Identifying pathogenic variants in the Follistatin-like 1 gene (FSTL1) in patients with skeletal and atrioventricular valve disorders.

BACKGROUND: Follistatin-like 1 (Fstl1) is a glycoprotein expressed throughout embryonic development. Homozygous loss of Fstl1 in mice results in skeletal and respiratory defects, leading to neonatal death due to a collapse of the trachea. Furthermore, Fstl1 conditional deletion from the endocardial/endothelial lineage results in postnatal death due to heart failure and profound atrioventricular valve defects. Here, we investigated patients with phenotypes similar to the phenotypes observed in the transgenic mice, for variants in FSTL1. METHODS: In total, 69 genetically unresolved patients were selected with the following phenotypes: campomelic dysplasia (12), small patella syndrome (2), BILU (1), and congenital heart disease patients (54), of which 16 also had kyphoscoliosis, and 38 had valve abnormalities as their main diagnosis. Using qPCR, none of 69 patients showed copy number variations in FSTL1. The entire gene body, including microRNA-198 and three validated microRNA-binding sites, were analyzed using Sanger sequencing. RESULTS: No variants were found in the coding region. However, 8 intronic variants were identified that differed significantly in their minor allele frequency compared to controls. Variant rs2272515 was found to significantly correlate (p < 0.05) with kyphoscoliosis. CONCLUSION: We conclude that pathogenic variants in FSTL1 are unlikely to be responsible for skeletal or atrioventricular valve anomalies in humans.

Follistatin-like 1 in development and human diseases.

Follistatin-like 1 (FSTL1) is a secreted glycoprotein displaying expression changes during development and disease, among which cardiovascular disease, cancer, and arthritis. The cardioprotective role of FSTL1 has been intensively studied over the last years, though its mechanism of action remains elusive. FSTL1 is involved in multiple signaling pathways and biological processes, including vascularization and regulation of the immune response, a feature that complicates its study. Binding to the DIP2A, TLR4 and BMP receptors have been shown, but other molecular partners probably exist. During cancer progression and rheumatoid arthritis, controversial data have been reported with respect to the proliferative, apoptotic, migratory, and inflammatory effects of FSTL1. This controversy might reside in the extensive post-transcriptional regulation of FSTL1. The FSTL1 primary transcript also encodes for a microRNA (miR-198) in primates and multiple microRNA-binding sites are present in the 3'UTR. The switch between expression of the FSTL1 protein and miR-198 is an important regulator of tumour metastasis and wound healing. The glycosylation state of FSTL1 is a determinant of biological activity, in cardiomyocytes the glycosylated form promoting proliferation and the non-glycosylated working anti-apoptotic. Moreover, the glycosylation state shows differences between species and tissues which might underlie the differences observed in in vitro studies. Finally, regulation at the level of protein secretion has been described.

Deletion of Fstl1 (Follistatin-Like 1) From the Endocardial/Endothelial Lineage Causes Mitral Valve Disease.

OBJECTIVE: Fstl1 (Follistatin-like 1) is a secreted protein that is expressed in the atrioventricular valves throughout embryonic development, postnatal maturation, and adulthood. In this study, we investigated the loss of Fstl1 in the endocardium/endothelium and their derived cells. APPROACH AND RESULTS: We conditionally ablated Fstl1 from the endocardial lineage using a transgenic Tie2-Cre mouse model. These mice showed a sustained Bmp and Tgfß signaling after birth. This resulted in ongoing proliferation and endocardial-to-mesenchymal transition and ultimately in deformed nonfunctional mitral valves and a hypertrophic dilated heart. Echocardiographic and electrocardiographic analyses revealed that loss of Fstl1 leads to mitral regurgitation and left ventricular diastolic dysfunction. Cardiac function gradually deteriorated resulting in heart failure with preserved ejection fraction and death of the mice between 2 and 4 weeks after birth. CONCLUSIONS: We report on a mouse model in which deletion of Fstl1 from the endocardial/endothelial lineage results in deformed mitral valves, which cause regurgitation, heart failure, and early cardiac death. The findings provide a potential molecular target for the clinical research into myxomatous mitral valve disease.