Cytogenetic and molecular analysis of a "masked" Philadelphia chromosome in chronic and blastic phases of chronic myeloid leukemia

Identification of a novel fusion gene, TTL, fused to ETV6 in acute lymphoblastic leukemia with t(12;13)(p13;q14), and its implication in leukemogenesis

ETS variant gene 6 (ETV6)/translocation, ETS, leukemia (TEL)-involving chromosomal translocations are frequently observed in various hematologic neoplasms. We describe here a novel ETV6-involving translocation, t(12;13)(p13;q14), found in the case of acute lymphoblastic leukemia, in which ETV6 fused with a previously unknown gene, named Twelve-thirteen Translocation Leukemia gene (TTL), at 13q14. TTL was weakly but ubiquitously expressed in normal human tissues as detected by reverse transcribed-PCR. Three TTL splicing forms were identified, TTL-T from a human testis cDNA library, with an open-reading frame of 402 bp encoding 133 amino acids (aa), and TTL-B1 and -B2 from a human brain cDNA library. These proteins have no homology to known proteins. In leukemic cells from the patient, both reciprocal fusion transcripts, ETV6/TTL and TTL/ETV6, were expressed. The predominant fusion transcript, TTL/ETV6-1, encodes a predicted 530 aa fusion protein containing 89 aa of the N-terminal TTL fusing to the helix-loop-helix domain and ETS-binding domain of ETV6. Although the function of TTL is yet to be elucidated, our findings will provide another insight into the molecular pathogenesis of leukemia having ETV6-involving translocations.

Myeloid- and lymphoid-specific breakpoint cluster regions in chromosome band 13q14 in acute leukemia

Abnormalities of chromosome band 13q14 occur in hematologic malignancies of all lineages and at all stages of differentiation. Unlike other chromosomal translocations, which are usually specific for a given lineage, the chromosomal translocation t(12;13)(p12;q14) has been observed in both B-cell and T-cell precursor acute lymphoblastic leukemia (BCP-, TCP-ALL), in differentiated and undifferentiated acute myeloblastic leukemia (AML), and in chronic myeloid leukemia (CML) at progression to blast crisis. The nature of these translocations and their pathologic consequences remain unknown. To begin to define the gene(s) involved on chromosome 13, we have performed fluorescence in situ hybridization (FISH) using a panel of YACs from the region, on a series of 10 cases of acute leukemia with t(12;13)(p12;q14) and 1 case each with "variant" translocations including t(12;13)(q21;q14), t(10;13)(q24;q14) and t(9;13)(p21;q14). In 8/13 cases/cell lines, the 13q14 break fell within a single 1.4 Mb CEPH MegaYAC. This YAC fell immediately telomeric of the forkhead (FKHR) gene, which is disrupted in the t(2;13)(q35;q14) seen in pediatric alveolar rhabdomyosarcoma. Seven of the 8 cases with breaks in this YAC were AML. In 4/13 cases, the 13q14 break fell within a 1.7-Mb YAC located about 3 Mb telomeric of the retinoblastoma (RB1) gene: all 4 cases were ALL. One case of myelodysplastic syndrome exhibited a break within 13q12, adjacent to the BRCA2 gene. These data indicate the presence of myeloid- and lymphoid-specific breakpoint cluster regions within chromosome band 13q14 in acute leukemia.

Fusion of ETV6 to the Caudal-Related Homeobox Gene CDX2 in Acute Myeloid Leukemia With the t(12;13)(p13;q12)

The t(12;13)(p13;q12) is a rare, recurrent translocation reported in a range of hematological malignancies. We have analyzed the molecular basis of this lesion in three patients with acute myeloid leukemia (AML), two of whom were known to have chromosome 12 breakpoints within the ETV6 gene. Fluorescence in situ hybridization (FISH) with ETV6 cosmids indicated that this gene was also disrupted in the third patient, while the normal ETV6 allele was retained. 3′ rapid amplification of cDNA ends (RACE) polymerase chain reaction (PCR) from bone marrow mRNA of this individual identified a novel sequence fused to ETV6 that was homologous to a region just upstream of the mouse CDX2 homeobox gene, the human homologue of which has previously been mapped to chromosome 13q12. PCR primers designed to amplify an ETV6-CDX2 fusion identified two major transcripts from this patient. First, a direct in-frame fusion between exon 2 of ETV6 and exon 2 of CDX2, and second, a transcript that had an additional sequence of unknown origin spliced between these same exons. Surprisingly, apparently normal CDX2 transcripts, usually expressed only in intestinal epithelium, were also detectable in cDNA from this patient. Neither normal nor fusion CDX2 mRNA was detectable in the two other patients with a t(12;13), indicating that this translocation is heterogeneous at the molecular level. Reverse transcription-PCR analysis showed that CDX2 mRNA, but not ETV6-CDX2 mRNA, was strongly expressed in 1 of 10 patients with chronic myeloid leukemia in transformation, suggesting that deregulation of this gene may be more widespread in leukemia. CDX2 is known to regulate class I homeobox genes and its expression in hematopoietic cells may critically alter the balance between differentiation and proliferation.

Fusion of ETV6 to the Caudal-Related Homeobox Gene CDX2 in Acute Myeloid Leukemia With the t(12;13)(p13;q12)

The t(12;13)(p13;q12) is a rare, recurrent translocation reported in a range of hematological malignancies. We have analyzed the molecular basis of this lesion in three patients with acute myeloid leukemia (AML), two of whom were known to have chromosome 12 breakpoints within the ETV6 gene. Fluorescence in situ hybridization (FISH) with ETV6 cosmids indicated that this gene was also disrupted in the third patient, while the normal ETV6 allele was retained. 3′ rapid amplification of cDNA ends (RACE) polymerase chain reaction (PCR) from bone marrow mRNA of this individual identified a novel sequence fused to ETV6 that was homologous to a region just upstream of the mouse CDX2 homeobox gene, the human homologue of which has previously been mapped to chromosome 13q12. PCR primers designed to amplify an ETV6-CDX2 fusion identified two major transcripts from this patient. First, a direct in-frame fusion between exon 2 of ETV6 and exon 2 of CDX2, and second, a transcript that had an additional sequence of unknown origin spliced between these same exons. Surprisingly, apparently normal CDX2 transcripts, usually expressed only in intestinal epithelium, were also detectable in cDNA from this patient. Neither normal nor fusion CDX2 mRNA was detectable in the two other patients with a t(12;13), indicating that this translocation is heterogeneous at the molecular level. Reverse transcription-PCR analysis showed that CDX2 mRNA, but not ETV6-CDX2 mRNA, was strongly expressed in 1 of 10 patients with chronic myeloid leukemia in transformation, suggesting that deregulation of this gene may be more widespread in leukemia. CDX2 is known to regulate class I homeobox genes and its expression in hematopoietic cells may critically alter the balance between differentiation and proliferation.

Molecular genetic characterization of the Philadelphia chromosome detected in reactor personnel highly exposed to radiation from the Chernobyl accident

Clonal del(22q) chromosome aberrations were coincidentally observed in highly exposed reactor personnel of the Chernobyl power plant accident in the course of retrospective biological dosimetry. These aberrant chromosomes were detected in PHA-stimulated cultures from peripheral blood after FPG staining and revealed a morphology similar to a Philadelphia chromosome. A rearrangement of the BCR gene on 22q11 could be confirmed in unstimulated peripheral blood by RFLP analysis from three of four del(22q) carrying cases. FISH analysis of the del(22q) carrying cases with BCR- and ABL-specific DNA probes additionally exhibited a BCR-ABL fusion in 5.2 to 9% of cells in unstimulated blood. Breakpoints within the BCR gene could be located either in the M-bcr or the m-bcr region and thus, a specific breakpoint region could not be detected in these four patients. Since typical clinical leukemic symptoms associated with the translocation (9;22)(q24;q11) could not be observed in these highly irradiated subjects (1.1 to 5.8 Gy), the role of this particular aberration in the development of a radiation-induced leukemia remains obscure.

Reciprocal translocation t(12;13)(p13;q14) in acute nonlymphoblastic leukemia: report and cytogenetic analysis of two cases

Two cases of acute nonlymphoblastic leukemia with a reciprocal translocation t(12;13)(p13;q14) are described. Both patients were male adults with a diagnosis of M0 FAB type. Beside standard cytogenetic analysis, we applied fluorescence in situ hybridization (FISH) in order to investigate the position of the RB gene with respect to the breakpoint at 13q14. Our results showed that the RB gene was proximal to the breakpoint, but, apparently, not split in either case.

Complex translocation involving Ph chromosome in a patient with typical chronic myelogenous leukemia

We report a cytogenetic study of a patient with chronic myelogenous leukemia (CML) who, while displaying a Philadelphia (Ph) chromosome, resulting from a standard t(9;22) at diagnosis, during the chronic phase (CP) showed disappearance of the Ph and occurrence of new chromosome changes, including a marker probably arising from a translocation involving chromosome 17 and the Ph. In situ hybridization confirmed the cytogenetic appearance and demonstrated that the breakpoint on the Ph marker occurred below the BCR-ABL fusion gene.