Billions of people suffer from androgenetic alopecia, alopecia areata, telogen effluvium, or hair loss caused by chemotherapy. Unfortunately, effective treatments targeting the underlying causes of these conditions—as well as age-related hair changes—remain limited.
Arguably, the most effective approach to hair restoration would be to reactivate hair follicles that have undergone miniaturization or damage, inducing them into a new anagen (growth) phase and maintaining their normal cyclical function. This could potentially be achieved by intervening at the genetic level through genetic engineering. As our understanding of hair follicle disorders advances, many experts believe that “gene therapies” could be designed for this purpose. Through targeted gene repair, both physicians and patients hope to ultimately win the “battle” against androgenetic alopecia.
Genetic engineering (gene therapy) involves manipulating specific genes that regulate hair follicle physiology, enabling the follicles to express desirable traits.
Candidate genes for such genetic interventions may include those that initiate the anagen phase, genes that prolong or maintain the follicle in the anagen stage, activating genes that increase follicle size, or inhibitory genes that limit the transition from anagen to catagen (regression phase). Moreover, in cases of complete follicle loss, inducing the formation of entirely new hair follicles—similar to processes observed during embryogenesis—could be invaluable.
While these possibilities sound promising, in reality, they currently remain a concept of science fiction, at least given our present knowledge and capabilities in the field of genetic engineering.
What is Genetic Engineering?
Genetic engineering is the technique of replacing genes within adult cells with the goal of inducing changes in cellular function. It is a technology still in its early stages of development, with very few successful applications to date. Specifically, the vast majority (96%) of ongoing genetic engineering studies remain in Phase I or II clinical trials.
Given the hope expressed by many patients and physicians that this branch of medicine may soon provide a solution for androgenetic alopecia (AGA), it is worthwhile to explore the theoretical potential of genetic engineering as a future treatment option for AGA and other hair follicle disorders.
Why Target the Hair Follicle?
The hair follicle is theoretically an ideal target for genetic engineering, both for treating dermatological disorders and systemic diseases. It is easily accessible and represents a visible, “safe” target, allowing for intervention without causing damage to the surrounding skin.
Delivering active agents to specific sites within the pilosebaceous unit opens up opportunities not only for addressing hair follicle-related conditions but also for broader applications such as gene therapy, immunotherapy, and systemic delivery of therapeutic substances.
Monogenic, recessive skin diseases such as epidermolysis bullosa, epidermolytic hyperkeratosis, lamellar ichthyosis, and X-linked ichthyosis have shown promising responses to recent advances in genetic engineering, suggesting it may play a role in their treatment.
Targeted cutaneous gene therapy may also be utilized for systemic treatments like passive immunization and vaccination. Studies by Shi et al. and Fan et al. demonstrated immune responses in mouse skin to proteins encoded by topically applied naked DNA. These findings highlight that local approaches can effectively target the skin’s dendritic antigen-presenting cells.
Additional systemic applications of skin gene therapy include treatments for hemophilia and growth hormone deficiency, leveraging epidermal keratinocytes as synthetic secretory cells.
However, the application of genetic engineering to treat androgenetic alopecia (AGA) remains an entirely different and far more complex challenge.
Technical Challenges
Genetic engineering is an incredibly promising scientific field, at least in theory. However, in laboratory reality, numerous technical challenges remain unresolved before successful gene therapy for conditions such as androgenetic alopecia (AGA) can be achieved. For its eventual clinical application, many obstacles must be overcome, and the safety of any intervention must be rigorously established.
In general, effective gene therapy involves the following stages:
- Identification of the target gene(s)
- Isolation of the target gene(s)
- Targeting the organ’s stem cells
- Delivery of the gene(s) to the stem cells
- Transfer of the gene(s) into the cell nucleus
- Integration of the gene(s) into the DNA
- Ensuring predictable therapeutic outcomes
- Ensuring prolonged therapeutic effect
- Ensuring long-term safety
Below, we will examine in detail the technical and functional requirements of each stage, both in general for genetic engineering and specifically in relation to the hair follicle, where data is available. The most important takeaway is that all these components are equally critical and indispensable — none can be left unresolved.
The “equation” of gene therapy is essentially a product: if any factor is missing (i.e., equals zero), the entire product collapses.
This can be illustrated as a “chain”: if any link is missing, the entire chain fails. The stages in genetic engineering, step by step, are as follows:
Gene Identification & Isolation in Genetic Engineering
These issues are addressed in the section “Etiology of Androgenetic Alopecia.” Briefly, it is important to remember that we do not yet know which or how many genes need to be targeted in order to reverse or prevent androgenetic alopecia (AGA). We understand that AGA is a polygenic condition, but the full range of causative genes—or even the key ones—remain unidentified..
Targeting the Organ’s Cells
Targeting the appropriate cells is critical for the success of gene therapy. If mature cells are targeted, any benefits are nullified, as these cells wear out and are replaced by new cells that carry the original DNA. To ensure a long-lasting effect, it is essential to target the stem cells of the respective organ. When targeting is successful, the modified stem cells subsequently produce altered transient amplifying cells, which in turn generate specialized cells permanently expressing the desired new traits.
Hair follicle stem cells have the capacity to produce most of the cells that compose the human epidermis. However, there are four cell populations that are equally likely to serve as the ideal target for gene introduction into the hair follicle. It is also possible that a combination of targeting strategies may be necessary. The populations of hair follicle stem cells reside in the following anatomical areas:
- The sebaceous gland
- The follicular infundibulum
- The bulge region
- The dermal papilla
At the same time, the hair follicle is perhaps the most metabolically active organ in the human body, exhibiting the fastest proliferation rate of any tissue. The continuous proliferation and development of the epidermis and hair follicle represent both an advantage and a challenge for genetic engineering. Viruses, which are commonly used as vectors in gene therapy, require cell proliferation to express the transgene and integrate it into the genome.
However, the cells in the epidermis and hair follicle responsible for intense proliferative activity (transient amplifying cells) have a short lifespan. Moreover, since stem cells divide infrequently, targeting these cells poses a significant challenge and requires a thorough understanding of their proliferative behavior. Another issue is that epidermal stem cells are few in number and divide slowly; therefore, they need to be “stimulated” to begin proliferating.
This stimulation can be achieved through dermabrasion, which removes the interfollicular epidermis and triggers rapid regeneration from the stem cells, as demonstrated in mouse experiments.
Genetic Engineering: Gene Delivery & Cell Proliferation
The most common method for delivering and introducing desired genes into target cells involves the use of viruses. Viruses possess the unique ability to recognize specific cells, infect them, insert their genetic material into the nucleus, and ultimately integrate it into the host cell’s genome.
After infection by a typical virus, the host produces proteins directed by the viral DNA, leading to the expression of various diseases. Most genetic engineering studies rely on retroviruses for integrating desired genes into cells, while alternative vectors include adenoviruses, adeno-associated viruses, lentiviruses, poxviruses, and herpesviruses. Viruses can be directly injected at the site of stem cells (in vivo method), or stem cells can be cultured, genetically modified by viruses in vitro, and then reintroduced into the body (ex vivo method).
Viruses used as gene delivery vectors for hair follicles must ensure:
- High transduction efficiency in both dividing and non-dividing cells
- Targeting of the appropriate cells
- Stable and regulatable expression of the transgene
- Minimal immune response
- Zero mutations in the endogenous DNA due to integration
- Easy, repeatable, and cost-effective large-scale virus production
In gene therapy, the viral DNA must first be attenuated to prevent disease, while retaining its ability to deliver DNA into target cells. The desired genes are “cloned” into the viral genome, and the viruses then introduce this genetic material into the target cells of the hair follicle.
Gene Delivery to the Hair Follicle
Recombinant genes are large and polar molecules that must be precisely “guided” to the nucleus of the target cell. Conventional drug administration methods, such as oral intake, are unsuitable for genetic engineering because the gastrointestinal tract resists the absorption of recombinant genes, proteins, and other complex molecules, breaking them down into smaller units (amino acids, monosaccharides, etc.) which lack therapeutic value.
Similarly, intravenous or subcutaneous injections cannot specifically target hair follicles and carry the risk of unintended integration of recombinant genes into other organs, including germline cells (gametes).
Transdermal delivery emerges as a promising solution for delivering recombinant genes to hair follicles. Transdermal administration offers significant advantages over traditional drug delivery methods, such as oral administration, which suffers from poor macromolecular bioavailability, or injections, which cause pain, risk accidental needle injury to the operator, and potential side effects due to transient high serum concentrations (bolus) and systemic infection.
Additional benefits of transdermal delivery include easy skin accessibility, a large application surface area, precise targeting (only the scalp), and the possibility of sustained release, since the skin can act as a reservoir. These advantages, combined with the rapid advancement of innovative macromolecular drugs, have driven significant progress in transdermal device development over the last decade.
However, major challenges remain in delivering genes to human epidermal cells, as the skin and stratum corneum have evolved to act as formidable barriers against entry of any substances or organisms—including viruses—into the body. The stratum corneum is penetrated by several epidermal appendages, whose roles in transdermal absorption were unclear until recently.
Studies over the past 20 years have shown that the hair follicle channel is an effective route for substances to penetrate the skin or the dermis itself. Historically, hair follicles were considered insignificant due to their occupying less than 0.1% of the skin surface. However, in 2011 this view shifted dramatically, when Lademann et al. demonstrated via clever experiments that selectively blocking hair follicle openings greatly reduced absorption, proving that hair follicle pores represent the primary route for absorption in the human scalp (appendageal route).
The hair follicle infundibulum is another key research focus: its deeper region forms an incomplete barrier with increased permeability, is densely populated by immune cells, and is surrounded by an extensive capillary network. The proximity to capillaries allows rapid systemic transport of substances, while the high immune cell density suggests hair follicles as potential local targets for systemic vaccination. These characteristics make the follicle a promising target for genetic engineering
Technologies for Enhancing Transdermal Gene Delivery
Numerous devices have been developed to enhance transdermal absorption, all of which could potentially be employed to facilitate gene penetration into human skin. These devices fall into two broad categories based on whether external energy is applied:
- Passive Methods
Passive techniques include chemical enhancers, emulsions, lipids, and biological methods such as peptide use. However, these are generally insufficient for delivering genes through intact skin. Various plasmid DNA delivery systems have been recently investigated for the pilosebaceous unit, including niosomes, microspheres, nanoparticles, nanoemulsions, and lipid nanocarriers. Polyethylenimine-DNA complexes were successfully used by Jan et al. (2012) for in vitro transfer of human telomerase reverse transcriptase into hair follicles.
Liposomes (lipoplexes) are also classified as passive carriers and have favorable attributes for local delivery: they are composed of naturally occurring human body components (lipids, sterols), are biodegradable, non-toxic, and non-allergenic. Lipoplexes have been used frequently for in vitro transfection of cells with plasmid DNA and may be effective in vivo. Factors influencing their efficacy include liposome/DNA ratio, absolute concentrations, and liposome composition.
- Active Methods
Active methods enhance transdermal transport by applying external forces or disrupting the skin barrier. Growing academic and industrial research efforts focus on active transdermal delivery devices, including microneedles, jet injectors, iontophoresis, electroporation, thermal energy, ultrasound, powder injection, keratin layer removal (ablation), tape stripping, biolistic injectors, and gene guns. These technologies offer promising means for the delivery of recombinant genes into the skin and hair follicles.
Integration of Genes into Hair Follicle DNA
Once a gene is selected based on its therapeutic potential, it must be effectively integrated into the DNA of hair follicle stem cells. The expression levels of a gene can be enhanced by increasing cellular uptake of the DNA and its subsequent transport to the nucleus.
Current research in targeted gene repair focuses on the use of single-stranded oligonucleotides (ssODNs) to induce genomic and phenotypic changes in experimental models. The gene repair efficiency achieved using modified ssODNs varies widely, ranging from less than 1% to over 40% depending on the target cell populations. These variations depend on the specific gene and clone targeted, ssODN modifications, length, cell type, and the experimental protocols employed.
Approaches aimed at stabilizing and increasing gene repair frequency include the induction of DNA breaks, chromatin remodeling, manipulation of the cell cycle, and inhibition of proteins known to suppress the gene repair reaction. Many researchers have attempted to develop therapeutic gene delivery systems for hair follicles using in vivo or ex vivo methods:
1. In Vivo Approaches
In vivo strategies involve direct gene delivery to intact skin and integration via viral or non-viral techniques:
- Viral in vivo methods: Plasmids or viral vectors deliver genes directly to hair follicle keratinocytes. This can be done through topical application of lipoplexed DNA or intradermal injection of viral carriers. Recombinant retroviral vectors enable long-term transgene expression due to their high efficiency in integrating genes into host DNA.
- Non-viral in vivo methods: Include direct injection of plasmid DNA or RNA:DNA oligonucleotides (RDO), particle bombardment, and topical application of naked DNA. However, non-viral methods have been shown to produce only transient gene expression. Key challenges of in vivo methods include transient gene expression, lower gene transfer efficiency compared to ex vivo methods, and potential immune responses against viral vectors.
2. Ex Vivo Approaches
Ex vivo methods start with the removal of cells from their natural environment, followed by in vitro infection with a viral vector, and then reintroduction into the recipient. This technique has higher potential for long-term gene expression because keratinocyte stem cells are precisely targeted and manipulated. However, it is more technically demanding and raises functional challenges related to stem cell transplantation, similar to hair follicle “cloning.”
Advantages of ex vivo strategies include:
- Ability to expand cell numbers in vitro
- Higher infection efficiency achieved in vitro
- Ability to screen for mutated cells before implantation
- Avoidance of immune reactions to viral vectors, since reintroduced cells do not produce viral proteins
Disadvantages include:
- Need for cell culture
- Loss of tissue architecture
- Possible alteration of cell phenotype
- Requirement for invasive transplantation, which may cause local scarring, injury, and ultimately hair loss
Predictable Outcome
At the experimental level, gene introduction into hair follicle cells was initially achieved by Li et al. (1993). Following their publication, numerous studies explored the possibility of genetically modifying hair follicles both in vitro and in vivo. More specifically, Cotsarelis et al. demonstrated that topical application of liposomes containing genes during the onset of the anagen phase was critical for the successful transfection of mouse and human hair follicles.
Alexeev et al. showed that an RNA-DNA oligonucleotide corrected a point mutation in the tyrosinase gene in mouse hair follicles, restoring melanin synthesis in vivo, albeit with very low efficiency. Zhao et al. demonstrated that skin explants from albino mice infected with the pLme/SN retrovirus produced melanin pigments in hair bulbs and shafts, indicating that the tyrosinase gene carrier was successfully transferred and expressed in melanocytes.
In a study by Saito et al., an effective technique was developed for genetic modification of hair shaft development using collagen to enhance gene transfer into hair follicles. This was possibly caused by partial digestion of the dermis, which exposed the follicle more fully to the GFP-adenovirus. However, none of these experiments have been conducted in vivo in humans, and to date, no clinical trials of genetic engineering targeting hair follicle disorders have been published or even planned.
The feasibility of applying the positive experimental results obtained thus far to human skin remains uncertain due to significant differences between the skin and hair follicles of laboratory animals and humans.
The main differences include:
- Mouse hair follicles have synchronized cycles during the first 2–3 months of life and, in the absence of proliferative stimuli, mainly reside in the telogen phase. In normal human scalp, 80–90% of follicles are in the anagen phase, about 10% are in telogen, and a small percentage (5–10%) are entering catagen or anagen.
- Human epidermis is thicker, human hair follicles are much larger, and the characteristics of dendritic epidermal cells differ from those of mice. Specifically, human scalp hair follicles can reach lengths of up to 5 mm compared to only about 1 mm in mice. The vibrissae follicles studied extensively are larger than typical pelage follicles and somewhat resemble human follicles, but they are highly unusual in structure and hair cycle properties.
Until extensive in vivo studies in humans are performed, the effectiveness and replicability of the experimental data remain uncertain.
Prolonged Action
In 1993 and later in 1995, pioneering studies by Li and Crystal respectively demonstrated that selective gene therapy targeting hair follicles in experimental animals is feasible by specifically targeting the matrix cells of the hair follicle with lac-Z reporter genes encapsulated in liposomes. In subsequent experiments using human skin grafted onto immunodeficient mice, the anagen hair follicle was identified as the primary target for gene transduction.
These results laid the foundation for the work of Alexeev et al., in which a chimeric oligonucleotide was used to correct the albino point mutation in the tyrosinase gene in mice. This correction heritably restored tyrosinase activity and subsequent melanin synthesis, resulting in the production of pigmented hairs in previously albino mice. The localized gene correction was maintained for three months, suggesting permanent modification of melanocyte precursors. The same research group had previously published an in vitro study showing that the identical chimeric oligonucleotide corrected the albino mutation in melanocyte cultures, again resulting in heritable melanin production.
In an in vivo study by Hoffman et al., the same chimeric oligonucleotide was administered via liposomes and intradermal injection in experimental animals. When the fluorescently labeled chimeric oligonucleotide was applied topically in a liposome formulation, fluorescence was primarily detected in hair follicles and the epidermis. Intradermal injection led to more effective but less targeted delivery, with fluorescent dye detected in both the skin and hair follicles.
These experimental data suggest that prolonged action is achievable in experimental animals and may also be possible in human hair follicles.
Short-term and Long-term Safety
For a gene therapy outcome to be considered valuable, it must be not only predictable but also safe in both the short and long term. The topical application of liposomes for targeting hair follicles has been shown, from the initial studies by Li et al., to be a safe procedure. A subsequent study by Domashenko et al. demonstrated that gene uptake following topical administration of a liposome-DNA mixture in skin xenografts of experimental animals could be optimized by appropriate liposome composition and skin preparation using depilation and retinoic acid, aiming to increase the number of anagen hair follicles.
In this experiment, “infected” cells were found only in hair follicles at the early anagen stage, while follicles in telogen, catagen, or full anagen stages were not infected. The infection efficiency with depilation and retinoic acid treatment was about 10±6% of the total number of hair follicles in the xenograft. Sato et al. used intradermal injection to introduce plasmid DNA or viral vectors into the dermal papilla of mouse scalp.
An adenoviral vector was used to deliver DNA into the skin of newborn mice. In mice receiving intradermal injection, rapid entry into the anagen phase occurred, along with increased hair follicle size and melanin production. The researchers concluded that transient, localized overexpression of SHh in the skin promotes entry into anagen and thus acts as a “biological switch.” Moreover, six months after SHh injection, the histological appearance of the skin remained normal, with no signs of pathological abnormalities such as hair follicle tumors or basal cell carcinoma associated with SHh structural overexpression during embryogenesis.
However, success in these genetic engineering experiments does not guarantee that application in humans is free of complications, and improper application has even led to human fatalities.
One major risk of genetic engineering is incorrect gene integration, which can cause harmful mutations and potentially lead to cancer. Additionally, viral vectors used for gene transfer and integration may infect healthy cells with their original viral properties, leading to unpredictable consequences.
Another concern is the inadvertent infection of the patient’s reproductive cells by transferred genes, potentially causing germline transmission to offspring. Other significant concerns include gene overexpression with harmful effects on the host, transmission of viral vectors from the host to the community, and induction of autoimmune diseases in response to viral vector or gene introduction. This last complication has already cost the lives of two adolescent volunteers, Jesse Gelsinger and Jolee Mohr, in gene therapy clinical trials and has been associated with leukemia in three children treated experimentally for X-linked severe combined immunodeficiency (X-SCID).
Unfortunately, truly safe gene therapies have yet to be developed. Currently, the FDA’s guidelines recommend gene therapy only as a last resort after failure of all other treatments and only to address severe and potentially fatal diseases.
Conclusions
Despite the scientific questions and ethical dilemmas arising from the application of gene therapy, the field will continue to develop, with a well-founded intuition that it will transform medicine more radically than any previous medical breakthrough—provided, of course, that patient safety is ensured and ethical standards are upheld.
Safety concerns related to genetic engineering experiments have indeed caused the initial enthusiasm for planning and approving studies to subside since 2009, with fewer gene therapy clinical trials being scheduled and approved. Furthermore, the entirety of gene research receiving essential government funding focuses exclusively on severe, life-threatening diseases and rare monogenic disorders for which developing new pharmacological treatments is not economically viable (orphan drugs). There is currently no provision for funding gene engineering research for any hair follicle diseases by governmental institutions or private companies worldwide.
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