532
signals on metaphase chromosomes. ScFISH probes are produced directly from
genomic DNA, and thus more quickly than by recombinant DNA techniques.
Single-copy probes have been developed for three chromosomal regions: the
CDC2L1 (chromosome 1p36), MAGEL2 (chromosome 15q11.2), and HIRA (chro-
mosome 22q11.2) genes. Abnormalities seen on metaphase chromosomes could be
characterized with scFISH probes at a resolution greater than previously possible.
Visualization of human chromosomes is a routine laboratory procedure but
commercially available FISH probes, covering 100–300 kilobases, or 100,000–
300,000 DNA bases, are too large for their targeted DNA. Single copy probes, by
contrast, are much smaller and more densely represented on a chromosome. They
can, therefore, detect smaller lesions in addition to being able to probe rare condi-
tions, whereas current clinically available probes principally detect relatively com-
mon abnormalities. Single copy probes thus can enable more precise treatments for
individuals, even differentiating between two patients suffering from what may
otherwise appear to be the same disease. In addition, because the single copy
probes are very small and derive directly from the genome sequences, they can
precisely localize chromosomal breakpoints based on which chromosome harbors
the hybridized signal.
Comparative Genomic Hybridization
Comparative genomic hybridization (CGH) is a modifi ed in situ hybridization tech-
nique, which allows detection and mapping of DNA sequence copy differences
between two genomes in a single experiment. In CGH analysis, two differentially
labeled genomic DNA (study and reference) are co-hybridized to normal metaphase
spreads. Chromosomal locations of copy number changes in the DNA segments of
the study genome are revealed by a variable fl uorescence intensity ratio along each
target chromosome. Since its development, CGH has been applied mostly as a
research tool in the fi eld of cancer cytogenetics to identify genetic changes in many
previously unknown regions. CGH may also have a role in clinical cytogenetics for
detection and identifi cation of unbalanced chromosomal abnormalities. It is now
possible to confi rm euploidy at the time of implantation in a woman undergoing
in vitro fertilization by using CGH with the pregnancy to ensure that the offspring
will be free from congenital disorders.
Representational Oligonucleotide Microarray Analysis
Representational oligonucleotide microarray analysis (ROMA) was developed by
arraying oligonucleotide probes designed from the human genome sequence, and
hybridizing with “representations” from cancer and normal cells to detect regions of
the genome with altered “copy number”. ROMA is used to describe copy number
changes in patients with chromosomal abnormalities and can defi ne cytogenetic
16 Personalized Management of Genetic Disorders