531interpretation of certain chromosome abnormalities have largely been overcome by
these new technologies, which include fl uorescence in situ hybridization (FISH),
comparative genomic hybridization (CGH), and multicolor FISH (M-FISH, SKY,
and Rx-FISH). Clinical applications include diagnosis of microdeletion and micro-
duplication syndromes, detection of subtelomeric rearrangements in idiopathic
mental retardation, identifi cation of marker and derivative chromosomes, prenatal
diagnosis of trisomy syndromes, and gene rearrangements and gene amplifi cation in
tumors. Molecular cytogenetic methods have expanded the possibilities for precise
genetic diagnoses, which are extremely important for clinical management of
patients and appropriate counseling of their families. Cytogenetics is dealt with in
more detail in a special report on this topic (Jain 2015b ).
FISH with Probes to the Telomeres
When clinicians see a child with developmental delay or mental retardation and any
kind of congenital anomaly, the fi rst thing they consider is chromosomes. Basic
chromosome analysis does not always catch genetic disorders. For example, the
1p36 deletion syndrome in which a deletion occurs on the telomere or tip of chromo-
some 1, is missed because genetic loss occurs on the most telomeric or distal band
of chromosome 1. But the syndrome has clinically recognizable aspects – facial
characteristics, seizures, mental retardation, hearing loss and slow developmental
growth. These signs have been correlated with 1p21-22 deletion by use of a refi ned
FISH technique with probes to the telomeres. Nearly half of the cases now diag-
nosed by use of refi ned FISH were previously missed by chromosome analysis that
was reported normal. Early diagnosis is important as it would lead to early interven-
tion and therapies, and help the patient and family deal with a particular disorder.
Single Copy FISH Probes
Current commercially produced DNA probes, while important and useful, are lim-
ited to primarily examining large sections of DNA in identifying relatively common
genetic disorders. Single copy FISH (scFISH) probes offer specifi city in hybridiz-
ing genetic chromosomes not heretofore available for identifying elusive strains of
inherited genetic diseases. Enzo Biochem Inc has acquired a license to the technol-
ogy from the Children’s Mercy Hospital & Clinics (Kansas City, MO) along with
rights to ~50 DNA probes developed at the University of Missouri, which have been
shown to identify a number of genetic diseases. scFISH probes were designed by
computational sequence analysis of ~100-kb genomic sequences, produced by long
PCR, then purifi ed, labeled, and hybridized individually or in combination to human
chromosomes. Preannealing or blocking with unlabeled, repetitive DNA is unnec-
essary, as scFISH probes lack repetitive DNA sequences. The hybridization results
are analogous to conventional FISH, except that shorter probes can be readily visu-
alized. Combinations of probes from the same region gave single hybridization
Molecular Diagnosis of Genetic Disorders