Simultaneous onset of familial hemophagocytic lymphohistiocytosis and T-cell large granular lymphocytic leukemia in an adult.

Hemophagocytic lymphohistiocytosis (HLH) is a severe hyperinflammatory syndrome. The familial form (fHL), associated with specific genetic alterations, typically manifests in childhood, while secondary forms arise due to triggers such as malignancies or infections. We present the rare case of adult-onset fHL in a 30-year-old female concurrent with T-cell large granular lymphocytic leukemia (T-LGL). After presenting with symptoms of HLH, a bone marrow biopsy revealed T-cell lymphoma, leading to CHOEP treatment. This diagnosis was later revised to T-LGL, prompting a switch to methotrexate for T-LGL treatment. Further genetic testing uncovered an unexpected homozygous missense mutation in UNC13D (p.R414C), associated with fHL type 3. A detailed family history revealed a consanguineous marriage between her parents, which likely increased the risk of the homozygous mutation. Additionally, it was discovered that the patient's sister had undergone allogeneic stem cell transplantation during childhood for an unspecified lymphoproliferative disorder, with the patient herself serving as the donor. This case highlights the rare occurrence of adult-onset fHL, triggered by the evolution of T-LGL as a possible contributing factor. It underscores the importance of early diagnosis and illustrates how familial history and persistent symptoms can necessitate genetic testing.