This method, termed comparative genomic hybridization (CGH), is a DNA-based method, which employs a competitive hybridization of differentially labelled DNA samples (for example DNA from the sample in green; chromosomally normal reference DNA in red) to normal metaphase chromosomes on a microscope slide. An excess of green fluorescence on a specific chromosome is indicative of chromosomal gain, whereas an excess of red fluorescence is indicative of chromosome loss. This technique was a pre-cursor to arrayCGH which improved the resolution of the assay by replacing the chromosomes with DNA clones spotted on a slide.
