Genetic Penetrance of Macular Telangiectasia Type 2.

Importance: The apparent genetic penetrance of macular telangiectasia type 2 (MacTel) is important for gene discovery studies and for clinical risk assessment of affected individuals' family members. Objective: To determine the genetic penetrance of MacTel. Design, Setting, and Participants: Descriptive cross-sectional study of patients with MacTel at a tertiary referral eye center. From 2008 to 2016, consecutive patients with MacTel were independently identified, and all of their available siblings and parents were recruited. Seventeen probands with MacTel were included in the study who satisfied the requirement of having at least 1 parent or sibling willing and able to participate. Data from these 17 families were included for the analysis of apparent genetic penetrance. Main Outcomes and Measures: Determination of MacTel genetic penetrance in probands' parents and siblings. Results: Of 80 study participants, 50 (62.5%) were women. The mean (SD) age of study participants with MacTel was 61.2 (14.0) years (range, 23-81 years) and without MacTel was 60.7 (16.4) years (range, 24-92 years). There were 17 MacTel probands, and there was a high rate of enrollment of living siblings and parents: 52 of 71 living siblings (73%) and 11 of 12 parents (92%). Of 52 enrolled siblings, 9 (17%) were affected. Of 11 enrolled parents, 3 (27%) had MacTel. Apparent genetic penetrance was calculated to be 0.35 (95% CI, 0.14-0.6) by sibling analysis and 0.55 (95% CI, 0.02-1.00) by parent analysis. Combining the sibling and parent analyses, the apparent penetrance was calculated to be 0.38 (95% CI, 0.19-0.57). Conclusions and Relevance: The genetic penetrance of MacTel in rigorously phenotyped multiple large families is described. Families such as these could be critical for successful identification of MacTel genes.