Familial hemophagocytic lymphohistiocytosis type 3 (FHL3) caused by deep intronic mutation and inversion in UNC13D

Familial hemophagocytic lymphohistiocytosis (FHL) is an autosomal recessive, often-fatal hyperinflammatory disorder. Mutations in PRF1, UNC13D, STX11, and STXBP2 are causative of FHL2, 3, 4, and 5, respectively. In a majority of suspected FHL patients from Northern Europe, sequencing of exons and splice sites of such genes required for lymphocyte cytotoxicity revealed no or only monoallelic UNC13D mutations. Here, in 21 patients, we describe 2 pathogenic, noncoding aberrations of UNC13D. The first is a point mutation localized in an evolutionarily conserved region of intron 1. This mutation selectively impairs UNC13D transcription in lymphocytes, abolishing Munc13-4 expression. The second is a 253-kb inversion straddling UNC13D, affecting the 3'-end of the transcript and likewise abolishing Munc13-4 expression. Carriership of the intron 1 mutation was found in patients across Europe, whereas carriership of the inversion was limited to Northern Europe. Notably, the latter aberration represents the first description of an autosomal recessive human disease caused by an inversion. These findings implicate an intronic sequence in cell-type specific expression of Munc13-4 and signify variations outside exons and splice sites as a common cause of FHL3. Based on these data, we propose a strategy for targeted sequencing of evolutionary conserved noncoding regions for the diagnosis of primary immunodeficiencies.