The finding could mean quicker diagnosis and offer a new target for treatment. The study appears today on the Nature Genetics journal Web site.

Children with Down syndrome are 10 to 20 times more likely as unaffected children to have leukemia. The most common form of leukemia in children with Down Syndrome is called acute megakaryoblastic leukemia (AMKL), a kind that's extremely rare in children who don't have Down Syndrome.

"This study, for the first time, defines a part of the molecular pathway leading to acute megakaryoblastic leukemia," says study director John Crispino, an assistant professor at the Ben May Institute for Cancer Research at the University of Chicago.

"Having three copies of chromosome 21 places children with Down syndrome at increased risk for leukemia, then this abnormality tips the balance toward AMKL," Crispino says.

The study focused on a gene called GATA1. The researchers looked at DNA from 75 people with various types of myeloid leukemia and DNA from 21 healthy people. They found that six of six people with Down syndrome and AMKL had a defect in the GATA1 gene. None of the other people in the study had an abnormal version of the gene.

GATA1 controls the expression of other genes, and normally regulates genes controlling production of red blood cells and platelets.

The abnormal GATA1 gene found in this study produces a protein with a piece missing from it. That makes the protein less able to regulate target genes. The result is similar to having no GATA1 protein at all, the study says.

"GATA1 is just part of the story, but it is a crucial early step that should lead us to the rest of the pathway," Crispino says.

More information

St. Jude's Children's Research Hospital has more on acute megakaryoblastic leukemia.