Paradoxical relationship between the degree of EGFR amplification and outcome in glioblastomas

Abstract

Glioblastoma (GBM) is the most common primary brain tumor in adults and often has amplification of the epidermal growth factor receptor (EGFR) gene. The value of EGFR as a prognostic marker in GBMs is unclear; some studies have shown an adverse correlation, whereas others have indicated a neutral or even favorable association with longer survival. Furthermore, EGFR-amplified GBMs are usually regarded as a single subgroup of tumors, although the range of EGFR copy number varies greatly. In this study, 532 GBMs were analyzed for EGFR amplification via fluorescence in situ hybridization at the time of initial diagnosis. Although there was no difference in survival by EGFR amplification (P = 0.33), stratification by the amount of EGFR amplification showed that, surprisingly, median survival was 39% longer in the high-amplifier group (EGFR:chromosome 7 ratio >20) compared to nonamplified GBMs (P = 0.03) and was 43% longer compared to GBMs with low to moderate EGFR amplification (EGFR:chromosome 7 ratio = 2 to 20; P = 0.0007). Stratifying by postsurgical treatment regimens, this difference was seen only when temozolomide (TMZ) was used; tumors without amplification and with high EGFR amplification both responded better to TMZ than those with low to moderate amplification (P = 0.01), whereas GBMs that had not been treated with adjuvant therapy nor with adjuvant therapy lacking TMZ showed no survival differences (P = 0.63 and 0.91, respectively). These results suggest that GBMs with EGFR amplification are a heterogenous group of tumors and that behavior might differ according to the degree of amplification, although not in a straightforward dose-response manner.

Figure 1. Optimal outcome-based EGFR:CEP7 cutoff between low-moderate and high-amplified GBMs

532 GBMs with known survival times were analyzed for EGFR amplification by FISH (see Methods). This plot displays results from a log-rank test comparing the survival curves of patients with EGFR ratios below the cut-off to those above it. The x-axis varies cut-off values and the y-axis shows the P-value from the resulting log-rank test. The vertical black line denotes the EGFR ratio cut-off with the minimum P-value, i.e. maximal separation between the two groups, equating to an optimal cut-off of 21.0 (for the sake of simplicity a cutoff of 20, which generated comparable P value, was used henceforth).

Figure 2. GBM survival varies according to degree of EGFR amplification

532 GBMs were pooled into three groups: (A) no EGFR amplification (EGFR:CEP7 < 2), (B) low-to-intermediate level of amplification (EGFR:CEP7 2-20), (C) high-level amplification (EGFR:CEP7 > 20). (D) Surprisingly, tumors with low-to-intermediate EGFR amplification had the shortest overall survival, followed by non-amplifiers and high-level amplifiers (P < 0.006 overall; *P = 0.0007 versus EGFR:CEP7 = 2-20 and P = 0.03 versus non-amplified. P = 0.09 between non-amplified and EGFR:CEP7 = 2-20. In (A-C), red signals = EGFR and green signals = centromeric enumeration probe for chromosome 7. GBM = glioblastoma.

Figure 3. Response to temozolomide differs according to degree of EGFR amplification

(A) Overall survival was equally poor among all three subgroups when no postsurgical adjuvant therapy was administered (P = 0.63). (B) Likewise, survival did not differ when adjuvant therapy was given but did not include temozolomide (P = 0.91). (C) However, when temozolomide was part of the adjuvant regimen, patients whose GBMs had low-to-moderate EGFR amplification had worse survival than non-amplified GBMs (P = 0.02) and GBMs with high EGFR amplification (P = 0.003).