High levels of genomic aberrations in serous ovarian cancers are associated with better survival

Abstract

Genomic instability and copy number alterations in cancer are generally associated with poor prognosis; however, recent studies have suggested that extreme levels of genomic aberrations may be beneficial for the survival outcome for patients with specific tumour types. We investigated the extent of genomic instability in predominantly high-grade serous ovarian cancers (SOC) using two independent datasets, generated in Norway (n = 74) and Australia (n = 70), respectively. Genomic instability was quantified by the Total Aberration Index (TAI), a measure of the abundance and genomic size of copy number changes in a tumour. In the Norwegian cohort, patients with TAI above the median revealed significantly prolonged overall survival (p<0.001) and progression-free survival (p<0.05). In the Australian cohort, patients with above median TAI showed prolonged overall survival (p<0.05) and moderately, but not significantly, prolonged progression-free survival. Results were confirmed by univariate and multivariate Cox regression analyses with TAI as a continuous variable. Our results provide further evidence supporting an association between high level of genomic instability and prolonged survival of high-grade SOC patients, possibly as disturbed genome integrity may lead to increased sensitivity to chemotherapeutic agents.

Figure 1. Examples of genomic profiles with low (left) and high (right) median Total Aberration Index (TAI).

(a.) Examples from the Norwegian and (b.) from the Australian cohort. The log2-transformed copy numbers of the chromosomes 1 to 23 are illustrated. The median was 0.135 for the Norwegian and 0.242 for the Australian cohort.

Figure 2. Frequency of copy number changes in serous ovarian carcinomas of two independent cohorts.

The frequencies of copy number alterations in serous ovarian cancers of two independent cohorts from Norway and Australia are illustrated. Regions with copy number gains are marked in red and regions with copy number losses are marked in green, respectively. (a) The frequency of copy number changes of 74 serous ovarian tumours of the Norwegian cohort were determined using 42k cDNA arrays. Several high frequency peaks are visible, including gains at regions on chromosome arms 1q, 3q, 8q, and 20q, and losses on chromosome arms 4q, 5q, 6 p, 8 p, 13, 16q, 18q, and the whole of the X chromosome. (b) The frequency of aberrations of 70 ovarian tumour samples of the Australian cohort, as measured by 50 k SNP Affymetrix arrays. All high frequency peaks of the Norwegian cohort are also identified in the Australian cohort, although some additional peaks appear in the Australian data, e.g. gains in 1 p and losses on chromosome arms 17 p and 22q. The two data sets show high consistency in the aberration pattern, despite differences in populations and analysis platforms (see also Figure 3).

Figure 3. Comparison of estimated log copy numbers in the two cohorts.

A total of 2923 genomic loci spaced 1Mb from each other were defined, and the average estimated log copy number was found at each loci and in each of the two study cohorts. The resulting set of 2923 pairs of averages is shown in the figure, suggesting considerable consistency between the two study cohorts.

Figure 4. Survival analysis in relation to genomic instability.

Kaplan-Meier survival curves illustrating progression-free survival (PFS) and overall survival (OS) time (in months) for serous ovarian cancers patients with Total Aberration Index (TAI) above and below the median in the Norwegian cohort (above) and the Australian cohort (below). Test results are based on log-rank tests. Note that high TAI implies a significant survival advantage, both with regard to progression-free survival and to overall survival in the Norwegian cohort, as well as for overall survival in the Australian cohort.