Clinical relevance of gain-of-function mutations of p53 in high-grade serous ovarian carcinoma

Abstract

Purpose: Inactivation of TP53, which occurs predominantly by missense mutations in exons 4-9, is a major genetic alteration in a subset of human cancer. In spite of growing evidence that gain-of-function (GOF) mutations of p53 also have oncogenic activity, little is known about the clinical relevance of these mutations.

Methods: The clinicopathological features of high-grade serous ovarian carcinoma (HGS-OvCa) patients with GOF p53 mutations were evaluated according to a comprehensive somatic mutation profile comprised of whole exome sequencing, mRNA expression, and protein expression profiles obtained from the Cancer Genome Atlas (TCGA).

Results: Patients with a mutant p53 protein (mutp53) with a GOF mutation showed higher p53 mRNA and protein expression levels than patients with p53 mutation with no evidence of GOF (NE-GOF). GOF mutations were more likely to occur within mutational hotspots, and at CpG sites, and resulted in mutp53 with higher functional severity (FS) scores. Clinically, patients with GOF mutations showed a higher frequency of platinum resistance (22/58, 37.9%) than patients with NE-GOF mutations (12/56, 21.4%) (p=0.054). Furthermore, patients with GOF mutations were more likely to develop distant metastasis (36/55, 65.5%) than local recurrence (19/55, 34.5%), whereas patients with NE-GOF mutations showed a higher frequency of locoregional recurrence (26/47, 55.3%) than distant metastasis (21/47, 44.7%) (p=0.035). There were no differences in overall or progression-free survival between patients with GOF or NE-GOF mutp53.

Conclusion: This study demonstrates that patient with GOF mutp53 is characterized by a greater likelihood of platinum treatment resistance and distant metastatic properties in HGS-OvCa.

Figure 1. Guanine and cytosine (GC) content, CpG site, and hotspot mutations in TP53.

Frequencies of TP53 mutations and their locations with GC content and CpG density. Most mutations were located in DNA binding domain (A). Hotspot mutations (five or more frequencies in 301 patients) were more likely to occur at CpG sites (p<0.001, chi-square test) (B). Functional severity scores of hotspot mutant p53 proteins (n=74) were much higher than those of p53 mutant proteins that occurred outside of the hotspots (n=111) (p=0.0074, Wilcoxon rank sum test) (C).

Figure 2. Expression of p53 mRNA, protein, mutation type, and survival in high grade serous ovarian carcinoma.

Expression levels of p53 mRNA and protein according to mutation type were present. Higher level of expression was present in in-frame deletion, in-frame insertion, and missense mutation (A). Higher expressions of p53 mRNA (p=0.0214, t-test) and protein (p=0.0054, t-test) were present in tumors with hotspot mutation (n=72 and n=54, respectively) than mutation in non-hotspot (n=113 and n=73, respectively) (B). In overall (C) and progression-free survival (D) according to functional severity (FS) scores for mutant p53 proteins in patients (pt) with high-grade serous ovarian carcinoma, pt with high FS score showed poorer prognosis but statistically not significant. In overall (E) and progression-free survival (F) according to mutation type, there were no survival differences. Log-rank test was used for survival analysis.

Figure 3. Clinical features and gain-of-function (GOF) mutations in p53.

Patients with GOF mutations showed a higher frequency of platinum resistance (22/58, 37.9%) than patients with no evidence (NE)-GOF mutations (12/56, 21.4%) (p=0.054, chi-square test) (A). Patients with GOF mutations showed a higher frequency of distant metastasis (36/55, 65.5%) than patients with NE-GOF (21/47, 44.7%) (p=0.035, chi-square test) (B). No differences in overall (C) and progression free survival (D) between patients with GOF and NE-GOF mutations were identified. Log-rank test was used for survival analysis.

Figure 4. Differential protein expression between gain-of-function (GOF) and no evidence of GOF (NE-GOF) mutations of p53.

Among 165 proteins, 11 significant differentially expressed proteins (p<0.05) and their fold changes were shown in Volcano plot (A). By the significance analysis of microarrays (SAM) method, only CTNNB1 was identified as a significant differentially expressed protein between GOF and NE-GOF mutant p53 proteins (B).