Palindromic amplification of the ERBB2 oncogene in primary HER2-positive breast tumors

Michael Marotta,Hisashi Tanaka,Takaaki Watanabe,Atsushi Niida,Armando E Giuliano,Jeffrey Johnson,Xiaojiang Cui,Taku Onodera,G Thomas Budd

doi:10.1038/srep41921

Abstract

Oncogene amplification confers a growth advantage to tumor cells for clonal expansion. There are several, recurrently amplified oncogenes throughout the human genome. However, it remains unclear whether this recurrent amplification is solely a manifestation of increased fitness resulting from random amplification mechanisms, or if a genomic locus-specific amplification mechanism plays a role. Here we show that the ERBB2 oncogene at 17q12 is susceptible to palindromic gene amplification, a mechanism characterized by the inverted (palindromic) duplication of genomic segments, in HER2-positive breast tumors. We applied two genomic approaches to investigate amplification mechanisms: sequencing of DNA libraries enriched with tumor-derived palindromic DNA (Genome-wide Analysis of Palindrome Formation) and whole genome sequencing (WGS). We observed significant enrichment of palindromic DNA within amplified ERBB2 genomic segments. Palindromic DNA was particularly enriched at amplification peaks and at boundaries between amplified and normal copy-number regions. Thus, palindromic gene amplification shaped the amplified ERBB2 locus. The enrichment of palindromic DNA throughout the amplified segments leads us to propose that the ERBB2 locus is amplified through the mechanism that repeatedly generates palindromic DNA, such as Breakage-Fusion-Bridge cycles. The genomic architecture surrounding ERBB2 in the normal genome, such as segmental duplications, could promote the locus-specific mechanism.

Highlights

Genome instability is an enabling characteristic by which tumor cells acquire unlimited proliferation and metastatic potential[1]
Because genomic segments would be unevenly inherited by daughter cells due to the random locations of breaks, repeating this cycle would lead to a population of cells with heterogeneous copy numbers (Fig. 1b and c)
Previous studies reported that a common pattern of copy number transitions of chromosome 17 is associated with ERBB2 amplification in HER2 positive breast tumors: 17p loss, 17q12-21.1 amplification with the loss of flanking regions, and distal 17q gain[36,37]

Summary

Introduction

Genome instability is an enabling characteristic by which tumor cells acquire unlimited proliferation and metastatic potential[1]. Because ERBB2 copy numbers are extensively heterogeneous between individual tumor cells (Fig. 2), BFB cycles are a strong candidate for the underlying mechanism. We found that fold-back inversions were distributed throughout the amplified regions, indicating that (1) inversions occur many times during the establishment of amplified genomic segments, and (2) fusion points may differ between individual tumor cells (breakpoint heterogeneity). These results strongly suggest that BFB cycles underlie the amplification of ERBB2

Methods

Results

Conclusion