For locally progressive mid-low rectal cancer, current guidelines recommend a combination of neoadjuvant chemoradiotherapy (nCRT), TME, and postoperative adjuvant chemotherapy. However, patients undergoing radical surgery for rectal cancer may face various risks, such as anastomotic leakage, temporary or permanent fistula, organ excision, and urinary damage and dysfunction. Research data show the following findings: the perioperative complication incidence of radical surgery for rectal cancer is about 20–30%[11, 12]; among patients undergoing surgery, no residual tumor cells are found in surgically resected specimens in 20–30% of the patients[13], i.e., pathological complete remission (pCR) is achieved following nCRT, and the prognosis of this group of patients is significantly better, with the 5-year overall survival rate of approximately 90%[14]. In general, whether clinical complete remission (cCR) is achieved after neoadjuvant therapy can be judged by rectal MRI, endoscopy, serological markers, and other auxiliary examinations[15, 16]. In recent years, domestic and foreign experts have obtained more refined diagnostic criteria for cCR determination based on extensive clinical data[17].
Currently, three treatment modalities are commonly used for patients with locally progressive mid-low rectal cancer who have achieved cCR in clinical evaluation after neoadjuvant therapy: standard TME surgery, the “wait-and-see” strategy, and local excision[18]. Patients undergoing TME after neoadjuvant therapy are exposed to surgical risks and postoperative complications, and patients with low rectal cancer need to have their anal sphincter resected and undergo permanent ostomy[19]. In effect, some rectal cancer patients are unable to accept ostomy and the risks of major surgery, and strongly request that the “wait-and-see” strategy be implemented after neoadjuvant therapy[20, 21]. Glynne-Jones et al. reviewed the cCR rates of rectal cancer patients who did not undergo surgery after neoadjuvant therapy in published journals over the past decade. The results revealed that low rectal cancer with small tumor sizes was easily subjected to relatively refined clinical evaluation, but not suitable for clinical evaluation of progressive rectal cancer[22]. Several meta-analyses of rectal cancer show local regeneration rates of 31–37% in progressive rectal cancer[23], indicating a high recurrence rate in high-risk patients with poor prognostic evaluation results of rectal cancer risks (including cT3c − d/T4, lateral lymph node metastasis, MRF involvement, and WMVI+), to whom the “wait-and-see” strategy is not recommended[24, 25]. A similar retrospective study was conducted by Professor Wang Xishan’s team. They found that positive lymph nodes or tumor residuals were found in 13.6% of ypT0N0 patients in 60 rectal cancer patients who underwent radical surgery after nCRT[26]. It can be seen that the cCR and pCR rates are low and that no high-level evidence has been obtained for the applicability and efficacy of the "wait-and-see" strategy[27]. Large-scale prospective studies are needed to confirm the long-term outcomes and to refine chemotherapeutic medication selection, cCR determination, and optimal follow-up protocols for the "wait-and-see" strategy[28]. Some experts and scholars suggest that local excision after neoadjuvant therapy for rectal cancer patients can balance the pros and cons of the first two regimens, which can effectively excise the "primary tumor" and clarify pathological T-stages, but still cannot solve such problems as lymph node positivity, organ preservation, surgical risks, and tumor recurrence[18].
Tom1L1 plays a role in the onset and development of multiple tumors in the studies that have been confirmed so far. Clément et al. found that Tom1L1 exerted pro-oncogenic activity in breast cancer, Tom1L1 and ERBB2 were co-expressed in breast cancer, overexpression of ERBB2 in human breast cancer resulted in tumor invasion and metastasis, and Tom1L1 regulated ERBB2-induced cellular invasion by interacting with TOLLIP to promote MT1-MMP membrane delivery[29, 30]. Sirvent et al. revealed a completely different function of Tom1L1 in colorectal cancer through mouse xenotransplantation experiments and cell transfection experiments; low expression of Tom1L1 slightly affected Src-induced proliferation of CRC cells in vitro but significantly reduced tumor growth in xenotransplanted nude mice; the results suggested that Tom1L1 promoted Src-induced tumor growth[31]. Studies have found that Tom1L1 interacts with clathrin heavy chain (CHC) in vivo (CHC is a structural component of coated vesicles); when Tom1L1 binds to CHC, it negatively regulates Src's mitotic and transforming activity by decreasing the expression level of CHC in cholesterol-rich microvesicles; when Tom1L1 does not bind to CHC, it relocates in the vesicles and promotes Src-driven DNA synthesis[32]. Therefore, we can assume that Tom1L1 (as a new substrate) binding to Src family protein tyrosine kinase can promote Src activity, which in turn promotes cancer cell proliferation, triggers growth invasion mechanism, and induces angiogenesis.
In recent years, miRNA has been widely mentioned as a more studied tumor serum marker with promising research prospects. miRNAs play important regulatory roles in various cellular processes such as cell proliferation, differentiation, and apoptosis, and can be used as a potential biological indicator in colorectal cancer screening[33, 34]. Among them, miR-21-5p is expressed significantly different before and after nCRT, and miR-21-5p overexpression is likely to predict good tumor regression response[35, 36]. Controversially, however, in several in vitro studies of CRC cell lines, miR-21-5p overexpression was found to induce chemotherapy resistance[37], and patients with poor prognosis also showed miR-21-5p overexpression[38]. Rectal cancer is highly heterogeneous, and single biomarkers and imaging evaluation methods are not sufficiently sensitive and specific to predict the efficacy of neoadjuvant chemotherapy for rectal cancer[39]. Therefore, for patients with locally progressive rectal cancer, finding an accurate evaluation method to improve cCR and pCR rates is an urgent issue to be addressed. If molecular markers could be found to effectively predict pCR after neoadjuvant therapy for rectal cancer and then to accurately evaluate the TRG grades of neoadjuvant therapy, patients with locally progressive mid-low rectal cancer could be provided with more accurate treatment options, thereby improving their survival and prognosis.
In this study, functional analysis of differential genes screened from the TCGA database suggests that Tom1L1 expression was significantly correlated with clinical tumor stages - lymph node metastasis. Further clinical validation was carried out. To be specific, qPCR was conducted to detect Tom1L1 expression in colorectal cancerous tissues and paracancerous normal tissues. The results suggested that the relative expression of Tom1L1 was higher in colorectal cancerous tissues than that in paracancerous normal tissues. The results of qualitative and semiquantitative analysis of Western blot assay and IHC suggested that the expression level of Tom1L1 protein was significantly higher in colorectal cancerous tissues than that in paracancerous normal tissues. For patients with locally progressive mid-low rectal cancer, there was a significant difference in the expression level of Tom1L1 before and after neoadjuvant therapy; a significant difference was also seen between different efficacy grades, which correlated with TRG grades to a certain extent. Furthermore, combined with pathological and histochemical results, it was confirmed that patients with locally progressive mid-low rectal cancer were more sensitive to neoadjuvant chemotherapy.
In conclusion, this experiment provides valuable biological indicators for the diagnosis and staging of colorectal cancer, as well as for efficacy prediction of neoadjuvant therapy through bioinformatics prediction screening and clinical validation. However, this study is only limited to the clinical level and has not investigated the carcinogenic mechanism in depth at the cellular level. Moreover, the number of patients with locally progressive mid-low rectal cancer is relatively small. Therefore, multicenter, large-sample, and multidisciplinary collaborations can be carried out in the future, which will be the direction of our team's future research.