T(11;22)(q23;q11.2) In acute myeloid leukemia of infant twins fuses MLL with hCDCrel, a cell division cycle gene in the genomic region of deletion in DiGeorge and velocardiofacial syndromes.
ABSTRACT: We examined the MLL genomic translocation breakpoint in acute myeloid leukemia of infant twins. Southern blot analysis in both cases showed two identical MLL gene rearrangements indicating chromosomal translocation. The rearrangements were detectable in the second twin before signs of clinical disease and the intensity relative to the normal fragment indicated that the translocation was not constitutional. Fluorescence in situ hybridization with an MLL-specific probe and karyotype analyses suggested t(11;22)(q23;q11. 2) disrupting MLL. Known 5' sequence from MLL but unknown 3' sequence from chromosome band 22q11.2 formed the breakpoint junction on the der(11) chromosome. We used panhandle variant PCR to clone the translocation breakpoint. By ligating a single-stranded oligonucleotide that was homologous to known 5' MLL genomic sequence to the 5' ends of BamHI-digested DNA through a bridging oligonucleotide, we formed the stem-loop template for panhandle variant PCR which yielded products of 3.9 kb. The MLL genomic breakpoint was in intron 7. The sequence of the partner DNA from band 22q11.2 was identical to the hCDCrel (human cell division cycle related) gene that maps to the region commonly deleted in DiGeorge and velocardiofacial syndromes. Both MLL and hCDCrel contained homologous CT, TTTGTG, and GAA sequences within a few base pairs of their respective breakpoints, which may have been important in uniting these two genes by translocation. Reverse transcriptase-PCR amplified an in-frame fusion of MLL exon 7 to hCDCrel exon 3, indicating that an MLL-hCDCrel chimeric mRNA had been transcribed. Panhandle variant PCR is a powerful strategy for cloning translocation breakpoints where the partner gene is undetermined. This application of the method identified a region of chromosome band 22q11.2 involved in both leukemia and a constitutional disorder.
Project description:Panhandle PCR amplifies genomic DNA with known 5' and unknown 3' sequences from a template with an intrastrand loop schematically shaped like a pan with a handle. We used panhandle PCR to clone MLL genomic breakpoints in two pediatric treatment-related leukemias. The karyotype in a case of treatment-related acute lymphoblastic leukemia showed the t(4;11)(q21;q23). Panhandle PCR amplified the translocation breakpoint at position 2158 in intron 6 in the 5' MLL breakpoint cluster region (bcr). The karyotype in a case of treatment-related acute myeloid leukemia was normal, but Southern blot analysis showed a single MLL gene rearrangement. Panhandle PCR amplified the breakpoint at position 1493 in MLL intron 6. Screening of somatic cell hybrid and radiation hybrid DNAs by PCR and reverse transcriptase-PCR analysis of the leukemic cells indicated that panhandle PCR identified a fusion of MLL intron 6 with a previously uncharacterized sequence in MLL intron 1, consistent with a partial duplication. In both cases, the breakpoints in the MLL bcr were in Alu repeats, and there were Alu repeats in proximity to the breakpoints in the partner DNAs, suggesting that Alu sequences were relevant to these rearrangements. This study shows that panhandle PCR is an effective method for cloning MLL genomic breakpoints in treatment-related leukemias. Analysis of additional pediatric cases will determine whether breakpoint distribution deviates from the predilection for 3' distribution in the bcr that has been found in adult cases.
Project description:We used panhandle PCR to clone the der(11) genomic breakpoint junction in three leukemias with t(4;11) and devised reverse-panhandle PCR to clone the breakpoint junction of the other derivative chromosome. This work contributes two elements to knowledge on MLL translocations. First is reverse-panhandle PCR for cloning breakpoint junctions of the other derivative chromosomes, sequences of which are germane to understanding the MLL translocation process. The technique revealed duplicated sequences in one case of infant acute lymphoblastic leukemia (ALL) and small deletions in a case of treatment-related ALL. The second element is discovery of a three-way rearrangement of MLL, AF-4, and CDK6 in another case of infant ALL. Cytogenetic analysis was unsuccessful at diagnosis, but suggested t(4;11) and del(7)(q21q31) at relapse. Panhandle PCR analysis of the diagnostic marrow identified a breakpoint junction of MLL intron 8 and AF-4 intron 3. Reverse-panhandle PCR identified a breakpoint junction of CDK6 from band 7q21-q22 and MLL intron 9. CDK6 encodes a critical cell cycle regulator and is the first gene of this type disrupted by MLL translocation. Cdk6 is overexpressed or disrupted by translocation in many cancers. The in-frame CDK6-MLL transcript is provocative with respect to a potential contribution of the predicted Cdk6-MLL fusion protein in the genesis of the ALL, which also contains an in-frame MLL-AF4 transcript. The sequences in these three cases show additional MLL genomic breakpoint heterogeneity. Each breakpoint junction suggests nonhomologous end joining and is consistent with DNA damage and repair. CDK6-MLL is a new fusion of both genes.
Project description:Leukemias with MLL gene translocations are a complication of primary cancer treatment with DNA topoisomerase II inhibitors. How early translocations appear during primary cancer treatment has not been investigated. We tracked the leukemic clone with an MLL gene translocation during neuroblastoma therapy in a child who developed acute myeloid leukemia. The karyotype of the leukemic clone showed del(11)(q23). We used panhandle PCR-based methods to isolate the breakpoint junction involving MLL and an unknown partner gene. Marrow DNA from neuroblastoma diagnosis and DNA and RNA from serial preleukemic marrows were examined for the translocation. The karyotypic del(11)(q23) was a cryptic t(11;17). GAS7, a growth arrest-specific gene at chromosome band 17p13, was the partner gene of MLL. Two different MLL-GAS7 fusion transcripts were expressed. The translocation was already detectable by 1.5 months after the start of neuroblastoma treatment. The translocation was not detectable in the marrow at neuroblastoma diagnosis or in peripheral blood lymphocyte DNAs of six normal subjects. GAS7 is a new partner gene of MLL in treatment-related acute myeloid leukemia. MLL gene translocations can be present early during anticancer treatment at low cumulative doses of DNA topoisomerase II inhibitors. Although MLL has many partner genes and most have not been characterized, panhandle PCR strategies afford new means for detecting MLL gene translocations early during therapy when the partner gene is unknown.
Project description:Two known recurrent constitutional translocations, t(11;22) and t(17;22), as well as a non-recurrent t(4;22), display derivative chromosomes that have joined to a common site within the low copy repeat B (LCR-B) region of 22q11.2. This breakpoint is located between two AT-rich inverted repeats that form a nearly perfect palindrome. Breakpoints within the 11q23, 17q11 and 4q35 partner chromosomes also fall near the center of palindromic sequences. In the present work the breakpoints of a fourth translocation involving LCR-B, a balanced ependymoma-associated t(1;22), were characterized not only to localize this junction relative to known genes, but also to further understand the mechanism underlying these rearrangements. FISH mapping was used to localize the 22q11.2 breakpoint to LCR-B and the 1p21 breakpoint to single BAC clones. STS mapping narrowed the 1p21.2 breakpoint to a 1990 bp AT-rich region, and junction fragments were amplified by nested PCR. Junction fragment-derived sequence indicates that the 1p21.2 breakpoint splits a 278 nt palindrome capable of forming stem-loop secondary structure. In contrast, the 1p21.2 reference genomic sequence from clones in the database does not exhibit this configuration, suggesting a predisposition for regional genomic instability perhaps etiologic for this rearrangement. Given its similarity to known chromosomal fragile site (FRA) sequences, this polymorphic 1p21.2 sequence may represent one of the FRA1 loci. Comparative analysis of the secondary structure of sequences surrounding translocation breakpoints that involve LCR-B with those not involving this region indicate a unique ability of the former to form stem-loop structures. The relative likelihood of forming these configurations appears to be related to the rate of translocation occurrence. Further analysis suggests that constitutional translocations in general occur between sequences of similar melting temperature and propensity for secondary structure.
Project description:Identifying translocations of the MLL gene at chromosome band 11q23 is important for the characterization and treatment of leukemia. However, cytogenetic analysis does not always find the translocations and the many partner genes of MLL make molecular detection difficult. We developed cDNA panhandle PCR to identify der(11) transcripts regardless of the partner gene. By reverse transcribing first-strand cDNAs with oligonucleotides containing coding sequence from the 5' MLL breakpoint cluster region at the 5' ends and random hexamers at the 3' ends, known MLL sequence was attached to the unknown partner sequence. This enabled the formation of stem-loop templates with the fusion point of the chimeric transcript in the loop and the use of MLL primers in two-sided PCR. The assay was validated by detection of the known fusion transcript and the transcript from the normal MLL allele in the cell line MV4-11. cDNA panhandle PCR then was used to identify the fusion transcripts in two cases of treatment-related acute myeloid leukemia where the karyotypes were normal and the partner genes unknown. cDNA panhandle PCR revealed a fusion of MLL with AF-10 in one case and a fusion of MLL with ELL in the other. Alternatively spliced transcripts and exon scrambling were detectable by the method. Leukemias with normal karyotypes may contain cryptic translocations of MLL with a variety of partner genes. cDNA panhandle PCR is useful for identifying MLL translocations and determining unknown partner sequences in the fusion transcripts.
Project description:We analyzed the der(11) and der(4) genomic breakpoint junctions of a t(4;11) in the leukemia of a patient previously administered etoposide and dactinomycin by molecular and biochemical approaches to gain insights about the translocation mechanism and the relevant drug exposure. The genomic breakpoint junctions were amplified by PCR. Cleavage of DNA substrates containing the normal homologues of the MLL and AF-4 translocation breakpoints was examined in vitro upon incubation with human DNA topoisomerase IIalpha and etoposide, etoposide catechol, etoposide quinone, or dactinomycin. The der(11) and der(4) genomic breakpoint junctions both involved MLL intron 6 and AF-4 intron 3. Recombination was precise at the sequence level except for the overall gain of a single templated nucleotide. The translocation breakpoints in MLL and AF-4 were DNA topoisomerase II cleavage sites. Etoposide and its metabolites, but not dactinomycin, enhanced cleavage at these sites. Assuming that DNA topoisomerase II was the mediator of the breakage, processing of the staggered nicks induced by DNA topoisomerase II, including exonucleolytic deletion and template-directed polymerization, would have been required before ligation of the ends to generate the observed genomic breakpoint junctions. These data are inconsistent with a translocation mechanism involving interchromosomal recombination by simple exchange of DNA topoisomerase II subunits and DNA-strand transfer; however, consistent with reciprocal DNA topoisomerase II cleavage events in MLL and AF-4 in which both breaks became stable, the DNA ends were processed and underwent ligation. Etoposide and/or its metabolites, but not dactinomycin, likely were the relevant exposures in this patient.
Project description:Constitutional translocations, typically involving chromosome 3, have been recognized as a rare cause of inherited predisposition to renal cell carcinoma (RCC) for four decades. However, knowledge of the molecular basis of this association is limited. We have characterized the breakpoints by genome sequencing (GS) of constitutional chromosome abnormalities in five individuals who presented with RCC. In one individual with constitutional t(10;17)(q11.21;p11.2), the translocation breakpoint disrupted two genes: the known renal tumor suppressor gene (TSG) FLCN (and clinical features of Birt-Hogg-Dubé syndrome were detected) and RASGEF1A. In four cases, the rearrangement breakpoints did not disrupt known inherited RCC genes. In the second case without chromosome 3 involvement, the translocation breakpoint in an individual with a constitutional t(2;17)(q21.1;q11.2) mapped 12 Kb upstream of NLK. Interestingly, NLK has been reported to interact indirectly with FBXW7 and a previously reported RCC-associated translocation breakpoint disrupted FBXW7. In two cases of constitutional chromosome 3 translocations, no candidate TSGs were identified in the vicinity of the breakpoints. However, in an individual with a constitutional chromosome 3 inversion, the 3p breakpoint disrupted the FHIT TSG (which has been reported previously to be disrupted in two apparently unrelated families with an RCC-associated t(3;8)(p14.2;q24.1). These findings (a) expand the range of constitutional chromosome rearrangements that may be associated with predisposition to RCC, (b) confirm that chromosome rearrangements not involving chromosome 3 can predispose to RCC, (c) suggest that a variety of molecular mechanisms are involved the pathogenesis of translocation-associated RCC, and (d) demonstrate the utility of GS for investigating such cases.
Project description:Constitutional translocations at the same 22q11.21 low copy repeat B (LCR-B) breakpoint involved in the recurrent t(11;22) are relatively abundant. A novel 46,XY,t(8;22)(q24.13;q11.21) rearrangement was investigated to determine whether the recurrent LCR-B breakpoint is involved. Investigations demonstrated an inversion of the 3Mb region typically deleted in patients with the 22q11.2 deletion syndrome. The 22q11.21 inversion appears to be mediated by low copy repeats, and is presumed to have taken place prior to translocation with 8q24.13. Despite predictions based on inversions observed in other chromosomes harboring low copy repeats, this 22q11.2 inversion has not been observed previously. The current studies utilize novel laser microdissection and MLPA (multiplex ligation-dependent probe amplification) approaches, as adjuncts to FISH, to map the breakpoints of the complex rearrangements of 22q11.21 and 8q24.21. The t(8;22) occurs between the recurrent site on 22q11.21 and an AT-rich site at 8q24.13, making it the fifth different chromosomal locus characterized at the nucleotide level engaged in a translocation with the unstable recurrent breakpoint at 22q11.21. Like the others, this breakpoint occurs at the center of a palindromic sequence. This sequence appears capable of forming a perfect 145 bp stem-loop. Remarkably, this site appears to have been involved in a previously reported t(3;8) occurring between 8q24.13 and FRA3B on 3p14.2. Further, the fragile site-like nature of all of the breakpoint sites involved in translocations with the recurrent site on 22q11.21, suggests a mechanism based on delay of DNA replication in the initiation of these chromosomal rearrangements.
Project description:Chromosomal rearrangements of the human MLL/KMT2A gene are associated with infant, pediatric, adult and therapy-induced acute leukemias. Here we present the data obtained from 2345 acute leukemia patients. Genomic breakpoints within the MLL gene and the involved translocation partner genes (TPGs) were determined and 11 novel TPGs were identified. Thus, a total of 135 different MLL rearrangements have been identified so far, of which 94 TPGs are now characterized at the molecular level. In all, 35 out of these 94 TPGs occur recurrently, but only 9 specific gene fusions account for more than 90% of all illegitimate recombinations of the MLL gene. We observed an age-dependent breakpoint shift with breakpoints localizing within MLL intron 11 associated with acute lymphoblastic leukemia and younger patients, while breakpoints in MLL intron 9 predominate in AML or older patients. The molecular characterization of MLL breakpoints suggests different etiologies in the different age groups and allows the correlation of functional domains of the MLL gene with clinical outcome. This study provides a comprehensive analysis of the MLL recombinome in acute leukemia and demonstrates that the establishment of patient-specific chromosomal fusion sites allows the design of specific PCR primers for minimal residual disease analyses for all patients.
Project description:We have demonstrated that the breakpoints of the constitutional t(11;22) are located at palindromic AT-rich repeats (PATRRs) on 11q23 and 22q11. As a mechanism for this recurrent translocation, we proposed that the PATRR forms a cruciform structure that induces the genomic instability leading to the rearrangement. A patient with neurofibromatosis type 1 (NF1) had previously been found to have a constitutional t(17;22) disrupting the NF1 gene on 17q11. We have localized the breakpoint on 22q11 within the 22q11-specific low-copy repeat where the breakpoints of the constitutional t(11;22)s reside, implying a similar palindrome-mediated mechanism for generation of the t(17;22). The NF1 gene contains a 195-bp PATRR within intron 31. We have isolated the junction fragments from both the der(17) and the der(22). The breakpoint on 17q11 is close to the center of the PATRR. A published breakpoint of an additional NF1-afflicted patient with a constitutional t(17;22) is also located close to the center of the same PATRR. Our data lend additional support to the hypothesis that PATRR-mediated genomic instability can lead to a variety of translocations.