Taysha Rett Syndrome Community Update - International Rett Syndrome Foundation

From IRSF:
2022 is a big year for Rett syndrome: the return of an in-person Family Summit, a successful EL-PFDD meeting to share the voice of the community with the FDA, ACADIA will submit a new drug application to the FDA to request approval of trofinetide as the first medication specifically targeting Rett syndrome, and now the first-ever gene therapy trial in Rett syndrome has been announced by Taysha Gene Therapies.

All this success is the culmination of decades of work, advocacy, and dedication from IRSF and all of you in the community. You and your loved one with Rett remain the focus of all the work we do. Your fundraising efforts allowed us to invest in promising researchers throughout the years. Back in 2007, IRSF saw promise in a young post-doc researcher named Steven Gray. We funded his work as a trainee and helped support his work in Rett syndrome gene therapy as he started his independent lab.

“We continue to explore approaches to treat Rett syndrome by gene therapy, and that future is looking brighter. It was a fellowship from IRSF in 2007 that started me working on gene therapy for Rett syndrome, for which I’ll always be thankful.” – Dr. Steven Gray

And now today, through continued dedication to gene therapy work, Dr. Gray’s work has been advanced by Taysha Gene Therapies to a Clinical Trial in adult women with Rett syndrome. IRSF is proud to be working with Taysha in their development program and we are excited to hear more about their Rett program at the ASCEND 2022 Rett Syndrome National Summit. We are honored to have Taysha as the presenting sponsor of this event.

Every advancement comes with more urgency and more questions. We stand with you today, tomorrow, and every day as we fight for a world without Rett syndrome.

March 30, 2022

Dear Rett Community,

We are pleased to share that upon review of the Clinical Trial Application (CTA) for the Phase 1/2 clinical trial of TSHA-
102, our investigational gene therapy for adult females with Rett syndrome, Taysha has received notification of a
“No Objections Letter” from Health Canada. This means that Taysha is cleared to proceed with the clinical trial.

As you are aware, in most patients with Rett, there is a mutation in the MECP2 gene that tells cells how to make a
protein called Methyl-CpG-binding protein 2 (MeCP2). This protein plays an important part in the development of the
brain throughout childhood, in maintaining normal brain function and communicating between nerve cells. In Rett
patients with a MECP2 gene mutation, the body does not make enough or properly make MeCP2.

TSHA-102 is designed to deliver a functional MECP2 gene to the brain in patients with Rett syndrome using a novel
technology platform, called miRARE, developed by Drs. Sarah Sinnet and Steven Gray of UT Southwestern
Medical Center. TSHA-102 will be tested in a Phase 1/ 2 trial to determine whether the functional MECP2 gene can
be packaged with our technology to replace MeCP2 and safely control the level of MeCP2 protein expression. Approval
of the TSHA-102 clinical trial plans by Health Canada is an important milestone, and we are encouraged by
the progress made towards initiating a clinical trial.

“Today’s CTA clearance is a pivotal moment for the Rett community, and it would not have been
possible without the tireless work of my Taysha Colleagues, our partners at UTSW, the Rett Clinical
and Scientific experts with whom we collaborate, and most importantly the Rett syndrome
community. Initiation of a clinical trial is a formative moment, and we are motivated by the
patients and families whom we serve and the potential of TSHA-102. Thank you to the patient
advocacy leaders for your partnership and the patients and caregivers for your continued support.”
-Suyash Prasad, MBBS, M.Sc., MRCP, MRCPCH, FFPM,
Pediatrician, Chief Medical Officer and Head of Research & Development at Taysha Gene Therapies

We are grateful for the continued support of the Rett syndrome community and our Rett advocacy partners. We
understand that you may have many questions about the trial and plans for future studies. We look forward to
providing further updates about the TSHA-102 clinical trial to the global patient organizations as new information
becomes available.