Association of the hypoxia-inducible factor-1α (HIF-1α) gene polymorphisms with prognosis in ovarian clear cell carcinoma

Background Ovarian clear cell carcinoma (OCCC) is the second most common ovarian cancer after serous carcinoma in Japan. OCCC has a more unfavorable clinical outcome due to a poor response to platinum-based chemotherapy, compared with serous carcinoma. Hypoxia inducible factor-1α (HIF-1α) is a key regulator of cellular response to hypoxia and plays an important role in tumor growth, and HIF-1α gene single-nucleotide polymorphisms (SNPs) adversely affect the outcome in some cancers. Herein, we investigated the association of the HIF-1α gene SPNs with clinical outcome in OCCCs. Eighty-nine patients with OCCC were recruited in whom pathological diagnosis was confirmed with surgically resected specimen. Results The SNPs of C1772T and G1790A in the HIF-1α gene occurred in 23.6 and 3.3% of the patients, respectively. In the univariate analysis, overall survival was associated with stage and surgical residual tumor but not with the SNPs C1772T, G1790A, C1772T and/or G1790A. In the multivariate survival analysis, a significant association was observed between outcome and FIGO stage and/or surgical residual tumor; however, no association was obtained between HIF-1α gene SNPs and these factors. Conclusion In conclusion, unlike the other cancers in which HIF-1α gene SNPs were demonstrated to be associated with the outcome, OCCC prognosis may not be affected by HIF-1α gene SNPs. Further studies need to be performed to clarify the association of HIF-1α expression with the unfavorable prognosis in OCCCs, in terms of transcriptional/translational activity, nuclear translocation of the protein, and protein degradation.


Background
Ovarian cancer is the leading cause of death among gynecological malignancies, as well as is the fourth most common malignancy in women in developed countries, following breast, lung, and colorectal cancer [1,2]. Each of the ovarian cancers, represented by serous carcinoma, endometrioid carcinoma, clear cell carcinoma, and mucinous carcinoma, are known to have specific clinicopathological features and molecular or genetic characteristics. In Japan, ovarian clear cell carcinoma (OCCC) is the second most common ovarian cancer, following serous carcinoma [3,4]. OCCC arises from endometriosis in 50-70% of the cases [5,6] and has a more unfavorable prognosis due to a poor response to platinum-based chemotherapy, compared with serous carcinoma [3,4].
HIF-1α is a key regulator of cellular response to hypoxia and plays an important role in tumor growth by trans-activating various genes that are related to regulation of angiogenesis, energy metabolism, survival, resistance to anti-tumor therapy, and cell survival, apoptosis, and proliferation [7][8][9]. In our previous studies of OCCC and other ovarian epithelial cancers, we found an increased nuclear expression of HIF-1α in OCCC and have identified the HIF-1α regulating factors [10,11].
This study was conducted to investigate the impact and susceptibility of HIF-1α gene SNPs (C1772T and G1790A) on the prognosis of OCCCs because there have been no reports to analyze the association of the SNPs with outcome. In particular, the two SNPs associated with transcriptional activity were the focus of the study because they were associated with transcriptional activity.

Discussion
HIF-1α expression represents an important biomarker in the evaluation of ovarian carcinoma prognosis [34]. In our study, OCCCs are characterized by a nuclear expression of HIF-1α compared to other histological types. It is believed that HIF-1α is one of the key factors closely associated with chemo-resistance or unfavorable OCCC prognosis [10,11]. Overexpression of HIF-1α may be attributed to transcriptional and/or translational activity, nuclear transition of the protein, and its degradation.
This study was conducted to assess whether there is an association of the HIF-1α gene SNPs with the prognosis and clinicopathological characteristics of OCCCs. A significant association was observed between prognosis and clinicopathological factors such as FIGO stage and surgical residual tumor. However, any variations of the SNPs were proven not to be associated with the prognosis. The previous studies of variable cancers with a focus on the relationship between HIF-1α gene SNPs and patient prognosis are summarized in Table 3 [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. OCCC patients had more frequent C1772T SNPs than the healthy Japanese population [19,[31][32][33] and many other carcinomas. OCCC prognosis as well as colorectal cancer [21,22], thymic malignancy [27], and cervical cancer [28,29] prognoses had no association with C1772T and G1790A SNPs. However, the T allele of C1772T and A allele of G1790A are a poor or good prognostic factor in several cancers [17,23]. The effects of HIF-1α SNPs on the prognosis with cancers are not uniform.
The C1772T SNP has been reported to increase HIF-1α protein expression in some cancers [13,15]. Twenty specimens, which were randomly selected out of the 89 OCCCs examined in this study, were subjected to immunohistochemical staining for HIF-1α. The results failed to show the associated between HIF-1α staining and presence of SNPs (data not shown). In our previous studies, OCCCs showed the highest frequency of HIF-1α, histone deacetylase (HDAC) 6, and HDAC7 compared to other ovarian epithelial cancer [10,11,35]. HDAC6 and HDAC7 induced not only HIF-1α transcriptional activity, but also stabilized HIF-1α protein via interaction with von Hippel Lindau and ubiquitin-independent proteasomal degradation of HIF-1α [36][37][38]. In OCCCs, post-translational modification may be more important for the HIF-1α expressions than upregulated transcription activity by HIF-1α gene SNPs.
Our study has several limitations. The sample size used in this study was small and the survival analysis was only performed with a few events. However, when considering the low incidence of OCCC, the present study included a relatively large number of patients. Secondly, normal controls were not recruited in the present study; instead, we compared the frequencies of HIF-1α SNPs using the normal Japanese population reported in the past studies [19,[31][32][33].

Conclusion
In conclusion, HIF-1α gene SNPs were demonstrated to be less significant as a prognostic marker in OCCCs. The precise mechanism of the association between the

Methods
Patient data and clinicopathological features (

Statistical analysis
Genetic polymorphisms and clinic pathological parameters were assessed using the Pearson chi-square test or the Fisher exact test. Univariable survival analysis was performed by the generation of Kaplan-Meier curves, and differences between the groups were assessed using the log rank statistic. Univariable and multivariable survival analyses were performed using the Cox proportional hazards model. SPSS v24.0 (SPSS Inc., Chicago, IL, USA) was applied for these all analyses. p values < 0.05 were considered significant.