Colorectal small cell carcinoma in ulcerative colitis with identical rare p53 gene mutation to associated adenocarcinoma and dysplasia.

Colorectal small cell carcinomas (SCCs) are rare tumors and are infrequently associated with ulcerative colitis (UC). We report a case of primary rectal SCC combined with adenocarcinoma arising in left-sided UC. Immunohistochemically, tumor cells were positive for chromogranin A, synaptophysin, and CD56 in the SCC but not in the adenocarcinoma. The patient simultaneously developed multiple lesions of adenocarcinoma and high-grade dysplasia in the sigmoid colon and rectum. To elucidate whether SCC might evolve from multipotential cells in dysplasia and/or adenocarcinoma, we examined the mutational status of TP53 and KRAS. The same clonality of these lesions including SCC was confirmed by the presence of an identical single nucleotide point mutation in TP53. KRAS mutation was not observed in these lesions. Thus, these lesions seem to have developed from the same origin. Long-standing inflammation leading to dysplasia might be responsible for the development of some SCCs in UC particularly when they are combined with dysplasia and/or adenocarcinoma.