Histological transformation to signet-ring cell carcinoma in a patient with clinically aggressive poorly differentiated adenocarcinoma of the ascending colon after response to chemotherapy plus cetuximab: a case report

Advances in chemotherapy, including anti-epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs), anti-vascular endothelial growth factor (VEGF) TKIs, inhibitors of v-raf murine sarcoma viral oncogene homolog B1 (BRAF) V600 kinase, or immune checkpoint inhibitors (ICIs) for metastatic colorectal cancers (CRCs), have provided benefits such as improvement in overall survival (OS) or progression-free survival (PFS) of patients over the past decade; however, there are still many cancers that are unresponsive or develop chemoresistance [1]. Liquid biopsy assessment during chemotherapy for CRCs has proved the acquisition of mutations in various oncogenes, including RAS or BRAF genes, tumor suppressor genes, or genes involved in the deoxyribonucleic acid (DNA) repair process, and the development of chemotherapy resistance [2‐4]; the acquisition of these mutations has been shown to occur even in chemosensitive tumors to some degree [5]. Alteration of chemosensitivity or tumor aggressiveness in response to chemotherapy has been reported; however, to the best of our knowledge, the occurrence of histological transformation seems to be extremely rare for CRCs, and the few existing case reports were from lung cancer and breast cancer [6‐9].

Signet-ring cell carcinoma (SRCC) is a rare histological type of CRC with an approximate incidence of 1%; SRCC is diagnosed when > 50% of tumor cells have prominent intracytoplasmic mucin, typically with displacement and molding of the nucleus [10]. It is associated with a scirrhous appearance, aggressive manifestation including a higher frequency of locoregional lymph node involvement, lympho-vascular invasion and perineural infiltration, peritoneal dissemination, and adverse prognosis [11‐14].

In this report, we describe the histological transformation of clinically aggressive scirrhous-type poorly differentiated adenocarcinoma (PDA) of the ascending colon to SRCC in almost all recurrent tumors that were confirmed by autopsy after response to chemotherapy plus cetuximab.

A 59-year-old female presented with whole abdominal pain and body weight loss of 4 kg and visited our hospital in September 2020. Computed tomography (CT) revealed wall thickening of the whole length of the ascending colon and aggressive metastasis to locoregional lymph nodes and those located around the pancreas head and body, paraaortic region, celiac artery, right prerenal space, and peri-left adrenal gland (Fig. 1). Colonoscopy was carried out, and the anal side margin of a circumferential tumor on the hepatic flexure was found; the endoscope was not able to pass through the tumor lesion due to significant narrowing (Fig. 2). A biopsy was taken. Tumor cells showed PDA-like histology with scant intracytoplasmic mucin, resembling the appearance of malignant lymphoma (Fig. 3A). Then, immunohistochemical staining was performed, revealing positive staining for CK AE1/AE3 and CDX2 and negative staining for CD45 (Fig. 3B–D). Thus, a diagnosis of PDA of the ascending colon was made. Genetic mutations in KRAS, NRAS, and BRAF were absent. Serum levels of tumor markers such as CEA and CA19-9 showed normal levels of 2.1 ng/mL and 20.5 U/dL, respectively. Considering the aggressive nature of the colon cancer accompanied by metastasized lymph nodes but no bowel obstruction, chemotherapy, namely, mFOLFOX6, was initially administered followed by 5 courses of mFOLFOX6 plus cetuximab. CT after chemotherapy showed an obviously decreased size of metastasized lymph nodes (Fig. 4). To achieve primary tumor excision to prevent recurrence and subsequent stenosis or ileal stoma construction, right hemicolectomy with palliative lymph node dissection was carried out in February 2021. During the operation, peritoneal or liver metastases were not found. However, around the pancreas, white scarring was significant, and thickening and stiffening were obvious from the root of the mesocolon to the peripancreatic area; furthermore, the mesocolon of the hepatic flexure was unable to be detached from the pancreas. The tumor obviously remained in the peripancreatic area, paraaortic region and other retroperitoneal areas. The patient showed uneventful recovery and was discharged on postoperative day 14.

Chemotherapy consisting of mFOLFOX6 plus cetuximab was started in March 2021. CT conducted 8 months after the operation revealed no metastases to lymph nodes or other organs, and the patient was judged to have a complete response (Fig. 5). Chemotherapy was continued until one year after the operation. CT conducted at that time showed no evidence of recurrence; thus, a complete response had been maintained. Chemotherapy was discontinued at the patient’s request. One month later, in March 2022, she was admitted due to obstructive jaundice, with a serum total bilirubin value of 25.04 mg/dL. Serum values of CEA and CA19-9 were 6.2 ng/mL and 30.2 U/mL, respectively. CT revealed choledochal obstruction due to a mass in the pancreas head accompanied by mild dilatation of the main pancreatic duct and dilatation of the intrahepatic bile duct (Fig. 6A). Bulky lymph nodes located in the peri-left adrenal region (Fig. 6B) and swollen lymph nodes in the para-aortic region (Fig. 6C) were observed. Endoscopic retrograde cholangiopancreatography (ERCP) was attempted; however, we failed to confirm the papilla of Vater due to duodenal constriction caused by tumor infiltration and exposure of tumors (Fig. 7). A biopsy was taken from the recurrent tumor of the constricted segment of the duodenum, and PDA with signet-ring cells was diagnosed. Jaundice was improved by percutaneous transhepatic biliary drainage (PTBD); however, cholangitis persisted. Before readministration of chemotherapy, we planned gastrojejunal anastomosis and conversion to internal drainage of the biliary tract in order to resume oral intake and improve cholangitis, and the operation was performed 3 weeks after PTBD, 1 year and 2 months after the prior operation. Exacerbation of the patient’s condition progressed, and 18 days after the second operation, the patient died of the recurrent tumor in April 2022.

With consent from the patient’s family, autopsy was conducted.

Despite improvements in OS or PFS of metastatic CRC patients due to advances in chemotherapy, tumors that are unresponsive or develop resistance remain a serious problem. During chemotherapy, alterations in various oncogenes, tumor suppressor genes, and genes involved in the DNA repair process have been detected by liquid biopsy; data from circulating tumor cells, circulating cell-free DNA, exosomes secreted from cancer cells, and epigenetic observations have been analyzed and compared to corresponding data collected prior to chemotherapy [17‐19]. It was reported that even in a chemosensitive tumor treated with cetuximab, mutations in oncogenes occurred to some degree [5]. Although such genetic alterations can confer chemoresistance, followed by aggressive biological mechanisms can occur, histological transformation of cancer cells seems to be an extremely rare phenomenon. To the best of our knowledge, there are no reports of histological transformation due to chemotherapy in CRC, whereas a few cases of histological transformation in lung cancer and breast cancer have been reported [6‐9]. The present case showed clinically aggressive scirrhous-type ascending colon cancer accompanied by locoregional and distant lymph node metastases at diagnosis and intrinsic chemosensitivity; this case was revealed to mainly consist of PDA but was not associated with extracellular accumulation of mucin or signet-ring cell components, except for a minute cluster in only a few lymphatic emboli in the surgical specimens of ascending colon tumors obtained after mFOLFOX6 plus cetuximab therapy. Chemotherapy plus cetuximab was continued postoperatively, and the tumors had disappeared at 8 months after the operation; this response was maintained for 4 additional months according to CT findings. Discontinuation of chemotherapy plus cetuximab 1 year after the operation resulted in immediate tumor recurrence and rapid expansion. Autopsy specimens revealed that almost all of the recurrent tumors consisted of signet-ring cell histology exhibiting histological transformation. Unlike the insufficient histological confirmation from biopsy specimens from a recurrent site suitable for sampling, the presented patient was autopsied, and many metastasized lesions were investigated; almost all recurrent tumors were replaced by SRCC. Only sporadic reports of histological transformation in lung cancer, such as small cell carcinoma evolving from lung adenocarcinoma, and breast cancer have been reported to be associated with chemotherapy combined with anti-EGFR TKIs, chemotherapy alone, immunotherapy, or hormone replacement therapy [6‐9].

Possible mechanisms of histological transformation have been suggested in previous literature, including pluripotent cancer stem cell differentiation and baseline heterogeneity of cancers [20, 21]. Regarding the baseline heterogeneity of the primary cancer, a histopathological diagnosis of PDA of the ascending colon was made, although scirrhous colon cancer showed a characteristic appearance common to SRCC histology of CRCs [22], and a signet-ring cell component was not observed in the primary colon tumor, except for a tiny cluster within a few lymphatic emboli, despite the evaluation of 5 mm slices along the whole length of the tumor. Possibly, the large tumor had a very small amount of SRCC between the sliced specimens, and selective outgrowth of the minute variant component occurred. However, preoperative chemotherapy reduced the primary tumor and metastasized lymph nodes, colectomy was carried out, PDA was proven, and it was not clear whether tiny clusters of SRCC were intrinsic or developed after chemotherapy. After surgical treatment and chemotherapy, biopsy specimens taken from the constricted duodenum also showed a mixture of signet-ring cells among PDA histology. Hematoxylin and eosin staining showed that most metastasized cancer cells had poorly differentiated morphology; however, AB-PAS staining revealed that the signet-ring-like cells had enriched cytoplasmic mucin, and other PDA-like cells also had more intracytoplasmic mucin than the original PDA cells. We observed the process of histological transformation to mature SRCC via immature signet-ring cell-like cells from original PDA cells and ultimately the replacement of PDA cells with signet-ring cells in almost all recurrent tumors.

Among CRCs, SRCC is reported to be diagnosed at a younger age than adenocarcinoma, with a right-sided predilection, large tumor size, scirrhous appearance, aggressive manifestation including a higher frequency of locoregional lymph node involvement, lymphovascular invasion and perineural infiltration, peritoneal dissemination, higher risk of recurrence, and incorrect prognosis [11‐14]. Pulmonary and hepatic metastases are uncommon; however, metastases to the brain and bone, including bone marrow, are more common [23]. In this case, the patient was a middle-aged woman with right-sided colon cancer with features similar to those of CRC-SRCC. Given the possibility of an MSI-high status, we investigated MMR protein expression by immunohistochemistry, and the result was MMR-proficient; thus, in this case, MMR did not seem to have an effect on the transformation to SRCC. In accordance with the WHO classification [10], among CRCs, PDA could be categorized as having a signet-ring cell component if present and might have affinity to SRCC; however, the present case scarcely had a component of signet-ring cells and had a histological appearance resembling malignant lymphoma confirmed by negative immunohistochemical staining for CD45. Distinct histological differences existed between these original and recurrent tumors, suggesting that decisive histological transformation occurred. In addition to reports that suggest the existence of an adenoma-SRCC sequence [24, 25], the molecular characteristics of SRCC have been reported to possibly arise from a separate genetic pathway, accompanied by a lower frequency of APC mutations; the presence of less common KRAS, BRAF, and PI3K mutations; frequent SMAD4 mutations; and ordinary MSI-H status [26]. These characteristics are distinct from the frequent alterations in the conventional colorectal adenoma-carcinoma sequence, including APC, KRAS, and PI3K alterations as well as infrequent SMAD4 mutations [12]. The present case showed alteration of the KRAS gene sequence after chemotherapy. Although KRAS mutation does not represent the typical genetic state of SRCC, it suggests that mFOLFOX6 plus cetuximab therapy might promote other oncogene mutations or epigenetic changes, causing the transformation of non-SRCC CRC to SRCC.

Finally, our ability to reveal the mechanisms underlying histological transformation in the presented case was limited because only one recurrent tumor specimen was obtained, which limited the assessment of KRAS, NRAS, and BRAF codon alterations, and additional tumor genomic analysis to identify the unique combination of mutations of various oncogenes or epigenes contributing to transformation to SRCC was not performed. Further accumulation of cases and investigations, including tumor genomic analysis, are necessary to elucidate the mechanisms of the histological transformation of non-SRCC CRC to SRCC.

We reported an extremely rare case of poorly differentiated scirrhous-type ascending colon cancer that exhibited transformation to SRCC histology after chemotherapy plus cetuximab. This case might suggest that mutations in various oncogenes or epigenes resulting from chemotherapy, especially regimens that include cetuximab, contribute to transforming non-SRCC CRC to SRCC histology and can promote the aggressive clinical progress characteristic of SRCC.