# A Case of a Novel Perforin Gene Variant in Severe Familial Hemophagocytic Lymphohistiocytosis Type 2 (FHL2)

## TL;DR

A 5-month-old boy with severe FHL2 was found to have a new PRF1 gene variant, A21V, which contributes to the disease when combined with another variant.

## Contribution

The study reports a novel PRF1 variant, A21V, and provides functional evidence of its pathogenicity in FHL2.

## Key findings

- The PRF1 A21V variant is novel and associated with reduced perforin expression in NK cells.
- The A21V/P16S compound heterozygous variant combination contributes to early-onset FHL2.
- Functional validation supports the pathogenic role of the P16S variant in the context of A21V.

## Abstract

Hemophagocytic lymphohistiocytosis (HLH) is a life‐threatening hyperinflammatory syndrome caused by excessive cytokine release from activated T cells and macrophages. Primary HLH, or familial HLH (FHL), results from genetic mutations affecting cytotoxic lymphocyte function.

We present a case of FHL Type 2 (FHL2) caused by compound heterozygous variants in the PRF1 gene, including one novel missense variant of p.Ala21Val (A21V). A 5‐month‐old boy presented with persistent fever, pancytopenia, coagulopathy, hepatosplenomegaly, and elevated ferritin, meeting the HLH‐2004 diagnostic criteria. Bone marrow revealed hemophagocytosis, and NK cell activity was markedly reduced. Genetic analysis identified compound heterozygous PRF1 variants: A21V and p.Pro16Ser (P16S). Flow cytometric analysis demonstrated markedly reduced PRF1 protein expression in the patient’s NK cells. The patient was treated with etoposide, dexamethasone palmitate, and cyclosporine, followed by cord blood transplantation. The patient has been in remission for over a year.

The PRF1 A21V variant has not been described in the public database or the literature and is therefore considered a novel pathogenic variant for FHL2 with functional validation. Although the PRF1 P16S variant has been previously reported in the heterozygous state in an adult patient with primary HLH, our findings provide functional and clinical evidence supporting a contributory role of the P16S variant in autosomal recessive early‐onset FHL2 when present in trans with the novel A21V variant.

We identified a previously unreported PRF1 variant, A21V, and provided the first functional evidence of impaired perforin expression associated with A21V/P16S, highlighting the importance of functional validation of rare PRF1 variants in FHL2.