Genetic Analysis of Macular Telangiectasia
Macular telangiectasia type 2, or MacTel, is an adult onset retinal disease that causes progressive loss of central vision, usually beginning between the 5th and 7th decades of life. Macular degenerative diseases comprise a large portion of the blinding diseases affecting people over the age of 50. The collective burden of vision loss and low vision includes the impact on patients in loss of independence and decline of self-reported quality of life, as well as significant resource allocation in medical and supportive care. There are no cures for macular diseases; currently available treatments may slow the progression in some cases, but patients remain at risk of losing their eyesight and the independence that vision affords. While macular telangiectasia is not as prevalent as other macular degenerative diseases, such as age-related macular degeneration, elucidating the causes of MacTel may reveal mechanisms that are common to other macular diseases, providing clues to guide research into possible treatment targets. In this work, I have assembled the first cohort of MacTel patients and their family members in collaboration with the MacTel Project, an international consortium of researchers and clinicians dedicated to studying this disease. I have identified and analyzed families with multiple affected relatives in which MacTel appears to be an inherited disease. The goal of this work has been to identify a genetic cause for the disease in these families. To that end, I used genotyping data from approximately 900,000 single nucleotide polymorphism per individual to create sets of genetic markers for linkage analysis. I analyzed these markers using two-point and multipoint linkage algorithms to search for a region of the genome that was inherited in conjunction with the phenotype more often than would be expected by chance alone, with the goal of finding a causative genetic variant. This analysis resulted in the identification of a region of chromosome 1 spanning approximately 15 million bases that is significantly linked to the MacTel phenotype. I analyzed recombination breakpoints in this region, as well as across the genome, to map chromosomal segments inherited identical by descent in siblings that express the MacTel phenotype. This approach provided a profile for each set of siblings of regions of the genome incompatible with inherited genetic disease by virtue of the fact that no alleles were shared identical by descent. I then compared these regions across families under the hypothesis that if MacTel is caused by the same gene in all families, the chromosomal region harboring a causative variant would fail to be excluded in any family. This analysis excluded much of the genome and refined the boundaries of the linkage interval on chromosome 1, but did not appreciably narrow the region of interest. In parallel with analysis of linkage in families, I sequenced 40 genes of interest as possible functional candidates, using Sanger sequencing in the probands of families with multiple affected relatives. I selected candidate genes based on gene function and expression, choosing genes with a plausible connection to the phenotype based on information in the literature. Thirteen of these genes were selected based on their location in the region of the maximum linkage score. I was unable to identify any variant that was consistent with being a causative variant for inherited MacTel.
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