151410
Description
正常なBCR遺伝子産物は160kDaのセリントレオニンキナーゼ活性をもつリン酸化タンパク質である。
BCR遺伝子は慢性骨髄性白血病と急性リンパ性白血病に見られるふたつのフィラデルフィア染色体の転座に関わるブレイクポイントである。
BCRとABL1のエキソンが合わさる。融合により二種類のキメラがん遺伝子産物（p210BCR-ABL, p185BCR/ABL）が作られる。、ABLチロシンキナーゼの活性化ががん遺伝子活性に必要である。
The normal cellular BCR gene encodes a 160-kD phosphoprotein associated with serine/threonine kinase activity. The BCR gene is the site of breakpoints used in the generation of the 2 alternative forms of the Philadelphia chromosome translocation found in chronic myeloid leukemia (CML; 608232) and acute lymphocytic leukemia (Groffen et al., 1984; Shtivelman et al., 1985; Hermans et al., 1987). These alternative breakpoints join different exon sets of BCR to a common subset of the exons of the ABL1 gene (189980) located on chromosome 9. This fusion results in 2 alternative chimeric oncogene products called p210(BCR-ABL) and p185(BCR-ABL). The activation of ABL tyrosine kinase activity is necessary for the oncogenic potential of the chimeric oncogene (Maru and Witte, 1991).
Laurent et al. (2001) reviewed the role of BCR alone or when joined with ABL in normal and leukemic pathophysiology.
Gene Structure
BCR遺伝子は23個のエキソンをもつ135kBに及ぶ22番染色体上にある。1番目のエキソンがセリントレオニンキナーゼ活性部位と少なくともふたつのSH2ドメインをもつ。
Heisterkamp et al. (1985) determined the structural organization of the BCR gene, which contains 23 exons and occupies a region of about 135 kb on chromosome 22. The first exon contains a unique serine/threonine kinase activity and at least two SH2 binding sites.
Stam et al. (1987) demonstrated that the BCR gene is oriented with its 5-prime end toward the centromere of chromosome 22.
Chissoe et al. (1995) determined that the BCR gene contains 23 BCR exons with putative alternative BCR first and second exons.
Mapping
The BCR gene is located on chromosome 22q11.2, the site of the translocation breakpoint in CML (Prakash and Yunis, 1984).
Croce et al. (1987) demonstrated that there are in fact 4 BCR genes, all located in the 22q11.2 band. By studying mouse-human hybrid cells with breakpoints at various sites in that region, they concluded that the order of loci is centromere--BCR2, BCR4, IGL--BCR1--BCR3--SIS. This linear order was confirmed by Budarf et al. (1988), who also confirmed that all of the BCR-like genes map proximal to the 22q11-q12 breakpoint of a t(11;22) in a Ewing sarcoma (612219).
Using an interspecific backcross, Justice et al. (1990) mapped the Bcr gene to chromosome 10 of the mouse.
Gene Function
BCR遺伝子産物は自己リン酸化活性と、いくつかのタンパク質基質をリン酸化する活性をもつ。また、プロテインキナーゼ活性、SH2結合ドメインをもつことから細胞内シグナル伝達経路で働いていると考えられる。
Maru and Witte (1991) demonstrated that purified BCR contains autophosphorylation activity and transphosphorylation activity for several protein substrates. A region encoded by the first exon was essential for this novel phosphotransferase activity. The findings suggested that the protein kinase and SH2-binding domains may work in concert with other regions of the molecule in intracellular signaling processes.
Diekmann et al. (1991) found that the C-terminal domain of BCR functions as a GTPase activating protein for p21(rac); see rac serine/threonine protein kinase (164730).
BCR/ABL Fusion Gene
慢性骨髄性白血病で長腕の一部が欠損したフィラデルフィア染色体が報告された。
Nowell and Hungerford (1960) identified the Philadelphia chromosome of chronic myeloid leukemia (a G group chromosome with part of its long arm missing). It was called the Philadelphia chromosome because it was thought to be useful to follow the practice of hemoglobinologists and name anomalous chromosomes after the city of discovery. It was presumed to be a deleted chromosome 21, the same chromosome as that which is trisomic in Down syndrome (190685). The elevated alkaline phosphatase activity in Down syndrome and depressed activity in CML was viewed as consistent with this interpretation. Using the improved definition provided by 'banding' methods, Rowley (1973) showed that in fact there is a translocation of the distal part of chromosome 22 (not 21) onto another chromosome, usually 9q.
Fitzgerald (1976) described a family in which a man and 2 of his 3 children had a Philadelphia-like chromosome t(11;22)(q25;q13). The fact that they did not have leukemia or other hematologic disorder was thought by Fitzgerald to relate to the finding that the break in chromosome 22 was distal to that in CML. In these cases it was at the q12-q13 interface, whereas in the Philadelphia chromosome it is at the q11-q12 band interface. Thus the 22q12 band may contain critical genetic material concerned with normal (and abnormal) myeloid proliferation.
デクラインらは1982年にフィラデルフィア染色体の形成の過程でアベルソンがん遺伝子を含む９番染色体が２２番に転座することを報告した。
De Klein et al. (1982) demonstrated that the Abelson oncogene is translocated from chromosome 9 to chromosome 22 in the formation of the Philadelphia chromosome.
Prakash and Yunis (1984) located the breakpoints in CML to subbands 22q11.21 and 9q34.1. Although the position of the breakpoint in chromosome 9 is quite variable, the breakpoint in chromosome 22 is clustered in an area called bcr for 'breakpoint cluster region.' Shtivelman et al. (1985) referred to bcr as a gene and stated that the ABL oncogene is transferred 'into the bcr gene of chromosome 22.' They found that an 8-kb RNA specific to CML is a fused transcript of the 2 genes. The fused protein is presumably involved in the malignant process. The protein has bcr information at its amino terminus and retains most but not all of the normal abl protein sequences. Since the breakpoint in 9 may be as much as 30 or 40 kb 5-prime to ABL, a large amount must be 'looped out' in the fusion process. The fusion protein has tyrosine kinase activity.
About 10% of patients with acute lymphocytic leukemia (ALL) have the translocation t(9;22)(q34;q11) indistinguishable from that of CML (608232). Erikson et al. (1986), however, found in 3 of 5 such cases of ALL that the bcr region was not involved and that the 22q11 chromosome breakpoint was proximal (5-prime) to the bcr region. Furthermore, the bcr and c-abl transcripts were of normal size in an ALL line carrying the t(9;22) translocation. The breakpoints of the t(9;22) CML, the t(9;22) of acute lymphocytic leukemia, and the t(8;22) of Burkitt lymphoma fall into 22q11 and are cytologically indistinguishable. By chromosomal in situ hybridization, however, they can be distinguished (Emanuel et al., 1984; Erikson et al., 1986). To this experience, Griffin et al. (1986) added observations on t(11;22), both constitutional and tumor-related (see 133450). In the hybrid gene of CML, the bcr contribution is 5-prime to the abl contribution. In the creation of the gene, splicing can occur across an interval as great as 100 kb. The product of the bcr/abl hybrid gene is a 210-kD protein. It was suggested that the hybrid gene and its product protein be designated PHL.
Mes-Masson et al. (1986) isolated overlapping cDNA clones defining the complete coding region for the hybrid protein generated from the ABL and BCR genes.
As a result of the CML translocation, the 3-prime BCR exons are removed and remain on chromosome 22 (Stam et al., 1987). The BCR and ABL genes are fused in a head-to-tail fashion.
Verhest and Monsieur (1983) found the Philadelphia chromosome in a case of essential thrombocytopenia.
Ganesan et al. (1986) found that the BCR gene was rearranged in 7 cases of Philadelphia chromosome-negative CML. In 5 cases hematologic findings were indistinguishable from those of patients with the Philadelphia chromosome.
Hermans et al. (1987) showed that a distinct and different fusion of BCR and ABL occurs in creation of the Philadelphia chromosome associated with acute lymphoblastic leukemia (ALL). Rubin et al. (1988) similarly reported a distinctive fusion in ALL.
Haas et al. (1992) demonstrated a 'parent of origin' bias in cases of Philadelphia chromosome-positive leukemia by use of unique specific chromosome band polymorphisms. They showed that the translocated chromosome 9 was always of paternal origin, whereas the translocated chromosome 22 was derived exclusively from the maternal copy. Preferential retention of paternal alleles has been found in sporadic tumors such as Wilms tumor (194070), rhabdomyosarcoma (268210), osteosarcoma (259500), and retinoblastoma (180200). The finding of preferential participation of the paternally derived chromosome 9 and the maternally derived chromosome 22 in the Ph-translocation strongly indicates that the chromosomal regions involved are imprinted. Feinberg (1993) reviewed 11 examples of preferential parental allelic alterations in cancer. Fioretos et al. (1994) were unable to find evidence for genomic imprinting of the human BCR gene. They identified a BamHI polymorphism in the coding region of BCR exon 1 and by RT-PCR assay showed that both BCR alleles are expressed in the peripheral blood cells of normal persons. Furthermore, Litz and Copenhaver (1994) used PvuII and MaeII restriction site polymorphisms in the BCR gene to study 3 cases of Philadelphia chromosome-positive CML. In all 3 cases, the rearranged allele was paternal in origin. Melo et al. (1994) could find no evidence of parental imprinting of the ABL gene and cited evidence which appeared to exclude imprinting of the BCR gene and to suggest that there is, in fact, no preferential involvement of the maternal BCR or paternal ABL alleles in the formation of the BCR-ABL fusion gene. Melo et al. (1995) further reviewed the evidence they interpreted as indicating that there is no parental bias in the origin of the translocated ABL gene and no evidence for genomic imprinting of ABL in CML. On the other hand, Haas (1995) argued that it still remains likely that ABL and BCR are imprinted.
From the sequence of 4 newly studied Philadelphia chromosome translocations and a review of several other previously sequenced breakpoints, Chissoe et al. (1995) could discern no consistent breakpoint features. No clear-cut mechanism for Philadelphia chromosome translocation was evident.
To block BCR/ABL function, Lim et al. (2000) created a unique tyrosine phosphatase by fusing the catalytic domain of SHP1 (604630) to the ABL binding domain of RIN1 (605965), an established binding partner and substrate for c-ABL and BCR/ABL. This fusion construct binds to BCR/ABL in cells and functions as an active phosphatase. It effectively suppressed BCR/ABL function as judged by reductions in transformation of fibroblast cells, growth factor independence of hematopoietic cell lines, and proliferation of primary bone marrow cells. In addition, the leukemogenic properties of BCR/ABL in a murine model system were blocked by coexpression of the fusion construct. Expression of the construct also reversed the transformed phenotype of a human leukemia-derived cell line. These results appeared to have direct implications for leukemia therapeutics and suggested an approach to block aberrant signal transduction in other pathologies through the use of appropriately designed escort/inhibitors.
Saglio et al. (2002) found that a patient with a typical form of CML carried a large deletion on the derivative chromosome 9q+ and an unusual BCR-BCL transcript characterized by the insertion, between BCR exon 14 and ABL exon 2, of 126 bp derived from a region located on chromosome 9, 1.4 Mb 5-prime to ABL. This sequence was contained in a bacterial artificial chromosome (BAC), which in FISH experiments on normal metaphases was found to detect, in addition to the predicted clear signal at 9q34, a faint but distinct signal at 22q11.2, where the BCR gene is located, suggesting the presence of a large region of homology between the 2 chromosome regions. A BLAST analysis of the particular BAC sequence against the entire human genome revealed the presence of a stretch of homology, about 76 kb long, located approximately 150 kb 3-prime to the BCR gene, and containing the 126-bp insertion sequence. Evolutionary studies using FISH identified the region as a duplicon, which transposed from the region orthologous to human 9q34 to chromosome 22 after the divergence of orangutan from the human-chimpanzee-gorilla common ancestor about 14 million years ago. Saglio et al. (2002) noted that sequence analyses reported as part of the Human Genome Project had disclosed an unpredicted extensive segmental duplication in the human genome, and the impact of duplicons in triggering genomic disorders is becoming more and more apparent. The discovery of a large duplicon relatively close to the ABL and BCR genes and the finding that the 126-bp insertion is very close to the duplicon at 9q34 open the question of the possible involvement of the duplicon in the formation of the Philadelphia chromosome translocation.
The arrest of differentiation is a feature of chronic myelogenous leukemia cells both in myeloid blast crisis and in myeloid precursors that ectopically express the p210(BCR-ABL) oncoprotein. Related to the underlying mechanisms of this arrest of differentiation, Perrotti et al. (2002) showed that expression of BCR-ABL in myeloid precursor cells leads to transcriptional suppression of the gene encoding granulocyte colony-stimulating factor receptor (CSF3R; 138971), possibly through downmodulation of C/EBP-alpha (116897), the principal regulator of granulocytic differentiation. Expression of C/EBP-alpha protein is barely detectable in primary marrow cells taken from individuals affected with chronic myeloid leukemia in blast crisis. In contrast, CEBPA RNA is clearly present. Further experiments by Perrotti et al. (2002) indicated that BCR-ABL regulates the expression of C/EBP-alpha by inducing heterogeneous nuclear ribonucleoprotein E2 (PCBP2; 601210).
RNA interference (RNAi) is a highly conserved regulatory mechanism that mediates sequence-specific posttranscriptional gene silencing initiated by double-stranded RNA (dsRNA) (Fire, 1999). Fusion transcripts encoding oncogenic proteins may represent potential targets for a tumor-specific RNAi approach. Scherr et al. (2003) demonstrated that small interfering RNAs (siRNAs) against BCR-ABL specifically inhibited expression of BCR-ABL mRNA in hematopoietic cell lines and primary CML cells.
Goldman and Melo (2003) tabulated BCR-ABL substrates and diagrammed the signal-transduction pathways affected by BCR-ABL.
To define oncogenic lesions that cooperate with BCR-ABL1 to induce ALL, Mullighan et al. (2008) performed a genomewide analysis of diagnostic leukemia samples from 304 individuals with ALL, including 43 BCR-ABL1 B-progenitor ALLs and 23 CML cases. IKZF1, encoding the transcription factor Ikaros (603023), was deleted in 83.7% of BCR-ABL1 ALL, but not in chronic phase CML. Deletion of IKZF1 was also identified as an acquired lesion at the time of transformation of CML to ALL (lymphoid blast crisis). The IKZF1 deletions resulted in haploinsufficiency, expression of a dominant-negative Ikaros isoform, or the complete loss of Ikaros expression. Sequencing of IKZF1 deletion breakpoints suggested that aberrant RAG-mediated recombination (see 179615) is responsible for the deletions. Mullighan et al. (2008) concluded that genetic lesions resulting in the loss of Ikaros function are an important event in the development of BCR-ABL1 ALL.
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 chronic myelogenous leukemia (CML; 608232) 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.
BCR/FGFR1 Fusion Gene
Demiroglu et al. (2001) described 2 patients with a clinical and hematologic diagnosis of CML in chronic phase who had an acquired t(8;22)(p11;q11). They confirmed that both patients were negative for a BCR-ABL fusion gene and that both had an in-frame mRNA fusion between BCR exon 4 and FGFR1 (136350) exon 9. Thus, a BCR-FGFR1 fusion may occur in patients with apparently typical CML. The possibility of successful treatment with specific FGFR1 inhibitors was suggested.
BCR/PDGFRA Fusion Gene
Baxter et al. (2002) reported the identification and cloning of a rare variant translocation, t(4;22)(q12;q11), in 2 patients with a CML-like myeloproliferative disease. An unusual in-frame BCR/PDGFRA (173490) fusion mRNA was identified in both patients, with either BCR exon 7 or exon 12 fused to short BCR intron-derived sequences, which were in turn fused to part of PDGFRA exon 12. Sequencing of the genomic breakpoint junctions showed that the chromosome 22 breakpoints fell in BCR introns, whereas the chromosome 4 breakpoints were within PDGFRA exon 12.
Molecular Genetics
Resistance to Tyrosine Kinase Inhibitors
Clinical studies with the Abl tyrosine kinase inhibitor STI571 in CML demonstrated that many patients with advanced-stage disease respond initially but then relapse. Through biochemical and molecular analysis of clinical material, Gorre et al. (2001) found that the drug resistance was associated with a reactivation of BCR-ABL signal transduction in all cases examined. In 6 of 9 patients, resistance was associated with a single amino acid substitution in a threonine residue of the Abl kinase domain known to form a critical hydrogen bond with the drug. This substitution (T315I; 189980.0001) was sufficient to confer STI571 resistance in a reconstitution experiment. In 3 patients, resistance was associated with progressive BCR-ABL gene amplification. Gorre et al. (2001) concluded that their studies provided evidence that genetically complex cancers retain dependence on an initial oncogenic event and suggested a strategy for identifying inhibitors of STI571 resistance.
Azam et al. (2003) stated that sequencing of the BCR-ABL gene in patients who relapsed after STI571 chemotherapy revealed a limited set of kinase domain mutations that mediate drug resistance. To obtain a more comprehensive survey of the amino acid substitutions that confer STI571 resistance, they performed an in vitro screen of randomly mutagenized BCR-ABL and recovered all the major mutations previously identified in patients and numerous others that illuminated novel mechanisms of acquired drug resistance. Structural modeling implied that a novel class of variants acts allosterically to destabilize the autoinhibited conformation of the ABL kinase, to which STI571 preferentially binds. The authors concluded that this screening strategy is a paradigm applicable to a growing list of target-directed anticancer agents and provides a means of anticipating the drug-resistant amino acid substitutions that are likely to be clinically problematic.
Resistance to tyrosine kinase inhibitors (TKIs) develops in virtually all cases of Philadelphia chromosome-positive acute lymphoblastic leukemia. Duy et al. (2011) reported the discovery of a novel mechanism of drug resistance that is based on protective feedback signaling of leukemia cells in response to treatment with TKIs. In Philadelphia chromosome-positive acute lymphoblastic leukemia cells, Duy et al. (2011) identified BCL6 (109565) as a central component of this drug-resistance pathway and demonstrated that targeted inhibition of BCL6 leads to eradication of drug-resistant and leukemia-initiating subclones.
