Implementing the human artificial chromosome gene therapy platform remains challenging, but continuous animal model research will advance the platform closer to clinical trials

A normal degree of ectopic gene expression, infinite retention in target cells without chromosomal integration, minimal risk of cell or neoplastic transformation, and minimal or no immunogenicity are all critical characteristics for vectors employed in gene therapy. HACs were produced and used as autonomous vectors to compensate for genetic defects in mouse and human cell cultures. Bottom-up human artificial chromosomes (HACs) were studied for functional transgene expression in vitro and in vivo mice models. The primary advantages of synthesized alphoid-HACs over top-down HACs are their defined and documented structure, as well as their relative simplicity of modification in adding numerous Cre-lox-type transgen loading sites. The HAC transfer method's efficacy has greatly increased in recent years. Despite significant progress in developing alphoid-HAC-based gene therapy models, the technology still has a number of drawbacks, including low HAC efficiency, complex repeated HAC alphoid-DNA structure, large DNA fragmentation difficulties outside eukaryotic cells, inefficient transfer of chromosomes to target cells, and variable mitotic stability. The quantity and quality of PSC-derived or reversibly immortalized stem/precursor cells that can transplant specific tissues are also critical determinants in the effectiveness of HAC-based tissue replacement therapies. Translating the HAC-based gene therapy platform remains difficult, but ongoing animal model research will move the HAC platform closer to clinical trials.

CENP-B creates alternative epigenetic chromatin states permissive for CENP-A or heterochromatin assembly

ABSTRACTCENP-B binds to CENP-B boxes on centromeric satellite DNAs (known as alphoid DNA in humans). CENP-B maintains kinetochore function through interactions with CENP-A nucleosomes and CENP-C. CENP-B binding to transfected alphoid DNA can induce de novo CENP-A assembly, functional centromere and kinetochore formation, and subsequent human artificial chromosome (HAC) formation. Furthermore, CENP-B also facilitates H3K9 (histone H3 lysine 9) trimethylation on alphoid DNA, mediated by Suv39h1, at ectopic alphoid DNA integration sites. Excessive heterochromatin invasion into centromere chromatin suppresses CENP-A assembly. It is unclear how CENP-B controls such different chromatin states. Here, we show that the CENP-B acidic domain recruits histone chaperones and many chromatin modifiers, including the H3K36 methylase ASH1L, as well as the heterochromatin components Suv39h1 and HP1 (HP1α, β and γ, also known as CBX5, CBX1 and CBX3, respectively). ASH1L facilitates the formation of open chromatin competent for CENP-A assembly on alphoid DNA. These results indicate that CENP-B is a nexus for histone modifiers that alternatively promote or suppress CENP-A assembly by mutually exclusive mechanisms. Besides the DNA-binding domain, the CENP-B acidic domain also facilitates CENP-A assembly de novo on transfected alphoid DNA. CENP-B therefore balances CENP-A assembly and heterochromatin formation on satellite DNA.