The role of chromosome 21 in hematology and oncology.

Newborns and children with Down syndrome (DS) often present with congenital transient leukemia and have an increased risk of acute myeloid leukemia and acute lymphoblastic leukemia. Thus, constitutional trisomy 21 represents an excellent model to study the origin and progression of leukemia. However, trisomy 21 can also occur as a somatic chromosome aberration leading to sporadic leukemia. During the 50 years, since the discovery of constitutional trisomy 21 in DS, we have also learned that this small chromosome 21, harboring about 300 genes, may be involved in numerous structural aberrations, e.g., translocations, deletions, and amplifications, in leukemias, lymphomas, and solid tumors. Moreover, genes located on chromosome 21 have been identified that play an important role in tumorigenesis. Somatic mutations of several of these genes have been shown to be associated with different solid tumors, but also constitutional mutations of a specific gene on chromosome 21 leading to myelodysplastic syndromes and acute myeloid leukemia have been described. In this review, the specific forms of myeloid leukemia as well as of acute lymphoblastic leukemia in children with DS will be presented and possible explanations for the paucity of solid tumors in DS will be given. Somatic numerical as well as structural chromosome 21 aberrations in association with leukemias will be described. Finally, the nature and function of specific genes, like RUNX1, TMPRSS2, and TFF, located in 21q, and their role in tumorigenesis will be exemplified.