ATSN-201 Gene Therapy in RS1-Associated X-linked Retinoschisis

The study is designed in three parts: a dose escalation phase (Part A), a dose expansion phase (Part B) and a randomized, controlled phase (Part C). In Part C of the study, eligible patients who enroll in this study will be randomly assigned to be treated with ATSN-201 or to have no treatment; subjects assigned to ATSN-201 will receive the drug as a one-time subretinal injection of ATSN-201 in one eye or both eyes, depending on whether only one or both eyes meet criteria for treatment. Subjects will have regular assessments for 1 year as part of the Main Study Period and additional assessments over the next 4 years as part of the Extension Study Period. Some subjects may have all their study visits at a surgery site. Some subjects may go to one study site (a medical site) to determine eligibility and another study site (a surgery site) to have surgery - including pre-operative care and approximately 1-week of post-operative care per treated eye. After the surgery, they will go back to the other study site (the medical site) to complete the follow-up visits. Subjects who do not receive treatment as part of the control group can choose to receive ATSN-201 in one or both eyes after the 1-year Main Study Period if eligible. If the subject is not eligible at the Month 12 visit, eligibility may be reassessed at the Month 18 visit.