80 Proposed Restoration of the ShoxA Box in Mosaic Turner Syndrome Patients Through CRISPR, HDR Pathway, and mRNA Delivery Methodologies: A Literature-Supported Approach

Faculty Mentor: Michael Werner (Biological Sciences, University of Utah)

Abstract

Turner Syndrome (TS) is one of the most enigmatic chromosomal disorders known to genetics – a condition defined not only from the loss of one X-chromosome, but by the deviation of genetic symmetry that orchestrates the development itself. Affecting approximately 1 out of 2,500 female births, TS emerges through a plethora of phenotypes, which arise from the complete loss or mosaicism of the X-chromosome. Notably, it is estimated that nearly 99% of Turner Syndrome conceptions do not survive to birth, underscoring the devastating development consequences of X-chromosome loss and the necessity for early-stage molecular intervention. Amidst the most critical genetic absences lies a specific portion within the pseudoautosomal region 1 (PAR1) Xp22.3 – the ShoxA gene. This portion of the X-chromosome is cordially labeled the “gene sink” as it is keystone in dictating cellular homeostasis, segmental growth patterns, and hormone secretion with the assistance of a variety of genes. Its deficiency or absence constitutes for a substantial amount of the Turner Syndrome phenotype; specifically at the juvenile age range, as it induces a cascade of developmental discrepancies that expand far beyond height dissimulation, mediating reproductive, cardiovascular, and neurocognitive systems – resulting in a diverse clinical spectrum as the young patients develop into adulthood and begin senescence.

Contemporary advances in CRISPR/Cas9 genome editing technology, the homology-directed repair (HDR) pathway, and mRNA-based delivery methodologies are redefining what was once considered irreversible genetic loss. By targeting the restoration of the ShoxA box within the Xp22.3 locus, these evolving technologies concurrently delineate a potential route towards the curative genome correction in mosaic TS patients that emphasizes both precise repair and high-fidelity molecular control. This dual-approach not only offers a framework for addressing chromosomal mosaicism, but also illuminates an emerging paradigm in cutting-edge, chromosomal-scale therapeutics, where architectural abnormalities may one day be restored at its roots rather than endured through management.

Scaffolding upon these foundational advancements, this research investigates how coupling CRISPR/Cas9 with the HDR pathway – delivered through transient mRNA expression – can accomplish sound and targeted restoration of the ShoxA gene in mosaic TS models. The focus of this project is on cell-line models as a fundamental first step towards clinical application on human patients, specifically through the quantification of expression in targeted phenotypes. This proposal aims to overcome one of the most persistent barriers in chromosomal editing: achieving high cellular compatibility and precision while minimizing indel formation and off-target activity. By initially working with Turners Syndrome – derived ovarian tissue cell lines, this study establishes a controlled environment to assess gene expression outcomes following ShoxA restoration – producing critical data that may later guide translation to human therapeutic applications, such as mitigating the likelihood of infertility in mosaic TS patients. By integrating HDR’s instinctive ability to facilitate exact sequence emendation and mRNA’s transient expression kinetics, this proposal contends a controllable, high- fidelity complex capable of recovering lost genetic function in the affected X-chromosome loci.

Moreover, this research analyzes the translational feasibility of these molecular interventions, emphasizing their capability to generate phenotypic rescue at the cellular level – specifically in endocrine and skeletal systems shaped by ShoxA insufficiency. Ovarian tissue, in particular, represents a key target for quantifying phenotypic rescue, given its relevance to hormone regulation and fertility in TS patients. The assimilation of human induced pluripotent stem cell (iPSC) models derived from TS patients will function as a proof-of-concept platform to evaluate the degree of genetic restoration and subsequent protein re-expression. Through the quantification of gene expression in these iPSC- derived ovarian cell models, this study aims to establish the molecular benchmarks necessary for eventual translation to aneuploid human patients. Cumulative findings from recent research may highlight the scope to which chromosomal-scale gene restoration can mitigate downstream phenotypic ramifications associated with partial monosomy.

The outcomes of this project transcend beyond Turner Syndrome, itself. The proposed dual-framework of CRISPR/HDR and mRNA-mediated delivery exemplifies a modular platform for remediating other chromosomal disorders characterized by haploinsufficiency or segmental deletions, alike. By optimizing the efficiency, precision, and transience of these respective mechanisms, this research aims to contribute toward the wider discipline of curative genomic medicine – a method that not only regulates the sequelae of chromosomal abnormalities but reinstates their molecular origin. Should the proposed framework prove successful, it stands to catalyze a paradigm shift in genomic medicine, redefining therapeutic intervention from symptom management toward curative chromosomal restoration.

Introduction/Background

The urgency of research in Turner Syndrome (TS) stems from the lack of parameters associated with the disease. The specific causation of TS is unknown – aside from the failure of chromosomes to separate during meiosis I or the failure of sister chromatids to separate during meiosis II. Thus, variation in the TS phenotypes originate from the heterogeneous patterns of X-chromosomes mosaicism during this separation process, which determine the degree of gene expression imbalance across body tissues. Therefore, it is almost guaranteed that a range of supplementary diseases and comorbidities will arise during the lifespan of an individual with TS. These include congenital cardiovascular malformations (i.e. coarctation of the bicuspid aortic valve and aorta), endocrine disorders akin to insulin resistance and hypothyroidism, gonadal dysgenesis resulting in infertility, and neurocognitive defects. Acknowledging that nearly all Turner Syndrome conceptions – approximately 99% – result in prenatal lethality, the few surviving case provide a rare and indispensable window into the mechanisms of X-linked gene compensation and chromosomal viability. These overarching exhibitions place an emphasis on the necessity for continued research in molecular and genomic mechanisms under the X-chromosome mosaicism and its downstream, systematic consequences.

Emerging evidence implies that Mosaic TS exhibits partial ShoxA gene expression in particular somatic tissues, such as gonadal and osteogenic lineages, conditional on the proportion and distribution of monosomic (45,X) versus euploid (46,XX) cells (Anastasiou et al. 2023). The partial preservation of ShoxA transcriptional activity provides a foothold for therapeutic molecular reconstitution, as residual gene expression suggests that necessary downstream regulatory networks and transcriptional architecture remain, at least, partially intact within certain cellular subpopulations. Therefore, the presence of tissue-specific ShoxA expression in mosaic TS affirms the rationale for genome-restorative therapy, as targeted repair or reintroduction could enhance the existing, functional alleles rather than requiring de novo activation across a span of tissues.

Given the substantial clinically-active variability and multisystemic involvement associated with Turner Syndrome, traditional genetically therapeutic methods – such as growth hormone supplementation or hormone replacement therapy – are primarily remedial and fail to focus on the underlying deficiencies in the genome. This restriction has catalyzed a shift in research toward molecular-level interventions that are aimed at the compensation or restoration for X-linked genetic loss. Contemporary advances in genome-editing technologies, specifically the CRISPR/Cas9 system coupled with homology directed repair (HDR), have produced new potential for the targeted correction or chromosomal abnormalities. These evolving developments offer an auspicious framework for investigating gene restoration approaches in TS, with an emphasis in regions such as Xp22.3, which harbors critical growth and developmental genes like ShoxA. As follows, the next section of this review will explore current gene-editing methodologies and their potential congruity in the correction of the genetic basis of Turner Syndrome.

Focus

This research project investigates the gene-editing therapies to restore genomic loss in Turner Syndrome (TS), a chromosomal disorder which results from the complete or partial monosomy of the X-chromosome. My focus is on engineering the targeted repair and gene replacement techniques using the CRISPR/Cas9 complex in conjugation with the HDR pathway, as well as evaluating the mRNA delivery mechanism to mitigate the effects of monosomy/mosaicism; specifically, through the restoration of the ShoxA gene in the pseudoautosomal region of Xp22.3. I am hypothesizing that restoring gene function/mitigating the effects of genetic loss in Turners Syndrome through targeted genomic editing of the ShoxA genomic region (Xp22.3) using the CRISPR/Cas9 complex in conjunction with high-fidelity DNA repair pathways (HDR) and transient mRNA delivery will yield measurable phenotypic recovery at a cellular level and provide a foundation for in-vivo, therapeutic intervention.

Research Objectives

Methodologies

Utilization of advancements in gene-editing technologies, specifically the aggregated, regularly interspaced, short palindromic repeats of the CRISPR/Cas9 system, have introduced an unprecedented capacity to correct abnormalities in the chromosome at a molecular level. CRISPR/Cas9 employs a singular guide RNA (sgRNA) to govern the Cas9 endonuclease to a chosen complementary genomic site. Here, it induces a double-strand DNA break (DSB) (Zhang et al., 2019). Following the cleavage, endogenous DNA mechanisms dictate the editing outcome – either through non-homologous end- joining (NHEJ), which is intrinsically error-prone and frequently results in unexpected deletions or insertions (indels) (Lieber, 2010), or through homology-directed repair (HDR), which allows the enabling of precise gene replacement/correction using an exogenous DNA template (Integrated DNA Technologies, n.d.).

HDR functions as a precise route for addressing partial monosomy and genomic loss observed in Turner Syndrome (TS), as it facilitates accurate reintegration or the repair of deleted loci, such as the ShoxA region located on the Xp22.3 region. This approach aims to precisely correct partial monosomy, a strategy that has thus far remained primarily theoretical in aneuploid disorders. Although, the efficiency of HDR in somatic and mosaic cell populations remains limited, predominantly due to its dependency of the cell-cycle phase and competition with the rapid, but less accurate, NHEJ pathway (Chen et al., 2021). Consequently, research has shifted its emphasis to the optimization of delivery systems and distinct repair environments to enhance HDR fidelity and yield.

Amid the most effective delivery systems, transient mRNA-based delivery of Cas9 and sgRNA and components has portrayed high editing efficiency while minimizing the risk of genomic integration and immune activation (Mohanraju et al., 2022). This method facilitates rapid protein expression and degradation cycles, curbing the duration of nuclease activity; thereby, decreasing the likelihood of off- target cleavage phenomena. Emerging evidence utilizing mRNA or Cas9 ribonucleoprotein complexes have effectively accomplished multigene editing and stable chromosomal correction in pluripotent stem cell models (Gong et al., 2023). Analogous chromosomal editing approaches, such as targeted Y-chromosome elimination in trisomy 21 using CRISPR/Cas9 and snRNA systems (Li et al., 2017), notably underscore the feasibility of translating these approaches to X-linked chromosomal abnormalities like TS. Notwithstanding, perpetual challenges remain evident, counting the high frequency of indel formation in mosaic populations (Anastasiou et al., 2023) and the lower efficiency of large-vector systems including human artificial chromosomes (HACs) in pluripotent stem cells (Kazuki & Oshimura, 2018).

Based upon these developments, from the current literature, the following experimental design proposes a CRISPR/Cas9-mediated restoration of the ShoxA gene in the pseudoautosomal region (Xp22.3), utilizing HDR to achieve high-fidelity DNA repair and mRNA delivery to secure transient, high-fidelity expression. This conjugated system is calculated to serve as a proof-of-concept for genomic restoration methodologies suitable for to Turners Syndrome and alternative chromosomal disorders associated with segmental genetic loss.

Proposed Experiment

Aim 1: Model Preparation

Objective: Pair CRISPR/Cas9 complex with the HDR pathway to edit and restore absent X- chromosome sequences.

Method: Perform the knock-in of a ShoxA mimetic gene that is flanked by homology arms using HDR in mouse models induced/born with X monosomy (Yin et al., 2014) and prepare for scaling to human iPSCs derived from Turner Syndrome patient(s).

Aim 2: Compare HDR and mRNA Delivery to Rescue ShoxA Expression in Knockout Mice Cell Lines

Objective: Quantify the phenotypic rescue in ShoxA-deficient mouse cell lines using HDR-mediated gene knock-in and transient mRNA delivery.

Design: A controlled experiment with four cell groups:

A: Untreated ShoxA knockout (negative control)

B: Wild-type (positive control)

C: mRNA-transfected cells (Pardi et al., 2018)

D: HDR knock-in cells (Song et al., 2016)

Assays: Using qPCR, I will evaluate the gene expression, confirm genome integration with Sanger sequencing, and assess the protein’s expression through Western blot. For preliminary analysis, this research project will use an autologous blood sample provided by the principal investigator for proof- of-concept experimentation.

Aim 3: Rescue of ShoxA Expression in Human Cell Lines

Objective: Extend comparison of HDR and mRNA delivery to human iPSC-derived ShoxA-deficient cell lines. These iPSCs will be differentiated into ovarian tissue-like cell lines, allowing direct quantification of gene expression modifications in cell types that are physiologically relevant to Turner Syndrome reproductive dysfunction and pathology. Ovarian-derived iPSC lines were chosen due to their high sensitivity to SHOX dosage and their explicit involvement in the reproductive and endocrine phenotypes of TS. These models enable the quantification of both transcription correction and downstream hormonal pathway renewal following the gene-editing process.

Design: Same four experimental groups as Aim 2.

Assays: qPCR, Sanger sequencing, Western blot. An autologous blood sample from the principal investigator will be used for proof-of-concept experimentation (Takahashi & Yamanaka, 2006). By quantifying differential ShoxA expression and downstream gene activation in these ovarian cell lines, this experiment aims to constitute a molecular ground for translation into in-vivo and eventually human therapeutics.

Aim 4: Translate Laboratory Gene Therapy into an Operable Clinical Approach for Turners Syndrome

Objective: Construct a protocol to deliver the ShoxA copies via intravenous injection (IV) of edited iPSCs in-vivo.

Operationalization Strategy: Begin with successful in-vitro editing and scale to in-vivo trials in murine model – eventually proposing early-phase human testing contingent on ethical and clinical review.

Long-Term Goal: Design a platform therapy for other chromosomal abnormalities who are characterized by regional deletions (Doudna and Charpentier, 2014).

Discussion

This research project utilizes a first-of-its-kind engineering framework, combining the CRISPR/HDR and translational mRNA methodologies, to provide phenotypic rescue in Turner Syndrome patients. Rather an applying silencing techniques that are novel strategies used in genetic knock-in technology (i.e. hormonal or symptomatic treatment), we are directly addressing gene loss at the root – targeting the genomic insufficiency that is responsible for many of the syndrome’s systematic manifestations.

By leveraging the precision of homology-directed repair (HDR) in conjunction with the transient nature of mRNA-based Cas9 delivery, this dual-approach aims to restore ShoxA gene expression with minimized genomic risk and increased accuracy. HDR embodies a highly refined correction mechanism, permitting targeted sequence replacement through homologous donor templates (Integrated DNA Technologies, n.d.). This methodology is distinctly advantageous for partial monosomy detected in Turners Syndrome, where the absence of pivotal pseudoautosomal genes, like ShoxA, disrupt endocrine balance and skeletal growth (Anastasiou et al., 2023). However, aside from its higher precision, HDR’s reliance on the S/G2 phase of the cell cycle and its competition with the faster but increasingly error-prone NHEJ pathway (Lieber, 2010), presents eminent efficiency objection.

The mRNA-based delivery system produces a complementary advantage by facilitating the transient Cas9 and sgRNA expression; thus, reducing the risks of randomized genomic integration and long- term nuclease persistence (Mohanraju et al., 2022). Past and current literature has demonstrated that Cas9 ribonucleoprotein or mRNA delivery promotes fidelity in gene-editing, while minimizing the activation of the immune system and off-target-cleavage (Gong et al., 2023). Therefore, the coalescence of HDR precision and mRNA’s temporal control produces potential for attaining high- fidelity correction in models of X-linked gene loss, while maintaining cellular integrity and restricting undesirable mutational results.

Tantamount chromosomal-editing initiatives, such as CRISPR/Cas9-mediated Y-chromosome elimination in trisomy 21 pluripotent stem cells (Li et al., 2017), reinforces the translational viability of adjacent interventions in X-chromosome monosomy. This body of research further confirms that large-scale chromosomal manipulation and discriminatory sequence restoration are achievable with modernized gene-editing systems. Furthermore, while larger genomic constructs like human artificial chromosomes (HACs) have displayed low transfer efficiency and destitute stability in human iPSCs (Kazuki & Oshimura, 2018), the modularity of CRISPR/HDR complexes allow for a more adaptable and direct method of gene correction and recovery.

Though this potential is evident, persistent challenges remain. The mosaicism inherent to Turner Syndrome suggests variability in the proportion of cells that undergo successful editing, leading to incomplete correction results or accidental indel formation (Anastasiou et al., 2023). Accordingly, optimizing HDR efficiency through the synchronized editing environments and well-defined delivery will be keystone for ensuring consistent phenotypic outcomes for a chosen gene. Evolving advancements in delivery systems, including lipid nanoparticle (LNP) or viral vector adaptation for Cas9 mRNA, may strengthen the therapeutic applicability of this approach.

From an immunological and ethical perspective, chromosomal correction through a CRISPR/HDR dual-approach raises unique considerations. Ethically, interventions must uphold a strict boundary between germline and somatic applications, as heritable edits could carry prevalent biological and societal implications. Immunologically, transient mRNA delivery mitigates continued nuclease exposure immune system activation but still requires careful dosage control to avoid accidental epigenetic effects or cytotoxicity. While the practicality of chromosomal-scale restoration poses extraordinary curative potential, it mandates vigilant oversight to ensure that such interventions are conducted with equity, transparency, and safety as guiding principles.

Conclusion and Future Works

In the final analysis, this investigation expands beyond the scope of Turner Syndrome. The coupled CRISPR/HDR and mRNA system proposed in this review represents a platform technology capable of addressing a spectrum of chromosomal deletion disorders characterized by segmental loss or partial monosomy. By developing a framework that prioritizes repair with the highest fidelity repair, transient nuclease activity, and cellular compatibility, this research project offers a scalable model for genomic restoration in genetically complex conditions. Proper execution of this system could lay the groundwork for future in-vivo therapeutic applications, implementing a path toward curative genome editing in chromosomal disorders previously thought to be inaccessible to molecular correction.

Bibliography