Abstract
Down syndrome (DS) arises from trisomy of human chromosome 21 (Hsa21) and is the most common cause of congenital heart defects (CHD). Approximately half of all DS births present with a form of CHD, a significant contributor to infant mortality in the condition. These defects result from aberrant heart septation during development. However, the developmental origins of CHD in DS remain poorly understood. The Dp1Tyb mouse model of DS, containing a duplication of a region of mouse chromosome 16 orthologous to a large stretch of Hsa21, recapitulates many CHD seen in human DS. This project aimed to characterise the cardiac development of Dp1Tyb embryos by identifying morphological and cellular behaviours driving the CHD phenotype.Key stages of chamber septation between E10.5 to E13.5 were 3D reconstructed from images in Dp1Tyb hearts to form a developmental timeline. This analysis revealed no significant differences in muscular septum growth nor the closure of the interventricular communication of the Dp1Tyb embryos. Investigation into the endocardial cushions, transient structures crucial to heart septation and valve formation, revealed dysmorphic outflow tract cushions (OFTC). Underlying this phenotype, the Dp1Tyb OFTC were comprehensively quantified through cellular packing, apoptosis and proliferation through serial sections. Dp1Tyb OFTC were found to have reduced cellular packing through its structure at both stages and a decrease in proliferating cells in the proximal region. The Dp1Tyb OFTC mesenchyme occupied larger section areas at its proximal anatomy while retaining its low cell density phenotype, suggesting more extracellular matrix in this region.
To determine if excess extracellular matrix impacted the material stiffness of the OFTCs, Atomic Force Microscopy was deployed to test the structure’s stiffness, which may be the first example of the technique on fresh OFTC tissue. Due to the preliminary nature of the experiment and results, there were no conclusive findings. Exploration of mechanosensitive-YAP nuclear expression, showed a reduction in nuclear translocation in distal regions of the Dp1Tyb OFTC, suggesting altered mechanosensitive pathways.
Overall, the work presented in this thesis begins to unravel the cellular behaviours underpinning OFTC development, its contribution to ventricular septation and how this is altered in the Dp1Tyb. The data identifies these structures as probable contributors to the formation of CHD in the DS mouse model Dp1Tyb. The thesis further covers and discusses possible dysregulated pathways and causative genes from Hsa21 that contribute to the DS phenotypes, in addition to avenues for future experimentation.
| Date of Award | 1 Oct 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jeremy Green (Supervisor) & Victor L. J. Tybulewicz (Supervisor) |