Expanding the clinical spectrum of biglycan-related Meester-Loeys syndrome

Abstract

Pathogenic loss-of-function variants in BGN, an X-linked gene encoding biglycan, are associated with Meester-Loeys syndrome (MRLS), a thoracic aortic aneurysm/dissection syndrome. Since the initial publication of five probands in 2017, we have considerably expanded our MRLS cohort to a total of 18 probands (16 males and 2 females). Segregation analyses identified 36 additional BGN variant-harboring family members (9 males and 27 females). The identified BGN variants were shown to lead to loss-of-function by cDNA and Western Blot analyses of skin fibroblasts or were strongly predicted to lead to loss-of-function based on the nature of the variant. No (likely) pathogenic missense variants without additional (predicted) splice effects were identified. Interestingly, a male proband with a deletion spanning the coding sequence of BGN and the 5' untranslated region of the downstream gene (ATP2B3) presented with a more severe skeletal phenotype. This may possibly be explained by expressional activation of the downstream ATPase ATP2B3 (normally repressed in skin fibroblasts) driven by the remnant BGN promotor. This study highlights that aneurysms and dissections in MRLS extend beyond the thoracic aorta, affecting the entire arterial tree, and cardiovascular symptoms may coincide with non-specific connective tissue features. Furthermore, the clinical presentation is more severe and penetrant in males compared to females. Extensive analysis at RNA, cDNA, and/or protein level is recommended to prove a loss-of-function effect before determining the pathogenicity of identified BGN missense and non-canonical splice variants. In conclusion, distinct mechanisms may underlie the wide phenotypic spectrum of MRLS patients carrying loss-of-function variants in BGN.

Fig. 1. Pedigrees of thirteen families with BGN variants identified in the current study.

Circle = female. Square = male. Diamond = unknown sex. Filled = patients with connective tissue features. Unfilled = unaffected or asymptomatic person. Strikethrough = deceased person. + = hemi-/heterozygous BGN variant carrier. (+) = heterozygous BGN variant carrier reported by proband. – = tested negative for BGN variant. Arrow = proband.

Fig. 2. Western Blot of BGN protein expression in skin fibroblasts of probands from families 1, 6, 9, 10, 17, and 18 as well as matched controls.

Intracellular proteins were isolated from skin fibroblast samples and the biglycan (BGN) protein content was visualized. Cyclophilin B (CypB) was used as a loading control. Family 17 and 18 were reported as family 4 and 5, respectively, by Meester et al.. Controls 1–4 were samples of age-matched male controls. The Western Blot was derived from one experiment and all lanes were processed in parallel.

Fig. 3. mRNA sequencing of skin fibroblasts of the proband from family 17 (17-II-1) shows BGN 5’-UTR hijacking by ATP2B3.

Family 17 was reported as family 4 by Meester et al. (2017). The control sample was age- and sex-matched.