Spatial transcriptomic analysis identifies a border zone with localized expression of brain natriuretic peptide in the inferobasal region of the left ventricle in patients with mitral valve prolapse
Authors: Boeckel J , Erbe S , Spampinato R , Meder B , Borger M , Laufs U , Dieterlen M , Lavall D 
Published: November 2023

Abstract

Background/Introduction
Mitral valve prolapse (MVP) is a common cause of mitral regurgitation (MR), adverse left ventricular (LV) remodeling and arrhythmias. MVP induces mechanical stretch to the inferobasal (IB) myocardium and papillary muscles. The underlying regional and molecular pathology of MVP-induced LV remodeling remains incompletely understood.

Purpose
The aim was to characterize the molecular effects of mechanical stretch by comparing the regional gene expression of the IB region of LV with the interventricular septum in patients with MVP.

Methods
Myocardial biopsies were obtained from the IB LV (i.e. between papillary muscles) and the interventricular septum during mitral valve repair surgery for MVP-induced primary MR. The degree of cardiac fibrosis was detected and quantified by histology. We used spatial transcriptomics (ST) that - in contrast to traditional analyses of tissue homogenates - allows regional expressional analysis.

Results
Automated clustering of ST results correlated with histological staining of fibrosis in the IB and septal regions (R=0.97, P&lt;0.0001), demonstrating good automated identification of fibrotic regions. The extent of fibrosis was similar between the inferobasal region and the septal control in the respective patients with MVP as determined by histological analysis and ST. The localized fibrotic areas of the inferobasal region showed decreased expression of cardiac myocytegenes such as RYR2, TNNI3, TNNC1 or TPM1, and increased expression of fibrous and EndMT markers such as COL3A1, BGN, SM22 or FN1. In the IB region, we identified a transition zone between healthy myocardium and fibrous tissue. This transition zone showed increased expression of the NPPB gene encoding brain natriuretic peptide (BNP), whereas NPPB was not increased in the septal region of the same patients. The upregulation of NPPB (6.4-fold, P=0.004) in the inferobasal region compared to the septal control was confirmed by qPCR in biopsies from patients with MVP. The identified transition zones were characterized by increased expression of genes such as IGFBP2 and LTBP2, which have been described to be associated with adverse LV remodeling in heart failure. Interestingly, the serum NT-proBNP levels (mean 276.8 ± 121.6 pg/ml) of the patients correlated with NPPB levels from the inferobasal region (R=0.76, P=0.01, N=9), whereas no correlation was found with NPPB levels from the septal control regions (R=0.17, P=0.65) of the same patients.

Conclusions
ST analysis in MVP identifies a transition zone of the inferobasal region of LV which is exposed to high mechanical stretch. This transition zone - but not the inferobasal fibrotic area nor the LV septum - is characterized by a specific increase in NBBP expression that correlates with increased NT-ProBNP serum concentrations. These special sequencing results indicate that mechanical stretch induces highly localized myocardial pathologies with systemic consequences.