Quantifying aberrantly methylated genes was a feasible method for the early detection of CRCs. Several biomarkers have been demonstrated excellent performance in CRC early detection [25, 27, 28], however, they often suffer the limitation with some positive samples miss detected, which directly affects their efficiency for CRC detection. In this study, we defined three CRC methylator groups, HH, HL, and LL based on the methylation status of SDC2 and TFPI2 and then assessed their characteristics of genomic instability, mutation load, patient age, and biological processes. These findings suggested that it would be reasonable and essential to define three methylator groups according to the methylation status of these targets and will benefit guiding the development of more effective methylated biomarkers.
Our results revealed that high-methylated SDC2 or TFPI2 occurred in more than 95% of CRCs, suggesting that their methylation status can perform well to discriminate CRCs from normal controls. Previous studies have demonstrated the good performance of methylated SDC2 and TFPI2 alone or in combination with other biomarkers for stool-based CRC detection [23, 43], however, the combination of these two targets has not been reported. The present study revealed to some extent that the dual-target showed huge potential for CRC detection, which will be helpful for the development of stool-based noninvasive or blood-based minimally invasive detection techniques in the future.
However, in terms of individual genes, nearly 10% of the samples had only one gene methylated, and here we defined them as the HL group. Further analysis showed that more samples harboring high-methylated TFPI2 and low-methylated SDC2 were from HL group (about 3 times than the samples with high-methylated SDC2 and low-methylated TFPI2). These results suggested that methylated TFPI2 occurs more frequently on CRCs, implying that combining TFPI2 with SDC2 would help improve the sensitivity of CRC detection. Previous studies have demonstrated that multi-target outperformed single target [44, 45], which was evidenced by this study.
In clinical practice, the HL and LL group CRCs will cut down the sensitivity of these biomarkers. Our findings indicated that HL group CRCs more likely originate from the left-sided colon. Many differences were observed between the proximal (right-sided) and distal (left-sided) CRCs. For example, right-sided cancers were reported an increased incidence of proximal migration, while it was inversed for rectosigmoid tumors [46]. Moreover, the incidence between proximal and distal CRCs also differs in age and gender [47]. These data reflect extensive distinctions in molecular pathogenesis between the two anatomical locations, which might generate a significant impact on tumorigenesis in these respective sides. Several studies have demonstrated that left-sided colon presents lower degrees of methylation than the right-sided colon, which was called the CpG island methylator phenotype, or CIMP, characterized by significant hyper-methylated CpG islands of tumor suppressors [48][49]. Therefore, tumor location would be an important factor of biological heterogeneity. In this study, we observed lower methylation levels of SDC2 in left-sided CRCs. Given that HL group CRCs were dominated by high-methylated TFPI2 and low-methylated SDC2, this might explain why they appeared more frequently in the left-sided colon. These results suggested a potential impact of tumor locations on the early detection of CRCs when adopting abnormal methylated DNA as biomarkers.
Additionally, we found a positive correlation between the methylation levels of SDC2 and TFPI2 and MSI scores in MSI-H CRCs, as well as lower mutation load and rare BRFA mutations in HL group CRCs. Approximately 10% of CRCs harbor mutated BRAF, and its mutations are associated not only with poor prognosis but also with less benefit in metastatic CRCs when treated by antibodies [50]. Therefore, the methylator phenotypes may serve as a stratification factor in clinical therapies. It’s been reported that CIMP tumors showed significant associations with BRAF mutations, MSI-H [49]. These results, on the other hand, confirmed that molecular events, such as epigenetic variations, instability, aberrant DNA mutations, and MSI, are coupled with each other.
Gene expression analysis identified methylator group-specific DEGs and functional annotation of LL-specific DEGs was suggested to focus on the biological process of ECM-receptor interaction, implying the potential alteration in molecular pathways in LL group CRCs. Interestingly, many studies have showed very important roles of SDC2 and TFPI2 in the interaction of extracellular matrix with cell plasma [15, 51]. Besides, we also observed a significant enrichment of 111 mutant genes on GO term of extracellular matrix organization, which might, from the other hand, imply the tight association between ECM-receptor interaction and the three methylator groups. Overall, these findings revealed the possible impact of ECM process on the performance of methylated SDC2 and TFPI2 in detecting CRCs.
Colorectal cancer is a disease with high heterogeneity, often divided into proximal (right sided) and distal (left-sided) cancer according to their anatomical locations. This classification is reasonable because of their distinctive embryonic derivation, which is the midgut and the hindgut for the proximal and distal colon, respectively [46, 52]. It might give rise to the altered methylations between the left- and right-sided colons, thus affecting the early detection of CRCs based on abnormally methylated DNA.
In conclusion, the current study demonstrated the possible association of CIMP phenotype, tumor location, and MSI with the dual-target in CRC early diagnosis, making us propose a possible diagram of patient characteristics between the three groups (Figure 7). In terms of genomic features, HH group CRCs are characterized by more often microsatellite instable (MSI), high mutation load, and frequent BRAF-mutated. HL group CRCs prefer to originate from the left-sided colon. Our observations also suggested that it should be considered when developing new methylation-based biomarkers for CRC detection in these respective sides.