Novel GDF2 Loss of Function Variant in a Family with HHT and PAVMs Expands the Phenotype Associated with BMP9 Dysfunction: American Thoracic Society International Conference 2020

Rationale: Hereditary hemorrhagic telangiectasia (HHT) is a multisystemic vascular dysplasia inherited as an autosomal dominant trait, characterized by nosebleeds, mucocutaneous telangiectasia, and arteriovenous malformations in major organs, including the lung (PAVM). The three known HHT genes are ACVRL1 (encoding ALK-1), ENG, or SMAD4, all members of the bone morphogenetic protein (BMP) signaling pathway. In contrast, haploinsufficiency for the BMP type-II receptor (BMPR2) causes pulmonary arterial hypertension (PAH). The major ligand for the ALK-1/ENG/BMPR2 receptor complex is BMP9, encoded by GDF2. To date, the majority of GDF2 gene damaging variants have been found in patients with PAH. Three variants were reported in unrelated probands with a vascular anomaly syndrome similar to HHT, but not meeting diagnostic (Curaçao) criteria. Methods: To identify novel HHT and PAVM genes, families were recruited via UK Genomic Medicine Centres. Whole genome sequencing and variant tiering was performed through Genomics England’s 100,000 Genomes Project. Plasma BMP9 levels were measured by ELISA. Functional effects of GDF2 variants were studied in vitro by site directed mutagenesis and overexpression in 293T cells. Results: A novel heterozygous GDF2 missense variant was identified in one HHT family and segregated with disease. The proband was severely affected, having presented in childhood with multiple PAVMs, frequent nosebleeds, and typical HHT telangiectasia, but was negative for ENG/ACVRL1/SMAD4. His sister and mother had three Curaçao criteria (nosebleeds, telangiectasia and an affected first degree relative). The variant was not previously seen in publicly-available exome sequencing cohorts, and is predicted deleterious. Plasma samples from this family showed significantly lower circulating BMP9 levels in the affected individuals compared to healthy controls (p=0.03), consistent with heterozygous loss of BMP9. Following in vitro overexpression in 293T cells, conditioned medium from the mutant plasmid showed no detectable mature BMP9 protein, compared with a high level of expression from the wildtype plasmid. Conclusions: These results suggest that GDF2 haploinsufficiency can be a rare cause of HHT and PAVMs. Functionally, however, the impact of this variant on BMP9 protein processing appears identical to the variants associated with PAH. Some phenotypic cross-over is already known, since a small subset of HHT patients develop PAH, while at least two PAH patients with BMPR2 mutations have been reported with PAVM. GDF2 now adds to this enigma, highlighting the importance of understanding potential modifier genes and second hits in the pathogenesis of both HHT and PAH.