Rett syndrome (RTT) is an X-linked dominant neurodevelopmental disorder primarily affecting female infants and young children, with an incidence of approximately 1 in 10,000 to 15,000 females. Characteristic clinical features include intellectual disability, loss of language skills, stereotypic hand movements, and gait disturbances. Affected individuals typically experience a period of normal development, followed by deceleration in head circumference growth between 6 to 18 months of age, and subsequent regression of acquired motor and cognitive abilities. Overt impairments in cognition and motor function generally emerge within 1 to 2 years. Mutations in the methyl-CpG-binding protein 2 (MECP2) gene are responsible for over 90% of RTT cases. MECP2 is a nuclear protein that binds methylated DNA to modulate gene transcription. MECP2 gene duplications lead to MECP2 duplication syndrome (MDS), while MECP2 deficiency disrupts central nervous system maturation, adversely affecting learning and memory, culminating in the clinical manifestations of RTT.

Current therapeutic strategies for RTT primarily revolve around gene supplementation using adeno-associated virus (AAV) vectors to deliver functional human MECP2 genes to compensate for the endogenous deficiency. However, the substantial size of the MECP2 gene surpasses the packaging capacity of most viral vectors, and overexpression of MECP2 poses a risk of severe neurological complications. These challenges have significantly impeded the progress of gene supplementation therapies. Consequently, the focus has shifted towards DNA/RNA editing approaches aimed at correcting MECP2 mutations and restoring physiological levels of MECP2 protein expression. Notably, several research groups have successfully employed CRISPR-based gene editing technologies to rectify MECP2 mutations in induced pluripotent stem cells (iPSCs) or patient-derived cells ex vivo ^[1-2]. Given the pivotal role of animal models in preclinical research, the development of humanized mouse models expressing the human MECP2 gene is crucial. These models facilitate the transition of gene therapy candidates—encompassing small nucleic acids, CRISPR-based editors, base editors, and RNA editing technologies—into clinical stages ^[3-4].

This strain is a humanized MECP2 gene mouse model, generated by replacing the endogenous mouse Mecp2 gene with the human MECP2 gene harboring the T158M mutation through embryonic stem cell targeting techniques. This mutation represents the most common human RTT-associated missense mutation in MECP2. Studies have shown that mice carrying this mutation recapitulate many clinical features of RTT ^[5].