The primary objectives of this study are to evaluate the safety of a single intrathecal (IT) dose of TSHA-102 in females with typical Rett syndrome, to select the TSHA-102 dose with the best benefit/risk profile based on the totality of safety and efficacy data and to evaluate the efficacy and safety of TSHA-102 at the selected dose.

REVEAL Part A (Phase 1/2) is an open-label safety and dose-finding study designed to evaluate the safety and preliminary efficacy of two dose levels of TSHA-102 to establish initial safety of TSHA-102 and select a safe and efficacious dose for further evaluation. Enrollment of 6 participants in Part A is complete.

REVEAL Part B (Phase 3) will evaluate the efficacy and safety of TSHA-102 at the dose level 2 determined in Part A in 15 females ages 6 to <22 years with typical Rett syndrome. TSHA-102 is designed to target the genetic root cause of Rett syndrome by regulating the expression of MECP2 in cells.

Each participant will be followed for the observation period of 5 years after TSHA-102 administration in Part A and B.

Inclusion Criteria:

• Females between the ages of 12 and <22 in Part A (closed) and females between the ages of 6 and <22 in Part B (pivotal cohort).
• Participant has a clinical diagnosis of classic/typical Rett syndrome with a documented pathogenic mutation of the methyl-CpG-binding protein 2 (MECP2) gene that results in loss of gene function.
• Participants must be willing to receive blood or blood products for the treatment of an AE if medically needed.
• Participants and parent/caregiver must agree to reside within easy access to the study site prior to the baseline visit and at least 3 months after TSHA-102 treatment

Exclusion Criteria:

• Participant has another neurodevelopmental disorder independent of the MECP2 loss-of-function mutation, or any other genetic syndrome with a progressive course.
• Participant has a history of brain injury that causes neurological problems or had grossly abnormal psychomotor development in the first 6 months of life.
• Participant has a diagnosis of atypical Rett syndrome or a MECP2 gene mutation that does not cause Rett syndrome.
• Participant requires invasive ventilatory support.

Note: Other protocol defined inclusion/exclusion criteria may apply

Central raters assessing the gain or regain of a developmental milestone from videos are blinded to the timing of each video.