title: Analysis of a breakpoint cluster reveals insight into the mechanism of intrachromosomal amplification in a lymphoid malignancy.
abstract: A distinct sub-group of B-cell precursor acute lymphoblastic leukemia, defined by intrachromosomal amplification of chromosome 21 (iAMP21), is restricted to older children and has been associated with a poor outcome. Accurate diagnosis is important for appropriate risk stratification for treatment. It could be improved by understanding the initiating mechanism. iAMP21 is characterized by amplification of a 5.1-24 Mb region of chromosome 21, which includes the RUNX1 gene. It is thought to arise through a breakage-fusion-bridge (BFB) mechanism. Breakpoints initiating BFB cycles were determined from recent array data from 18 patients. Three occurred within the PDE9A gene. Other patients with breakpoints in PDE9A were identified by fluorescence in situ hybridization and molecular copy number counting. Sequencing defined a 1.7 Kb breakpoint cluster region, positioned 400 bp distal to an extensive region enriched for CA repeats with the potential to form Z-DNA. None of the rearranged sequences showed the inverted repeat structure characteristic of BFB; instead PDE9A was fused to intergenic regions of chromosome 21 or to genes on other chromosomes. These observations indicated that previously unrecognized complex events, involving microhomology-mediated end joining, preceded or accompanied initiation of the BFB cycle. A chi-like heptomer, CCTCAGC, contained four of the breakpoints, two within PDE9A and two within partner Alu-repeat sequences. This heptomer was closely homologous to a breakpoint hotspot within the TCF3 gene, suggesting involvement of a common novel recombinogenic mechanism that might also contribute to the recombinogenic potential of Alu repeats. These findings provide insight into potential mechanisms involved in the formation of iAMP21.
authors.affiliation: Leukaemia Research Cytogenetics Group, Northern Institute for Cancer Research, Newcastle University, Sir James Spence Institute, Royal Victoria Infirmary, Newcastle-upon-Tyne, UK.
authors.forename: Paul B
authors.initials: PB
authors.lastname: Sinclair
authors.forename: Helen
authors.initials: H
authors.lastname: Parker
authors.forename: Qian
authors.initials: Q
authors.lastname: An
authors.forename: Vikki
authors.initials: V
authors.lastname: Rand
authors.forename: Hannah
authors.initials: H
authors.lastname: Ensor
authors.forename: Christine J
authors.initials: CJ
authors.lastname: Harrison
authors.forename: Jonathan C
authors.initials: JC
authors.lastname: Strefford
mesh_terms: D015105:3',5'-Cyclic-AMP Phosphodiesterases / Q000235:genetics; D020087:Alu Elements / Q000235:genetics; D001483:Base Sequence; D019457:Chromosome Breakage; D002891:Chromosomes, Human, Pair 21 / Q000235:genetics; D055028:Comparative Genomic Hybridization; D056915:DNA Copy Number Variations / Q000235:genetics; D004260:DNA Repair / Q000235:genetics; D043542:DNA, Z-Form / Q000235:genetics; D005784:Gene Amplification / Q000235:genetics*; D017353:Gene Deletion; D050939:Gene Fusion / Q000235:genetics; D023061:Gene Order; D015321:Gene Rearrangement / Q000235:genetics; D018398:Homeodomain Proteins / Q000378:metabolism; D006801:Humans; D008969:Molecular Sequence Data; D015452:Precursor B-Cell Lymphoblastic Leukemia-Lymphoma / Q000235:genetics* / Q000503:physiopathology*; D016415:Sequence Alignment
chemical_list: D043542:DNA, Z-Form; D018398:Homeodomain Proteins; C064658:RAG-1 protein; D015105:3',5'-Cyclic-AMP Phosphodiesterases; C116137:PDE9A protein, human