Sensitive chromosome probes recently discovered by a University of Torontogeneticist will make it easier to detect certain types of genetic and prenatal diseases, aswell as being used to determine paternity and provide forensic evidence in criminalProbes are short pieces of DNA which bind to, and actually pinpoint, particularsites on a chromosome. Because these new probes are actually repeated hundreds orthousands of time at a particular site, they are much more sensitive than previouslyOf the 23 pairs of human chromosomes, Dr. F.
H. Willard has discovered repeatedprobes or markers for six plus the gender determining X and Y chromosomes. “Whatwe’re trying to decide now is whether to isolate probes for the other chromosomes, orwhether we should utilize the eight we have,” he says. Dr.
Willard is currently negotiating with an American company to developprenatal diagnostic tests, which, because the current tests are time consuming andtechnically difficult to do, are restricted to women over 35 and those who have a familyhistory of chromosomal abnormalities. Prenatal tests using Willard’s probes would bemuch simpler and faster to perform and could be available to all pregnant women whowish to take advantage of the technology. Current prenatal testing involves growing fetal cells in vitro and examining them,over one or two months, to see if there are two copies of a particular chromosome, whichis normal, or one or three, which is abnormal. A test using Willard’s probes would requireonly a few cells and a few days to detect abnormalities. “I don’t think it’s beyond therealm of possibility that these kinds of tests could eventually be done by an obstetricianin the office during the early stages of pregnancy,” he adds.
The determination of gender is another possible use for the probes. Manydiseases, such as Duschene’s muscular dystrophy, show up on the X chromosome,manifesting only in boys. Willard thinks it is possible to develop a test which wouldquickly indicate the fetus’ sex. This would benefit parents whose only option is to haveConfirming gender in children with ambiguous genitalia is another medicalreason for using the test. A quick examination of the X and Y chromosomes of the childwould indicate whether genetically the child is male or female.
As yet, Willard has been unable to develop a probe for chromosome 21. Down’sSyndrome results from three copies of chromosome 21 (trisomy 21). “I think we’ll knowwithin a year whether a test to detect trisomy 21 is feasible, ” he says hopefully. The other six chromosome probes which Willard has developed do notimmediately lend themselves to diagnostic tests, except for certain cancers, he says. “Wehave a probe for chromosome 7 and we know that trisomy 7 is a signal for certain typesof cancer.
Chromosome abnormalities of all kinds are a signpost of tumors. “Theoretically, an oncologist could use a chromosome probe test to examine tissue andobtain a reading for a specific cancer. ” It wouldn’t suggest a mode of therapy,” he pointsout, “but would be a speedy test and would have prognostic implications for the kind ofAs a basic research tool, Willard’s probes could be used to develop a geneticlinkage map for human chromosomes. “It’s important to know the location of genes in thehuman genome, particularly disease genes. The leading approach to try to sort out diseasegenes is to use genetic linkage.
Because our sequences are at the centromere it wouldThe third application for the probes is in forensic biology. Willard believes hismarkers are as unique to each each individual as are fingerprints. According to thegeneticist, it will be possible to make a DNA ‘fingerprint’ from blood or sperm, whichcould be used as evidence in rape or murder cases. “We haven’t yet done the analysiswhich confirms that our probes are DNA fingerprints, but once we do, we will makethem available for development into tests. ” As research progresses in all these areas, Willard hopes to collaborate with otherdepartments at the U of T to conduct clinical trials. His work is funded by the March ofDimes, the Hospital for Sick Children Foundation and the Medical Research Council.
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