Mechanisms of topoisomerase I (TOP1) gene copy number increase in a stage III colorectal cancer patient cohort

PLoS One. 2013;8(4):e60613. doi: 10.1371/journal.pone.0060613. Epub 2013 Apr 5.

Abstract

Background: Topoisomerase I (Top1) is the target of Top1 inhibitor chemotherapy. The TOP1 gene, located at 20q12-q13.1, is frequently detected at elevated copy numbers in colorectal cancer (CRC). The present study explores the mechanism, frequency and prognostic impact of TOP1 gene aberrations in stage III CRC and how these can be detected by fluorescent in situ hybridization (FISH).

Methods: Nine CRC cell line metaphase spreads were analyzed by FISH with a TOP1 probe in combination with a reference probe covering either the centromeric region of chromosome 20 (CEN-20) or chromosome 2 (CEN-2). Tissue sections from 154 chemonaive stage III CRC patients, previously studied with TOP1/CEN-20, were analyzed with TOP1/CEN-2. Relationships between biomarker status and overall survival (OS), time to recurrence (TTR) in CRC and time to local recurrence (LR; rectal cancer only) were determined.

Results: TOP1 aberrations were observed in four cell line metaphases. In all cell lines CEN-2 was found to reflect chromosomal ploidy levels and therefore the TOP1/CEN-2 probe combination was selected to identify TOP1 gene gains (TOP1/CEN-2≥1.5). One hundred and three patients (68.2%) had TOP1 gain, of which 15 patients (14.6%) harbored an amplification (TOP1/CEN-20≥2.0). TOP1 gene gain did not have any association with clinical endpoints, whereas TOP1 amplification showed a non-significant trend towards longer TTR (multivariate HR: 0.50, p = 0.08). Once amplified cases were segregated from other cases of gene gain, non-amplified gene increases (TOP1/CEN-2≥1.5 and TOP1/CEN-20<2.0) showed a trend towards shorter TTR (univariate HR: 1.57, p = 0.07).

Conclusions: TOP1 gene copy number increase occurs frequently in stage III CRC in a mechanism that often includes CEN-20. Using CEN-2 as a measurement for tumor ploidy levels, we were able to discriminate between different mechanisms of gene gain, which appeared to differ in prognostic impact. TOP1 FISH guidelines have been updated.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cell Line, Tumor
  • Chromosomes, Human, Pair 2 / genetics
  • Chromosomes, Human, Pair 20 / genetics
  • Cohort Studies
  • Colorectal Neoplasms / diagnosis
  • Colorectal Neoplasms / enzymology
  • Colorectal Neoplasms / genetics*
  • Colorectal Neoplasms / pathology*
  • DNA Topoisomerases, Type I / genetics*
  • Female
  • Gene Dosage / genetics*
  • Humans
  • In Situ Hybridization, Fluorescence
  • Male
  • Middle Aged
  • Neoplasm Staging
  • Prognosis

Substances

  • DNA Topoisomerases, Type I

Grant support

This study was supported by the Danish Agency for Science, Technology and Innovation through the Industrial PhD program. Nils Brünner and Hans Jørgen Nielsen were supported by The Danish Council for Strategic Research, Simon Fougner Hartmanns Family Foundation, IMK Almene Foundation, Kathrine og Vigo Skovgaards Foundation, Tømrermester Johannes Fog Foundation, Fabrikant Einar Willumsens Memorial Trust, Danish Cancer Society, The Danish Medical Research Council, The Hede Nielsen Foundation, Director Ib Henriksens Foundation, Sawmill owner Jeppe Juhl and Wife Ovita Juhl Foundation, The Kornerup Fund, The Aase and Ejnar Danielsen Fund and The Aage and Johanne Louis-Hansen Fund. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.