Sequential lymphoid neoplasms mimicking relapse of mediastinal grey zone lymphoma in a patient with germline variants in EP300 and PTPRK

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Abstract
Sequential lymphoid neoplasms are rare and diagnostically challenging, with a variable clonal relationship. Here we present the case of a 59-year-old woman with mediastinal grey zone lymphoma (MGZL), who, shortly after achieving complete remission, developed a subsequent cutaneous neoplasm. Comparative molecular studies were performed using high-throughput sequencing (HTS), which demonstrated that both neoplasms shared high-allele-frequency E P300 :p.P2333L and P TPRK :p.R532K variants. In addition, each neoplasm harboured private, mutually exclusive variants— NFK BIA :p.R245Sfs*39 and B TG2 :c.142+5G>C in the MGZL and JA K1 :p.Q562* and J A K1 :p.G1097D in the skin. Integrating the clinical course with HTS, we diagnosed lymphomatoid papulosis and excluded relapse of the MGZL. Interestingly, the shared variants were likely germline rather than clonal, suggesting they created a permissive background predisposing to two independent transforming events along the B- and T-cell lineages. Molecular studies may help to reveal the genetic basis of composite lymphomas and to resolve diagnostically discordant, complex presentations.

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