Evidence for a novel mechanism for gene amplification in multiple myeloma: 1q12 pericentromeric heterochromatin mediates breakage‐fusion‐bridge cycles of a 1q12∼23 amplicon

Abstract

Summary Gene amplification is defined as a copy number (CN) increase in a restricted region of a chromosome arm, and is a mechanism for acquired drug resistance and oncogene activation. In multiple myeloma (MM), high CNs of genes in a 1q12~23 amplicon have been associated with disease progression and poor prognosis. To investigate the mechanisms for gene amplification in this region in MM, we performed a comprehensive metaphase analysis combining G-banding, fluorescence in situ hybridization, and spectral karyotyping in 67 patients with gain of 1q. In six patients (9%), evidence for at least one breakage-fusion-bridge (BFB) cycle was found. In three patients (4%), extended ladders of 1q12~23 amplicons were identified. Several key structures that are predicted intermediates in BFB cycles were observed, including: equal-spaced organization of amplicons, inverted repeat organization of amplicons along the same chromosome arm, and deletion of sequences distal to the amplified region. The 1q12 pericentromeric heterochromatin region served as both a recurrent breakpoint as well as a fusion point for sister chromatids, and ultimately bracketed both the proximal and distal boundaries of the amplicon. Our findings provide evidence for a novel BFB mechanism involving 1q12 pericentromeric breakage in the amplification of a large number of genes within a 1q12~23 amplicon.

Jumping translocations of 1q12 in multiple myeloma: a novel mechanism for deletion of 17p in cytogenetically defined high-risk disease

“…32 We and others have previously speculated that the cause of these duplications and jumping translocations of 1q may be region-specific hypomethylation of the 1q12 pericentromeric heterochromatin. [18][19][20][21][22] The 1q12 pericentromeric region is a distinctive region in the human genome in that it is the largest single block of late-replicating, highly repetitive satII/III DNA that is known to contain unstable segmental duplications (low copy repeats). It is thought that a 5-azacytidine fragile site at 1q12 may play a role in the incomplete replication (delayed condensation), breaks, and triradial configurations of 1q12.…”

“…Based on the present and previous findings, 18,22,27 we propose a model for the JT1q12 in the subclonal evolution of cytogenetically defined high-risk disease. Risk stratification of subclones is founded on the current IMWG 2 iFISH probe recommendations.…”

“…22 The aim in this study was to clarify the impact of JT1q12 on CNAs involving RCs. We have identified a novel type of the JT1q12 aberration in which the same JT1q12 sequentially translocates from one RC to a second RC and, in some cases, focally amplifies a segment of the original RC that becomes part of the second translocation.…”

Evidence of an epigenetic origin for high-risk 1q21 copy number aberrations in multiple myeloma

“…Probes for chromosome loci 1q21(CKS1B), 1p13 (AHCYL1), and 2p24 (MYCN) were prepared as previously described. 22 Probes for pericentromeric regions including 1q12, sat III DNA (red), 9q11 (a sat, aqua), and 16q11 (sat II, aqua) were acquired from Vysis and used according to manufacturer's instructions (Vysis, Downers Grove, IL). The SKY probe mixture and hybridization reagents were prepared by Applied Spectral Imaging (Carlsbad, CA) and applied as previously described.…”

“…The SKY probe mixture and hybridization reagents were prepared by Applied Spectral Imaging (Carlsbad, CA) and applied as previously described. 22 Original magnification was 31000 for all G-band and fluorescence in situ hybridization (FISH) images and 3630 for SKY.…”