Children with Down syndrome are 10 to 20 times as likely as unaffected children to develop leukemia. They most commonly develop a type known as acute megakaryoblastic leukemia (AMKL), which is extremely rare in children without Down syndrome.
"This study, for the first time, defines a part of the molecular pathway leading to acute megakaryoblastic leukemia," said John Crispino, Ph.D., assistant professor in the Ben May Institute for Cancer Research at the University of Chicago and director of the study. "Having three copies of chromosome 21 places children with Down syndrome at increased risk for leukemia, then this abnormality tips the balance toward AMKL."
"This is a rare malignancy," added co-author Michelle Le Beau, Ph.D., professor of medicine at the University, "but a great deal of what we now know about the molecular basis of cancer has come from disorders like this. 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."
Unlike most studies, which begin with a disease then search for the genetic trigger, this one began with a suspect gene. Crispino's laboratory had been interested in a gene called GATA1 for years because it played a role in the maturation of blood cells.
Dr. Crispino hypothesized that GATA1 might be mutated or dysregulated in leukemia. He contacted Le Beau, an expert on the genetics of leukemia. After they identified a patient with Down syndrome who had a mutation in GATA1 and had acute megakaryoblastic leukemia in a small pilot study they began searching for other patients with childhood leukemia and an abnormal copy of this gene.
When they looked at DNA from 75 patients with various types of myeloid leukemia and 21 healthy people, they found that six out of six patients with Down syndrome and acute megakaryoblastic leukemia had an alteration in GATA1. None of the other patients surveyed had an abnormal version of this gene.
GATA1 is a transcription factor; it controls the expression of other genes. It normally functions to regulate genes that control the production of red blood cells and platelets, which enable the blood to carry oxygen and to clot. Previous studies in mice had shown that the loss of GATA1 caused the cells that give rise to platelets to proliferate excessively.
The abnormal GATA1 gene, found in the leukemia patients, produces a protein with a piece missing. The incomplete protein appears to be far less effective in regulating target genes, resulting in an outcome that is similar to having no GATA1 protein at all.
"GATA1 is just part of the story," said Crispino, "but it is a crucial early step that should lead us to the rest of the pathway." The authors suspect that it requires several gene abnormalities working in tandem to cause full-fledged acute megakaryoblastic leukemia and they are searching for the other genes that combine with GATA1 to trigger this disease.
Funding for this study was provided by the Burroughs Wellcome Foundation, the Aplastic Anemia and MDS International Foundation, the Cancer Research Foundation and the Picower Foundation. Additional authors include Joshua Wechsler, Marianne Greene and John Anastasi of the University of Chicago; Michael McDevitt of Johns Hopkins University; and Judith Karp of the University of Maryland.