Abstract
Down Syndrome (DS) is one of the most common human forms of human aneuploidy occurring in 1 in 700 live births. It is caused by a third copy of human chromosome 21 (Hsa21). One of the many phenotypes of DS is craniofacial dysmorphology characterised by micrognathia, brachycephaly, and mid-facial hypoplasia. The genetic cause of the dysmorphology is unknown but is likely due to an increased dosage of one or more of the ~230 protein-coding genes on Hsa21. The identification of such causative genes will lead to a better understanding of DS aetiology, morphogenetic mechanisms and normal skull development, as well helping to find potential therapeutic targets.Initially I analysed the craniofacial skeleton of the Dp1Tyb mouse model of DS which has a duplication of a region of Mmu16 containing 148 Hsa21-orthologous genes. This was extended to analysis of a series of mouse strains with shorter duplications of sub-regions of this 148-gene region on Mmu16 to search for causative dosage-sensitive genes. Both a landmark-based and novel landmark-free method were used to look for morphometric changes in adult skulls from these mouse strains. In Dp1Tyb mice, the high-resolution landmark-free computational pipeline detected the same dysmorphologies as the landmark-based method, but revealed additional DS-specific dysmorphologies, including changes in the skull base. These led to the discovery of aberrant fusion of the synchondroses, a phenotype that had not previously been characterized in mouse models of DS but had previously been observed in individuals with DS.
Genetic mapping of the DS phenotypes using morphometric analysis of the mapping panel demonstrated that causative genes are located within four distinct sub-regions of Mmu16, implying that increased dosage of at least four different genes causes the craniofacial dysmorphology. One of these causative genes was confirmed by genetic rescue experiments to be Dyrk1A, a gene previously implicated in neuronal DS phenotypes.
To understand the aetiology of the phenotype, Dp1Tyb skulls from earlier developmental stages were analysed. These indicated that bones of neural crest (NC) origin were much more severely dysmorphic than others, suggesting an NC origin for the phenotypes as a whole. Histological analysis of the NC-derived frontal bones at embryonic day 13.5 revealed a deficit in cell numbers. In contrast to previous reports in other models of DS, I found no defect in migration of Dp1Tyb neural crest cells. However, it remains possible that defects in NC delamination, differentiation or proliferation may be causing the craniofacial dysmorphology. I also detected a Dyrk1a-independent decrease in osteogenic marker gene expression, suggesting this as at least one contributory mechanism.
Overall, the work in this thesis introduces a new morphometrics pipeline, maps the craniofacial DS phenotype to four specific chromosomal sub-regions, identifies Dyrk1a as one of the causative genes and implicates neural crest deficits as the key mechanism. Candidate causative genes and their relationships to other DS phenotypes are discussed.
| Date of Award | 1 Oct 2021 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jeremy Green (Supervisor) & Victor L. Tybulewicz (Supervisor) |