Gene therapy is a promising approach to the treatment of various inherited diseases, but its development is complicated by a number of limitations of the natural promoters used. The currently used strong ubiquitous natural promoters do not allow for the specificity of expression, while natural tissue-specific promoters have lowactivity. These limitations of natural promoters can be addressed by creating new synthetic promoters that achieve high levels of tissue-specific target gene expression. This review discusses recent advances in the development of synthetic promoters that provide a more precise regulation of gene expression. Approaches to the design of synthetic promoters are reviewed, including manual design and bioinformatic methods using machine learning. Examples of successful applications of synthetic promoters in the therapy of hereditary diseases and cancer are presented, as well as prospects for their clinical use.

The liver is a primary metabolic hub and a pivotal target for gene therapy, owing to its capacity for protein secretion, role in metabolic homeostasis and immune tolerance. Liver-directed gene therapies are used to treat numerous inherited metabolic disorders and coagulation factor deficiencies including hemophilia (A and B), Crigler–Najjar syndrome, mucopolysaccharidoses, phenylketonuria, Fabry, Gaucher, Wilson and Pompe diseases. The efficacy and safety of liver-directed gene therapy rely on the use of strong tissue-specific promoters. To date, there are many different liver-specific promoters used in preclinical and clinical studies, including novel completely synthetic promoters. This review provides a comprehensive analysis of the design, engineering and application of liver-specific promoters. Furthermore, we discuss fundamental principles of gene expression regulation in the liver and the physiological and immunological characteristics that make it a suitable target organ for gene therapy delivery.

INTRODUCTION. The creation of synthetic adeno-associated virus (AAV) vectors during gene therapy development is a labour-intensive and expensive process. The optimal solution to minimise the time and costs associated with gene therapy development lies in the improvement of methods aimed at assessing AAV vector biodistribution and transduction efficiency in vivo.AIM. This study aimed to develop a new bioinformatics-based assessment method for synthetic AAV vector libraries to analyse AAV vector biodistribution and transduction efficiency in vivo.MATERIALS AND METHODS. The production of synthetic AAV vectors involved assigning AAV serotype-specific barcodes (12-nucleotide tags flanked at the 5' end with a sequence encoding the green fluorescent reporter protein). Plasmids carrying unique barcodes were propagated in competent Escherichia coli XL10-Gold cells and used to create two AAV libraries: L1 with a viral genome count of 10¹⁰ and L2 with a viral genome count of 10¹¹. AAV production involved HEK293T cell transfection. L1 and L2 library vectors were administered to C57Bl/6N mice by intravenous injection. DNA and RNA were isolated from transduced organs for analysis by next-generation sequencing. The obtained data on DNA and RNA barcode quantities in different murine organs were analysed to assess the biodistribution and transduction efficiency of synthetic AAVs. Barcodes were identified by aligning them to the expected sequences and counted. The resulting values were normalised to the quantity of barcodes in the original library.RESULTS. Seven viral constructs based on different AAV serotypes were created as part of two AAV libraries. Six of the AAV serotypes were synthetic (sAAV1, sAAV2, sAAV3, sAAV4, sAAV5, and sAAV6). Sequencing of murine organ samples revealed significant quantities of DNA barcodes from both AAV libraries in all organs except the brain. For the L1 library, RNA barcodes were detected at a sufficient level in 4 organs, including the skeletal muscles, the heart, the liver, and the adrenal glands. For the L2 library, in addition to the listed organs, sufficient RNA-barcode levels were observed in the gonads and the kidneys. According to transduction efficiency analysis based on RNA barcode levels adjusted for DNA barcodes, sAAV5 was considered the most promising variant for gene therapy of liver-related diseases, whereas sAAV2 and sAAV6 were recognised as holding the most promise for adrenal diseases.CONCLUSIONS. The developed bioinformatics-based assessment method for synthetic AAV vector libraries can analyse AAV vector biodistribution and transduction efficiency in the body. The presented approach has the potential for selecting optimal AAV vectors for specific organs and tissues in further gene therapy development.

Coagulation factor VIII (FVIII) is a key therapeutic protein for hemophilia A, an inherited bleeding disorder caused by FVIII deficiency or dysfunction. The development and production of FVIII-based therapeutics remains challenging due to the intrinsic complexity of FVIII biosynthesis, folding, secretion, and functional stability. Advances in the understanding of the FVIII life cycle, as well as studies of its natural homologs, have created opportunities to develop improved FVIII variants. In parallel, therapeutic strategies to restore FVIII activity have evolved from replacement therapy with exogenous FVIII to gene-based delivery of a functional F8 copy. Among these approaches, AAV-mediated gene therapy has emerged as a clinically viable platform with a generally favorable safety profile, prompting the development of multiple therapeutic candidates for hemophilia A. This review summarizes established and current knowledge of FVIII structure, biosynthesis, and immunology in the context of developing effective and clinically applicable AAV-mediated therapies for hemophilia A. Particular attention is given to engineered FVIII variants and the latest clinical findings in AAV gene therapy, as well as to long-term efficacy and safety.