Genetic and phenotypic heterogeneity in autosomal recessive retinal disease

Molecular genetics has transformed our understanding of disease and is gradually changing the way medicine is practiced. Genetic mapping provides a powerful approach to discover genes and biological processes underlying human disorders. Recent advances in DNA microarray and sequencing technology have significantly increased the power of genetic mapping studies and have ushered in a new era for biomedicine. In this thesis, linkage analysis (including homozygosity mapping), exome sequencing and candidate gene sequencing have been utilised to genetically dissect autosomal recessive retinal disease. Subsequently, clinical findings from patients found to be similar in terms of molecular pathology have been pooled. DNA and basic phenotypic data from over 500 unrelated individuals were available for the project. Disease-causing variants in three genes that have not been previously associated with human recessive disorders are reported: (a) biallelic mutations in TRPM1 abrogate ON bipolar cell function and cause complete congenital stationary night blindness; (b) biallelic mutations in KCNJ13, a gene encoding an inwardly rectifying potassium channel subunit cause Leber congenital amaurosis; (c) biallelic mutations in PLA2G5, a gene encoding group V phospholipase A2, cause benign fleck retina. The consequences of mutations in these and other disease-related genes (RDH5, GRM6, KCNV2, OAT and SAG) on retinal structure (spectral domain optical coherence tomography, fundus autofluorescence imaging) and visual function (electrophysiology, perimetry testing) have been studied; features that may have mechanistic relevance have been identified. Additionally, DNA sequence variation of a highly polymorphic gene (C2ORF71), recently associated with photoreceptor degeneration, has been studied and quantified in patient and control samples. Basic bioinformatics tools to analyse genomic data have been developed (bash, perl, python and R programming languages). Overall, results presented in this thesis contribute to an understanding of Mendelian retinal disease that is not only observational but also mechanistic

Phenome-wide Mendelian randomisation analysis identifies causal factors for age-related macular degeneration

Background: Age-related macular degeneration (AMD) is a leading cause of blindness in the industrialised world and is projected to affect >280 million people worldwide by 2040. Aiming to identify causal factors and potential therapeutic targets for this common condition, we designed and undertook a phenome-wide Mendelian randomisation (MR) study. Methods: We evaluated the effect of 4591 exposure traits on early AMD using univariable MR. Statistically significant results were explored further using: validation in an advanced AMD cohort; MR Bayesian model averaging (MR-BMA); and multivariable MR. Results: Overall, 44 traits were found to be putatively causal for early AMD in univariable analysis. Serum proteins that were found to have significant relationships with AMD included S100-A5 (odds ratio [OR] = 1.07, p-value = 6.80E−06), cathepsin F (OR = 1.10, p-value = 7.16E−05), and serine palmitoyltransferase 2 (OR = 0.86, p-value = 1.00E−03). Univariable MR analysis also supported roles for complement and immune cell traits. Although numerous lipid traits were found to be significantly related to AMD, MR-BMA suggested a driving causal role for serum sphingomyelin (marginal inclusion probability [MIP] = 0.76; model-averaged causal estimate [MACE] = 0.29). Conclusions: The results of this MR study support several putative causal factors for AMD and highlight avenues for future translational research

Pathogenic variants in IMPG1 cause autosomal dominant and autosomal recessive retinitis pigmentosa

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Retinal Structure and Function in Achromatopsia : Implications for Gene Therapy

Purpose: To characterize retinal structure and function in achromatopsia (ACHM) in preparation for clinical trials of gene therapy. Design: Cross-sectional study. Participants: Forty subjects with ACHM. Methods: All subjects underwent spectral domain optical coherence tomography (SD-OCT), microperimetry, and molecular genetic testing. Foveal structure on SD-OCT was graded into 5 distinct categories: (1) continuous inner segment ellipsoid (ISe), (2) ISe disruption, (3) ISe absence, (4) presence of a hyporeflective zone (HRZ), and (5) outer retinal atrophy including retinal pigment epithelial loss. Foveal and outer nuclear layer (ONL) thickness was measured and presence of hypoplasia determined. Main Outcome Measures: Photoreceptor appearance on SD-OCT imaging, foveal and ONL thickness, presence of foveal hypoplasia, retinal sensitivity and fixation stability, and association of these parameters with age and genotype. Results: Forty subjects with a mean age of 24.9 years (range, 6e52 years) were included. Disease-causing variants were found in CNGA3 (n [ 18), CNGB3 (n ¼ 15), GNAT2 (n ¼ 4), and PDE6C (n ¼ 1). No variants were found in 2 individuals. In all, 22.5% of subjects had a continuous ISe layer at the fovea, 27.5% had ISe disruption, 20% had an absent ISe layer, 22.5% had an HRZ, and 7.5% had outer retinal atrophy. No significant differences in age (P ¼ 0.77), mean retinal sensitivity (P ¼ 0.21), or fixation stability (P ¼ 0.34) across the 5 SD-OCT categories were evident. No correlation was found between age and foveal thickness (P ¼ 0.84) or between age and foveal ONL thickness (P ¼ 0.12). Conclusions: The lack of a clear association of disruption of retinal structure or function in ACHM with age suggests that the window of opportunity for intervention by gene therapy is wider in some individuals than previously indicated. Therefore, the potential benefit for a given subject is likely to be better predicted by specific measurement of photoreceptor structure rather than simply by age. The ability to directly assess cone photoreceptor preservation with SD-OCT and/or adaptive optics imaging is likely to prove invaluable in selecting subjects for future trials and measuring the trials’ impact