Targeting Chromosome Loss in Down Syndrome Cells

Down syndrome (DS) is the most common viable trisomy in humans, with several thousand affected infants born in the United States every year. Patients with DS have a wide spectrum of clinical problems dominated by cardiac and developmental abnormalities. Up to 10 percent of DS infants experience a transient myeloproliferative disorder that can progress to a rare form of acute megakaryocytic leukemia, whereas pre-school-age children with DS have an abnormally high risk of developing acute precursor B-cell leukemia. DS patients with leukemia present unique management challenges owing to a combination of organ vulnerability and increased susceptibility to drug toxicity. Progress in understanding the hematopoietic defects and the molecular basis of leukemogenesis of DS has been hampered by a lack of a faithful murine disease model and the problems inherent in comparing results obtained using cells from different patients.

The recent study by Li and colleagues from the University of Washington in Seattle now provides evidence for the feasibility of deriving strictly diploid, induced pluripotent stem cells (iPSCs) from the trisomic fibroblasts of DS patients. The paper is at once a technical tour de force and an illustration of the effectiveness of combining targeted genome engineering approaches with induced pluripotency. The investigators initially generated iPSCs that were confirmed to be trisomic for chromosome (chr) 21. Next, they used an adeno-associated viral vector to insert a dual selectable marker into a gene located on chr 21 via homologous recombination. By using a strategy that selects against the vector-bearing chr 21, clones that had lost a complete copy of chr 21, and were hence disomic for chr 21, were generated. Subsequent experiments showed that the derived chr 21 diploid iPSCs had a higher proliferative rate than their trisomy chr 21 counterparts, but in vitro hematopoietic differentiation was not consistently different.

As noted above, patients with DS have a several hundred-fold increased risk of developing megakaryocytic myeloid leukemia in infancy and acute lymphocytic leukemia in early childhood. In another recent paper, this one by MacLean and colleagues from Dana-Farber Cancer Institute in Boston, the authors used isogenic disomic pluripotent stem cells to investigate differences in hematopoiesis between stem cells with diploid chr 21 and stem cells with trisomy chr 21.¹  The Dana-Farber group did not engineer their diploid chr 21 cell lines as did the University of Washington group, but instead isolated spontaneous disomic revertants from human embryonic stem cell and iPSC clones. Differences in immunophenotype and a significant increase in progenitor colony formation by chr 21 trisomic cells compared with chr 21 disomic cells was observed, consistent with developmental dysregulation of hematopoiesis in DS individuals.