Search

Although approximately 1 in 500 individuals carries a reciprocal translocation, little is known about the mechanisms that result in their formation. We analyzed the sequences surrounding the breakpoints in three unbalanced translocations of 1p and 9q, all of which were designated t(1;9)(p36.3;q34), to investigate the presence of sequence motifs that might mediate nonhomologous end joining (NHEJ). The breakpoint regions were unique in all individuals. Two of three translocations demonstrated insertions and duplications at the junctions, suggesting NHEJ in the formation of the rearrangements. No homology was identified in the breakpoint regions, further supporting NHEJ. We found translin motifs at the breakpoint junctions, suggesting the involvement of translin in the joining of the broken chromosome ends. We propose a model for balanced translocation formation in humans similar to transposition in bacteria, in which staggered nicks are repaired resulting in duplications and insertions at the translocation breakpoints.

Interchromosomal insertional translocations are rare chromosome rearrangements with an incidence of about 1:80,000 live births. We report on the clinical and cytogenetic findings of a newborn baby with partial trisomy 10q22-10q24 due to a maternal insertional translocation 15;10. Partial trisomy of the long arm of chromosome 10 is a distinctive chromosome aberration characterized by prenatal-onset growth retardation and craniofacial, skeletal, and other somatic anomalies. Most cases are unbalanced products from reciprocal chromosome translocations, and insertional translocations are rarely involved. The proband was initially referred because of severe intrauterine growth retardation, and fluorescence in situ hybridization (FISH) using painting probes confirmed the maternal balanced (15;10) insertion.

Approximately one in 500 individuals carries a reciprocal translocation. Balanced translocations are usually associated with a normal phenotype unless the translocation breakpoints disrupt a gene(s) or cause a position effect. We investigated breakpoint junctions at the sequence level in phenotypically normal balanced translocation carriers. Eight breakpoint junctions derived from four nonrelated subjects with apparently balanced translocation t(1;22)(p36;q13) were examined. Additions of nucleotides, deletions, duplications, and a triplication identified at the breakpoints demonstrate high complexity at the breakpoint junctions and indicate involvement of multiple mechanisms in the DNA breakage and repair process during translocation formation. Possible detailed nonhomologous end-joining scenarios for t(1;22) cases are presented. We propose that cryptic imbalances in phenotypically normal, balanced translocation carriers may be more common than currently appreciated.

Deletion of chromosome 1p36 is the most commonly observed terminal deletion in humans with a frequency of 1 in 5,000 in the general population. In contrast, 22q13 duplications are rare and only a few cases have been reported. Unbalanced translocations resulting in monosomy 1p36 and a trisomy of 22q13.3 are, thus far, unreported in the literature. Here we present the clinical data and the results of array CGH and FISH analysis of four patients with unbalanced translocations t(1;22)(p36;q13) inherited from unrelated balanced translocation carrier parents. The sizes of the imbalances ranged from 0.12 Mb to nearly 10 Mb. One balanced translocation carrier parent had disruption of the period homolog 3 (PER3) gene and reported sleep disturbances. Overall, patients tended to have more features consistent with deletion of 1p36 than duplication of 22q.

Approximately one in 500 individuals carries a reciprocal translocation. Of the 121 monosomy 1p36 subjects ascertained by our laboratory, three independent cases involved unbalanced translocations of 1p and 9q, all of which were designated t(1;9)(p36.3;q34). These derivative chromosomes were inherited from balanced translocation carrier parents. To understand better the causes and consequences of chromosome breakage and rearrangement in the human genome, we characterized each derivative chromosome at the DNA sequence level and identified the junctions between 1p36 and 9q34. The breakpoint regions were unique in all individuals. Insertions and duplications were identified in two balanced translocation carrier parents and their unbalanced offspring. Sequence analyses revealed that the translocation breakpoints disrupted genes. This study demonstrates that apparently balanced reciprocal translocations in phenotypically normal carriers may have cryptic imbalance at the breakpoints. Because disrupted genes were identified in the phenotypically normal translocation carriers, caution should be exercised when interpreting data on phenotypically abnormal carriers with apparently balanced rearrangements that disrupt putative candidate genes.

Patients with t(11;19) leukemias have one of two different translocations resulting in gene rearrangement with the MLL gene on 11q23. The ELL (eleven-nineteen lysine-rich leukemia) gene at 19p13.1, is rearranged in individuals with t(11;19) and acute myeloid leukemia, while the ENL (eleven-nineteen leukemia) gene at 19p13.3, is rearranged most frequently in those patients with t(11;19) and B-cell acute lymphoid leukemia. However, AML and rare T-cell ALL patients have been reported with the ENL gene rearrangement. At the cytogenetic level of resolution, it is not always possible to distinguish which of the two genes is involved in these translocations and precise molecular classification may be necessary for appropriate chemotherapy regimens to be devised and administered. We therefore developed a dual-color FISH (fluorescent in situ hybridization) assay using BACs (bacterial artificial chromosomes) for ELL and ENL. ELL is directly labeled in spectrum orange and ENL is directly labeled in spectrum green and the probes are co-hybridized. The strategy for the assay is to assess the interphase nuclei of patients for gain of an orange signal, indicating an ELL gene rearrangement (3O2G signal pattern), or gain of a green signal, indicating an ENL gene rearrangement (3G2O signal pattern). The gain of signal is due to splitting of the respective locus during the translocation of distal 19p to 11q23. As constructed, the probes showed a 100% specificity for the respective translocations, based on metaphase and interphase analysis of ELL and ENL t(11;19) positive patients; all of the patients were diagnosed with myeloid leukemia. In addition, further comparative validation was performed using the commercially available dual-color breakapart MLL probe (Vysis). Analysis of the ELL positive AML patients with the MLL probe showed the expected abnormal interphase signal pattern (1O1G1Fusion), confirming MLL gene rearrangement. However, the MLL pattern for the single patient with an ENL gene rearrangement and AML showed an alternate interphase pattern, indicating an alternate MLL breakpoint. Analysis of additional patients is required to determine if those with AML and ENL gene rearrangements routinely have a different MLL breakpoint; this difference could potentially contribute to a myeloid disease presentation. In order to establish clinically valid cut-offs for residual disease analysis, interphase validation was performed. Probe specificity was 100%, with a sensitivity of greater than 97% for both loci. This results in a normal cut-off (plus 2 S.D.) of 0.9/200 abnormal cells for ENL gene rearrangement and 2.18/200 abnormal cells for ELL gene rearrangement. In this laboratory’s use, the probe set is slightly more sensitive for disease detection than the commercially available MLL gene probe (Vysis), which has a sensitivity of 96% and a normal cut-off of 1.84/200 cells. This probe set allows quick distinction between ELL and ENL gene rearrangements in patients with acute leukemia.