This study showed that DFS might be influenced by the mutation burden, independent to clinical factors, or tumor burden in synchronous and solitary CRC. The genetic profiles revealed that RAS and BRAF mutations were associated with more pronounced effects than MSI on DFS. In this study, old age was the only risk factor for OS, regardless of the mutation profile and pathologic stage, and OS was not associated with MSI status in synchronous CRC. By contrast, a previous study suggested that, due to the concordance between MSI status and synchronicity, older individuals are more likely to develop multiple cancers through the MSI pathway, secondary to a widespread CpG island methylator phenotype (CIMP) and silencing of the MMR gene MLH1 by genetic and or environmental factors (6, 33). However, the systematic review of studies comparing synchronous and solitary CRC revealed a lack of published data, as well as much heterogeneity in genetic and survival information, with unclear associations of clinical factors and genetic profiles with the prognosis of synchronous CRC (Table 5). Thus, we consider that the association between genetic mutations and the prognosis of synchronous CRC patients remains unclear and is open to debate.
As a commonly identified genotype in multiple CRCs, MSI arising from promoter methylation of the biallelic hMLH1 gene differs to that arising from the HNPCC pathway (34). In sporadic CRC, the risk of synchronicity was higher (2.14-fold) in patients with MSI than in patients with MSS, but there were no relationships between clinical features and the MSI genotype (35). Global hypermethylation of colorectal epithelium, as independent events, could increase the frequency of multiple CRCs in older age instead of developing from a predisposition to cancer in patients with sporadic MSI CRC (20). In terms of the oncologic outcomes, the association between MSI status and greater tumor burden in synchronous CRC suggests that synchronicity and BRAF mutations are risk factors for OS in patients with MSS CRC, but the disease-specific survival of MSI CRC patients was unaffected by synchronicity in a stage-adjusted analysis (33). By contrast, a prospective cohort study found that the overall mortality was greater in synchronous CRC patients than in solitary CRC patients, and the authors reported that multiple colon cancers arose through the serrated pathway, which is characterized by high frequencies of BRAF mutations, CIMP-high, and MSI-high (6). Another study reported contradictory results regarding the rarity of BRAF c.1799T4A mutation in synchronous advanced malignancies as a stage-independent predictor of poor prognosis in association with MSS, which is incompatible with the various epigenetic defects of synchronous CRCs (33).
Poor oncologic outcomes of synchronous CRC, in terms of the OS, DFS, and cancer-specific survival (CSS), were reported in a previous study using a matched-pairs analysis (36) (Table 5). Following disease relapse, it is important to select the most appropriate molecular-targeted drug by performing biomarker analysis (12). However, the greater and heterogenous mutation burden of paired tumors makes it difficult to identify the most appropriate target, resulting in poor prognosis of patients with relapsed synchronous CRC. Adjuvant therapeutic strategies have not yet been established for relapsed synchronous CRC, and we are still dependent on the clinical guidelines for solitary CRC. For synchronous CRCs within the same patient, it has been reported that paired lesions display heterogeneity in canonical genes, including APC, KRAS, TP53, and PIK3CA, together with a high frequency of mutations, compared with solitary CRC. Therefore, when drugs such as vemurafenib and dabrafenib, which target the BRAF mutation pV600E, are used to treat one lesion, the other lesion might be unresponsive due to the heterogeneous mutation profile of paired synchronous CRCs (37). Those molecular profiles develop independently, and lesions present with different gene copy numbers resulting in unique gene signatures in each lesion, combined with clonal mutations at different loci and accumulated timing (4). When treating patients with BRAF-mutated synchronous CRC, in particular, the MSI status and genetic heterogenicity of the paired tumors should be considered rather than the tumor burden or clinical stage. According to a systematic review of patients with BRAF-mutated CRC, MSS was associated with worse prognosis than MSI, but the clinical stratification by MSI testing and heterogeneity of genetic mutations have not been established for patients with BRAF-mutated synchronous CRC (38). Physician should also be aware that the poor prognosis of synchronous cancers might be independent of genetic factors such as BRAF mutations, MSI-high, and CIMP-high due to unidentified molecular events caused by the genetic or environmental background (6). Several markers, such as the transcriptional effector RPL22, a candidate gene involved in nodal/transforming growth factor-β and the ribosomal protein–murine double minute 2 (MDM2)–p53 signaling pathway (4), as well as different methylation rates of CACNA1G, NEUROG1, and CDKN2A (p16) (1), might confound analyses of the prognosis of synchronous CRC.
Some studies have also demonstrated similar or better prognosis of synchronous CRC patients compared with solitary CRC patients, regardless of CIMP status and KRAS or BRAF mutations (1). This prognostic pattern was observed in several studies that lacked genetic information (8),(9),(11),(13) (Table 5). Although there was no clear explanation for this finding, intensive perioperative colonoscopy detected associated adenomas that are more prone to progress into multiple colorectal cancers in old patients and slow growing tumors with an uncharacterized predisposition (11),(13),(39). In addition, advanced surgical procedures could achieve comparable long-term outcomes for synchronous CRC patients, regardless of whether they underwent resection of more than two regions or extensive resection of a single region (3).
This study has some limitations, including a small sample size due to the exclusion of synchronous CRC patients without genetic information, which reduced the statistical power. The absence of data regarding CIMP status and germline mutation of MMR genes might also introduce bias in terms of assessing whether the synchronous CRCs were sporadic or Lynch-associated tumors. Furthermore, there were no data for palliative therapy for synchronous CRC patients with distant recurrence.
In conclusion, this study showed that the oncologic outcomes might differ according to the mutation burden characterized by RAS, BRAF, and MSI between synchronous CRC and solitary CRC. Furthermore, RAS and BRAF mutations were associated with worse DFS compared with MSI status, independently of clinical factors, stage, and tumor burden. Our systematic review highlighted a lack of data and much heterogeneity in the genetic characteristics and survival outcomes of synchronous CRC relative to that of solitary CRC. These factors make it difficult to predict the prognosis of synchronous CRC and complicate the decision-making process when selecting the most appropriate target drug following relapse of synchronous CRC.