Description
ABL1がん限遺伝子は細胞質と核に局在し、細胞の分化、細胞分裂、細胞の接着、ストレス応答などにかかわるチロシンキナーゼをコードする。ウイルス感染や染色体の構造変化によりABL1遺伝子が変化すると、慢性骨髄性白血病にみられるような悪性の形質転換が生じる
The ABL1 protooncogene encodes a cytoplasmic and nuclear protein tyrosine kinase that has been implicated in processes of cell differentiation, cell division, cell adhesion, and stress response. Alterations of ABL1 by chromosomal rearrangement or viral transduction lead to malignant transformation, as in chronic myeloid leukemia (CML; 608232) (summary by Barila and Superti-Furga, 1998).
Cloning
ABLタンパク質は145kDaあり、受容体と関係のなりチロシンキナーゼである。1番目のエキソンの選択的スプライシングにより、6キロまたは7キロのmRNAを転写している。1aエキソンを含むときは核に局在し、1bエキソンが含まれる場合はN末端のグリシンがミリスチル化され、膜に局在する様になる。
The 145-kD ABL protein is a nonreceptor tyrosine kinase. The ABL gene is expressed as either a 6- or 7-kb mRNA transcript based on alternatively spliced first exons. When the N-terminal region of the ABL protein is encoded by exon 1a (ABL1A), the protein is believed to be localized in the nucleus, while when encoded by exon 1b (ABL1B), the resulting N-terminal glycine would be myristylated and thus postulated to direct that protein to the plasma membrane (see review by Chissoe et al., 1995).
Gene Structure
第一エキソンは二種類から洗濯され、2-11は共通である。エキソン1bは1aの200kb上流にある。
The ABL gene contains 2 alternatively spliced exons, exons 1a and 1b, spliced to the common exons 2-11. Exon 1b is approximately 200 kb 5-prime of exon 1a (Shtivelman et al., 1986).
BCR遺伝子とABL遺伝子の塩基配列を調べた結果、ABLのエキソン2-11と選択的な1番目のエキソンは
Chissoe et al. (1995) sequenced the complete BCR gene and greater than 80% of the ABL gene. Included were the common ABL exons 2-11 and the alternative ABL first exons, as well as a novel gene 5-prime to the first ABL exon and an open reading frame with homology to an EST within the BCR fourth intron. One of the most striking observations from analyzing the BCR and ABL gene regions was their extremely high density, 38.83 and 39.35%, respectively, of Alu homologous regions. This contrasted with the previously calculated 4% level that was based on estimates of approximately 500,000 Alu elements in the human genome.
Mapping
Heisterkamp et al. (1982) assigned the human cellular homolog of the Abelson strain of murine leukemia virus to chromosome 9. Goff et al. (1982) mapped the mouse homolog on chromosome 2, which bears other homology of synteny to human chromosome 9. (Strictly speaking, 'murine' refers to the rodent family Muridae, which includes both rats and mice. By common practice, however, the term is used almost exclusively for mice.) By in situ hybridization, Trakhtenbrot et al. (1990) showed that the murine c-abl oncogene on chromosome 2 lies closer to the centromere than the region commonly deleted in radiation-induced murine myeloid leukemias.
Gene Function
The DNA-binding activity of ABL is regulated by CDC2-mediated phosphorylation (116940), suggesting a cell cycle function for ABL (Kipreos and Wang, (1990, 1992)).
Welch and Wang (1993) showed that the tyrosine kinase activity of nuclear ABL is regulated in the cell cycle through a specific interaction with RB1 (614041). A domain in the C terminus of RB1 binds to the ATP-binding lobe of the ABL tyrosine kinase, resulting in kinase inhibition. The RB1-ABL interaction is not affected by the viral oncoproteins that bind to RB1. Hyperphosphorylation of RB1 correlates with release of ABL and activation of the tyrosine kinase in S phase cells. The nuclear ABL tyrosine kinase can enhance transcription, and this activity is inhibited by RB1. Thus, nuclear ABL is an S phase-activated tyrosine kinase that might participate directly in the regulation of transcription.
Feller et al. (1994) described the SRC homology domains SH2 and SH3 as molecular adhesives on many proteins involved in signal transduction. They reviewed the interactions of ABL and CRK (164762) as a model of SH2 and SH3 interaction. Barila and Superti-Furga (1998) presented evidence for an intramolecular inhibitory interaction of the SH3 domain with the catalytic domain and with the linker between the SH2 and catalytic domain. Site-directed mutations in each of these 3 elements activated c-Abl. Mutations in the linker caused a conformational change of the molecule and increased binding of the SH3 domain to peptide ligands. Individual mutation of 2 charged residues in the SH3 and catalytic domain activated c-Abl, while inhibition was restored in the double reciprocal mutant. Barila and Superti-Furga (1998) proposed that regulators of c-Abl will have opposite effects on its activity depending on their ability to favor or disrupt these intramolecular interactions.
Using competition experiments, Han et al. (1997) demonstrated specific binding of the proline-rich region of RIN1 (605965), a RAS inhibitor, to the SH3 domain of ABL. By coimmunoprecipitations and kinase assays, they demonstrated that the N-terminal end of RIN1 is phosphorylated by ABL in vitro.
Gong et al. (1999) demonstrated that the amount of p73 protein (601990) is normally increased in cells treated by the cancer chemotherapeutic agent cisplatin. However, this induction of p73 was not seen in cells unable to carry out mismatch repair and in which the nuclear enzyme c-Abl tyrosine kinase was not activated by cisplatin. The half-life of p73 was prolonged by cisplatin and by coexpression with c-Abl tyrosine kinase. The apoptosis-inducing function of p73 was also enhanced by the c-Abl kinase. Mouse embryo fibroblasts deficient in mismatch repair or in c-Abl did not upregulate p73 and were more resistant to killing by cisplatin. Gong et al. (1999) concluded that c-Abl and p73 are components of a mismatch repair-dependent apoptosis pathway which contributes to cisplatin-induced cytotoxicity. Agami et al. (1999) demonstrated that the apoptotic activity of p73-alpha requires the presence of functional, kinase-competent c-Abl. Furthermore, p73 and c-Abl association is mediated by a PxxP motif in p73 and the SH3 domain of c-Abl. Agami et al. (1999) found that p73 is a substrate of the c-Abl kinase and that the ability of c-Abl to phosphorylate p73 is markedly increased by gamma-irradiation. Moreover, p73 was phosphorylated in vivo in response to ionizing radiation. These findings define a proapoptotic signaling pathway involving p73 and c-Abl. Yuan et al. (1999) demonstrated that c-Abl binds to p73 in cells, interacting through its SH3 domain with the C-terminal homooligomerization domain of p73. c-Abl phosphorylated p73 on a tyrosine residue at position 99 both in vitro and in cells that had been exposed to ionizing radiation. Yuan et al. (1999) concluded that c-Abl stimulates p73-mediated transactivation and apoptosis. This regulation of p73 by c-Abl in response to DNA damage was also demonstrated by a failure of ionizing radiation-induced apoptosis after disruption of c-Abl-p73 interaction.
Using a yeast 2-hybrid screen, Cong et al. (2000) identified PSTPIP1 (606347) as an ABL-interacting protein. PSTPIP1 had been identified originally as a binding protein of the PEST-type protein tyrosine phosphatases (PTPs, e.g., PTP-PEST; 600079) by Spencer et al. (1997). Cong et al. (2000) showed that PSTPIP1 was phosphorylated by ABL. Growth factor-induced PSTPIP1 phosphorylation was diminished in ABL null fibroblasts. PSTPIP1 was able to bridge ABL to the PEST-type PTPs. Several experiments suggested that the PEST-type PTPs negatively regulate ABL activity: ABL was hyperphosphorylated in PTP-PEST-deficient cells; disruption of the ABL-PSTPIP1-PEST-type PTP ternary complex by overexpression of mutants increased ABL phosphotyrosine content; and PDGF (see 190040)-induced ABL kinase activation was prolonged in PTP-PEST-deficient cells. The authors concluded that dephosphorylation of ABL by PSTPIP1-directed PEST-type PTPs represents a novel mechanism by which ABL activity is regulated.
Pluk et al. (2002) reported inhibition of the catalytic activity of purified ABL in vitro, demonstrating that regulation is an intrinsic property of the molecule. They showed that the interaction of the N-terminal 80 residues with the rest of the protein mediates autoregulation. This N-terminal 'cap' is required to achieve and maintain inhibition, and its loss turns ABL into an oncogenic protein and contributes to deregulation of BCR-ABL.
In mouse muscle and cultured myotube cells, Finn et al. (2003) showed that Abl1 and Abl2 (164690) are concentrated at the postsynaptic neuromuscular junction and are mediators of postsynaptic acetylcholine receptor (AChR) clustering downstream of agrin (103320) and MuSK (601296) signaling. The authors suggested that the Abl kinases influence cytoskeletal regulatory molecules important for synapse assembly and remodeling.
Schindler et al. (2000) reported the crystal structure of the catalytic domain of ABL, complexed to a variant of STI571, a small-molecule inhibitor of ABL that is effective in the treatment of chronic myelogenous leukemia. Critical to the binding of STI571 is the adoption by the kinase of an inactive conformation, in which a centrally located 'activation loop' is not phosphorylated. The conformation of this loop is distinct from that in active protein kinases, as well as in the inactive form of the closely related SRC kinases. Schindler et al. (2000) concluded that compounds that exploit the distinctive inactivation mechanisms of individual protein kinases can achieve both high affinity and high specificity.
Hantschel et al. (2003) found that the ABL1B variant is activated by phosphotyrosine ligands. Ligand-activated ABL was particularly sensitive to the tyrosine kinase inhibitor STI571 (imatinib). The authors showed that the SH2 domain-phosphorylated tail interaction in SRC kinases is functionally replaced in ABL by an intramolecular engagement of the N-terminal myristoyl modification with the kinase domain. Functional studies coupled with structural analysis defined a myristoyl/phosphotyrosine switch in ABL that regulates docking and accessibility of the SH2 domain. Hantschel et al. (2003) concluded that this mechanism offers an explanation for the observed cellular activation of ABL by tyrosine-phosphorylated proteins and for the intracellular mobility of ABL, and it provides insights into the mechanism of action of STI571.
Nagar et al. (2003) studied crystal structures of ABL that showed that the N-terminal myristoyl modification of ABL1B binds to the kinase domain and induces conformational changes that allow the SH2 and SH3 domains to dock onto it. Autoinhibited ABL formed an assembly that was similar to that of inactive SRC kinases but with specific differences that explained the differential ability of STI571 to inhibit the catalytic activity of ABL, but not that of SRC.
Biochemical Features
Hantschel et al. (2005) determined the nuclear magnetic resonance (NMR) structure of the F-actin-binding domain (FABD) of ABL. The FABD forms a compact bundle of 4 antiparallel alpha helices, which are arranged in a left-handed topology in solution. The putative nuclear export signal found in this region is part of the hydrophobic core and is nonfunctional in the intact protein. Helix alpha-III contains critical residues responsible for F-actin binding and cytoskeletal association. Hantschel et al. (2005) concluded that these interactions represent a major determinant for both BCR-ABL and c-ABL localization.
ABL/BCR Fusion Gene
A t(9;22) translocation occurs in greater than 90% of chronic myelogeneous leukemia (CML; 608232), 25 to 30% of adult and 2 to 10% of childhood acute lymphoblastic leukemia (ALL; 613065), and rare cases of acute myelogenous leukemia. The translocation results in the head-to-tail fusion of the BCR (151410) and ABL genes (see review of Chissoe et al., 1995).
De Klein et al. (1982) demonstrated that the ABL gene is translocated from chromosome 9 to chromosome 22 in the formation of the Philadelphia chromosome. This indicated that the translocation is reciprocal and suggested a role for the ABL gene in the generation of CML. Collins and Groudine (1983) showed amplification of ABL sequences some 4- to 8-fold in K-562, a Philadelphia chromosome-positive cell derived from a patient with CML in blast crises. Furthermore, the lambda light chain immunoglobulin genes were amplified in K-562, but the kappa genes showed no amplification. Whereas in Burkitt lymphoma of the t(8;22) type the lambda light chain genes are translocated to chromosome 8, they remain on chromosome 22 (i.e., on the Philadelphia chromosome) in CML (Selden et al., 1983). Heisterkamp et al. (1983) found that the breakpoint in 9q in CML is only 14 kb upstream from the beginning of the ABL oncogene. Konopka et al. (1985) presented evidence that the translocation of the ABL oncogene in Ph1-positive CML results in the creation of a chimeric gene leading to production of an abnormal Abl protein with tyrosine kinase activity. Konopka et al. (1985) suggested that this protein probably plays a key role in the malignant transformation. Acute nonlymphocytic leukemia associated with t(6;9)(p23;q34) is not accompanied by alteration in the ABL oncogene (Westbrook et al., 1985), despite the same location of the breakpoint at 9q34 and the frequent association with basophilia as in CML.
In about 10% of patients with acute lymphocytic leukemia, patients carry a 9;22 translocation cytogenetically indistinguishable from the Philadelphia chromosome of CML. Clark et al. (1986) demonstrated, however, that Philadelphia chromosome-positive ALL cells express unique Abl-derived tyrosine kinases of 185 and 180 kD that are distinct from the Bcr, Abl-derived P210 protein of CML. Kurzrock et al. (1987) found a novel Abl protein product in Philadelphia chromosome-positive acute lymphoblastic leukemia. They suggested that alternative mechanisms of activation of Abl exist and that a different mechanism may apply in human acute lymphoid leukemia as opposed to myeloid malignancies.
Bernards et al. (1987) showed by pulsed field gel electrophoresis (PFGE) that a 5-prime exon of the ABL gene lies at least 300 kb upstream of the remaining ABL exons. The very long intron is a target for translocations in chronic myeloid leukemia. Whereas in chronic myeloid leukemia the ABL gene is translocated from chromosome 9 to the center of the BCR gene on chromosome 22 to produce a chimeric BCR-ABL RNA translated into a protein of molecular weight 210 kD, Fainstein et al. (1987) found that in acute lymphocytic leukemia, ABL is translocated into the 5-prime region of the BCR gene. The consequence of this is the expression of a fused transcript in which the first exon of BCR is linked to the second ABL exon. This transcript encodes a 190-kD protein kinase.
By RT/PCR, Melo et al. (1993) detected ABL-BCR mRNA in cells from 31 of 44 BCR-ABL positive CML patients and in 3 of 5 CML cell lines. Thus, in many patients with CML, 2 reciprocal fusion genes are expressed. Of the 34 positive samples, 31 had classic t(9;22)(q34;q11) translocations; in 3 samples, there were no Philadelphia and/or 9q+ chromosomes.
Melo et al. (1994) concluded that the normal ABL gene is not imprinted and cited evidence resolving the question of imprinting of the BCR gene in favor of nonimprinting. They felt that their ABL data and the published BCR data excluded the possibility of a parental bias in the origin of the Philadelphia (Ph) chromosome and cited observations suggesting that there is, in fact, no preferential involvement of maternal BCR or paternal ABL alleles in the formation of the BCR-ABL fusion gene of Ph-positive CML patients, in apparent contradiction to the reported cytogenetic evidence (Haas et al., 1992).
Chissoe et al. (1995) analyzed 4 new Philadelphia chromosome translocation breakpoints from chronic myelogenous leukemia patients and several other previously sequenced breakpoints but could find no consistent breakpoint features. No clear-cut mechanism for Philadelphia chromosomal translocation was evident.
Arico et al. (2000) reviewed the medical records of 326 children and young adults with Ph-positive ALL at ages ranging from 0.4 to 19.9 years (median, 8.1). Unlike the usual type of ALL, Ph-positive cases have a poor prognosis. The authors found that transplantation of bone marrow from an HLA-matched related donor is superior to other types of transplantation and to intensive chemotherapy alone in prolonging initial complete remissions.
Zhao et al. (2009) demonstrated that the loss of Smoothened (Smo; 601500), an essential component of the hedgehog pathway (see 600725), impairs hematopoietic stem cell renewal and decreases induction of CML by the BCR-ABL1 oncoprotein. Loss of Smo causes depletion of CML stem cells, which propagate the leukemia, whereas constitutively active Smo augments CML stem cell number and accelerates disease. As a possible mechanism for Smo action, Zhao et al. (2009) showed that the cell fate determinant Numb (603728), which depletes CML stem cells, is increased in the absence of Smo activity. Furthermore, pharmacologic inhibition of hedgehog signaling impairs not only the propagation of CML driven by wildtype BCR-ABL1, but also the growth of imatinib-resistant mouse and human CML. Zhao et al. (2009) concluded that hedgehog pathway activity is required for maintenance of normal and neoplastic stem cells of the hematopoietic system and raised the possibility that the drug resistance and disease recurrence associated with imatinib treatment of CML might be avoided by targeting this essential stem cell maintenance pathway.
Resistance of Bcr-Abl-positive leukemic stem cells (LSCs) to imatinib treatment in patients with chronic myeloid leukemia (CML; 608232) can cause relapse of disease and might be the origin for emerging drug-resistant clones. Dierks et al. (2008) identified Smo as a drug target in Bcr-Abl-positive LSCs. They showed that Hedgehog signaling is activated in LSCs through upregulation of Smo. While nullity for Smo does not affect long-term reconstitution of regular hematopoiesis, the development of retransplantable Bcr-Abl-positive leukemias was abolished in the absence of Smo expression. Pharmacologic Smo inhibition reduced LSCs in vivo and enhanced time to relapse after end of treatment. Dierks et al. (2008) postulated that Smo inhibition might be an effective treatment strategy to reduce the LSC pool in CML.
Using xenografting and DNA copy number alteration (CNA) profiling of human BCR-ABL1 lymphoblastic leukemia, Notta et al. (2011) demonstrated that genetic diversity occurs in functionally defined leukemia-initiating cells and that many diagnostic patient samples contain multiple genetically distinct leukemia-initiating cell subclones. Reconstructing the subclonal genetic ancestry of several samples by CNA profiling demonstrated a branching multiclonal evolution model of leukemogenesis, rather than linear succession. For some patient samples, the predominant diagnostic clone repopulated xenografts, whereas in others it was outcompeted by minor subclones. Reconstitution with the predominant diagnosis clone was associated with more aggressive growth properties in xenografts, deletion of CDKN2A (600160) and CDKN2B (600431), and a trend towards poorer patient outcome. Notta et al. (2011) concluded that their findings linked clonal diversity with leukemia-initiating cell function and underscored the importance of developing therapies that eradicate all intratumoral subclones.
Tyrosine Kinase Inhibitors
Because tyrosine kinase activity is essential to the transforming function of BCR-ABL, Druker et al. (2001) reasoned that an inhibitor of the kinase may be an effective treatment for CML. They found that the tyrosine kinase inhibitor STI571 was well tolerated and had significant antileukemic activity in patients with CML in whom treatment with standard chemotherapy had failed. This experience demonstrated the potential for the development of anticancer drugs based on the specific molecular abnormality present in a human cancer.
Druker et al. (2001) found that STI571 was well tolerated and had substantial activity in the blast crises of CML and in Philadelphia chromosome-positive acute lymphoblastic leukemia. The response was less satisfactory in the ALL group. Goldman and Melo (2001) also illustrated the likely mode of action of STI571.
Imatinib (Gleevec, Novartis, Basel, Switzerland), formerly referred to as STI571, was approved by the Food and Drug Administration in May 2001 for the treatment of CML that is refractory to interferon therapy and in February 2002 for the treatment of gastrointestinal stromal tumors (606764), which can be caused by mutations in the KIT gene (164920) (Savage and Antman, 2002). In both cases the agent works as an inhibitor of specific protein tyrosine kinases.
