A novel pathogenic variant located just upstream of the C‐terminal Ser423‐X‐Ser425 phosphorylation motif in SMAD3 causing Loeys–Dietz syndrome

Abstract Objective Loeys–Dietz syndrome (LDS) is a heritable disorder of connective tissue closely related to Marfan syndrome (MFS). LDS is caused by loss‐of‐function variants of genes that encode components of transforming growth factor‐β (TGF‐β) signaling; nevertheless, LDS type 1/2 caused by TGFBR1/2 pathogenic variants is frequently found to have paradoxical increases in TGF‐β signaling in the aneurysmal aortic wall. Here, we present a Japanese LDS family having a novel SMAD3 variant. Methods The proband was tested via clinical, genetic, and histological analyses. In vitro analysis was performed for pathogenic evaluation. Results The novel heterozygous missense variant of SMAD3 [c.1262G>A, p.(Cys421Tyr)], located just upstream of the C‐terminal Ser423‐X‐Ser425 phosphorylation motif, was found in this instance of LDS type 3. This variant led to reduced phospho‐SMAD3 (Ser423/Ser425) levels and transcription activity in vitro; however, a paradoxical upregulation of TGF‐β signaling was evident in the aortic wall. Conclusions Our results revealed the presence of TGF‐β paradox in this case with the novel loss‐of‐function SMAD3 variant. The precise mechanism underlying the paradox is unknown, but further research is warranted to clarify the influence of the SMAD3 variant type and location on the LDS3 phenotype as well as the molecular mechanism leading to LDS3 aortopathy.