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Microduplication, Not Microdeletion, May Be Root of Common Chromosomal Defects

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A multi-center team of researchers has identified a previously misdiagnosed or undiagnosed genetic syndrome caused by microduplication.

According to the team, the discovery is important because it “provides evidence for a class of new genetic errors” as a precondition leading to disease; most often, microdeletion is studied and associated with disease.

Specific symptoms of the syndrome, which does not yet have a name, may include slightly misshapen faces and unusually widely spaced eyes, eyebrows placed higher than usual, and long, narrow faces, with irregularly shaped ears. Some patients have impaired hearing and speech, malfunctioning spleen and thymus gland, or heart defects. Other symptoms include faulty immune function and degrees of mental impairment.

Scientists from New York, North Carolina, Maine, California, and Georgia reached their conclusions after studying the position of genes in chromosomes relativeto the placement of eyebrows on the faces of 653 patients referred to the Mayo Clinic for evaluation of a known syndrome, called DiGeorge, that shares some of the traits with the new syndrome.

According to the scientists, the new syndrome was “often misdiagnosed” as DiGeorge, which has many of the same symptoms. However, they said, this syndrome is genetically distinct. There study appears in the November American Journal of Human Genetics.

Studying low-copy repeats in these 653 patients, the team found 13 that showed microduplication within chromosome 22. By contrast, DiGeorge patients microdelete at this site. Syed Jalal, a medical geneticist and professor at the Mayo Clinic College of Medicine and the study’s chief author, said it is not known how common the new syndrome is, but suggested “it’s likely” that it is more common than DiGeorge, which occurs in one in 4,000 or 5,000 Caucasians.

Knowing that chromosome 22 was vulnerable to rearrangement, the researchers “theorized that rearrangement would lead to deletion as often as it did duplication.”

“Surprisingly,” said Jalal, “there was no real evidence for this.” In the past, just three cases of microduplication disorders have been described in medical literature, the researchers report. One reason perhaps is that routine chromosome analysis can “easily miss” this duplication. The team said that fluorescent DNA probes may be a more useful tool.

“Microduplication now needs to be considered and investigated when some features or symptoms overlap with commonly occurring microdeletion disorders,” Jalal in a statement released by the Mayo Clinic last week.

— KL

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