Neuroendocrine and mucinous differentiation in signet ring cell carcinoma of the stomach: Evidence for a common cell of origin in composite tumors

Composite tumors are rare neoplasms containing a mixture of 2 different cellular components present in roughly equal proportions. It is hypothesized that composite tumors arise from a multipotential stem cell with subsequent bidirectional differentiation. We present an unusual composite tumor of the stomach composed equally of signet ring cell carcinoma and low-grade neuroendocrine carcinoma. Twenty-one additional patients with signet ring cell carcinomas of the stomach were studied to determine the prevalence of neuroendocrine differentiation by morphology and immunohistochemistry for synaptophysin and chromogranin A. Immunohistochemistry for mucins 5AC and 2 was performed to assess for divergent differentiation toward foveolar and intestinal mucin phenotypes, respectively, and to evaluate for any potential relationship with neuroendocrine differentiation. We found morphologic evidence of neuroendocrine carcinoma in 4 (19%) of 21 consecutive signet ring carcinomas. E-cadherin immunostaining was subsequently performed on these 4 tumors plus the index case. All 5 tumors demonstrated concordance between the signet ring and neuroendocrine components. There was no distinct relationship to mucin 5AC/mucin 2 profiles, with the exception that all 11 intramucosal signet ring cell carcinomas from 4 patients with germ line cadherin 1 gene mutations were composed exclusively of mucin 5AC+ signet ring cells that lacked intestinal mucin and neuroendocrine differentiation. The concordant E-cadherin status in the neuroendocrine and signet ring cell tumor components and the frequent admixture of mucin 5AC+ cells with foveolar differentiation and mucin 2+ cells with intestinal differentiation may support the hypothesis that composite tumors arise from a common stem cell with bilineage or multilineage differentiation.