Characterising Diabetic Cardiomyopathy Using Cardiac MRI and Transcriptomics

Patients with type 2 diabetes mellitus (T2D) have an increased and premature risk of cardiovascular disease. In the presence of preceding myocardial pathologies, concomitant T2D results in a more exaggerated phenotype and worse prognosis.

Cardiac magnetic resonance (CMR) and phosphorus magnetic resonance spectroscopy (31P-MRS) are advanced imaging modalities which allow for detailed characterisation of the myocardium in the presence of health and disease. Furthermore ‘transcriptomics’ offer an important unbiased approach to study the molecular phenotypes of disease, allowing for a deeper understanding of which genes (and at what level) are activated or suppressed, as well as their activity and expression, within a cell or tissue, in a detailed way.

The aim of this thesis was to combine imaging techniques with RNA sequencing to understand the changes to the myocardium in the presence of T2D. CMR and 31P-MRS were used to define the impact of co-morbid T2D across the spectrum of aortic valve stenosis (AS), and heart failure with reduced ejection fraction (HFrEF), particularly with regards to the alterations in the myocardial energetic state, perfusion, structure and function. RNA extraction and sequencing was performed both on myocardial tissue samples taken at the time of valve replacement surgery and from bloods taken at the time of CMR for the diagnosis of causation of new HFrEF. Myocardial transcriptional differences between patients with severe AS with and without T2D, blood transcriptional differences between HFrEF patients with and without T2D, and transcriptomic correlates of CMR parameters with these concomitant cardiac diseases were sought.
Compared to controls, with increasing AS severity, there was progressive increase in left ventricular (LV) concentricity and a stepwise decline in cardiac function measured via global longitudinal shortening (GLS). Indices of cardiac perfusion (stress myocardial blood flow (MBF) and myocardial perfusion reserve (MPR) were only significantly reduced in patients with Severe-AS compared to controls, with significant differences also detected between patients with Moderate-AS and Severe-AS. Layer specific perfusion imaging revealed that stress MBF and MPR in the endocardial and epicardial myocardial layers were all significantly reduced in both Moderate-AS and Severe-AS. Myocardial energetics were reduced in both AS groups. In the presence of T2D these changes appeared to be more pronounced.

Principal components (PC) were used to represent RNA-seq results correlated with numerous CMR markers pre and post valve replacement. PC3 was negatively correlated with the pre-operative markers of myocardial contraction left atrial and left ventricular ejection fraction (LAEF and LVEF respectively), (p< 0.001), and positively with the perfusion parameters rest MBF (endo/epi) ratio and stress MBF (endo/epi) ratio respectively. Interestingly, the post-operative correlations in PC3 for LAEF and rest MBF (endo/epi) ratio were directionally opposite, and GLS correlated with PC1 and PC2. The reasons for these differential associations are unclear and require further exploration.

In another patient cohort of newly diagnosed patients with HFrEF, PC1 was correlated with LV mass, LV end-diastolic volume index and the visual presence or absence of inducible ischaemia in patients with Severe-AS going for valve replacement surgery. PC1 also correlated with gender, and four of the five genes most associated with PC1 in loading plots were found on sex chromosomes; XIST (on X), UTY, KDM5D, and RPS4Y1 (on chromosome Y); HLA-B, one of the human leukocyte antigens involved in antigen recognition also substantially contributed to PC1. Correlations between PCs and CMR parameters were also noted.

Differences in the blood transcriptome of people with HFrEF with versus without T2D were observed, in addition to many blood transcriptomic correlates of cardiac function and disease. Further exploration of these may help to define the biological phenomena underpinning CMR traits.

The proof-of-concept work in this thesis showed that advanced imaging techniques can be used alongside transcriptomics to define the phenotypic characteristics of heart disease in different forms with concomitant T2D. Further work is needed to identify potential biomarkers from the differentially expressed genes (DEGs) that can be used for prognostication in this high-risk population.