The
Bcr-Abl oncogene is generated by a reciprocal t(9;22)(q34;q11) chromosome translocation known as
Philadelphia chromosome (
Ph), which fuses varying amounts of the
breakpoint cluster region (
Bcr) gene on chromosome 22 with sequences upstream of the second exon of
cellular Abl (
cAbl) gene on chromosome 9. Depending on the amount of
Bcr sequences fused, three different Bcr-Abl fusion proteins may be produced with molecular masses of 185 kilodalton (Kd) (p185
Bcr-Abl), 210 Kd (p210
Bcr-Abl), and 230 Kd (p230
Bcr-Abl) [
1‐
3]. p210
Bcr-Abl expression is a causative event in over 95% of human chronic myelogenous leukemia (CML) cases, while p185
Bcr-Abl is found in 60–80% of
Ph-positive B cell acute lymphocytic leukemia (
Ph+ B-ALL) cases [
3‐
5]. Development of the Abl tyrosine kinase inhibitor (TKI) imatinib and other second-generation TKIs, dasatinib and nilotinib, has revolutionized the treatment of
Ph+ leukemia, with remarkable rates of sustained complete cytogenetic remission and disease-free survival for CML patients at the chronic phase [
6]. However, relapse is often observed in the patients with
Ph+ B-ALL or advanced CML due to the persistence of leukemic progenitor cells and accumulation of additional mutations that result in drug resistance [
6‐
8]. A major challenge in the treatment of
Ph+ leukemia has been in developing novel therapies for patients who are resistant to TKI-based therapy.
The hematopoietic stem/progenitor cells isolated from
Ph+ leukemia patients exhibit multiple abnormalities of cytoskeletal function such as increased motility, altered adhesion, and decreased response to stromal cell-derived factor 1α (SDF-1α) [
9‐
11]. These abnormalities may play a critical role in the progression of leukemia, since altered adhesion and mobility may contribute to premature release of leukemic stem/progenitor cells from bone marrow and accumulation and infiltration of these cells in peripheral hematopoietic tissues such as blood, spleen, and liver. Abnormal actin remodeling may also contribute to the deregulation of leukemic progenitor cell proliferation and survival [
11]. Bcr-Abl oncoproteins exert their oncogenic potential in cooperation with additional cytoplasmic and nuclear effectors such as those involved in the regulation of mitogenic and apoptotic pathways [
1,
5]. They are also capable of binding to cytoskeleton proteins and other proteins involved in the regulation of cell adhesion and migration [
1,
5,
12]. Among these proteins is the Abl interactor 1 (Abi1) [
13], a key regulator of Rac-dependent actin polymerization [
14,
15]. Abi1 is present in cells as a complex with
WASP-family verprolin-homologous (WAVE) proteins, Nck-associated protein (Nap), specifically Rac-associated (Sra) protein, and hematopoietic stem progenitor cell 300 (Hspc 300) [
14,
16‐
18]. The macromolecular complex, named WAVE regulatory complex (WRC), regulates initiation of actin polymerization in response to signal transduction from membrane receptors to small GTP-binding proteins and PI3 kinase (PI3K) [
19‐
21]. In addition to the interactions with Abl, WAVE and Nap, Abi proteins were also found to interact with a variety of other signaling molecules that are involved in the control of cell proliferation, apoptosis, cytoskeletal functions, receptor signaling, endocytosis, and trafficking [
19,
21‐
29]. Despite the importance of Abi1 in intracellular signaling, its role in cancer and leukemia development remains unclear. Previously, we have shown that the knockdown of Abi1 expression by sequence-specific small hairpin RNA (shRNA) inhibited p185
Bcr-Abl-stimulated cell adhesion and migration in vitro and impaired p185
Bcr-Abl-induced leukemogenesis in vivo [
30,
31]. In these studies, however, the leukemogenesis was delayed but not eliminated, possibly due to incomplete Abi1 depletion [
30]. In addition, studies by Chorzalska et al. suggest that the low expression of Abi1 may associate with drug resistance of Bcr-Abl-positive leukemic cells, whereas Juskevicius et al. reported that relapsing diffuse large B cell lymphoma (DLBCL) more commonly displayed gains of a cluster of genes including Abi1 [
32,
33]. More recently, Chorzalska et al. reported that bone marrow-specific knockout of Abi1 induces myeloproliferative neoplasm [
34]. Studies in other cancer cells involving the role of Abi1 in cancer development in vitro and in vivo are also contradictory. While the studies of breast cancer and colorectal carcinoma cells support a role of Abi1 in breast cancer and colorectal cancer development in vitro and in vivo [
35‐
37], other studies suggest that Abi1 may function as a tumor suppressor in prostate cancer and gastric carcinoma development [
38‐
40]. To determine the role of Abi1 in p185
Bcr-Abl-positive leukemia development, we set to completely deplete its expression in p185
Bcr-Abl-positive leukemic cells using CRISPR/Cas9-mediated gene editing. Here, we report that Abi1 is involved in regulation of the Bcr-Abl signaling to downstream pathways including mitogen-activated protein kinases (MAPK) and PI3K-Akt pathways. The complete depletion of Abi1 not only inhibits Bcr-Abl-induced abnormal actin polymerization, cell proliferation, and cell migration in vitro, but also inhibits leukemogenesis in vivo. Moreover, the inhibition of Bcr-Abl-induced leukemia by Abi1 deficiency is independent of the sensitivity of these cells to imatinib, as the imatinib-tolerant p185
Bcr-Abl cells also require Abi1 for development of leukemia in vivo.