Researchers in Chicago have isolated the genetic defect that causes leukemia to occur more often in children with Down syndrome than in other children.

The findings are detailed in a study conducted by the Ben May Institute for Cancer Research at the University of Chicago and appear in the Aug. 12 issue of Nature Genetics.

The scientific discovery centres on a gene called GATA1, mutations of which, researchers say, lead to the development of acute megakaryoblstic leukemia (AMLK), a form of bone-marrow cancer almost exclusive to Down children.

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Scientists have long been aware that those children are 10 to 20 times as likely to develop leukemia as children without Down syndrome, but the reason had always been a mystery, a co-author of the study said.

"AMLK is a rare malignancy," Dr. Michelle Le Beau said. "Our finding pinpoints a specific pathway that leads to this kind of cancer, offers a method for rapid and precise diagnosis, and suggests more focused ways to treat this disease."

The hope, Dr. Le Beau said, is that additional research on GATA1 mutations will result in a more effective treatment for leukemia in Down children, that work too often hamstrung by the absence of compatible bone-marrow donors.

Down children have three copies of chromosome 21 instead of the usual two, and that abnormality is what puts them at greater risk of developing mutations on those genes.

Under normal functioning, GATA1 controls the characteristics of the genes responsible for the production of red blood cells and platelets. Previous studies indicated that the absence of GATA1 caused platelets to divide excessively, said another of the report's authors, Dr. John Crispino.

A mutated GATA1 gene - the kind found in Down children with leukemia - causes the same proliferation of platelets and so opens the door to leukemia, he said.

By a simple screening for GATA1 mutations, said Dr. Crispino, doctors will now be able to identify more quickly those Down babies likely to develop leukemia. That early diagnosis will in turn allow them to begin treatment sooner and to extend the lives of children.