Prospective study design and data analysis in UK Biobank

Population-based prospective studies, such as UK Biobank, are valuable for generating and testing hypotheses about the potential causes of human disease. We describe how UK Biobank's study design, data access policies, and approaches to statistical analysis can help to minimize error and improve the interpretability of research findings, with implications for other population-based prospective studies being established worldwide.</p

The role of capillary pericytes in cerebral blood flow changes during ischaemia and in Alzheimer’s disease

Recently, it was found that brain capillary pericytes play a significant role in controlling cerebral blood flow. Pericytes may therefore also play an important pathological role when cerebral blood flow is compromised, both acutely, as occurs during ischaemic stroke, and chronically, as occurs in Alzheimer’s disease. In this thesis I have investigated the role of brain capillary pericytes as follows. (1) Ischaemia: Pericytes have been suggested to constrict capillaries and subsequently die in rigor during ischaemia, making them a therapeutic target after acute stroke when a long-lasting decrease of cerebral blood flow occurs despite re-opening of the occluded artery. I confirmed that pericytes constrict and die in rigor during ischaemia and demonstrated that the L-type Ca2+ channel blocker nimodipine inhibits ischaemia-induced pericyte constriction. (2) Alzheimer’s disease (AD): Vascular compromise occurs early in AD and amyloid β (Aβ) has been shown to reduce cerebral blood flow. In the cerebral vasculature most resistance is in capillaries, so Aβ might primarily act on contractile capillary pericytes. I used live and fixed human tissue to establish disease-relevance, and rodent experiments to define mechanism, to show that Aβ constricts brain capillaries at pericyte locations. Applying soluble Aβ oligomers to live human cortical tissue constricted capillaries by 25%. Using rat cortical slices this was shown to reflect Aβ evoking capillary pericyte contraction, with an EC50 of 4.7 nM, by generating reactive oxygen species and activating endothelin ET-A receptors. In freshly-fixed diagnostic biopsies from human patients investigated for cognitive decline, mean capillary diameters were 8.1% less in patients showing Aβ deposition than in patients without Aβ deposition. For patients with Aβ deposition, capillary diameter was 31% less at pericyte somata than away from the somata, predicting a halving of blood flow. Constriction of capillaries by Aβ will contribute to the cerebral energy deficit occurring in AD, which promotes further Aβ generation

Genetics of Cerebral Small Vessel Disease

Cerebral small vessel disease (SVD) is a leading cause of stroke and vascular dementia. The majority of cases are sporadic, occurring in the elderly hypertensive population. However, there also exist patients with familial disease. The most common form is Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy (CADASIL), caused by mutations in the NOTCH3 gene. In recent years, other genes have also been found to cause familial SVD, such as COL4A1/A2, HTRA1, FOXC1 and TREX1. Genome wide association studies (GWAS) have also revealed loci associated with sporadic SVD strokes and its related features. This thesis explores the genetic basis of SVD primarily from the angle of the ‘one gene, one disease’ hypothesis. We explore the phenotype of familial SVD using CADASIL as a prototype. We next adopt a candidate gene approach to rare variant discovery using high throughput sequencing (HTS) techniques in two forms: 1) a multi-gene sequencing panel to examine the presence of rare variants in a cohort of 993 presumed-sporadic, early-onset SVD stroke patients, and 2) whole genome sequencing in 118 pedigrees with suspected familial SVD. We also evaluate the prevalence of known disease-causing mutations in the general population using a cohort of whole genome sequenced non-SVD patients, and other control databases. We demonstrate that a few presumed-sporadic SVD stroke patients may in fact have familial disease that was not previously diagnosed. We show that known and novel rare variants in candidate genes are found in our cohort of familial SVD patients, and suggest a possible role for rare variants in genes associated with related phenotypes and sporadic disease in this cohort. Finally, we identify known disease-causing variants in relatively high frequencies in the population, and show that conclusions on the pathogenicity of variants based on allele frequency and functional analyses may sometimes be misguided, thus highlighting the current limitations we face in the clinical interpretation of variants identified on HTS. In recent years genetic studies have revealed that pathways in different familial diseases are likely to converge in the pathogenesis of sporadic disease. Further uncovering the genetic basis of undiagnosed cases of familial SVD may shed light on the mechanisms underlying the sporadic form of disease, and may in turn drive the identification of potential therapeutic targets