UNCOVERING A NOVEL AND ELUSIVE SERPING1 DELETION IN HEREDITARY ANGIOEDEMA WITH PLASMINOGEN AND MASP-1 MODIFIERS

Introduction

Hereditary angioedema (HAE) is an autosomal dominant genetic disorder caused by mutations in the C1 esterase inhibitor gene, SERPING1. Mutations in this gene lead to overproduction of bradykinin, resulting in debilitating swelling attacks. Despite over 748 variants identified in SERPING1, genetic diagnosis of some patients with unknown HAE-causing mutations (U-HAE) but clear HAE clinical presentations are elusive.

Methods

We obtained blood from three family members previously tested with low C4 and C1-inhibitor levels. DNA samples were library-prepped and NGS sequenced. MLPA was performed to assess exon-level copy number variation for SERPING1. A customized bioinformatics workflow was developed to detect the percentage of soft-clipped NGS reads. We designed custom forward and reverse primers to amplify and sequence SERPING1 exon 6.

Results

NGS and MLPA failed to uncover pathogenic variants or large genomic rearrangements in SERPING1. NGS analysis revealed a PLG gene missense variant (p.Gly106Trp), and a MASP1 missense variant (p.Pro607Leu) in the proband and mother but not in the son. Bioinformatic analysis detected a high rate of soft clipping in SERPING1 exon 6 and a subsequent heterozygous 56bp deletion was discovered by Sanger sequencing in all three subjects.

Conclusion

Variants may go undetected even after NGS and MLPA. We propose that a systematic approach to U-HAE analysis, incorporating soft clipping as part of an overall strategy, would be effective in identifying a small percentage of causal variants in approximately 5% of C1-INH-HAE cases where no mutation is found by standard NGS procedures, especially when there is a high clinical suspicion of a familiar disorder.