HAPLN3 p.T34A contributes to incomplete penetrance of moyamoya disease in Chinese carrying RNF213 p.R4810K.

Background and purpose: The penetrance of the RNF213 p.R4810K, a founder mutation of moyamoya disease (MMD), is estimated to be only 1/150–1/300. However, the factors affecting its penetrance remain unclear. Therefore, our study aims to identify modifier genes associated with the incomplete penetrance of this founder mutation. Methods: Whole‐exome sequencing (WES) was performed on 36 participants, including 22 MMD patients and 14 non‐MMD controls with RNF213 p.R4810K mutation. Fisher's exact test was used to assess the presence of genetic variants that differed significantly between MMD patients and non‐MMD controls. In order to exclude false‐positive results, another 55 carriers were included to perform Fisher's exact test for the selected sites in the WES discovery stage. Subsequently, human brain microvascular endothelial cells were transfected with wild‐type and mutant HAPLN3 for tube formation assays and western blotting to explore the impact of candidate genes on angiogenesis. Results: Analysis of variants from WES data revealed a total of 12 non‐synonymous variants. Through bioinformatics analysis, the focus was on the HAPLN3 p.T34A variant with a significant p value of 0.00731 in Fisher's exact test. Validation through Sanger sequencing confirmed the presence of this variant in the WES data. In vitro experiments revealed that silencing HAPLN3 and transfecting HAPLN3 p.T34A significantly enhanced tube formation and increased the relative protein expression of vascular endothelial growth factor in endothelial cells. Conclusions: These results suggest that HAPLN3 may function as a modifier gene of RNF213 p.R4810K, promoting the development of MMD and contributing to the incomplete penetrance of MMD founder mutations. [ABSTRACT FROM AUTHOR]