Taysha Gene Therapies Announces Pivotal Part B Trial Design Details for TSHA-102 in Rett Syndrome Enabled by IRSF Natural History Data and Positive Clinical Data from Part A of the REVEAL Adult/Adolescent and Pediatric Trials Evaluating TSHA-102

About this update from Taysha Gene Therapies, Inc.

Natural history data analysis established patients ≥ six years of age are in developmental plateau, with a ~0% likelihood of gaining/regaining developmental milestones across the core functional domains of Rett syndrome Written alignment from FDA supports single-arm, open label pivotal trial with primary endpoint of developmental milestone gain/regain in the developmental plateau population (≥ 6 years, intend N=15) with each patient serving as their own control 100% of patients in REVEAL Part A (N=10, 6-21 years) gained/regained ≥ one developmental milestone post-TSHA-102 with concordant improvements across multiple outcome measures; high dose consistently outperformed low dose, with dose-dependent effects deepening over time No treatment-related SAEs or DLTs following low dose and high dose of TSHA-102 FDA advised the Company to submit pivotal Part B trial protocol and SAP as an amendment to the IND application, which is expected to occur in the current quarter; pivotal trial initiation activities anticipated in Q3 2025 DALLAS , May 28, 2025 (GLOBE NEWSWIRE) -- Taysha Gene Therapies, Inc. (Nasdaq: TSHA) (Taysha or the Company), a clinical-stage biotechnology company focused on advancing adeno-associated virus (AAV)-based gene therapies for severe monogenic diseases of the central nervous system (CNS), today announced details of its planned pivotal Part B trial design for TSHA-102 following written alignment from the U.S. Food and Drug Administration (FDA). Additionally, the Company announced positive clinical data from Part A of the REVEAL Phase 1/2 adolescent/adult and pediatric trials evaluating TSHA-102 in Rett syndrome. The alignment reached with the FDA was supported by the Company’s analysis of the International Rett Syndrome Foundation’s (IRSF) longitudinal Rett syndrome natural history study data, as well as clinical data from the ongoing REVEAL Phase 1/2 trials. “Our rigorous analysis of the robust natural history study dataset demonstrated that after six years of age, the likelihood of achieving defined developmental milestones across the core functional domains of Rett syndrome is highly improbable. Therefore, it is quite striking that we observed a 100% responder rate following treatment with TSHA-102, with all pediatric, adolescent and adult patients across varying disease severity gaining or regaining one or more developmental milestone that represents activities of daily living that are important to caregivers and clinicians. We believe this objective and clinically meaningful primary endpoint has the potential to redefine treatment expectations and expand the possibilities of gene therapy for patients with Rett syndrome,” said Sean P. Nolan , Chairman and Chief Executive Officer of Taysha. “We believe aligning with the FDA on key elements of our proposed pivotal trial design validates these important findings that underpin our pivotal trial design and strengthen our conviction in the transformative potential of TSHA-102. Importantly, this progress sets us on an efficient and expeditious path to potentially deliver TSHA-102 to patients and families suffering from this devastating disease with high unmet need. We plan to submit the pivotal trial protocol and SAP as an amendment to the IND application this quarter and anticipate initiating this pivotal program in the third quarter of 2025.” Dr. Jeffrey Neul , M.D., Ph.D., Rett Specialist who served as Administrative Head of the Rett Syndrome Natural History Study added, “This innovative analysis and cumulative incidence models of the Rett syndrome natural history dataset help shape our understanding of the disease trajectory by establishing that the likelihood of gain and regain of developmental milestones is predictable after six years of age. Importantly, these data allow us to objectively measure how transformative therapies impact functional aspects of the disease that are essential to activities of daily living. I believe these insights uncovered via the collaboration of academic researchers and a patient advocacy group with industry partner, Taysha, will be instrumental in shaping the future of therapeutic development for Rett syndrome.” Laura Hameed , Chief Executive Officer of the International Rett Syndrome Foundation (IRSF), added, “On behalf of IRSF, I want to thank the families, clinicians, and researchers whose dedication made the Rett syndrome Natural History Study possible. Their participation provided a deeper understanding of how Rett progresses over time, insights that are now helping shape meaningful clinical outcome measures and could someday lead to new treatments. We’re pleased to support Taysha’s efforts to build on this foundation and are hopeful about the progress it represents for families living with Rett syndrome.” Analysis of IRSF’s Longitudinal Rett Syndrome Natural History Study Data Supported Taysha’s Pivotal Trial Design for TSHA-102 N = ~1100 females with confirmed Rett syndrome diagnosis, with up to 14 years follow-up, representing the largest available Rett syndrome natural history study dataset Developed age- and time-based cumulative incidence models of longitudinal natural history data across 28 developmental milestones in the core functional domains of fine motor, gross motor and communication Findings demonstrated patients ≥ six years of age have reached a developmental plateau, with an exceedingly low (0% to

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