Gastric-type endocervical adenocarcinoma in situ as the presenting feature in a mosaic STK11 pathogenic variant carrier with a Peutz-Jeghers syndrome child.

We present the first documented case of gastric-type endocervical adenocarcinoma in situ in a mosaic STK11 pathogenic variant carrier, who delivered a child with classic Peutz-Jeghers syndrome (PJS). A 53-year-old woman presented with persistent watery vaginal discharge for 2 years. Histopathology confirmed gastric-type endocervical adenocarcinoma in situ. Familial genetic investigation was initiated after her son's diagnosis of PJS with an apparent de novo STK11 germline variant [NM_000455.4:c.842del (p.Pro281Argfs*6)]. Comprehensive STK11 screening via Sanger sequencing of peripheral blood from both parents showed no pathogenic variants. Following multidisciplinary tumor board review, ultra-deep next-generation sequencing was performed on multiple tissue specimens. Molecular analysis revealed low-level mosaicism for the familial STK11 variant in cervical tissue with no detectable mutations in blood, saliva, urine, or ovarian stroma. This case demonstrates three key clinical insights: (1) parental mosaicism may underlie apparent de novo PJS cases, (2) tissue-specific STK11 mosaicism can manifest as localized neoplastic transformation without classic PJS manifestations, and (3) ultra-deep sequencing may be considered in genetic counseling paradigms for parents of children with "de novo" cancer predisposition syndromes. These findings highlight the importance of considering mosaic phenomena in tumor prevention for hereditary cancer syndrome families.