Genetic Pathways in Therapy-Related Leukemia: FHL2 Cooperates with del(5q).

Therapy-related myelodysplastic syndrome (t-MDS) and acute myeloid leukemia (t-AML) are late complications of cytotoxic therapy used in the treatment of both malignant and non-malignant diseases. The most common subtype of t-MDS/t-AML (~75% of cases) develops after exposure to alkylating agents, and is characterized by loss of a whole chromosome 5 and/or 7, or a deletion of the long arms of these chromosomes [-5/del(5q), -7/del(7q)]. Patients who develop t-MDS/t-AML in this setting typically show a latency of 5 years from alkylating agent exposure, and present with cytopenias and myelodysplasia. In the University of Chicago’s series of 306 patients with t-MDS/t-AML, 64 (21%) patients had abnormalities of chromosome 5, 85 (28%) patients had abnormalities of chromosome 7, and 65 (21%) patients had abnormalities of both chromosomes 5 and 7 (Smith et al. Blood102:43, 2003). Survival times of these t-AML patients are short (median 8 mos), and new therapeutic approaches are needed. Genetic pathways in t-MDS/t-AML are poorly-understood. Monosomy 7/del(7q) has been associated with activating mutations of the RAS pathway and methylation silencing of the CDKN2B (p15INK4B) gene, whereas -5/del(5q) is associated with TP53 mutations and a complex karyotype. To identify novel molecular pathways involved in the pathogenesis of t-MDS/t-AML, we previously performed gene expression profiling of CD34+ hematopoietic progenitor cells from t-AML patients, and identified two major groups, characterized by a -7/del(7q) or normal karyotype (Group A) or -5/del(5q) (Group B) (Qian et al., Proc Natl Acad Sci USA99:14925, 2002). Patients with a -5/del(5q) have a higher expression of genes involved in cell cycle control (CCNA2, CCNE2, CDC2), checkpoints (BUB1), or growth (MYC), and loss of expression of ICSBP. In addition, FHL2 is upregulated in this subgroup. FHL2 is a multi-functional LIM domain protein that has been implicated as a transcriptional co-activator or co-repressor mediating proliferation and apoptosis in a tissue-specific fashion. Interacting partners of FHL2 include WT1, b-catenin, PLZF, and the androgen receptor. In subsequent studies, we determined that hematopoietic cells express a novel isoform of FHL2 (termed B-FHL2), that contains a unique 5′-UTR, but encodes the same peptide, suggesting that FHL2 is regulated by a different promoter in hematopoietic cells. To examine the effects of enhanced expression of FHL2 on hematopoiesis, we transduced primary bone marrow cells from mice treated with 5-FU with retroviral vectors expressing FHL2 (MIGR1-FHL2, MSCV-FHL2). Up-regulation of FHL2 enhanced proliferation of myeloid progenitor cells, and self-renewal capacity of hematopoietic cells in vitro. To determine if up-regulation of FHL2 in vivo leads to alterations in lineage-specific differentiation, we used retroviral transduction/transplantation techniques. Expression of FHL2 resulted in increased numbers of colony-forming cells and GM colonies, but not BFU-E or GEMM colonies. Recipient mice followed for up to 12 months were characterized by enhanced myelopoiesis and partially blocked B lymphopoiesis, with no detectable effect on T cell development. Expression of FHL2 did not induce leukemia in transplanted mice. These results suggest that FHL2 is involved in hematopoiesis. Furthermore, upregulation of FHL2 results in increased stem cell and progenitor cell self-renewal; however, leukemogenesis may require the acquisition of secondary cooperating mutations.