The gold standard for RM treatment is surgical resection(Campbell et al., 2009; Ljungberg et al., 2015). However, with the advancement of modern medicine, the increase in the detection rate and operation rate of small RMs has not reduced the mortality rate(Hollingsworth et al., 2006; Sun et al., 2011). Therefore, AS was recommended as a strategy for managing small RMs(Campbell et al., 2009; Ljungberg et al., 2015). However, there have been few studies on the natural history of sporadic ccRCC and VHL-associated RCC reported in the global literature. The present study aimed to compare the natural history of sporadic ccRCC and VHL-associated RCC combined with immunohistochemical analysis. Compared to sporadic ccRCC, we found that patients with VHL-associated RCC were younger, more likely to develop bilateral tumors, and had a lower histological grade at onset, which was consistent with the conclusions of previous studies(Maher et al., 2011; Ben-Skowronek and Kozaczuk, 2015). In addition, our study revealed that the sex ratio in patients with VHL-associated RCC was more equal, while the proportion of males in the sporadic group was higher, which was closer to the sex ratio of sporadic ccRCC shown in a previous large-scale epidemiological study(Siegel et al., 2020). There was no significant difference in the initial tumor size between the two groups, regardless of whether the tumor size was measured by MTD or volume. The sporadic group received AS for a longer period than the VHL group. We believe that this might be due to selection bias, as most sporadic ccRCC patients receiving AS had surgical contraindications. This conjecture is supported by the fact that the final average tumor size of the sporadic group at the last follow-up was larger than that of tumors in the VHL group.
A previous meta-analysis showed that the mean LGR of sporadic small RMs was 0.28 cm/yr, and the growth rate of RCC diagnosed pathologically was approximately 0.4 cm/yr(Chawla et al., 2006). Our study only included patients diagnosed with ccRCC through pathology; the mean LGR was 0.91 cm/yr, and the median was 0.61 cm/yr. This result more accurately reflects the growth rate of ccRCC. In the VHL group in our study, the mean LGR was 0.49 cm/yr, and the median was 0.30 cm/yr, which was similar to that described in previous reports(Jilg et al., 2012; Zhang et al., 2012). We further compared the tumor growth rate of the two groups and found that the mean LGR and mean VGR of the sporadic group were significantly faster than those of the VHL group, which was consistent with previous findings that VHL-associated RCC is more indolent(Neumann et al., 1998; Zhang et al., 2012). We also found that VHL-associated RCC had zero or even negative growth, which was not observed for sporadic ccRCCs. Compared with sporadic ccRCC, the LGR distribution of VHL-associated RCC was more concentrated, which indirectly indicates that the heterogeneity of sporadic ccRCC was greater than that of VHL-associated RCC.
Mutations in VHL, PBRM1, SETD2, and BAP1 cause loss of function of the encoded proteins in ccRCC. A study based on whole-genome sequencing found that PBRM1 and BAP1 mutations are largely mutually exclusive in ccRCC(Peña-Llopis et al., 2012). Schraml et al. reported that there was a significant correlation between the expression levels of PBRM1, H3K36me3 and BAP1 based on the tissue chip technique(Bihr et al., 2019). However, our study used immunohistochemical staining to analyze the expression levels of VHL, PBRM1, H3K36me3, and BAP1 in all tumors of the two groups and revealed that there was no correlation between any of them. We hypothesize that the reason for the different results in our study is that immunohistochemistry cannot accurately reflect gene mutations, and the results of immunohistochemical staining were also affected by the time of tissue fixation and conditions of wax block storage.
Which factors are predictors of the tumor growth rate have always been the focus of research on the natural history of renal tumors, but there are still no widely accepted conclusions. This study revealed that in the sporadic ccRCC group, the patient’s age, sex, and initial size of the tumor had no effect on the tumor growth rate, which was consistent with the results of most other studies, including our previous studies(Chawla et al., 2006; Crispen et al., 2008; Zhang et al., 2015). However, histological grade was correlated with the tumor growth rate, and the higher the grade was, the faster the growth rate was. In the VHL group, the initial size of the tumor was an influencing factor for the growth rate, and other VHL disease-related factors, such as subtypes and mutation types, were not correlated with the tumor growth rate. We believe that these differences further prove that VHL-associated RCC and sporadic ccRCC are two different tumors, even though they are both classed as ccRCCs by histology. Loss of VHL protein is known to contribute to the initiation and progression of VHL disease-associated tumors as well as certain sporadic tumors, including ccRCC. Schraml et al. reported that PBRM1 was the second most frequently mutated gene in RCC after VHL, and loss of PBRM1 was correlated with advanced tumor stage, low differentiation grade and worse patient outcome(Pawłowski et al., 2013). However, Kilic et al. reported that weak PBRM1 and VHL expression was significantly associated with higher Fuhrman grade, but only weak VHL expression was associated with a higher pT stage and with decreased patient overall survival times, but PBRM1 expression did not affect the overall survival outcome(Högner et al., 2018). Our study revealed that the expression of VHL and PBRM1 was correlated with tumor histological grade; lower VHL expression was associated with higher tumor histological grade, but this trend was not observed for PBRM1. Moreover, we found that low VHL expression and negative PBRM1 were both correlated with the tumor growth rate and may have had additive effects in sporadic ccRCC. It is possible that VHL and PBRM1 will become sensitive predictors of the growth rate of sporadic ccRCC.
Due to the limitations of retrospective design and sample size, prospective studies with a larger sample size are required to verify these results. Moreover, this study only qualitatively measured the protein expression level through immunohistochemistry; thus, the results remain to be further verified at the quantitative level.