Background: Hemophagocytic lymphohistiocytosis (HLH) is a rare and life-threatening disease in which cells of the immune system are overactivated, leading to uncontrolled inflammation and tissue destruction. Inherited or familial forms of HLH (FHL) are further classified into FHL1 to 5, based on the underlying genetic etiology. The most common form, FHL2, is associated with mutations in the PRF1 gene encoding perforin, a pore-forming glycoprotein required for natural killer and cytotoxic T cell-mediated apoptosis. Importantly, diagnosis of FHL can be challenging, particularly in late-onset cases in which presentation is delayed beyond the first years of life.

Aim: We report the essential role of whole exome sequencing in the diagnostic work-up of a patient with complex, late-onset FHL.

Methods: A comprehensive retrospective chart review was performed.

Results: Our patient presented at 11 years of age with recurrent fever, hepatosplenomegaly, and pancytopenia. In the following years, she was admitted to hospital on multiple occasions, including twice for febrile neutropenia, and once for febrile cytopenia. Serial immune evaluation revealed features of immune dysregulation. While HLH was suspected, she did not fulfil the diagnostic criteria. Initial genetic work-up involving a targeted primary immunodeficiency gene panel identified only a single novel variant of uncertain significance, c.T374C (p.I125T) in PRF1. Subsequently, research-based whole exome sequencing was performed which revealed a second variant, c.C272T (p.A91V), in the same gene. The expanded genetic findings, a set of compound heterozygous missense mutations in PRF1, strengthened the diagnosis of FHL. She later fulfilled the diagnostic criteria for HLH.

Conclusion: Whole exome sequencing identified compound heterozygous mutations in the PRF1 gene in a patient with late-onset FHL.

Statement of Novelty: We report the use of whole exome sequencing to identify compound heterozygous mutations in PRF1, including a novel p.I125T variant not previously identified in FHL.