Low frequency of BRAF and KRAS mutations in Chinese patients with low-grade serous carcinoma of the ovary

Ovarian serous carcinoma is the most common ovarian malignancy. In 2004, Malpica et al. described a novel two-tier grading system to classify ovarian serous carcinoma as high-grade serous carcinoma (HGSC) or low-grade serous carcinoma (LGSC) [1]. In this system, a carcinoma is categorized-based primarily on the degree of nuclear atypia and secondarily on its mitotic rate. LGSC and HGSC are characterized by different clinicopathological and molecular features [1‐5]. The two-tier grading system is now widely accepted and was adopted in the WHO classification system for female reproductive organ tumors (2014 version) [6]. Unlike HGSC, LGSC is more common in young patients and is associated with chemo-resistance and longer overall survival (OS). Typically, LGSC is also associated with KRAS and BRAF mutations that target specific cell signaling pathways [7‐11].

Several reports have demonstrated that KRAS and BRAF mutations occur at a frequency of 16–54% and 2–33%, respectively, in ovarian LGSC [12‐14]. The cohort in these studies included Caucasian and African individuals and a small cohort of individuals of other races. With the exception of a report by Cho that described 20 cases of Korean patients, few data are available regarding these mutations in Asian patients [15]. A previous study suggested that mutations in BRAF and KRAS contributed to the development of LGSC [16]. Patients with these mutations appear to have shorter survival [17]. Other reports have shown that BRAF or KRAS mutations in patients may be associated with an improved prognosis or favorable trends [12, 18, 19]. Therefore, the prognostic significance of KRAS and BRAF mutations remains controversial.

The purpose of this study was to evaluate the incidence of BRAF and KRAS mutations in patients with LGSC and to evaluate the prognostic significance of KRAS or BRAF mutations in LGSC in a Chinese population. In addition, we explore the association between KRAS and BRAF mutations and various clinical and pathological features.

The frequencies of KRAS and BRAF mutations vary from 16 to 54% and 2–33%, respectively, in different reports in ovarian LGSC [12‐14]. When whole-genome sequencing is applied, point mutations are much less common in LGSC of the ovary, and BRAF and KRAS are the most frequent mutations [13]. In our cohort, 28.13% (9/32) of the samples had KRAS mutations, while 6.25% (2/32) had BRAF mutations. This frequency was similar to that reported by Wong (19% KRAS and 2% BRAF) [12], Cho (30% KRAS and 10% BRAF) [15], Grisham (15.8% KRAS and 5.3% BRAF) [19], Farley (41% KRAS and 6% BRAF) [8] and Gershenson (22.8% KRAS and 3.8% BRAF) [18]. However, these results differed from the data published by Singer (26.7–54% for KRAS and 33% for BRAF) [16], which may be due to the small sample size in Singer’s study, which was only half the number of patients in our study. Nevertheless, BRAF mutation was a rare alteration in LGSC, and the frequency in most reports was lower than 10%.

In our cohort, we did not find a statistically significant difference in OS and DFS between patients harboring KRAS or BRAF mutations and patients with no KRAS or BRAF mutations. In addition, we combined cases of BRAF and KRAS mutations because there were only two cases with a BRAF mutation, and the outcome was similar to that of previous studies in that no statistically significant association was found between patients with tumors harboring KRAS or BRAF mutations and survival (Fig. 2a and b). Several reports have indicated that patients with KRAS or BRAF mutations may have a better prognosis than wild-type patients with ovarian LGSC [12, 18, 19]. Grisham et al. reported that the presence of a BRAF mutation in LGSC was associated with earlier stage disease and improved prognosis. Gershenson et al. reported that a KRAS or BRAF mutation may serve as a favorable prognostic factor and have a significant impact on outcomes in women with metastatic LGSC of the ovary or peritoneum. In our cohort, we also observed a favorable trend in prognosis with KRAS/BRAF mutation, even in the KRAS G12D mutation, but there was no significant difference. The possible explanations include selection bias due to limited sample sizes, geographic differences, ethnic heterogeneity, and short follow-up periods. Future studies with larger patient groups would provide more accurate information regarding these related links. Gershenson’s study included 79 LGSC cases diagnosed between 1975 and 2009 and had a long follow-up period. Similarly, there were 75 serous tumors (56 SBT and 19 LGSC) in Grisham’s study, with a median follow-up period of 35.9 months (0.8–129.3 months). However, the mean follow-up period was 30 months (9–87 months) in our cohort, which might not have been long enough to observe survival differences. Admittedly, this difference may be related to the smaller sample size in our study. In addition, typically, BRAF mutations have been more closely associated with SBT, whereas all the cases in our study were LGSC. Larger cohorts and longer follow-up periods will hopefully further clarify this issue. The results of targeting hotspot genes were limited compared with the results obtained by using a sequencing discovery phase for these two genes, indicating that it was highly likely that we missed additional case-specific mutations in our study population.

We observed that the prognostic significance of the KRAS mutation type was not significantly different, but there was a favorable trend toward better OS in patients with KRAS G12D mutation than in those with KRAS G12 V mutation or wild-type KRAS. This finding was similar to that of Tsang’s study [17], which reported that KRAS G12D mutation was associated with better OS in recurrent LGSC. We speculate that the sample size and shorter follow-up period is the bottleneck to obtaining significant differences. A similarly strong trend toward poor prognosis was also reported in lung adenocarcinoma with KRAS G12 V mutations [20].

KRAS and BRAF mutations are the most common mutations in LGSC, and both involve the mitogen-activated protein kinase (MAPK) pathway, as indicated by the effects of selective MEK inhibitors on recurrent or metastatic LGSC patients. However, no correlation was found between KRAS and BRAF mutations and therapeutic responses in the previously cited report [8]. With the development of next-generation sequencing technology, more abnormalities have been observed. These include NRAS mutations (26%), which occur at higher rates than either KRAS (21%) or BRAF (16%) mutations [21], and a 15-nucleotide deletion in the MAP2K1 gene, which encodes MEK1 [22, 23]. Hence, additional studies focused on mutations in LGSC should be performed.

In summary, we have demonstrated a low frequency of KRAS or BRAF mutation in Chinese patients with LGSC of the ovary. Our data have indicated that BRAF mutation is a very rare event and KRAS mutation is more common than BRAF mutation in our cohort. Patients with KRAS G12D mutation may have a more favorable outcome trend than other patients, but no statistically significance identified. Further studies with large cohorts are necessary to determine the prognostic value of KRAS G12D mutation.