Chromosome rearrangements not as random as believed

As the human genome gradually yields up its secrets, scientists are finding some genetic events, such as rearrangements in chromosomes, are less random than they had previously thought. Originating as structural weaknesses in unstable stretches of DNA, abnormal chromosomes may, rarely, result in a disabling genetic disease one or two generations later.

A report in the Feb. 17 issue of Science by genetics researchers at The Children's Hospital of Philadelphia and the University of Pennsylvania analyzes genetic predisposition to the translocation t(11;22), a swapping of genetic material between chromosomes 11 and 22. They found an unexpectedly high frequency of new translocations in the DNA of sperm samples from healthy individuals. (For technical reasons, egg cells cannot be tested for translocations.)

By and large, these translocations are not cause for alarm, because those men and their offspring are highly unlikely to be affected. Approximately one sperm in 100,000 has the rearrangement, so the chance of an affected sperm fertilizing an egg is exceedingly low. However, when that does occur, the child has a constitutional t(11;22) translocation--all of their cells carry the rearrangement. That child, in turn, while otherwise healthy, often has fertility problems as an adult. Further, if this adult does become a parent, his or her offspring may have an abnormal chromosomal composition and suffer a disabling multisystem disease.

The disorder, called supernumerary der(22) t(11;22) or Emanuel syndrome, results from the havoc caused by an extra chromosome composed of parts of chromosome 11 and chromosome 22. Like Down syndrome, it is a trisomy, a disease caused by an extra chromosome. Patients with the condition usually have mental deficiency, growth retardation, a heart defect, cleft or abnormal palate, and other symptoms.

Although the heart defects were often lethal in the past, advances in heart surgery are allowing children with Eman

Contact: John Ascenzi
Children's Hospital of Philadelphia

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