Use of Chromosome Engineering To Model a Leukemia-Associated 7q22 Deletion in the Mouse.

Abstract

Identifying tumor suppressor genes from intervals that are deleted in human hematologic malignancies such as 5q, 7q, 9q, and 20q has proven extremely challenging, and several laboratories have implicated haploinsufficiency as a likely mechanism. Chromosome engineering, which involves performing sequential rounds of gene targeting to insert loxP sites at flanking loci in mouse embryonic stem (ES) cells and using Cre recombination to delete the intervening sequences, was recently used to successfully interrogate the 1p36 interval in human solid tumors (Cell 128(3):459–75, 2007). Monosomy 7 and deletion 7q [del(7q)] are among the most common cytogenetic alterations found in myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Cytogenetic analysis of patients who developed myeloid disorders with del(7q) uncovered a 2.5 Mb commonly deleted segment (CDS) within 7q22 (Blood 88(6):1930–5, 1996), suggesting that this region plays an important role in leukemogenesis. To investigate the in vivo consequences of somatic loss of this interval, we generated a 5A3flox mouse model that harbor loxP sites flanking a ∼2 Mb interval on mouse chromosome 5A3 that is syntenic to the human 7q22 CDS. We intercrossed these mice with the interferon inducible Mx1-Cre transgenic strain, and injected these mice with polyinosinic-polycytidylic acid (pIpC) to delete the region in the hematopoietic compartment. The desired recombination is relatively inefficient; however, hematopoietic cells with loss of this region persist in the stem/progenitor compartment for over 1 year and are transplantable. We neither observed a block in differentiation nor clonal outgrowth of mutant hematopoietic cells, suggesting that additional mutations are necessary to initiate leukemia. We initiated AML in these mice by introducing additional genetic lesions using retroviral insertional mutagenesis, and we are characterizing these leukemias to study the effects of the 5A3 deletion on leukemogenesis and to clone cooperating genes. Chromosome engineering is a robust strategy for modeling leukemia-associated deletions in vivo, and for interrogating how loss of a specific interval alters hematopoietic growth. We are using the 5A3 strain to analyze candidate myeloid tumor suppressor genes from chromosome 7q and to uncover genes and pathways that cooperate in leukemogenesis.